Download Detector Development and Test Facility Commissioning for SuperCDMS Joseph Fox by

Detector Development and Test Facility Commissioning for
SuperCDMS
by
Joseph Fox
A thesis submitted to the
Department of Physics, Engineering Physics and Astronomy
in conformity with the requirements for
the degree of Master of Applied Science
Queen’s University
Kingston, Ontario, Canada
May, 2011
c Joseph Fox, 2011
Copyright Abstract
SuperCDMS, the next stage of the Cryogenic Dark Matter Search (CDMS), uses cylindrical germanium crystals as particle detectors to measure phonon and ionization signals resulting from particle
interactions. The aim of CDMS is to identify and measure interactions from dark matter particles
(WIMPs). Phonons produced during a particle interaction are absorbed by sensors on the detector
surface and are measured through the change in the sensors’ temperature dependent resistance.
Electrodes on the detector surface create an electric field causing charges released during an interaction to drift through the detector and produce an ionization signal. Surface events, which are
interactions that occur within a few µm from the electrodes, cause a reduced ionization signal due
to diffusion of some of the initially hot charge carriers into the electrode. Because the ability of
CDMS to discriminate between a WIMP interaction and background radiation is based on the ratio
of phonon to ionization energies, surface events cause a signal similar to a WIMP interaction and
are currently the largest source of background.
A detector test facility at Queen’s University has been commissioned to characterize detectors
and test new detector technology. Multiple detectors have been characterized and many tungsten
samples have been measured. Two sets of experiments were performed to test new detector designs.
To possibly reduce surface events, an insulating layer was deposited on a germanium detector beneath the electrode to prevent back diffusion of charge into the electrode. To possibly simplify the
phonon sensor production process, different cryogenic glues were used to attach silicon wafers with
a tungsten film to the crystal surface and phonon propagation through these glues was measured.
The most effective cryogenic glue for coupling tungsten samples to CDMS detectors was found to
be Araldite epoxy. Both experiments were successful at measuring interactions. Energy calibrations
were performed on both charge and phonon sensors. Further research is required to determine the
success of reducing surface events with an insulating layer.
i
Acknowledgements
I must first thank my parents. No matter the circumstances, their devotion, commitment and
support to my education has been unwavering. This thesis was not an attempt to gain a degree nor
to advance my career. It was my hope to make a contribution to our understanding of the universe
and to obtain a better understanding for myself. My parents understood this well and have never
turned their backs when I needed help or assistance.
I am extremely indebted to Dr. Wolfgang Rau. Being one of his first graduate students while
starting the CDMS collaboration at Queen’s Universty proved to be a lot of work for him, yet he
was always there to help. I’ll never forget all the times we had to stay in Stirling until 1 AM closing
up the cryostat or making sure the quick cool loop was working. This thesis required skills in many
different areas including electronics, cryogenics, using specialized CDMS hardware and software,
data analysis, hardware design and many more things. It also required knowledge in the areas of
cosmology, particle physics, solid state physics, low temperature physics... He helped teach me everything and was there to help me with every step along the way.
I need to thank Shuo Liu for all the work we did together in building the test facility at Queen’s.
Until we split off to focus on our own tasks to complete our theses, we spent just less than a year
working side by side on measuring light ouput from liquid scintillators, opening and closing the cryostat time after time, building the test facility and creating twisted pairs of old wiring by dancing
around the target room. I was lucky to have someone with me along the lengthy journey.
While still being the primary student responsible for the test facility, my job became a lot easier
when Carlos Martinez entered our group. It was a huge help to have a postdoc who was proficient
ii
in the field and was always there to lend a hand. I was also fortunate to find someone who I could
blame if anything ever went wrong. Thank you for everything.
Dr. Robert Knobel was extremely helful with the deposition experiment and HFl etching experiment. He was also an invaluable resource for our issues with the cryostat. So many small experiments required the use of his equipment and he was more than generous in helping me along the way.
I need to thank David Bearse for contribuiting so much to the test facility and setting up many of
my experiments. From building the cleanroom with Robert Gagnon, to teaching me how to solder,
to helping produce much of the custom hardware required at Queen’s, his help is greatly appreciated.
Chuck Hearns and Gary Contant have both helped immensely with designing and creating much
of the hardware required in the test facility.
I’d like to thank Dr. Geoff Lockwood for his help with the wirebonder, Dr. James Stotz for his
help with the glass cutter, Steve Gillen with his help in designing the wiring and connectors and Dr.
Gregory Jerkiewicz for his help with the surface profiler.
Many others helped along the way including Patrick Nadeau, Dr. Phillipe Di Stefano and other
graduate students and summer students with the CDMS collaboration. I couldn’t have done this
without any of you.
iii
Contents
Abstract
i
Acknowledgements
ii
Table of Contents
iv
List of Tables
x
List of Figures
xi
Glossary
xvi
1
Chapter 1 Introduction
1.1
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.2
Dark Matter Evidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
1.2.1
Galactic Rotation Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
1.2.1.1
Modified Newtonian Dynamics . . . . . . . . . . . . . . . . . . . . .
4
Gravitational Lensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.2.2.1
Bullet Cluster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
Cosmic Microwave Background . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Dark Matter Candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
1.3.1
MACHOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
1.3.2
Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
1.3.3
Axions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
1.3.4
WIMPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
Dark Matter Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
1.2.2
1.2.3
1.3
1.4
iv
1.4.1
Collider Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
1.4.2
Indirect Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
1.4.3
Direct Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
1.4.3.1
Nuclear Recoil Discrimination . . . . . . . . . . . . . . . . . . . . .
16
1.4.3.2
Detection Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
1.4.3.3
Phonon Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
1.4.3.4
Ionization Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
1.4.3.5
Scintillation Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
Direct Detection Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
1.4.4.1
CRESST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
1.4.4.2
Xenon 100 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
1.4.4.3
DEAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
1.4.4.4
PICASSO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
1.4.4.5
EDELWEISS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
1.4.4.6
DAMA/LIBRA . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
Detection Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
1.4.4
1.4.5
26
Chapter 2 The CDMS Experiment
2.1
CDMS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
2.1.1
26
Setup at Soudan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2
CDMS ZIP Detectors
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
2.3
Phonons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
2.3.1
QET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
2.3.2
Phonon Channel Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
2.4
Charge and Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
2.5
Cold Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
2.6
Reduced Charge Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
2.6.1
Surface Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
2.6.2
Detector Deneutralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
2.7
Latest Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
2.8
SuperCDMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
2.8.1
43
iZIP Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
v
44
Chapter 3 Queen's Test Facility
3.1
3.2
3.3
3.4
3.5
The Dilution Refrigerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
3.1.1
Classic Dilution Refrigerator . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
Dry Dilution Refrigerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
3.2.1
Pulse Tube Cooler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
3.2.2
Cooling Process and Mixture Circulation . . . . . . . . . . . . . . . . . . . .
47
Cryostat Commissioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
3.3.1
Thermometry calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
3.3.2
Cooling Power Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
3.3.3
Grounding Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
3.3.4
Pulse Tube Cooler Noise Tests . . . . . . . . . . . . . . . . . . . . . . . . . .
56
Warm Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
3.4.1
Breakout Box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
3.4.2
DCRC board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
Unique Properties of Queen’s Test Facility . . . . . . . . . . . . . . . . . . . . . . . .
57
3.5.1
Cold Hardware Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
58
3.5.2
Cryostat Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
58
3.5.2.1
Four Wire Measurement . . . . . . . . . . . . . . . . . . . . . . . . .
58
3.5.2.2
Resistance Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
3.5.2.3
Custom Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
3.5.3
Custom Designed Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
3.5.4
Contamination Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
3.5.4.1
Clean Room . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
3.5.4.2
Purge Cabinet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
68
Chapter 4 TES and Detector Testing
4.1
Detector Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
4.1.1
IbIs Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
69
4.1.2
Detector Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
4.2
SQUET Card Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
4.3
TES Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
4.3.1
76
Tungsten Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vi
4.3.2
TES Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
4.3.3
TES Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
Chapter 5 SiO 2 Deposition Experiment
80
5.1
Deposition System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
80
5.2
Slide Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
82
5.2.1
Deposition Method 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
5.2.2
Deposition Method 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
5.2.3
Deposition Method 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
84
5.2.4
Deposition Method 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
5.2.5
Deposition Method Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .
88
Deposition Thickness Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
5.3.1
Measurement Using Deposition Method 2 . . . . . . . . . . . . . . . . . . . .
90
5.3.2
Measurement Using Deposition Method 3 . . . . . . . . . . . . . . . . . . . .
92
5.3.3
Maximum SiO2 Thickness Measurement . . . . . . . . . . . . . . . . . . . . .
93
Germanium Crystal Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
5.4.1
Deposition Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
5.4.2
Test Depositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
5.4.3
Final Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
98
5.4.3.1
99
5.3
5.4
Corroding Surfaces
. . . . . . . . . . . . . . . . . . . . . . . . . . .
102
Chapter 6 Composite Phonon Detector
6.1
Composite Phonon Sensors in the CRESST Experiment . . . . . . . . . . . . . . . . 102
6.2
TES Chip Tc Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.3
Silicon Chip Detector
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
6.3.1
IbIs Measurements and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.3.2
Trace Data Measurement and Analysis . . . . . . . . . . . . . . . . . . . . . . 107
6.3.2.1
Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.3.2.2
Alpha Source Measurements . . . . . . . . . . . . . . . . . . . . . . 108
6.3.2.3
Gamma Source Measurements . . . . . . . . . . . . . . . . . . . . . 108
6.4
Cryogenic Glues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
6.5
Crystal Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
vii
114
Chapter 7 Custom Detector Experiment
7.1
Initial Difficulties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
7.2
Charge Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
7.3
7.2.1
Detector Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
7.2.2
Initial Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7.2.3
Charge Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.2.4
Energy Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
7.2.5
Deneutralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
7.2.6
241
7.2.7
Charge Results Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Am Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
Phonon measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
7.3.1
IbIs Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
7.3.2
Pulse Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
7.3.3
Phonon-Charge Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
7.3.4
Phonon Sensor Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
7.3.5
7.3.4.1
Phonon Sensor B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
7.3.4.2
Phonon Sensors C and D . . . . . . . . . . . . . . . . . . . . . . . . 135
Phonon Pulse Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
138
Chapter 8 Conclusion & Recommendations
8.1
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
8.2
Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
References
140
Appendices
146
Appendix A Deposition Procedure
147
A.1 HF Etch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
A.2 Germanium Crystal Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
Appendix B Custom Designed Hardware
149
viii
Appendix C IbIs Measurements
154
C.1 G31 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
C.2 S7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
Appendix D CDMS Detector Production Process
ix
159
List of Tables
3.1
Comparison between classic and dry dilution refrigerators . . . . . . . . . . . . . . .
50
3.2
Measurements of cooling powers at different temperatures . . . . . . . . . . . . . . .
54
4.1
Tc values for two CDMS detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
4.2
Shunt and Parasitic Resistance of SQUET cards ZC-17 and ZC-35. . . . . . . . . . .
75
4.3
A list of measured films and corresponding transition temperatures. . . . . . . . . .
79
5.1
Comparison and results of various deposition methods . . . . . . . . . . . . . . . . .
88
5.2
Measured thicknesses from deposition method 2. Error is defined by the standard
deviation about the mean. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
90
5.3
Measured thicknesses from deposition method 3 . . . . . . . . . . . . . . . . . . . . .
95
5.4
Measurement of the SiO2 layer with maximum thickness . . . . . . . . . . . . . . . .
96
6.1
Summary of chip properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
7.1
Calibrations obtained from histograms of charge signal . . . . . . . . . . . . . . . . . 122
7.2
A table of typical pulse shapes observed from the four different sensors. . . . . . . . 137
x
List of Figures
1.1
Rotation curve from galaxy NGC6503. . . . . . . . . . . . . . . . . . . . . . . . . . .
4
1.2
Examples of two µ(a/a0 ) terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.3
Mass distribution in the bullet cluster. . . . . . . . . . . . . . . . . . . . . . . . . . .
8
1.4
Power spectrum of the cosmic microwave background from WMAP . . . . . . . . . .
10
1.5
Neutron flux in various underground experimental laboratories. . . . . . . . . . . . .
16
1.6
Specific heat of various materials at cryogenic temperatures . . . . . . . . . . . . . .
18
1.7
Specific heat of germanium as a function of temperature . . . . . . . . . . . . . . . .
18
1.8
Schematic of a CRESST detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
1.9
The annual modulation curve observed by DAMA/LIBRA . . . . . . . . . . . . . . .
23
1.10 Exclusion curves for spin-independent interactions from the latest experiments. . . .
24
1.11 Exclusion curves for spin-dependent interactions from the latest experiments. . . . .
25
2.1
Electron vs. nuclear-recoil discrimination. . . . . . . . . . . . . . . . . . . . . . . . .
27
2.2
Experiment shielding of the CDMS experiment at Soudan, Minnesota. . . . . . . . .
28
2.3
CDMS ZIP detector mounted in the copper detector housing. . . . . . . . . . . . . .
29
2.4
Schematic of the layout of a CDMS detector. . . . . . . . . . . . . . . . . . . . . . .
30
2.5
Schematic of a transition curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
2.6
Detailed sketch of the sensors on a CDMS ZIP detector. . . . . . . . . . . . . . . . .
32
2.7
Schematic of the phonon readout electronics. . . . . . . . . . . . . . . . . . . . . . .
33
2.8
Schematic diagram of the ionization readout circuit. . . . . . . . . . . . . . . . . . .
34
2.9
The CDMS cold hardware.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
2.10 The DIB board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
2.11 The CDMS stripline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
2.12 Sketch of a surface event due to a back diffusion of charge. . . . . . . . . . . . . . . .
37
xi
2.13 Comparison between phonon pulse shapes due to bulk and surface events . . . . . .
38
2.14 Discrimination between different events using the timing parameter . . . . . . . . . .
39
2.15 Deneutralization of a detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
2.16 Latest results from CDMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
2.17 Visualization of the electric field caused by the interleaved electrodes. . . . . . . . .
43
3.1
Phase diagram of a 3 He-4 He mixture.
. . . . . . . . . . . . . . . . . . . . . . . . . .
45
3.2
Classic dilution refrigerator schematic . . . . . . . . . . . . . . . . . . . . . . . . . .
47
3.3
The liquid nitrogen cold trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
3.4
The cryostat at the QTF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
3.5
Factory calibration for the base temperature thermometer . . . . . . . . . . . . . . .
52
3.6
Calibration of mixing chamber thermometer using a cobalt thermometer. . . . . . .
53
3.7
Results from the cooling power tests . . . . . . . . . . . . . . . . . . . . . . . . . . .
54
3.8
Cross-calibration of cryostat temperature readout between a grounded and non-grounded
setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
The breakout box at the QTF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
3.10 The DCRC board connected to the breakout box. . . . . . . . . . . . . . . . . . . . .
58
3.11 The CDMS tower mounted to the base plate of the cryostat . . . . . . . . . . . . . .
59
3.12 Four wire measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
3.13 Resistance bridge and readout/control interface . . . . . . . . . . . . . . . . . . . . .
61
3.14 The first attempt at wiring in the cryostat . . . . . . . . . . . . . . . . . . . . . . . .
62
3.15 Connectors used for custom wiring. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
63
3.16 Cold plate heat sink used on custom wiring. . . . . . . . . . . . . . . . . . . . . . . .
64
3.17 Wiring readout channels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
3.18 Niobium wiring transition curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
3.19 The QTF cryostat mounted inside the cleanroom . . . . . . . . . . . . . . . . . . . .
67
4.1
Model curve overlaying the transition curves of multiple TES sensors in parallel. . .
69
4.2
Schematic of the TES biasing circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
4.3
A simulation of an IbIs curve for a phonon sensor . . . . . . . . . . . . . . . . . . . .
70
4.4
Raw data from an IbIs measurement of G31. . . . . . . . . . . . . . . . . . . . . . .
72
4.5
IbIs curves for G31 sensor A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
3.9
xii
4.6
Critical currents measured for detector G31 when going from normal resistance to
superconducting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
4.7
Four tungsten samples at the QTF. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
4.8
Two different sample holders for TES samples. . . . . . . . . . . . . . . . . . . . . .
78
4.9
Transition curve of a TES sensor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
5.1
The Thermionics Laboratory 3kW RCL linear e-gun ion beam evaporator . . . . . .
81
5.2
The second method of deposition described in three steps. . . . . . . . . . . . . . . .
85
5.3
The third method of deposition described in three steps. . . . . . . . . . . . . . . . .
86
5.4
The fourth method of deposition described in three steps. . . . . . . . . . . . . . . .
87
5.5
Dektak 8 Stylus Profiler. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
5.6
Scan position for layers deposited using method 2 . . . . . . . . . . . . . . . . . . . .
90
5.7
Two measured profiles of layers deposited on a glass slide using deposition method 2.
91
5.8
A crystal formation and its thickness measurement. . . . . . . . . . . . . . . . . . . .
92
5.9
Scratches and measurements performed on a glass slide using Deposition Method 3.
93
5.10 Thickness profile of a scratch through a 100 nm Al surface . . . . . . . . . . . . . . .
94
5.11 Deposition performed to measure the maximum thickness of SiO2 . . . . . . . . . . .
95
5.12 Sample scan of the maximum thickness measurement (scan 3) . . . . . . . . . . . . .
96
5.13 The holder used for deposition on the germanium crystal
. . . . . . . . . . . . . . .
97
5.14 Thickness of SiO2 layers deposited on the crystal. . . . . . . . . . . . . . . . . . . . .
98
5.15 The glass plate with the layer of SiO2 that flaked off the surface. . . . . . . . . . . .
99
5.16 The top side of the detector placed in the detector housing after the deposition. . . . 100
5.17 The detector surfaces after some aluminum has flaked off . . . . . . . . . . . . . . . 101
6.1
Transition curves of all four chips. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
6.2
Two TES chips placed inside a detector housing to be used as detectors. . . . . . . . 105
6.3
Combined IbIs plot for chip 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.4
IbIs trace for chip 1 with an alpha source. . . . . . . . . . . . . . . . . . . . . . . . . 106
6.5
Three types of noise observed in measurements of chip detectors . . . . . . . . . . . 107
6.6
A typical pulse caused by an alpha interaction with the TES. . . . . . . . . . . . . . 109
6.7
A histogram of the alpha energies based on the rail length of the trace. . . . . . . . . 110
6.8
A typical pulse from a gamma interaction. . . . . . . . . . . . . . . . . . . . . . . . . 111
xiii
6.9
Two histograms of gamma pulse energies based on pulse area and pulse height. . . . 111
6.10 The stripline cold ends with Hardman epoxy used as a reinforcement. . . . . . . . . 112
6.11 Bottom side of the detector after phonon sensors were glued . . . . . . . . . . . . . . 113
7.1
Charge pulse before and after being filtered . . . . . . . . . . . . . . . . . . . . . . . 116
7.2
Histogram of pulse heights from
7.3
Pulse height decay over 8 minutes for the half of the detector with 300 nm and 1000
137
Cs . . . . . . . . . . . . . . . . . . . . . . . . . . 117
nm of SiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
7.4
The template charge pulse used in this analysis. . . . . . . . . . . . . . . . . . . . . . 119
7.5
A 60 keV and an 8 keV pulse with a template fit . . . . . . . . . . . . . . . . . . . . 119
7.6
Charge signal calibration from a
137
7.7
Charge signal calibration from a
57
7.8
Calibration of voltage readout to charge signal energy . . . . . . . . . . . . . . . . . 122
7.9
Pulse height decay over 35 minutes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Cs source. . . . . . . . . . . . . . . . . . . . . . . 120
Co source. . . . . . . . . . . . . . . . . . . . . . . 121
7.10 Deneutralization of detectors over 30 minutes with ± 3V . . . . . . . . . . . . . . . . 124
7.11 Deneutralization of detectors over 30 minutes with ± 6V. . . . . . . . . . . . . . . . 125
7.12 A histogram of the pulse heights from a
241
Am source. . . . . . . . . . . . . . . . . . 126
7.13 Low energy charge peaks in CDMS detector T3Z2 . . . . . . . . . . . . . . . . . . . 127
7.14 Sample IbIs curve from sensor A of the custom detector taken at 63 mK . . . . . . . 129
7.15 Simulation of an IbIs curve for sensors with a very wide transition region. . . . . . . 130
7.16 Typical pulse shapes observed from the four phonon sensors. . . . . . . . . . . . . . 131
7.17 Analytic fit to a railing charge pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
7.18 Calibration of phonon energies for sensors A and B using charge energy. . . . . . . . 134
7.19 Comparison of sensors A and B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7.20 Comparison of sensors A and B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
7.21 Comparison of sensors A and D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
B.1 Heat sink for the custom wiring connector on the 4 K plate . . . . . . . . . . . . . . 149
B.2 Heat sink for the custom wiring connector on the base plate . . . . . . . . . . . . . . 150
B.3 Heat sink used for the custom wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
B.4 Plate for mounting chip detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
B.5 Holder for the germanium crystal deposition . . . . . . . . . . . . . . . . . . . . . . . 153
xiv
C.1 IbIs curves for G31 sensor A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
C.2 IbIs curves for G31 sensor B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
C.3 IbIs curves for G31 sensor C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
C.4 IbIs curves for G31 sensor D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
C.5 IbIs curves for S7 sensor A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
C.6 IbIs curves for S7 sensor B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
C.7 IbIs curves for S7 sensor C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
C.8 IbIs curves for S7 sensor D
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
D.1 First stage of the sensor production process. . . . . . . . . . . . . . . . . . . . . . . . 159
D.2 Second stage of the sensor production process. . . . . . . . . . . . . . . . . . . . . . 160
D.3 Third stage of the sensor production process. . . . . . . . . . . . . . . . . . . . . . . 160
D.4 Fourth stage of the sensor production process. . . . . . . . . . . . . . . . . . . . . . . 160
D.5 Fifth stage of the sensor production process. . . . . . . . . . . . . . . . . . . . . . . . 161
xv
Glossary
CDMS - Cryogenic Dark Matter Search
CMB - Cosmic Microwave Background
DCRC - Digital Control and Readout Card. Warm electronics for reading charge and phonon
signals from CDMS detectors.
DIB - Detector Interface Board. Component of CDMS cold hardware mounted in the detector
housing
IbIs - Ib (bias current) vs Is (sensor current). Used for measuring resistance of phonon sensors
ICM - Intracluster Medium. Gas which comprises the majority of the baryonic mass of a galaxy
cluster
MACHO - Massive Compact Halo Objects. Dead stars and large planets that can be found in the
galactic halo.
MOND - Modified Newtonian Dynamics. Possible alternative theory to dark matter to explain
galactic rotation curves
Tc - Transition temperature
TES - Transition Edge Sensor
QET - Quasiparticle-trap-assisted Electrothermal-feedback Transition-edge-sensors. Phonon sensor
configuration used in CDMS
QTF - Queen’s Test Facility
SQUET - SQUID and FET card used by CDMS to readout both the phonon and charge channels
SQUID - Superconducting Quantum Interference Device. Component of phonon channel readout
circuit
WIMP - Weakly Interacting Massive Particles. A leading dark matter candidate
ZIP - Z-sensitive Ionization and Phonon detector. Detectors used by CDMS
xvi
Chapter 1
Introduction
1.1
Overview
In chapter 1 of this thesis, the nature of dark matter and the rationale for dark matter detection experiments such as the Cryogenic Dark Matter Search (CDMS) will first be explained. It
will describe the evidence for the existence of dark matter and will outline the various dark matter
candidates that have been proposed. It will describe the basic principles of direct detection of dark
matter and will discuss several experiments which demonstrate different approaches to detecting
dark matter.
In chapter 2, the CDMS experiment and the underlying physics will be described in detail. The
detector technology and data analysis method will be outlined. Electronics, hardware, latest results
and future goals of the experiment will be reviewed.
In chapter 3, the CDMS test facility which has been commissioned at Queen’s University (Queen’s
Test Facility, QTF) will be described. The method in which the cryostat is used to cool detectors to
cryogenic temperatures will be explained and the unique properties of the QTF will be illustrated.
The cryostat was commissioned through various tests and results from these tests will be provided.
Custom hardware and wiring built for the cryostat will also be outlined.
1
The main goal of the test facility is to test and characterize future CDMS detectors before they
are used in the CDMS experiment. In chapter 5, detector tests performed at Queen’s University will
be reviewed and the success of the test facility to characterize detectors will be demonstrated. Test
results of both tungsten sensors (used in detector development) and CDMS hardware at Queen’s
University will be described.
A set of experiments performed to reduce background interactions due to surface events (explained in section 2.6.1) were completed. Chapter 6 describes a deposition experiment that was
performed to measure the capability of a thin SiO2 layer that was deposited on a crystal to insulate
and prevent charge drift into the electrode. Using an evaporator, many depositions on glass slides
were carried out to understand the deposition properties of the evaporator and to calibrate the deposition thickness guage. Results from these tests will be shown and the ability to successfully deposit
a thin insulating layer of SiO2 will be demonstrated. Experiments measuring deposition accuracy
and uniformity will also be described. The determination of which SiO2 thicknesses were deposited
on the germanium crystal will be explained. The success of the final deposition experiment on the
germanium detector will be reviewed as will problems faced during the process.
A simpler method of producing phonon sensors relative to the current method employed by
CDMS was tested and is described in chapter 7. Instead of a complicated method based on deposition, photolithography, and H2 O2 etching, tungsten films deposited on silicon chips were glued
to a germanium crystal to be used as phonon sensors. Initial characterization of these sensors will
be shown. A proof of concept experiment, using the sensors as detectors before being glued onto
the crystal, will be explained. Justifications for the various glues used to couple the sensors to the
detector will be given and the layout of the detector with the new phonon sensors will be shown.
Finally, experiments performed on the custom detector after the deposition and gluing of phonon
sensors will be described in chapter 8. Accurate calibrations of the charge signal will be demonstrated as well as detailed measurements of how the charge signal evolves over time. Calibration of
the phonon sensors will then be demonstrated using the previously found calibration of the charge
signal. The experiment resulted in finding the most effective glue for the CDMS experiment and
proved that detectors can operate with insulating layers deposited beneath the electrode.
2
1.2
Dark Matter Evidence
Evidence has been building since the 1930s pointing to a universe where approximately 85% of
the matter is non-luminous and cannot be accounted for in the standard model of particle physics
[24]. While theoretical models like supersymmetry and higher dimensions can explain observations,
none have been confirmed through experiment.
Currently, dark matter can be observed through its gravitational effects on galactic and extragalactic scales. Starting in the 1930s, Fritz Zwicky measured the masses of galaxies in the Coma
Cluster through the light output [62]. Then he measured the velocities and used the virial theorem to
calculate the mass of the cluster. The masses derived from the two estimates were off by more than
two orders of magnitude, indicating a large amount of mass not accounted for by the light observed
from the cluster. This anomaly is considered to be a key piece of evidence for the existence of dark
matter, but it did not receive much attention until the 1970s when Vera Rubin studied peripheral
star orbits of spiral galaxies.
1.2.1
Galactic Rotation Curves
The first conclusive evidence for dark matter comprising the majority of the mass within galactic halos came from studying rotational curves of spiral galaxies. Assuming that the majority of
the galactic mass was located in the visible disk, stars located outside the main disk should have
a decreasing rotation velocity as a function of radius to remain in stable orbits as the force from
Newtonian Gravity goes as 1/r2 . General Relativistic effects occur at high speeds and strong gravitational fields which are not relevant when studying stars in galactic halos. The variation of stellar
orbit speeds as a function of their distance can be used to probe the mass distribution of the galactic
halo. In the late 1960s and early 1970s, Vera Rubin studied edge-on spiral galaxies and found that
the velocity of stars did not reduce in the halo as predicted by Newtonian gravity [59]. An example
of a rotation curve can be seen in Figure 1.1.
3
Figure 1.1: Rotation curve from galaxy NGC6503 [6]. Dots with error bars correspond to observed
star orbital velocities as a function of radius from the centre of the galaxy. The two lines labeled
disk and gas are expected contributions to the rotational velocity from the baryonic matter located
in the disk and in the gas of the galaxy. The line labeled halo makes up for the difference between
the baryonic matter and the observed rotation. This is the expected contribution from the dark
matter. The solid line is a fit to the velocity profile using Modified Newtonian Dynamics (see
section 1.2.1.1).
1.2.1.1
Modified Newtonian Dynamics
Upon studying rotation curves, more unexplained problems arose. Newton’s laws are successful in
predicting rotation speeds up to approximately 50% of the optical radius of the galaxy. Deviations
from Newton’s laws appeared in the rotation curves at very low accelerations and the deviations
appeared to occur at a specific acceleration. Mordechai Milgrom published a paper in 1983 suggesting
that a modification of Newton’s laws (called Modified Newtonian Dynamics or MOND) could be
made at low accelerations which can explain galactic rotation curves [53]. He proposed including
a term in Newton’s Law of Gravity known as µ(a/a0 ) with the condition that µ(a/a0 >> 1) = 1.
a0 is a proposed constant of physics on the order of 2 × 10−10 m/s2 . Therefore at any accelerations
much greater than a0 , the equation is multiplied by 1 and the Newtonian case remains. For smaller
4
accelerations, µ(a/a0 << 1) = a/a0 returns the observed results in galaxy rotation curves. It is
proved by including the extra factor in Newton’s law of gravity:
GM m
r2
maµ(a/a0 ) =
(1.1)
Canceling m and substituting centripetal acceleration:
v2
GM
µ(a/a0 ) = 2
r
r
(1.2)
At accelerations normally observed, µ(a/a0 ) = 1 and Kepler’s Law is retrieved:
v2
GM
×1= 2
r
r r
GM
v=
r
(1.3)
(1.4)
Consequently, the MOND term renders Newton’s laws for regular accelerations.
At very small accelerations, which are only found at very large radii from the centre of the galaxy,
µ(a/a0 ) = a/a0 . Substituting this into Newton’s Law of Gravity:
GM
r2
GM
a2 = a0 2
r
a(a/a0 ) =
(1.5)
(1.6)
Substituting centripetal acceleration:
v4
GM
= a0 2
r2
r
(1.7)
v 4 = a0 GM
(1.8)
At very small accelerations, the velocity approaches a constant value independent of radius. This
result is observed through the flattening of rotation curves at large radii. Examples for µ(a/a0 ) include: µ(a/a0 ) =
1
1+a0 /a
or µ(a/a0 ) = √
1
.
1+(a0 /a)2
below (see Figure 1.2).
5
Examples of the µ(a/a0 ) function can be seen
Figure 1.2: Examples of two µ(a/a0 ) terms. Both functions approach 1 as a becomes large relative
to a0 and a/a0 for a << a0 .
From this modification to Newton’s law of gravity, the force of gravity does not affect orbital
rotation speed at very low accelerations. These accelerations are on the order of a0 , which are too
small to be measured on Earth or the solar system, but would affect stars in the galactic halo.
Generally, MOND has been successful at reproducing rotational curves of stars in the galactic
halo as seen above in Figure 1.1, but various issues did arise with MOND. If a0 is a constant of
nature, then it should be identical for all galaxies. However, the a0 value for different galaxies
varies. In addition, there is other evidence for dark matter which cannot be explained by MOND
(see section 1.2.2.1).
1.2.2
Gravitational Lensing
The ability of matter to alter space-time is a fundamental in general relativity. Because light
moves through space-time, the path of light is not necessarily a straight line in Cartesian space and
can be bent due to large gravitational fields. Gavitational lensing is the distortion of light from a
6
distant light source by a mass closer to the observer. The manner in which the light is deflected can
be used to study mass properties of the body that deflects the light, known as the lens. One of the
best known cases of gravitational lensing providing evidence for dark matter is the bullet cluster.
1.2.2.1
Bullet Cluster
The bullet cluster consists of two galaxy clusters which collided. Studies of the bullet cluster
which concluded in 2006 provided very strong evidence for dark matter [27]. Every galaxy cluster
has three important components: stars, gas (intracluster medium, ICM) and dark matter. When the
two clusters collide, the galaxies do not interact. Because their mass is so concentrated in relatively
small volumes (compared to that of the galaxy cluster), they pass by each other. The ICM is spread
throughout the cluster and would collide. All possible dark matter candidates (see section 1.3) are
either spread out and weakly interacting or concentrated into small volumes like stars. Either way,
they would be collisionless. The mass of the ICM is approximately 5 times the mass of the galaxies
excluding the dark matter [51]. Three studies have been performed on the bullet cluster in order to
obtain information about the mass distribution in the bullet cluster:
1. Using optical telescopes, the galaxy populations can be obtained and the mass of the cluster
can be estimated.
2. The ICM is very hot (between 107 and 108 K) and emits radiation in the x-ray spectrum. The
x-ray emission of the galaxy can be studied and the distribution of the ICM can be found.
3. A full mass profile of the entire system can be obtained through gravitational lensing and the
centre of mass of the two clusters can be found.
The majority of the mass was found at the position of the galaxies and not the ICM, despite the
fact that the ICM is much more massive than the galaxies. A detailed analysis of this system was
performed and the finding can be found in Figure 1.3.
All versions of MOND require that gravity comes from the centre of mass of the system. From
the bullet cluster, it is clear that the majority of mass in the system is not where the majority of
baryonic matter is located. Dark matter can cause the centre of mass to be displaced from the centre
of mass of the baryonic matter. It is difficult to reconcile MOND with this evidence.
7
Figure 1.3: Mass distribution in the buller cluster [27]. Two galaxy clusters can be seen on the left
and right side. The green spots which are labeled CM show the centre of mass of each galaxy
cluster based on gravitational lensing. The XR labeled spots are the centre of mass of the baryons
determined by x-ray observations of the ICM. Black lines, blue and red lines are various contour
maps of surface density. The XR and CM spots do not overlap to eight standard deviations.
1.2.3
Cosmic Microwave Background
Initially after the Big Bang, the universe was too hot to allow electrons and protons to combine
[7], which caused an opaque plasma that interacted with photons. Once the universe expanded and
cooled, the energy of the particles dropped below the ionization energy of hydrogen and the electrons
and protons formed hydrogen atoms. The universe then became transparent to the photons. After
billions of years, these photons became red-shifted with the expansion of the universe and can now
be observed in the microwave spectrum.
The universe was not completely smooth after the big bang and clumps of matter and dark
matter formed. Roughness is defined as the difference in density between the high density and low
density parts of the universe. As the universe expanded, the more dense areas would cause matter
to clump and the less dense areas would become voids, increasing the roughness. As matter clumps,
it gains energy from its gravitational potential and increases in temperature and pressure. This then
causes the dense gas to expand reducing the density within that area. The expansion of the baryonic
8
matter due to increase in pressure and temperature in higher density areas prevents roughness in the
universe from increasing. Dark matter also clumps but does not interact with itself, which prevents
any expansion and causes an increase of roughness. While the baryonic gas undergoes the process
of contracting and expanding, the gas in phase with the contracting dark matter will be hotter and
the gas out of phase with dark matter will be colder.
Once the universe cooled enough to allow formation of hydrogen atoms, known as recombination,
the photons experienced an enormous increase in scattering length from that with the electron-proton
plasma. First, the energy of matter in areas of low density decreased below the recombination energy, so the photons decoupled earlier. Photons from higher density areas decoupled later. As the
universe increased in size, photons red-shifted, but since the photons from the lower density areas
decoupled first, they have been red-shifted for a longer time and are currently at lower energy. All
photons released during recombination are in the microwave spectrum and are known as the cosmic
microwave background. The roughness of the early universe can be determined by studying the
variations in energy of the photons from the microwave background.
Many different experiments were performed to study the scales on which the anisotropy existed
and the magnitude of the temperature fluctuations. Results from the WMAP experiment can be
seen in Figure 1.4.
9
Figure 1.4: Power spectrum of the cosmic microwave background from WMAP [24]. The third
harmonic of the spectrum, which has a height similar to the second harmonic is predicted by
WIMP models. A universe without dark matter would have lower amplitude harmonics at higher
multiple moments.
The power spectrum is the measure of the scales on which the oscillations occur. If all variations
in the power spectrum occurred on really tiny scales and no large trends occurred on larger scales,
then there would be a lower value for anisotropy power at a low multipole moment and a higher
value at a larger multipole moment. From the magnitude and position of the peaks, information
can be attained about the amounts of baryonic matter in the universe, the amounts of non-baryonic
matter in the universe and the total density of the universe.
Consistent with other evidence for dark matter, the cosmic microwave background shows that
the non-baryonic matter outweighs the baryonic matter by a factor of about five. This mass is
significantly less than the critical density and only comprises about 27% of the universe [24]. The
rest of the universe is not in the form of matter and is known as dark energy [7].
10
1.3
Dark Matter Candidates
After understanding that most of the matter in the universe is not visible, various candidates
for dark matter arose from theories in the fields of cosmology and particle physics. By performing
experiments to test these theories, the dark matter problem can be solved and physics beyond the
standard model can be discovered.
1.3.1
MACHOs
One dark matter candidate is baryonic matter clumped in non-luminous bodies. These are known
as Massive Compact Halo Objects (MACHOs). Types of MACHOs include: black holes, neutron
stars, white dwarfs and brown dwarfs. All of these stars do not undergo fusion and do not radiate
light that can be easily detected. By having a large distribution of these objects in the halo, they
could account for the anomaly in galactic rotation curves.
Microlensing attempts to detect MACHOs by observing a light change from a distant star caused
by the MACHO acting as a lens. Microlensing experiments and evidence for non-baryonic dark
matter has ruled out the possibility of MACHOs comprising all the dark matter [56].
1.3.2
Neutrinos
Neutrinos are standard model particles that have been observed and have many qualities that
would be expected from a dark matter particle. Because neutrino flavour oscillations have been
demonstrated [38], neutrinos must have a mass. Neutrinos however, have been ruled out as the
dominant component of dark matter for two reasons:
1. The mass limits and known quantities that exist for neutrinos cannot account for the amount
of missing mass observed [7].
2. Neutrinos freeze out (stop interacting with baryonic matter) very early after the big bang.
The later the freeze out occurs, the lower the energy of the particles at the freeze out and
the more likely the particles will clump together and increase the roughness in the universe.
Because neutrinos freeze out at relativistic speeds, they would not clump together and there
would be very little roughness in the universe. Atoms would not have fallen into dark matter
11
halos and galaxies would have never formed. In short, neutrinos or hot dark matter candidates
in general, could not cause the roughness observed in the universe today. [57]
1.3.3
Axions
Axions are particles that were predicted in the 1970s as a possible solution to the strong CP
problem [55]. These particles would not freeze out at relitivistic speeds like neutrinos, have mass
and would have a low interaciton cross-section. Various experiments exist to attempt to detect
axions. One example is the Axion Dark Matter Experiment (ADMX), where a microwave cavity
with a strong magnetic field is used to attempt to detect axions [41]. If an axion were to cross into
the cavity, it could interact with the magnetic field and decay into photons. These photons can be
measured through an antenna.
1.3.4
WIMPs
Weakly Interacting Massive Particles (WIMPs) are currently the leading candidate for dark matter
particles [28]. From cosmological observations and theoretical models, many WIMP properties have
been deduced:
• They must have mass;
• They cannot interact via the strong or electromagnetic force or they would have already been
observed;
• Theoretical models predict that they interact through a weak force, either the weak nuclear
force or a force on a similar scale; [50] and
• They must have undergone a freeze-out at non relativistic speeds.[57]
Combining all known properties, multiple candidates have been found in various theoretical
models. These candidates all have the properties necessary to be defined as a WIMP and could
be detected through a weak force interaction. Multiple theories leading to physics beyond the
standard model have predicted a WIMP including higher dimensions [29] and supersymmetry [50].
Consequently, the existence of dark matter is strong evidence for physics beyond the standard model.
12
Supersymmetry has provided one of the most promising candidates for a WIMP, the neutralino
[50]. The interaction rate of the force carrier particles would be related to the size of the nucleus
(spin-independent) or the spin content of the nucleus (spin-dependent). For spin independent interactions, the interaction cross-section is proportional to the number of nucleons squared. Consequently,
heavier atoms are more likely to observe a WIMP interaction than light ones. For spin-dependent
interactions, the interaction cross-section is proportional to the spin content of the nucleus. The
dark matter detection experiments listed below are more sensitive to spin-independent interactions,
with the exception of PICASSO which is more sensitive to spin-dependent interactions.
1.4
Dark Matter Detection
All the evidence for dark matter so far comes from astronomy and cosmological observations, on
scales the size of the galaxy or larger. No conclusive experiments have been performed to confirm
the existence of a dark matter particle. The detection of dark matter particles would allow us to
estimate important properties such as interaction cross-section, its mass and its distribution in the
galaxy. Three different experimental methods can be performed in order to detect and study the
properties of a WIMP, should it exist: collider experiments (section 1.4.1), indirect detection (section
1.4.2) and direct detection (section 1.4.3).
1.4.1
Collider Experiments
In collider experiments, the goal would be to produce the dark matter particle in the collider.
Since the interactions occur at a rate on the order of one billion per second [30], WIMPs can be
created, even if they have a very low cross-section. If the energy of the collider is lower than what
is required to create the particle, then the particle would never be created. Once it can be created
while conserving all the laws of physics, then it will. Detectors placed around the area where the
interaction will occur can detect hadrons and leptons, but cannot detect a WIMP or a neutrino. If
a large energy and momentum deficit is found in an interaction and the result is repeatable, it is
strong evidence for a particle beyond the standard model. The mass can then be determined from
the energy and momentum deficit. However, in collider experiments a particle with a mass greater
than the beam energy cannot be observed and the stability of the particle cannot be determined.
13
1.4.2
Indirect Detection
Some models predict that the neutralino is a Majorana particle (it is its own anti-particle) [28].
Consequently, dark matter particles can interact with each other and annihilate, giving off two photons at a specific energy equal to their mass or any particle-antiparticle pair. This is highly unlikely
to happen in the galactic halo as the density is low, but dark matter could interact with baryonic
matter (e.g. the sun), lose energy and be gravitationally bound. Eventually, it will continue to
interact and spiral to the core of the sun where the dark matter particles would accumulate. There,
two dark matter particles could annihilate.
There are many benefits to indirect detection over direct detection (described in section 1.4.3).
Direct detection uses detectors on Earth, so if dark matter is very clumpy and the Earth is in a
volume with very a low relative density of dark matter, then interactions will not be found on Earth.
Experiments performed by looking into space are not confined to the flux of dark matter particles
through the Earth. An exact measurement of the mass of the WIMP can be determined from the
high energy particles emitted from the annihilation. Two examples of indirect detection experiments
are discussed below.
The IceCube experiment looks to detect neutrinos that would be emitted from a WIMP annihilation [48]. Located at the south pole, long arrays of light detectors are placed in the glacier up to 2.5
km beneath the surface. There, most background of cosmic rays would be sheilded from the first 1.5
km of ice from the surface. Interactions between the neutrinos and the ice would release Cherenkov
light which can then be measured by the light detectors. Limits set by the IceCube experiment are
shown below in Figure 1.11.
The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) experiment uses a satellite orbiting Earth to measure high energy cosmic rays [36]. If two WIMPs
would annihilate in the galactic halo, antiprotons or positrons could be created at very high energies. By detecting particle and anti-particle fluxes at high energies from the galactic halo, a dark
matter signal can be detected. A positron excess has been observed, however this is consistent with
anti-proton production in the galaxy from cosmic rays.
14
1.4.3
Direct Detection
In direct detection experiments such as CDMS, the goal is to measure an interaction between a
WIMP and an atom in a detector. Direct detection is a more reliable method of detecting dark
matter than indirect detection since there is a much better understanding of radioactive background
on Earth than sources of radiation from outer space. The two most significant problems with these
experiments are the low cross-section of the WIMP relative to the large amount of background radiation which can disguise itself as a WIMP interaction and the low interaction energy.
Supersymmetric models predict a cross-section on the scale of at most a few events/kg/year [50].
Given that ambient and cosmic radiation would produce events on the rate of 1 event/second without any radiation shielding, a method of limiting the radiation and discriminating between WIMP
events and background events is necessary.
In order to reduce cosmic radiation, experiments are done in deep underground mines or beneath
mountains. Currently, some of the most prominent laboratories for astroparticle physics experiments
are SNOLAB (Canada), Gran Sasso (Italy) and Kamioka (Japan) at a depth of 6.0, 3.2 and 2.8 km
of water equivalent, respectively. A list of some of the most prominent mines by depth can be seen
in Figure 1.5. Even at these depths, high energy muons can penetrate and affect the detector. The
deeper the mine, the smaller the chances of cosmic radiation affecting the detector.
In all locations, trace amounts of many radioactive atoms including uranium, thorium and radon
can be found. These elements can cause significant amounts of radiation to enter the detector. To
protect the detectors from this ambient radiation, shielding is placed around the detector. Materials
such as lead are used as shielding to protect against gamma and beta radiation. Lead-210 is found in
trace quantities inside lead and has a half-life of 22.3 years. Consequently, depleted lead is especially
valuable to use as shielding since it protects against gamma and beta particles without producing
radiation itself. Neutrons can be released by either (α,n) reactions or spontaneous fission of trace
uranium in the lab wall rock. In order to moderate neutrons, light atoms are used. This is usually
in the form of water or polyethylene shielding because the hydrogen atoms slow down the neutrons
through elastic scattering. Water is much less expensive to obtain, but polyethylene is more effective
at moderating neutron radiation due to a greater density of hydrogen atoms.
15
Figure 1.5: Neutron flux in various underground experimental laboratories [52]. All the high
energy neutrons accounted for in this plot, are caused by muons interacting with the lab wall rock
or objects within the lab. As depth increases, the ability of muons to penetrate into the laboratory
decreases causing a reduction in the neutron rate. Neutrons are relevant as they can cause
nuclear-recoils.
1.4.3.1
Nuclear Recoil Discrimination
When performing a simple calculation of the kinematics, a WIMP interaction energy would vary
greatly depending on whether it interacted with an electron or a nucleus of an atom. Assuming
WIMP speeds of approximately 270 km/s relative to the Earth (speeds of objects orbiting in the
halo) and a WIMP mass much greater than an electron or a nucleus, an estimate of the maximum
energy from an interaction can be made. Consequently, an elastic collision will cause the target
to recoil backward at the same relative speed at which the collision occured. A simple derivation
renders equation 1.9:
∆E =
1
m(2v)2
2
(1.9)
where ∆ E is energy transferred to the atom, m is the mass of the particle (either an electron or a
nucleus), and v is the speed of 270 km/s or 9 × 10−4 c.
16
This results in an interaction of approximately 1 eV when colliding with a 511 keV/c2 electron
or a 100 keV interaction for a nucleus of 50 GeV/c2 . It is difficult to build large detectors that
are sensitive down to 1 eV. Consequently, any WIMP signal observed would come in the form of a
nuclear-recoil.
Because most background radiation (gammas and betas) is ionizing radiation, it causes electronrecoil events. By discriminating between electron and nuclear-recoil events, a WIMP interaction can
be distinguished from background. By using this method, the most problematic type of radiation
is neutrons since they also produce nuclear-recoils and could be mistaken for a WIMP signal. Consequently, neutrons must be moderated to prevent a nuclear-recoil with a similar energy to what is
expected from a WIMP interaction. In many experiments, two signals are measured and the ratio
of the two signals can be used as a discrimination tool against electron-recoils. [10]
1.4.3.2
Detection Signals
Energy scales of dark matter particle interactions with nuclei are not large enough to break up
a nucleus or to produce detectable standard model particles. However a nuclear-recoil can produce
three types of signals (dependent on the material): phonon (section 1.4.3.3), ionization (section
1.4.3.4) and scintillation (section 1.4.3.5).
1.4.3.3
Phonon Signal
Phonon signals occur from lattice vibrations created by a recoil in a crystal. Specific heats differ
between materials and varies as a function of temperature, this can be seen in Figure 1.6.
With the exception of a few materials, the general trend is that the specific heat decreases with
temperature. Specifically for the CDMS experiment, the important material is germanium. The
variation of the specific heat of germanium as a function of temperature, as can be seen in Figure
1.7.
17
Figure 1.6: Specific heat of various materials at cryogenic temperatures [61]. The general trend
shows that specific heat decreases with a reduction in the temperature.
Figure 1.7: Specific heat of germanium as a function of temperature [8]. Because the specific heat
decreases as a function of temperature, an interaction at a lower temperature results in a greater
increase of temperature for a given interaction energy.
18
As the temperature decreases, the amount of energy required to change the temperature decreases. Consequently, an interaction at a given energy at low temperature would result in a more
drastic temperature change than it would at higher temperatures. In experiments where phonon
signals are observed, cryogenic equipment is required to measure these signals. Thermal signals cannot be used alone to discriminate between different types of events (nuclear-recoil or electron-recoil),
but can give an accurate measurement of the total energy deposited in the crystal.
1.4.3.4
Ionization Signal
When measuring ionization signals, a voltage is placed across the detector. Once an interaction
occurs, charge is released. Usually it would randomly move and eventually recombine with the atom
from which it was released. By placing an electric field across the material, the charge drifts to
the electrodes and can be captured there. The ionization energy only measures how much charge is
released. For electron-recoil events at a given energy, there is more charge released than for nuclearrecoil events at the same energy making the observed signal dependent on the interaction type [44].
1.4.3.5
Scintillation Signal
When an interaction occurs, electrons around an atom are energized to an excited state. As they
relax, light may be emitted at an energy equal to the difference between the excited state and ground
state. As more energy is distributed to the electrons in electron-recoil events, more scintillation light
is observed from an electron-recoil event at a given energy than a nuclear-recoil event.
1.4.4
Direct Detection Experiments
Many different experiments with different technologies are attempting to detect a WIMP interaction. A few examples of direct detection experiments will be discussed with a focus on leading
experiments in the field, other cryogenic experiments and fellow collaborators from SNOLAB. Results from the experiments are summarized in section 1.4.5. The CDMS experiment will be described
in detail in chapter 2.
19
1.4.4.1
CRESST
The Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) experiment,
located at Gran Sasso, uses both phonon and scintillation signals are measured in CaWO4 crystals
held at approximately 10 mK [26]. A schematic of a CRESST detector can be seen in Figure 1.8.
Figure 1.8: Schematic of a CRESST detector. The centre grey square is the CaWO4 crystal which
is surrounded by a housing depicted by the light gray square. A tungsten TES thermometer
(explained in section 2.3) is connected to the crystal to measure energy from phonon signals. A
silicon-on-sapphire (SOS) wafer is placed on top of the crystal with another TES attached, which
is used to measure energy from scintillation.
The phonon sensor is coupled to the crystal and all energy released the form of phonons is absorbed into the sensor and measured as a phonon signal. Above the crystal, there is a silicon-on
sapphire (SOS) wafer which absorbs photons released through scintillation and increases in temperature. Another TES thermometer measures this temperature increase. In order to maximize
photon collection, a reflective housing is placed around the setup so a higher percentage of photons
are absorbed by the SOS wafer.
20
With every interaction, scintillation light is released dependent on how many electrons are excited through the interaction. If the interaction is an electron-recoil, more energy will be released
through the electrons and more light will be given off through scintillation. By measuring the ratio
of phonon and scintillation signals, electron and nuclear-recoil events can be discriminated.
1.4.4.2
Xenon 100
The Xenon 100 experiment, located at Gran Sasso, uses a cylindrical vessel containing liquid
xenon beneath xenon gas which has an electric field applied across the entire vessel [4]. Photomultiplier tubes are set up both above and below the cylindrical vessel. When a particle interacts
with the liquid xenon, scintillation light and charge are released. The initial light is measured by
the photomultiplier tubes and the charge drifts towards the liquid gas interface. The charges are
extracted from the liquid phase into the gas phase through a strong electric field and are accelerated
in the gas phase. A second scintillation signal is then produced proportional to the amount of charge
released in the initial interaction. By combining the measurements of scintillation and ionization,
electron-nuclear-recoil discrimination can be achieved.
1.4.4.3
DEAP
The Dark Matter Expeirment using Argon Pulse-shape discrimination (DEAP 3600) experiment,
located at SNOLAB, uses a spherical vessel containing 3600 kg of liquid argon [32]. Approximately
250 photomultiplier tubes are placed around the vessel which measure any scintillation light released
from a particle interaction. Differing pulse shapes for electon and nuclear-recoils allow discrimination
against background radiation. The DEAP 3600 detector is currently under construction at SNOLAB.
1.4.4.4
PICASSO
The Project in CAnada to Search for Supersymmetric Objects (PICASSO) attempts to detect
WIMPs through their interaction with a superheated liquid [40]. C4 F10 molecules have a boiling
point of −1.7 ◦ C at atmospheric pressure, but can be superheated and remain liquid at room tem-
21
perature. The fluorine nucleus has a high spin content and consequently, PICASSO is sensitive
to spin-dependent WIMP interactions. If a nuclear-recoil occurs with a fluorine atom and enough
energy gets deposited, the C4 F10 can undergo a phase transition from liquid to gas and a bubble will
form. Microphones can be placed along the surface of the detector and sound signals can be used
to measure the shockwaves from the rapid phase transition. PICASSO is sensitive to the energy
deposited per track length and conseqently, beta and gamma interactions are not recorded as they
deposit energy over a longer tracklength. Recently, a method has been determined to discriminate
alpha interactions from nuclear-recoils using the sound signal. [25]
1.4.4.5
EDELWEISS
The EDELWEISS experiment has a similar concept to CDMS (explained in section 2) where a
germanium crystal is used to measure both phonon and ionization signals due to a WIMP interaction
[22]. EDELWEISS uses a different method of measuring phonon signals than CDMS. Germanium
wafers are transformed into effective thermometers for low temperature experiments by a process
called Neutron Transmutation Doping (NTD) [61]. Germanium wafers are irradiated by thermal
neutrons from a nuclear reactor. Neutrons can then be captured in germanium atoms creating a
p-type doped germanium. These NTDs are glued onto the germanium crystals and are used as
thermometers to measure phonon energy.
1.4.4.6
DAMA/LIBRA
The DArk MAtter (DAMA/LIBRA) experiment, which is located at Gran Sasso, uses NaI crystals to measure scintillation signals from WIMP interactions and have claimed to have observed
a WIMP signal [39]. DAMA/LIBRA is not capable of discriminating between electron-recoil and
nuclear-recoil events, but instead looks for a modulation in the scintillation signals on an annual
basis. This comes from the revolution of the Earth around the Sun and the revolution of the Sun
around the galaxy. During the summer, the Earth and the Sun are both moving in the same direction around the galaxy, so the Earth would have a greater velocity in the summer than the winter
relative to the galaxy. Thus, the dark matter signal during the summer should have a higher rate
than that of the winter. This modulation signal can be seen in Figure 1.9.
22
Figure 1.9: The annual modulation curve observed by DAMA/LIBRA [39]. The total interaction
rate observed by DAMA/LIBRA as a function of time oscillates on an annual basis. Background
interactions are expected to be constant on an annual basis, so a residual plot of recorded events
demonstrates an annual modulation of a signal as expected from WIMP interactions.
This claim of a dark matter signal is in dispute and cannot be reconciled with the absence of a
signal in other experiments like CDMS and Xenon 100 [47].
1.4.5
Detection Limits
The goal of all dark matter direct detection experiments is to observe an interaction caused by
a WIMP. If no interaction is observed, then WIMP cross-sections can be ruled out depending on
the sensitivity of the experiment. A graphical representation of the experiment sensitivity can be
represented in exclusion curves as seen in Figures 1.10 and 1.11. These curves include limits set by
leading experiments as well as theoretical predictions for WIMP cross-sections from supersymmetric
models. The limits set by the DAMA/LIBRA experiment and how other experiments have excluded
that region can be seen.
23
Figure 1.10: Exclusion curves for spin-independent interactions from the latest experiments [46].
The maroon region is the range of possible cross-sections and WIMP masses as determined by the
DAMA/LIBRA experiment [39]. The CRESST experiment, in pink, [26] rules out most of the
region of possible WIMP masses and cross-sections claimed by DAMA/LIBRA. Both the CDMS
[44] and Xenon 100 [21] experiments exclude the possibility of a WIMP signal as claimed by
DAMA/LIBRA. The range of possible WIMP cross-sections and masses from some supersymmetry
models are the light blue and light green regions [31]. Both CDMS and Xenon 100 are starting to
rule out ranges within theoretical models.
24
Figure 1.11: Exclusion curves for spin-dependent interactions from the latest experiments [46]. The
region of possible WIMP masses and cross-sections from the DAMA/LIBRA results can be seen in
magenta [39]. The PICASSO experiment, in black, has ruled out much of the region [40]. COUPP,
an experiment with a similar concept as PICASSO, has completely ruled out the DAMA/LIBRA
results for spin-dependent cross-sections [23]. In blue, the IceCube experiment is more sensitive
than PICASSO and COUPP, but only at higher energies [48]. Supersymmetric Models, in red and
gray, have yet to be probed by experiments sensitive to spin-dependent WIMP interactions [31].
25
Chapter 2
The CDMS Experiment
2.1
CDMS Introduction
The Cryogenic Dark Matter Search (CDMS) collaboration consists of more than 75 scientists at 19
different universities across the world. The CDMS experiment is one of the leading direct detection
experiments searching for dark matter. By measuring phonon and ionization energies, electron-recoil
and nuclear-recoil events can be discriminated to 1 in 4000. [44] An example of the discrimination
ability can be seen in Figure 2.1.
2.1.1
Setup at Soudan
In order to search for a rare interaction, background interactions must be reduced as much as
possible. The experiment is located in Minnesota at the Soudan Underground Laboratory, which is
700 m below the surface [52]. This depth helps to reduce cosmic muons interactions near the detector. These interactions may cause a neutron to be released which would undergo a nuclear-recoil
interaction in the CDMS detector.
Besides cosmic radiation, radioactive decays from the laboratory can cause events in the detector
and the respective particles must be shielded as well. In order to protect the setup from ambient
radiation, many layers of shielding are used.
The cryostat is connected to the detector volume which is located outside the entire setup. This
26
Figure 2.1: Electron vs. nuclear-recoil discrimination. When plotting the ionization and phonon
energies of an interaction, two clear bands are formed. The band with a higher ionization is caused
by electron-recoils and the band with lower ionization is caused by nuclear-recoils. This can be
used to discriminate between electron-recoil and nuclear-recoil events.
allows the detectors to be cooled while protecting them from radioactivity from the cryostat. The
outermost layer of shielding is a muon veto shield. A high energy muon can penetrate the polyethylene layer, interact with another material the in setup, and create a neutron which may interact
with the detectors through a nuclear-recoil. In order to avoid this, the veto can tag any event that
coincides with a muon entering the setup.
Inside the muon veto, there is the main layer of polyethylene shielding to attenuate any neutrons
caused by fission or (α,n) reactions in the laboratory. A layer of lead is then placed within the
polyethylene shielding to attenuate any gamma particles that might enter the setup.
210
Pb is found
in trace abundance but decays with a half-life of approximately 22 years and releases low energy
gamma rays which could then interact with the detectors. Inside the outer layer of lead, 2000 year
old lead, which is depleted of
210
Pb, is used. This entire setup can be seen in Figure 2.2.
27
Figure 2.2: Experiment shielding at Soudan, Minnesota.[16] Outside the entire setup is the
cryostat which cools down the icebox where the detectors are located through the coldfinger.
Moving in toward the detector, there is a muon veto, outer polyethylene layer, outer lead shielding,
inner depleted lead shielding, inner polyethylene layer, the icebox and the detectors.
High energy muons can interact with the lab wall rock and produce high energy neutrons which
enter the setup undetected by the muon veto. The energy of these neutrons is high enough that it
can make it through the outer layer of polyethylene and interact with the lead, creating more low
energy neutrons. To attenuate these neutrons, another thinner layer of polyethylene is placed inside
the lead. Inside all shielding is the icebox which houses the detectors.
2.2
CDMS ZIP Detectors
The CDMS Z-sensitive Ionization and Phonon (ZIP) detectors are cylindrical germanium crystals,
with a diameter of 7.62 cm and a height of 1 cm. Figure 2.3 shows a picture of such a detector.
28
Figure 2.3: CDMS ZIP detector mounted in the copper detector housing. The side of the detector
seen shows the phonon channel.
On one side of the detector there are two concentric aluminum electrodes used to measure the
charge. The inner charge channel (Qi) detects bulk events. The electrode around the rim is the
outer charge channel (Qo) and is used to cut out events that occur near the rim of the detector
because the electric field is non uniform at the edges of the electrodes. The phonon sensor on the
other side is divided into four quadrants. A schematic of the detector makeup can be seen in Figure
2.4.
2.3
Phonons
Lattice vibrations occur when an interaction occurs with an atom in the crystal. Phonons propagate throughout the crystal and are eventually absorbed into a tungsten film on the crystal surface.
The tungsten film works as a transition edge sensor (TES) where it is held on the edge of its superconducting transition curve. By absorbing a small amount of energy, the tungsten undergoes a
small increase in temperature, resulting in a large change in resistance [42]. A schematic can be
seen in Figure 2.5. The change in resistance is then measured through the phonon readout circuit
discussed in section 2.3.2.
29
Figure 2.4: Schematic of the layout of a CDMS detector. The left side shows the two charge
channels, Qinner and Qouter. The right side shows the phonon sensor. It is divided into four
quadrants and based on signal magnitude and timing between the different quadrants, the
interaction location can be determined.
2.3.1
QET
In CDMS, the phonons are absorbed into the TES before they thermalize. By having a fast
phonon signal, pulse shape can be measured and used as a discrimination tool between bulk and
surface events (see section 2.6.1). This can be done by coating the entire surface with tungsten
and maximizing the contact area between the germanium crystal and the TESs. However, this also
increases the heat capacity and reduces the energy resolution of a given event. In order to solve
this problem, tungsten filaments are incorporated into a Quasiparticle-trap-assisted Electrothermalfeedback Transition-edge-sensors (QETs). Aluminum fins are layered on the germanium crystal and
absorb phonons caused by interactions [54]. The aluminum fins allow the sensor to have a large
area without having a large heat capacity in order to be able to effectively absorb phonons before
they thermalize. The phonon energy absorbed breaks up the cooper-pairs in the superconducting
aluminum and is converted into quasiparticle excitations. These excitations diffuse until they are
trapped in the tungsten TESs [42]. A schematic of the QET layout can be seen in Figure 2.6.
30
Figure 2.5: Schematic of a transition curve. The gray line is the resistance of a TES as a function
of temperature. While in the transition region, a small increase in temperature causes a large
change in resistance, making the TES a very sensitive low temperature thermometer. Transition
temperatures are generally between 50-100 mK and resistances on average are between 1-3 Ω.
2.3.2
Phonon Channel Electronics
The electronics for the phonon readout circuit can be seen in Figure 2.7.
The bias current, (Ib ) is adjusted so that the TES is on its transition edge. Once a phonon signal
enters the TES, its resistance significantly increases and the current through the TES and input coil
is reduced accordingly. This change in current affects the magnetic field caused by the coil which
is coupled to a Superconducting Quantum Interference Device (SQUID). The SQUID amplifier circuit attempts to maintain a constant magnetic flux. Consequently, a current in the feedback coil is
induced to cancel the magnetic field caused by the input coil. The feedback coil has a factor of ten
times fewer turns than the input coil so that the current is amplified by a factor of ten. The current
is then measured through a voltage across a feedback resistor.
31
Figure 2.6: Detailed sketch of the sensors on a CDMS ZIP detector [5]. The bottom right shows
the two charge channels as seen in Figure 2.4. The bottom middle is a side view of the detector.
The bottom left is a view of the phonon sensor, which is divided into 4 quadrants each, with 37
dies. Top left side is a closeup view of one of the dies consisting of 28 QETs. All QETs in one
phonon sensor are connected in parallel. The top right side is a closeup of a QET. The grey
horizontal stripes are aluminum fins which absorb the phonons and transfer the energy into the
thin tungsten film. The dark vertical structure is the tungsten TES.
An intrinsic effect when measuring the phonon signal is known as SQUID jumps. The voltage
drop across the SQUID changes periodically with the magnetic field. Since the feedback circuit keeps
the voltage drop constant, a quick change in input current can lead to the feedback loop stabilizing
at the next cycle of the voltage vs. magnetic field curve. This leads to a constant offset in the
output. Examples of these SQUID jumps can be seen in Figure 4.4.
2.4
Charge and Electronics
Aside from depositing energy in the form of phonons, energy is also absorbed in the germanium
crystal through the liberation of electrons. When an electron-hole pair is created, a valence electron
is released from the bound state into the conducting band of the crystal. Initially, after the interaction, the electron-hole pairs undergo quasi-diffusive motion close to the area of interaction. An
32
Figure 2.7: Schematic of the phonon readout circuit [18]. The left side of the circuit is where the
signal is formed. The sensor current, Is , varies when the TES changes temperature and has a
change in resistance. The right side of the circuit is where the signal is read. It is amplified
through the SQUID and the feedback coil, creating a current 10 times greater than Is through the
feedback coil. This current causes a voltage drop through the 1 kΩ resistor which is read.
electric field is generated across the crystal by applying a voltage (typically 3V) across aluminum
layers on either side of the crystal. Once the electron-hole pairs thermalize, they drift across the
crystal instead of recombining. A schematic of the electronics readout circuit can be found in Figure
2.8.
The separated charge creates a voltage in the coupling capacitor (CC ) in series between the
detector and the JFET. The amplifier adjusts the charge on the feedback capacitor (CF ) to force
the voltage at the gate to 0. The voltage readout is V = Q/C, so a small capacitor, CF is used
to measure the output voltage as opposed to CC . The voltage adjustment is the signal which is
readout. The feedback capacitor is then discharged through the feedback resistor (RF ).
2.5
Cold Hardware
The cold hardware consists of all the equipment and electronics that houses the detectors and
transmits the readout signals from 40 mK (the temperature of the detectors) up to room tempera-
33
Figure 2.8: Schematic diagram of the ionization readout circuit. Charge collected in the detector
causes a voltage increase in the coupling capacitor, CC . In order to force the voltage at the gate of
the amplifier to 0, the charge of the feedback capacitor, CF is adjusted. The voltage required to
adjust the CF is readout.
ture, where it is read out with the warm electronics. A picture of all the cold hardware assembled
together can be seen in Figure 2.9.
The detectors are mounted in a copper housing which prevents any thermal radiation from affecting the sensors. A detector interface board (DIB board) is located within the detector housing
and provides bonding pads to wirebond the phonon and charge sensors to leads that transmit the
signal through the copper housing. A picture of the DIB board can be seen in Figure 2.10.
The sidecoax is a cold hardware component that connects to the DIB board outside the copper
housing and transmits the signal to the next cold hardware component, the tower. The copper housing, DIB board and sidecoax are all at 40 mK, the same temperature as the detector. The tower is
designed to transmit the signal from base temperature to 4 K without creating a thermal link, which
would prevent the cryostat from getting cold. At the end of the tower, the signal is transmitted
through a SQUET card, which contains the majority of the cold electronics. The SQUET card contains the SQUID, as explained in section 2.3.2, to read the phonon signal and the FET, as explained
in section 2.4, to read the charge signal. The cold hardware structure (detector housing, sidecoax,
tower) is frequently referred to as the tower. Once the charge and phonon signals are amplified in
34
the SQUET card, they are brought to the breakout box (described in section 3.4.1) through custom
wiring called striplines. The striplines consist of thin copper wiring with external kapton covers.
Their duty is to transmit the signal with as little resistance as possible while not producing a strong
thermal contact. Nevertheless, they are one of the largest heat loads in the CDMS setup. A picture
of a stripline can be seen in Figure 2.11.
Figure 2.9: The CDMS cold hardware. At the bottom of the stack are the detectors. A side coax
cable takes the signal from the detectors and brings it to the tower. The tower allows for transition
of the electrical signal while the hardware goes from one stage of the cryostat to the next,
preventing a heat contact. At the end of the tower, SQUET cards are plugged to amplify the
signal. Striplines (see Figure 2.11) are connected to the SQUET cards and bring the signal to the
breakout box (see Figure 3.9) where it can be readout by warm electronics.
35
Figure 2.10: The DIB board. A total of 10 bond pads are located on one side of the DIB board
(the side that can be seen in the picture) and two bond pads are located on the other side. 8 of the
10 bond pads are for the 4 phonon sensors (2 bond pads each). The second position from the right
is not a bond pad but instead holds two LEDs used for detection neutralization. The other two
bond pads provide the current for the LED signal.
Figure 2.11: The CDMS stripline. As labeled in the picture, the cold end of the stripline is
connected to the SQUET card at 4K and the warm end is connected to the breakout box at room
temperature.
2.6
2.6.1
Reduced Charge Events
Surface Events
If an interaction occurs sufficiently close to the electrode, multiple effects result in a reduced
measurement of ionization energy causing an electron-recoil to appear like a nuclear-recoil. Surface
events are currently the most significant source of background in CDMS. The first important effect
is back diffusion of charge into the electrode before the charges separate and drift. Consequently,
a smaller amount of charge drifts and a smaller ionization signal is recorded. The second effect is
caused by the interaction due to a mirror charge formed by the conducting electrode. If a charge is
36
sufficiently close to the electrode, the force caused by the mirror charge would be strong enough to
counter the electric field placed across the crystal since the electric field does not vary with distance
from the electrode and the force caused by the mirror charge goes as 1/(2r)2 . If a significant amount
of charge is released within this region, it will not drift and will not be measured. The volume in
which a surface event can occur is known as the dead layer. A sketch of how a surface event would
cause a reduced charge signal is shown in Figure 2.12.
Figure 2.12: Sketch of a surface event due to a back diffusion of charge. The bulk event (left) has a
charge cloud created from an interaction which eventually separates and the electrons and holes
drift towards opposite electrodes. The surface event (right) has the charge cloud entering the
electrode. Many electrons and holes that would have separated and drifted were trapped in the
electrode and were not read with the ionization signal, which causes a reduced signal.
The dead layer is reduced by depositing an amorphous silicon layer on the germanium crystal
which prevents charges from back-drifting into the electrode. To remove all remaining events within
the dead layer, the phonon pulse shape is studied [43]. A graph comparing pulse shapes between
bulk and surface events can be seen in Figure 2.13.
A timing parameter is defined based on the rise time of the phonon signal and the difference
in the start time between the charge signal and the phonon signal. This parameter is used to discriminate between bulk and surface events as shown in Figure 2.14. Removing events based on the
timing parameter requires discarding events from approximately 50% of the detector volume. This
increases the discrimination ability from 1 misidentified event in 104 to roughly 1 in 106 . [44]
37
Figure 2.13: Comparison between phonon pulse shapes due to bulk and surface events.[5] Surface
events experience a much faster rise-time due to phonon interactions in the surface metal layer.[44]
This difference can be used to identify surface events.
2.6.2
Detector Deneutralization
As interactions occur in the detector, charges are separated by the electric field and drift toward
the electrodes. Ideally, the charges would drift into the electrode leaving the detector neutral, but
this is not always the case. The charges sometimes get trapped on the surface or in the bulk of
the crystal. This buildup of charge creates an electric field counter to the initial one applied and
weakens the net electric field. An image of this effect can be seen in Figure 2.15.
Initially, when a charge cloud is formed, some charges recombine within the charge cloud. Eventually, the charges are separated by the electric field and drift toward the electrodes. While drifting,
the electrons are separated from the holes and they do not recombine. As the buildup near the electrode increases, the electric field becomes weaker and the rate at which the charges are separated
decreases. Consequently, more charge recombines within the initial charge cloud before it can be
38
Figure 2.14: Discrimination between different events using the timing parameter. The y-axis is the
ionization yield or the ionization energy divided by the recoil energy. The red events are caused by
a gamma source and have a high ionization yield. Black events which can be seen at much a lower
ionization yield are beta events which do not penetrate the crystal and interact at the surface. The
blue events are nuclear-recoils which must be discriminated from the electron-recoil events. Using
the timing parameter discussed in the text, nuclear-recoil events and surface events can be
discriminated.
separated and a smaller amount of charge drifts towards the surface. The resulting reduced charge
signal causes the electron-recoil events to appear like nuclear-recoil events. In order to avoid this
problem, LEDs are placed in the detector housing. Once the detectors are no longer neutral, the
detector is grounded by setting the bias voltage to 0 V and the LEDs are flashed creating lots of
electron-hole pairs. The charges which have built up recombine with these newly formed pairs and
the detector is neutralized again.
2.7
Latest Results
Calibration data are studied to define bands in which nuclear and electron-recoils occur on the
phonon energy vs. ionization energy plot (see Figure 2.1). Once the bands are defined, events are
removed from the data such as surface events (section 2.6.1), muon coincident events (section 2.2)
or data taken when the detectors were not operating properly. This analysis is performed without
looking at the data to be used in the final results [44]. In the latest measurement period, a total of
39
Figure 2.15: Deneutralization of a detector. As interactions occur and charges separate, some are
trapped at the interface between the crystal and the electrode or in the bulk of the crystal. These
eventually build up a counter electric field and make the applied field weaker.
612 kg-days of data was collected and two candidate events were identified. Figure 2.16 shows the
data from the two detectors in which the events occurred. When displaying the results, the convention is to display the interactions similar to Figure 2.1, except the y-axis shows the ratio between
ionization signal and recoil energy, converting the diagonal bands as in Figure 2.1 into horizontal
bands. Therefore the y-axis is defined as the ionization yield and is plotted as a function of the
interaction energy . The latest results from the CDMS experiment can be seen in Figure 2.16.
40
Figure 2.16: Latest results from CDMS. Ionization yield plots of the two detectors (T1Z5 on top
and T3Z4 on bottom) where nuclear-recoil events were observed [44]. The lower bands in each plot
define the nuclear-recoil region and the upper bands define the electron-recoil region to two
standard deviations. Black dots represent events which were removed by the CDMS analysis. Grey
filled circles are events which were not removed. In both detectors, one nuclear-recoil was observed.
Figure 2.16 shows the data from the two detectors in which the events were located. The expected background was 0.8 events and the probability of observing two or more events was 23% due
to background alone. A WIMP signal cannot be claimed from these results. However, they produced
the most sensitive limits on spin-independent WIMP cross sections for WIMP masses greater than
42 GeV at the time the data was released. These results have since been surpassed by the Xenon
100 experiment.
A low energy analysis was also performed where events under 10 keV were studied. Discrimination capabilities are decreased at lower energies, but competitive results were still obtained for low
mass WIMPs [45].
41
2.8
SuperCDMS
As the experiment continues, new technologies will be implemented by the CDMS collaboration
in order to increase sensitivity and the experiment will change from CDMS II to SuperCDMS [20].
Changes planned include:
• Implementation of a new phonon and charge sensor layout (iZIP);
• Use of larger detectors;
• Moving the experiment from its current location at Soudan, Minnesota to SNOLAB in Sudbury,
Ontario; and
• Use of a dry dilution refrigerator instead of a traditional dilution refrigerator.
Detector thickness will be increased from 1 cm to 2.54 cm. Because surface events occur at the
top and bottom of the detector which are not increasing in area, the surface-area to volume ratio
decreases making the dead layer a smaller fraction of the total mass. Increasing the thickness will
also increase detector mass and the total exposure.
Although the neutron background does not yet affect the experiment to a level that it is a large
concern for background radiation, as exposure is increased, the likeliness of a nuclear-recoil due to
a neutron increases. This problem can be reduced by moving to a deeper mine. SNOLAB has three
times more shielding than Soudan against cosmic radiation. By moving to SNOLAB, the cosmogenic
neutron background can be reduced by two orders of magnitude (see Figure 1.5). SNOLAB is more
equipped to meet the cleanliness requirements of low background experiments as it is designed as
one large cleanroom, as opposed to Soudan which contains a cleanroom only where the cryostat
and shielding are set up. By having everything contained in a cleanroom, the experiment is better
protected against dust particles which could settle on the detector and cause background radiation.
Classic cryostats require refilling the liquid cryogens (LN2 and LHe) regularly and this must be
done on site. Dry dilution refrigerators (see section 3.2) use new technology to keep the cryostat
cold, which eliminates the need for on-site maintenance and allows for the cryostat to be operated
remotely. Helium supplies are also expensive and eliminating the requirement to continually purchase large amounts of liquid helium can reduce expenses over long periods of time.
42
2.8.1
iZIP Technology
The new iZIP technology alters the geometry of the electrode to improve surface event discrimination ability, while the older technology involves a constant aluminum electrode placed across the
detector surface to create a constant electric field through the detector. An interleaved electrode has
recently been developed which still has a constant electric field through the bulk of the detector, but
a much different field at the surface [35]. A sketch of the electric field generated by the interleaved
electrode can be seen in Figure 2.17.
Figure 2.17: Visualization of the electric field caused by the interleaved electrodes. In the bulk of
the crystal, field lines are uniform and identical to what would be found from a uniform
non-interleaved electrode. At the surface, field lines are curved and would prevent charge from
drifting across the crystal.
Because of the non-uniform field lines at the surface, charge signals at either end of the crystal
would give different readings. Events that occur in the bulk would cause a uniform separation of
electrons and holes and both electrodes would measure them drifting. By using the iZIP technology,
an asymmetry measured in the charge channels can be used to discriminate against surface events.
43
Chapter 3
Queen’s Test Facility
As new detectors are produced for the SuperCDMS experiment, each detector needs to be studied at a test facility to determine its phonon sensor properties and demonstrate that it will work
properly [17]. A detector test facility has been installed and commisioned at Queen’s University for
this purpose.
The Queen’s Test Facility (QTF) was built for detector testing and development. The core of
the QTF is a dilution refrigerator used to cool the detectors down to operational temperatures of
approximately 40 mK. The cryostat at the QTF is a new dry dilution refrigerator which uses a pulse
tube cooler instead of the liquid cryogens (LN2 and LHe) as used in traditional dilution refrigerators.
The intention is to operate a dry dilution refrigerator for SuperCDMS instead of a classic dilution
refrigerator. Dry dilution refrigerators have only been made commercially available in the past few
years and understanding and troubleshooting problems associated with this new technology is a goal
of the QTF.
A nitrogen purge cabinet was installed to store detectors. It prevents contamination from radon
gas and any oxidation of the copper housing. The QTF is currently the only CDMS test facility
built inside a permanent cleanroom. The cleanroom protects detectors from contamination caused
by radioactive particles found in dust.
44
3.1
The Dilution Refrigerator
Of all cryostat technologies that can reach temperatures below 1 K, the dilution refrigerator is
the most powerful cryostat and the only one capable of indefinitely maintaining a minimum (base)
temperature [61]. The dilution refrigerator is based on the unique properties of a 3 He-4 He mixture
below 876 mK. The phase diagram of a 3 He-4 He mixture can be seen in Figure 3.1.
Figure 3.1: Phase diagram of a 3 He-4 He mixture [13]. Below 876 mK, the mixture separates into
two phases: a 3 He rich phase (right side) and a 3 He dilute phase (left side).
As the 3 He-4 He mixture is cooled, it splits into two phases, a 3 He rich phase and a 3 He dilute
phase. The unusual properties of the 3 He dilute phase (left side in the above phase diagram) are the
reason why this is called a dilution refrigerator. As the temperature is lowered towards 0 K, the 3 He
dilute phase approaches a concentration of 6.6% 3 He. The two phases separate by gravity but 3 He
in the dilute phase evaporates at a much faster rate than 4 He when pumped on. By pumping on the
3
He dilute phase, 3 He atoms are removed from the dilute phase. Due to a gradient in concentration
between the rich and dilute phases, 3 He atoms are forced to cross a barrier from the rich phase into
the dilute phase in a vessel called the mixing chamber. This process is endothermic, so that even as
absolute zero is approached, there is still cooling power.
45
3.1.1
Classic Dilution Refrigerator
A schematic of a classic dilution refrigerator can be seen in Figure 3.2. In the traditional dilution
cryostat, cooling is performed at 5 different stages: 77 K, 4 K, 1 K, 600 mK and 10 mK. First, liquid
nitrogen is used to reach a first stage of 77 K. Liquid helium is then used to reach a second stage of
4 K. Temperatures down to 1 K can be achieved inside the so-called 1 K pot, by evaporative cooling
of 4 He or by expansion of the 3 He-4 He mixture through an impedance. Below 1 K, there is very
little evaporation from the 4 He in the mixture and only the 3 He evaporates in a substantial amount.
Evaporative cooling by pumping on the mixture which is located inside the so-called still leads to
temperatures between 600 and 900 mK. The lowest temperature stage (base plate) is cooled by the
crossing of 3 He atoms between the rich phase into the dilute phase of the mixture in the mixing
chamber as explained in section 3.1.
3.2
Dry Dilution Refrigerator
Liquid cryogens can be difficult to refill and require frequent replacing. Consequently, classic dilution refrigerators require regular maintenance to remain in a cold state. In recent years, the pulse
tube cooler technology has been improved to maintain low temperatures (70 K and 4 K) which can
be used to replace liquid cryogens.
3.2.1
Pulse Tube Cooler
Pulse tube coolers, which use the compression and expansion of helium to cool down to 4 K, are
increasingly being used to replace liquid cryogens. The advantage of pulse tube coolers over any
other coolers is that there are no moving parts in the cold stage of the cycle. Moving parts at the
cold stage can cause mechanical failures, heat the inside of the cryostat and add noise to the system
[61].
46
Figure 3.2: A classic dilution refrigerator schematic [61]. The black line on the left side indicates
where the mixture is pumped into the mixing chamber. It passes through heat exchangers at every
step along the way and is expanded through multiple impedances. The mixture then condenses in
the mixing chamber, located at the bottom of the figure, where it undergoes phase separation. The
dilute phase is located on the bottom due to the mass difference between 4 He and 3 He. The dilute
phase resides between the mixing chamber and the still where it is pumped on. Most gas which
evaporates from the still is 3 He, causing 3 He to cross from the rich phase to the dilute phase in the
mixing chamber.
3.2.2
Cooling Process and Mixture Circulation
Cylindrical cans are attached to the top three plates and an outer vacuum can is attached around
the entire cryostat. First, the outer vacuum can must be evacuated to prevent heat links between
47
different stages in the cryostat caused by air, which would prevent the cryostat from cooling down
properly. In the classic dilution refrigerator, an inner vacuum can is placed around the stages below
4 K (explained below). Cans are placed between every stage down to the still plate in order to act as
radiation shields. If warmer temperatures, such as 4 K or 70 K, radiated onto the base plate, they
would prevent it from cooling down. By attaching the cans, each stage affects only the adjacent
stage and does not affect the colder stages.
The 3 He-4 He mixture can only start condensing once the 4 K plate drops below 10 K. Before this
happens, the stages below 4 K need to be cooled down along with the 4 K plate. In a classic dilution refrigerator, an inner vacuum can is placed around the lower stages. A small amount of helium
gas is placed in the inner vacuum can to create a heat link between the LHe bath and the lower stages.
Since the dry dilution refrigerator does not have a LHe bath or an inner vacuum can, a different
method is used. The mixture is pumped through a steel tube (known as the precool loop) which is
heat sunk at the 70 K and 4 K stages. The cooled mixture is then pumped down to the lower stages
where it removes heat and is pumped out of the cryostat. Once the 4 K stage and lower stages
are all below 10 K, all of the mixture must be removed from the precool loop to remove any heat
connection between the different stages. Once the precool loop is completely emptied, the mixture
can be pumped back into the circulation loop of the cryostat to begin condensing.
To prevent any residual air in the mixture from causing a blockage inside the cryostat, the mixture is pumped through a liquid nitrogen cold trap before entering the cryostat, which causes any air
to condense before the mixture enters the cryostat. A picture of the cold trap is shown in Figure 3.3.
The mixture then passes through heat exchangers at the 70 K and 4 K plates and then expands
through an impedance, causing it to condense and accumulate in the mixing chamber. Once the
mixture is completely condensed, a turbo pump can be turned on which reduces the pressure in the
still line (where the dilute phase is pumped out from the 600 mK stage), increases the circulation
rate and lowers the mixing chamber temperature down to the base temperature. This is the state
in which the cryostat is left throughout the remainder of the run until it is warmed up.
48
Figure 3.3: The liquid nitrogen cold trap. It consists of metal cylinder filled with activated carbon.
The cylinder is placed in a liquid nitrogen bath to cause any non-helium gas to condense. There is
a large cross section area in the cold trap, so that any condensed or frozen gas does not affect the
flow rate. If the gas were to enter the cryostat and condense in the inlet line, it would cause a
blockage, prevent the mixture from circulating and prevent the cryostat from reaching a proper
base temperature.
Once the run is complete, the mixture is collected back into the tank and the pulse tube cooler
is turned off. The cryostat is then left until it warms up to room temperature by thermal radiation.
The warmup process can be sped up by introducing an exchange gas into the cryostat.
Table 3.1 shows a comparison of the classical and dry dilution refrigerators.
3.3
Cryostat Commissioning
Figure 3.4 shows a picture of the Vericold Dultion Refrigerator from Oxford Instruments located
at the QTF with all radiation shields removed.
49
Table 3.1: Comparison between classic and dry dilution refrigerators
Temperature Reached
77 K / 4 K
Precool
1K
600 mK
10 mK
Maintenance
Noise
Classic Cryostat
LN and LHe
Exchange gas
Evaporative cooling
of 4 He (1 K pot)
Dry Dilution Cryostat
Pulse tube cooler
Precool loop
Expansion of He mixture
through an impedance
(Joule-Thompson cooler)
Evaporative cooling of 3 He
Forcing 3 He from the rich phase to the
dilute phase of a 3 He-4 He mixture
Requires refilling
Less maintenance and
cryogens
easier to run remotely
Low general noise
The pulse tube cooler may
but high noise
cause electronic noise, acoustic
during a refill
noise and vibrations
Proper cryostat operation needed to be ensured before using the cryostat to perform detector
tests and sensor measurements. A few runs were devoted to ensure that the cryostat reached base
temperatures below 10 mK, had the necessary cooling power (200 µW at 100 mK) and that the
thermometry calibration was correct. Thermometers in the cryostat are semiconductors that have
a sensitive, repeatable and reliable change in resistance as a function of temperature. A four wire
measurement (explained in section 3.5.2) is used to measure the resistances of various thermometers
placed throughout the cryostat. The QTF cryostat came with precalibrated thermometers which
needed to be verified.
3.3.1
Thermometry calibration
The most important thermometer is the mixing chamber thermometer located on the base plate
(the coldest part of the cryostat), which measures the temperature of the samples and detectors.
The mixing chamber thermometer only had a reliable calibration down to 20 mK as seen in Figure
3.5. In order to demonstrate that the QTF cryostat reached a base temperature lower than 10 mK,
a reliable thermometer at temperatures below 20 mK was required to cross calibrate the mixing
chamber thermometer.
50
Figure 3.4: The cryostat at the QTF. Five plates can be seen on the cryostat. From top to bottom:
70 K, 4 K, 600 mK (still plate), 100 mK (cold plate) and 10 mK (base plate). Each plate is gold
plated copper. There is a two stage pulse tube cooler located at the 70 K and 4 K plates. Heat
exchangers exist at every step along the way. The mixing chamber is a metal cylinder that can be
seen on the base plate.
A cobalt thermometer that works by measuring the radioactive decay of
unstable and decays to
60
60
Co was used.
60
Co is
Ni through a beta decay and releases gamma rays in the process. This de-
cay is anisotropic and the magnitude of anisotropy is dependent on the temperature. Consequently,
by measuring the spatial distribution of the gamma rays released in the
60
Co decay, the Co sample
can be used as a temperature reference. [58]
A
60
Co sample was placed on the base plate along with the mixing chamber thermometer to be
used as a cross calibration. Results comparing the initial calibration of the mixing chamber thermometer with the actual temperature (based on the
51
60
Co calibration) can be seen in Figure 3.6.
Figure 3.5: Factory calibration for the base temperature thermometer. The final data point is
located at exactly 8 mK and 70 kΩ and cannot be trusted. No reliable calibration existed below 20
mK.
3.3.2
Cooling Power Tests
Besides having a base temperature lower than 10 mK, the cryostat was specified to have a cooling
power of 200 µW at 100 mK. This cooling power is measured by putting a current through a resistor
on the base plate which puts in a constant heat load. When the input power is at 200 µW, the
cryostat should be able to maintain a tempreature of 100 mK or less.
One method of adjusting the cooling power is by heating the still plate. As explained in section
3.1, 3 He from the dilute phase of the mixture at the still is pumped out and circulated around and
back into the cryostat. The rate at which 3 He crosses the boundary from the rich phase into the
dilute phase is dependent on the rate of 3 He evaporation in the still. By heating the still plate, the
52
Figure 3.6: Calibration of mixing chamber thermometer using a cobalt thermometer. As can be
seen, the factory calibration of the resistance thermometer is unreliable at low temperatures,
assuming a correct measurement from the cobalt thermometer. For cross-calibration at low
temperatures, a linear fit was used. At higher temperatures, the cobalt thermometer is less
accurate so that the mixing chamber thermometer was assumed to be correct. Therefore, a linear
fit y=x was used to calibrate between the temperature measured from the resistance and the
actual temperature. Between the two ranges, polynomial fits were used to ensure continuity and to
approximate the data.
evaporation rate increases and causes a higher rate of 3 He to cross the rich-dilute phase boundary
in the mixing chamber, which results in a greater cooling power on the base plate. However, this
process also heats up the still plate, and increases the temperature of mixture when passed through
the heat exchanger and consequently causes a higher heat load on the base plate. In order to measure the cooling power, both the heaters on the base plate and still plate needed to be adjusted to
determine the maximum cooling power at a given temperature.
53
From these tests, a minimum temperature was determined as a function of input power. These
results are summarized in Table 3.2 and plotted in Figure 3.7. The cooling power at 100 mK was
slightly better than the specifications.
Table 3.2: Measurements of cooling powers at different temperatures
Input Power
(µW)
0
5
10
25
50
75
100
150
200
250
300
Minimum
Base
Temperature (mK)
10
20
25
36
50
60
69
84
96
107
118
±
±
±
±
±
±
±
±
±
±
±
Error
(mK)
1
1
1
1
1
1
1
1
1
1
1
Still Power Required (µW)
0
2000
3500
3500
3500
3500
3500
3500
3500
3500
3500
±
±
±
±
±
±
±
±
±
±
±
Error
(µW)
250
250
250
250
250
250
250
250
250
250
250
Figure 3.7: Results from the cooling power tests. At 0 and 200 µW of input power, the base
temperatures were 10 mK and 96 mK, respectively. This met the specifications of the cryostat.
54
3.3.3
Grounding Tests
It was noticed while commissioning that the temperature measured by the thermometer depended
on the grounding state of the cryostat. When a lead was connected from the cryostat support frame
to the outer vacuum can, the measured temperature changed significantly. Cryostat temperatures
were measured with and without a ground connection from the outer vacuum can to the support
structure while the rest of the configuration was unchanged. A calibration between grounded and
non-grounded states can be seen in Figure 3.8.
Figure 3.8: Cross-calibration of cryostat temperature readout between a grounded and
non-grounded setup. At higher temperatures, the reading is independent of whether or not the
ground is connected. At lower temperatures, grounding the cryostat causes a reading significantly
lower than the temperature without grounding.
After more hardware, like striplines and custom wiring (see sections 2.5 and 3.5.2.3), were installed inside the cryostat, additional heat loads increased the lowest reachable temperatures to
above 20 mK. Consequently, adjusting temperature measurements was not necessary at warmer
temperatures because the calibration was reliable and grounding the cryostat did not have a noticeable effect on temperatures above 20 mK.
55
3.3.4
Pulse Tube Cooler Noise Tests
Many tests were performed to determine the electronic noise introduced by the pulse tube cooler
and how it affects detector performance. The rotary valve, a valve used by the pulse tube cooler to
control helium flow, was identified as the main source of noise. To isolate the noise caused by the
rotary valve, it was switched off for short periods of time. However, leaving it off for too long would
warm the 4 K plate and prevent the mixture from condensing in the mixing chamber.
Detectors performed well while the pulse tube cooler was running. Grounding the outside can of
the cryostat changed the noise caused by the pulse tube cooler. Once the optimal grounding scheme
was found, there was no significant difference in noise when the pulse tube cooler was connected or
disconnected. Nevertheless, a filter was placed in series with the power supply for the rotary valve.
The filter slightly reduced noise from the rotary valve, but the detectors operated adequately even
without the filter.
3.4
3.4.1
Warm Electronics
Breakout Box
The striplines bring the signal from the SQUET card (see section 2.5) to the breakout box, a room
temperature vacuum interface. The breakout box is a cylindrical vacuum tight vessel, with a flange
containing 8 male-male 50 pin sub-D connectors. On the inside of the sub-D connectors, striplines
or custom wiring is connected. Outside of the breakout box, either custom room temperature wiring
connects the cryostat wiring to the resistance bridge for four wire measurements (see section 3.5.2.2)
or a DCRC board (see section 3.4.2) is connected to the detectors through the striplines. The QTF
has the breakout box mounted to the top of the cryostat. A picture of the breakout box can be seen
in Figure 3.9.
56
Figure 3.9: The breakout box at the QTF. 8 male-male 50 pin sub-D connectors are welded to a
flange to allow an electrical connection while maintaining an interface between room pressure and
vacuum. Connected to the bottom left side of the breakout box, custom room temperature wiring
can be seen which connects to the resistance bridge (described in section 3.5.2.2).
3.4.2
DCRC board
The DCRC accomplishes many tasks required to read CDMS detectors including: warm electronics of the readout circuit, signal digitizer, trigger control and computer interface. A picture of the
DCRC board connected at the QTF can be seen in Figure 3.10. After extensive testing, this new
board was able to take data with all four phonon sensors and both charge channels simultaneously.
3.5
Unique Properties of Queen’s Test Facility
Many attributes of the QTF are different from other CDMS test facilities. The manner in which
the cold hardware is mounted and the equipment is stored is unique to the QTF. Custom work was
also performed on the cryostat including the installation of custom wiring (section 3.5.2.3) and the
design of hardware (section 3.5.3) to be used for measuring superconducting tungsten films.
57
Figure 3.10: The DCRC board connected to the breakout box. The ethernet cable is used to
connect the DCRC board to the internet. The twisted pair connected to the end of the DCRC
board are used as a power supply to the board. The red alligator clip on top of the board is used
to ground the board to the breakout box.
3.5.1
Cold Hardware Setup
At the QTF, the cold hardware is mounted in the cryostat differently than most other CDMS test
facilities or at Soudan. The cold hardware is mounted as shown in Figure 3.11.
3.5.2
Cryostat Wiring
To perform tests on samples in the cryostat, extra wiring had to be installed. This allowed for
measurement of TES samples, adding extra thermometers and adding extra heaters.
3.5.2.1
Four Wire Measurement
In order to measure the resistance of any sample in the cryostat, a four wire measurement was
required. Some samples measured had extremely low resistances and the resistance of the wires leading to that sample could be orders of magnitude larger than that of the sample itself. Consequently,
measuring the resistance across the wiring would not be a successful means to determine the resistance of the sample. Four wire measurements can avoid this problem. The method of performing a
general four wire measurement is seen in Figure 3.12.
58
Figure 3.11: The CDMS tower mounted to the base plate of the cryostat. Six 1/4 inch diameter
copper rods were used to mechanically and thermally connect the CDMS cold hardware to the
base plate. The cold hardware was assembled as described in section 2.5.
3.5.2.2
Resistance Bridge
In order to perform the four wire measurements, the LakeShore 370 AC Resistance Bridge, as
seen in Figure 3.13, was used.
The scanner attached to the resistance bridge automatically scans through the channels while the
resistance bridge performed four wire measurements on each channel. Two factors are controlled by
the software: scan time and bias voltage. When switching between different channels, the resistance
bridge unfortunately sends a current spike into the sensor on which it is to perform a measurement.
59
Figure 3.12: Four wire measurement. Four wires, labeled V+, V−, I+ and I− are connected to
opposite sides of a sample as shown. A current is sent through the I+ and I− wires. The V+ and
V− wires have no current going through them, but can measure a voltage drop across the sample.
Thus, the resistance of the sample can be measured independently of the resistance of the wiring
by R = VI . Twisted-pair wires are used to reduce electromagnetic interference.
This current spike heats the sensor and consequently changes the measurement. Increasing the measurement time allows the sensor to stabilize. The bias voltage also affects the temperature, where a
higher bias voltage would heat up the sensor causing a different measurement. However, when the
bias voltage is too low, the measurement becomes less accurate and has a higher noise level. Scan
time and bias voltage has to be optimized depending on the measured sensor and the temperature.
3.5.2.3
Custom Wiring
Various types of wiring were required at different stages in the cryostat in order to perform measurements on samples. The two main considerations when choosing wiring were resistance and heat
conductivity. In order to have an electrical connection between two places at the same temperature,
copper wiring is used as it has low resistance and its heat conductivity is irrelevant. From room
temperature down through 70 K and 4 K, a high resistance wiring is used because it has lower
heat conductivity and will reduce the heat load on those lower plates. From 4 K down to the base
temperature, superconductive niobium wiring is used. Niobium becomes superconductive below 9.2
K and has almost no resistance between these plates when cold. Superconducting materials also
have very poor heat conduction making them ideal for low temperature wiring. [14]
60
Figure 3.13: Resistance bridge (top) and readout/control interface (bottom). The resistance bridge
on top is connected to the four wire measurements. Wiring below the electronics box leads into the
cryostat to perform four wire measurements. The screen capture seen below is the resistance bridge
control software. The squares indicate which thermometers will be measured. Measurements
usually take between 5 to 10 seconds. Once a measurement is completed, the resistance is displayed
along with the temperature obtained from a calibration used by the software. The resistance bridge
continues on to measure the next channel. Measurement times and values are saved in a log file.
Two different attempts were made to set up wiring. The first attempt used niobium and Evanohm
wiring available at Queen’s University. The Evanohm and niobium wires were spun into 8 twisted
pairs. The wires were heat sunk at each temperature stage by wrapping them around copper posts
screwed into plates at the respective stages. These wires were then glued to the posts with General
Electric varnish (GE varnish). To connect the different wires at the 4 K and base plates, custom
designed connectors were used as shown in Figure 3.14.
61
Figure 3.14: The first attempt at wiring in the cryostat. Wires were heat sunk by wrapping them
around copper posts and gluing them with GE varnish. Electrical connections were caused by
clamping wires with screws to opposite sides of copper tubes.
Many problems arose in attempting to install wiring in this manner. Screwing the two wires
into a connector was not simple and mechanical connections are difficult to use in order to maintain
an electrical connection for these wires. Also, either due to degradation of the wires from age or
because GE varnish partially dissolved the insulation around the wires, shorts between the wires
and the cryostat frequently arose. In order to improve on this situation, commercial twisted pair
wire bundles and plug-in connectors were used to replace the screw connectors.
The same setup with high resistance wiring, superconductive wiring and copper wiring, as previously used was repeated. Heat sinks for the wiring and connectors were designed in accordance with
geometric specifications of the cryostat as shown in Figure 3.15. Heat sinks which did not require
any gluing were also designed as shown in Figure 3.16. Room temperature wiring from the breakout
box to the resistance bridge was twisted pair copper wiring. A schematic of the wiring which was
installed can be found in Figure 3.17.
62
Figure 3.15: Connectors used for custom wiring. Top: 4 K plate connector, constantan wiring is
connected coming from room temperature and niobium wiring is connected going to the lower
plates. Bottom: base plate connector, niobium wiring is connected coming from upper plates and
copper wiring is connected going to sensors on the base plate.
Superconductive wiring is ideal for conducting electricity and insulating against heat; however,
if the used niobium wire is not superconducting, its fairly high resistance limits the measurements.
Therefore, the resistance of the installed niobium wiring was measured as a function of temperature.
The niobium wiring was plugged in at the 4 K plate and shorted at the baseplate such that the
roundtrip resistance of two wires connected in parallel could be measured. Throughout this test,
the transition temperature and residual resistance of the niobium wiring were found. Even at the
lowest temperature ( 4 K at the top end), a finite resistance of 6 Ω was found. A heat link between
the 70 K plate and the 4 K plate connector through the high resistance wiring, can heat the top end
of the niobium wiring causing it to remain normal conducting. Results from the test can be seen in
Figure 3.18.
63
Figure 3.16: Cold plate heat sink used on custom wiring. This heat sink design was used on all
plates where the wiring was clamped to copper plates attached to the cryostat.
Figure 3.17: Wiring readout channels. Two lines close together indicate a twisted pair. Two
twisted pairs exist for every channel to perform a four-wire measurement. Four channels required
custom wiring from the resistance bridge, through the breakout box and down to the 4 K plate.
Two channels already existed of preinstalled wiring from the resistance bridge to the 4 K plate
using a high resistance wiring. All extra channels from 4 K to the base plate consisted solely of
niobium wiring that was installed at Queen’s University. Any connection between two components
at the same temperature was done using copper wiring.
64
Figure 3.18: Niobium wiring transition curve. When cooling down, all plates below 4 K have
similar temperatures to the 4 K plate. Once they drop below 10 K, there is a rapid transition on
all plates. When warming up, once the pulse tube is stopped, the 4 K plate rises above 10 K
within a few minutes, while the base plate contains the mixture and remains below 4 K for a
longer time. Consequently, while the 4 K plate is hotter, parts of the wiring are still under the
transition temperature. In the warm-up curve, the niobium wiring resistance increases steup by
step as different plates rise above the transition temperature.
3.5.3
Custom Designed Hardware
For the purposes of wiring and sensor testing, copper structures and heat sinks were created for
the Queen’s Test Facility. This custom hardware includes:
• Heat sinks for the custom wiring at every stage;
• Heat sinks for the wiring connectors;
• Sample holders for the TES sensors; and
• Plates to mount sensors into CDMS hardware. (see section 6.3.)
Schematics for all these parts can be found in Appendix B.
65
3.5.4
Contamination Control
3.5.4.1
Clean Room
The QTF is the first test facility for the CDMS experiment has been installed in a permanent cleanroom. Equipment required for the mixture circulation is problematic for placement in a cleanroom
as it takes up a significant amount of space and some equipment (like the vacuum pump) releases
dirty exhaust which is not acceptable in cleanrooms. The only parts of the cryostat required to
be placed inside the clean room are the five plates. Since all the circulation lines are attached on
top of the supporting plate (above the 70 K plate), the supporting plate was implemented in the
cleanroom ceiling. This arrangement allows all the circulation, readout equipment and computer
interface to be placed outside the cleanroom. A picture of the entire setup can be seen in Figure 3.19.
3.5.4.2
Purge Cabinet
An acrylic purge cabinet was installed inside the clean room. The air inlet line into the cabinet
was connected to the vent line of a 200 L liquid nitrogen dewar. The evaporating nitrogen gas
from the dewar is sent into the purge cabinet. The outlet is a copper tube placed into a bottle
of mineral oil. The mineral oil does not evaporate and is an easy indicator of the flow rate into
the purge cabinet based on the visible bubbles coming from the copper tube. The mineral oil also
prevents back diffusion of air from the outlet into the purge cabinet. The gas being flushed into
the purge cabinet is essentially oxygen and radon free since these elements are found in negligible
quantities within liquid nitrogen. This prevents oxidation of the copper hardware used in CDMS
and radioactive contamination of SuperCDMS detectors while stored awaiting testing.
66
Figure 3.19: The QTF cryostat mounted inside the cleanroom. The steel frame with the clear
plastic walls of the cleanroom can be seen. Mounted inside the cleanroom is the cryostat, which is
mostly closed. The red can connected is the top part of the of the outer vacuum can. The
aluminum can is the radiation shield connected to the 70 K plate. All circulation inlets and outlets
that are connected to the cryostat are located on top, outside of the cleanroom.
67
Chapter 4
TES and Detector Testing
4.1
Detector Characterization
As explained in section 2.3.1, every phonon sensor has 37 dies per sensor and 28 QETs per die,
resulting in 1036 QETs per sensor. It is not always possible to create the entire phonon sensor with
a consistent Tc across the surface. In order for the phonon sensor to work, all the TESs must be
simultaneously held in the transition region between superconducting and normal conducting.
If the TES sensors are fully in the superconducting region or fully in the normal region, an
increase in temperature would not cause a change in resistance. The method of biasing all TESs in
parallel, as seen in Figure 4.1, is successful at forcing all TESs to simultaneously be in the transition
region.
However, if there is too much of a discrepancy between the lowest and the highest Tc , then, even
when one is fully normal, another one may still be superconducting. As long as one of the TES
sensors is still superconducting, the phonon readout will not work properly, since this one TES shorts
the whole sensor and no change in resistance is measured. If the TESs with a high Tc are forced
into the transition region through a higher bias, the sensors with the lower Tc will be normal, thus
reducing the sensitivity of the sensor. If variations in Tc are known, they can be adjusted through
ion implantation [15].
68
Figure 4.1: Model curve overlaying the transition curves of multiple TES sensors in parallel [9].
The dots indicate the resistance of each TES for a specific applied current. In this state, an applied
bias is causing all the TESs to be in the transition region simultaneously, although the sensors
have different transition temperatures ranging over approximately 10 mK. Initially, the TES with
the lowest Tc will start transitioning from the applied current. Once this happens, its resistance
increases and the current mostly goes through the other TESs which are still superconducting.
This increase of current in these TESs would cause other TES samples to go superconducting
forcing more current into the TES samples with the highest Tc . Eventually, they will all be held on
the transition curve.
4.1.1
IbIs Measurements
IbIs measurements involve adjusting the bias current (Ib ) and measuring the current going through
the sensor (Is ). The schematic of this circuit is shown in Figure 4.2.
IbIs tests are performed by holding a detector sensor at a specific temperature and varying the
bias current, starting from very high positive currents down through very low negative currents.
When performing this test, the sensor starts in the normal region at a high positive bias current.
As the current drops, the sensor transitions into the superconducint region and goes normal again
at high negative current. A simulation of what an IbIs curve would look like can be seen in Figure 4.3.
69
Figure 4.2: Schematic of the TES biasing circuit. (This is identical to the left side of Figure 2.7.) Ib
is the applied current to the entire loop. Is is the component of the current going through the TES.
Figure 4.3: A simulation of an IbIs curve for a phonon sensor. The three different regions:
superconducting, transition and normal are labeled. The diagram shows that as the temperature
increases, the critical current decreases. The transition temperature is defined as the temperature
at which it is normal for 0 bias current. For this sensor it would be 70 mK.
70
The simulation was created by modeling the resistance of a sensor as a function of Ib . At low
currents, the sensor is superconducting, at high currents the sensor is normal and in the transition
region a cosine wave is used to model a smooth resistance change as a function of Ib . Equation 4.4
(derived below) was used to calculate Is as a function of Ib and the sensor resistance.
The general equation for the IbIs measurement can be calculated as follows:
Let Ish be the current through the shunt resistor
Rsh be the shunt resistance
Rs be the sensor resistance
Vsh be the voltage drop across the shunt resistor
Vs be the voltage drop across the sensor
The bias current splits between the two branches of the circuit (in Figure 4.2):
Ib
= Is + Ish
(4.1)
The voltage drop across both sides of the circuit is equal:
Vsh
= Vs
Ish
=
Is Rs
Rsh
(4.2)
(4.3)
Combining equations 4.1 and 4.3 and solving for Is
Is
=
Ib
s
1 + RRsh
(4.4)
To determine the resistance of the TES equation 4.4 can be rearranged:
Rs
= Rsh (
Ib
− 1)
Is
(4.5)
When the TES is superconducting (Rs = 0), all of the applied current goes through the sensor;
thus, Is = Ib which is a straight line with a slope of 1 in the IbIs plot.
71
When the sensor is normal,
Rs
Rsh
>> 1; thus Is = Ib RRsh
, which gives a straight line with a very
s
small slope as seen in the normal region of Figure 4.3. By obtaining a full IbIs profile, the phonon
sensor is fully characterized [19]. Two CDMS detectors, named G31 and S7, were characterized at
the QTF.
4.1.2
Detector Measurements
In order to demonstrate the capability of the QTF to measure and characterize detectors, two
detectors, G31 and S7, were tested. An example of the raw data collected from an IbIs sweep of
G31 can be seen in Figure 4.4.
Figure 4.4: Raw data from an IbIs measurement of G31. The jumps seen around an Ib of 100 µA
are SQUID jumps caused by readout electronics.
72
SQUID jumps are an intrinsic feature of the readout circuit as explained in section 2.3.2. The
SQUID jumps for each curve were not identical and introduced y-offsets in different areas. In order
to combine the IbIs plots at different temperatures into one plot to compare critical currents, the
jumps had to be removed and the plots had to be centred. All the IbIs plots for a given sensor were
then combined into one. A combined plot for G31 sensor A can be seen in Figure 4.5 .
Figure 4.5: IbIs curves for G31 sensor A. As temperature increases, the critical current decreases
until it disappears at 86 mK. This is the critical temperature, Tc , above which the sensor is always
normal conducting, independent of the current.
The IbIs curves always have an asymmetry due to self-heating: when in the normal region, an
applied current heats the sensor and prevents it from transitioning. When already in the superconducting region, no heating occurs and the sensor can stay superconducting at higher currents.
While the superconducting resistance is close to 0, there is always a parasitic resistance. This
resistance can be found from the superconducting slope of the IbIs curve using equation 4.5.
73
Ib
Is
is the inverse of the slope of the IbIs curve. As long as the Rsh is known, the superconducting
and normal conducting resistances can be calculated. Measurements were made of Rsh and can be
found in section 4.2.
IbIs measurements were also performed on a silicon detector (S7) and rendered similar results.
All IbIs curves for detectors G31 and S7 can be found in Appendix C. Figure 4.6 demonstrates how
the critical current varies as a function of temperature.
Figure 4.6: Critical currents measured for detector G31 when going from normal resistance to
superconducting. As temperature increases, the current required to cause a sensor to go normal
decreases. When the Tc is reached, the critical current goes to 0.
Table 4.1 lists the Tc values of the measured G31 and S7 sensors. These values were slightly
different than what was measured previously at other test facilities. This may be an indication that
the thermometry at the QTF was not accurate to more than a few mK at temperatures greater than
50 mK.
74
Table 4.1: Tc values for two CDMS detectors
Detector
G31
S7
4.2
Phonon
Channel
A
B
C
D
A
B
C
D
Tc (mK)
99
107
99
86
93
89
92
92
SQUET Card Measurements
The nominal value of the shunt resistor used in the CDMS readout electronic circuit (see Figure
4.2) is 20 mΩ and the coil in series with the TES is assumed to have no resistance. Both of these
values were tested by modifying the wiring in the tower to perform four wire measurements on the
SQUET card. There are two SQUET cards available at the QTF and results from tests on both
cards are listed in Table 4.2.
Table 4.2: Shunt and Parasitic Resistance of SQUET cards ZC-17 and ZC-35.
SQUET
Card
ZC-17
ZC-35
Phonon
Channel
A
B
C
D
A
B
C
D
Rsh (mΩ)
at 800 mK
20.2
13.7
14.1
19.6
21.8
16.3
13.7
17.3
Rcoil (mΩ) at
800 mK
0.3
0.7
2.5
1.1
2.5
2.8
8.5
3.3
75
Rsh (Ω) at
291 K
14.1
10.1
9.7
11.8
14.2
11.4
8.4
11.8
Rcoil (kΩ)
at 291 K
13.5
0.7
28.6
14.8
7.1
0.6
15.1
7.5
Rsh and Rcoil values at 800 mK are accurate to ±0.2 mΩ. Rsh at 291 K was accurate to ±0.1
Ω and Rcoil at 291 K was accurate to ±0.1 kΩ. The measured values for the sunt resistors range
from 14 mΩ to 22 mΩ. This range is surprisingly large given that the measurement uncertainty is
much less than 1 mΩ. There was also a fairly strong correlation between the resistances at room
temperature and 800 mK.
4.3
4.3.1
TES Testing
Tungsten Films
Four tungsten samples on silicon wafers were shipped to the QTF from the CDMS collaborators
at the University of Santa Barbara. A picture of the four sensors can be seen in Figure 4.7. In order
to test TESs, sample holders were designed as seen in Figure 4.8.
GE varnish was always used to glue the TES samples to the sample holders because it conducts
heat well at cryogenic temperatures, insulates against electrical currents and is soluble in alcohol, so
that the sample is easy to remove. The bonding pads are 2 mm × 3 mm × 5 mm insulating material
with a copper surface on top. The readout wiring was soldered to half of the bondpad while the
other half of the copper surface was left for wirebonding to the TES.
4.3.2
TES Measurement Setup
Once the sample holders were made, the TES samples were mounted on the holders at the base
plate of the cryostat. Different problems with the setup of the cryostat prevented TES samples from
transitioning.
The TES samples were initially placed on the sample holder without any cover. While the sample had a strong heat link to the base plate, thermal radiation from the 600 mK stage was able to
warm the tungsten film. Small boxes were created out of copper tape and taped to the base plate
around the samples. This ensured that only thermal radiation at base temperature would affect
the tungsten film. An experiment was later conducted to verify this, where two TES samples that
76
Figure 4.7: Four tungsten samples at the QTF. Starting from top left going counter clockwise, the
samples are L23#1, L23#2, L23#3 and L23#4.
had previously been measured were placed in the cryostat, one with a cover and one without. The
sample with a cover transitioned and the sample without a cover did not.
Initially, high frequency noise warmed the sensors. This was resolved by connecting a 1 nF Pi
50 pin sub-D filter between the room temperature wiring and the breakout box. A similar problem
existed with the detectors, where the phonon sensors did not become superconducting unless the
filter was connected between the breakout box and the DCRC board.
In order to measure a transition properly, the bias current from the resistance bridge was controlled carefully. A lower bias voltage caused resistance measurements to be less accurate, while a
higher bias voltage warmed the sensors and prevented them from transitioning. In order to achieve
accurate measurements, multiple attempts at different bias currents were generally required to determine the optimal bias current.
77
Figure 4.8: Two different sample holders for TES samples. The top image shows the sample holder
with two chips glued onto it using GE varnish and mounted on the base plate. Wirebonds are
connected between two sides of the TES and the bonding pads. The bonding pads are glued to the
sample holder. Four copper wires in two twisted pairs are soldered onto the bonding pads to
perform a four wire measurement. The copper wires are then connected to the wire connector seen
in Figure 3.15.
4.3.3
TES Measurements
An example of a TES transition measured at the QTF can be seen in Figure 4.9. In the transition
curve, the Tc while cooling down is lower than when warming up. This effect can be attributed to
the same phenomenon described in section 4.1.2, where the normal conducting sensor self-heats and
does not go superconducting. This self-heating does not affect the sensor when it is superconducting
because there is no heating in the sensor when there is no resistance. A table of measured transition
temperatures of TES samples can be found in Table 4.3.
L23#3 a - b and L23#4 a - b are chips that were cut from the initial L23#3 and L23#4 chips to
be used in the composite detector experiment (see section 6).
78
Figure 4.9: Transition curve of a TES sensor. This sensor showed a relatively sharp transition
curve. A difference can be seen in transition temperatures between increasing and decreasing
temperature, similar to the effect explained in section 4.1.2.
Table 4.3: A list of measured films and corresponding transition temperatures. L23#3 a-b and
L23#4 a-b are samples that were cut from L23#3 and L23#4, respectively. Measurements were
accurate to approximately 1 mK.
Chip Label
L23#1
L23#2
L23#3
L23#3-a
L23#3-b
L23#4
L23#4-a
L23#4-b
Tc (mK)
56
54
61
61
58
55
57
56
79
Chapter 5
SiO2 Deposition Experiment
As explained in section 2.6.1, surface events are currently the largest source of background in
CDMS. Furthermore, discrimination against surface events requires an analysis that reduces the
nuclear recoil detection efficiency by 50%.
By depositing a thin insulating layer of SiO2 , charge released during a surface event will be
trapped at the surface of the crystal and will not drift into the electrode. This insulating layer
would prevent a reduced charge signal from surface events which result in measurements similar to
those from nuclear-recoil events.
5.1
Deposition System
For the deposition of thin layers of SiO2 and Al on a substrate, the Thermionics Laboratory 3kW
RCL linear e-gun ion beam evaporator, as shown in Figure 5.1, was used [1].
The substrate is placed on a sample holder located at the top of the bell jar. The materials for
deposition (SiO2 and Al) are placed in water-cooled crucibles, 30 cm below the sample holder. In
order to prevent the deposited material from being affected by gas molecules inside the bell jar, deposition must occur in a low-pressure environment. Using a roughing pump and a cryopump, the bell
jar is evacuated to 2 × 10−6 torr. A tungsten filament is then heated to the point of thermionic emission by an electric current. The free electrons are accelerated away from the tungsten filament and
80
Figure 5.1: The Thermionics Laboratory 3kW RCL linear e-gun ion beam evaporator. In the centre
of the top of the picture is the bell jar with an implosion guard. On the left side on the ground is
the roughing pump which evacuates the bell jar from atmospheric pressure to 3×10−2 torr.
Beneath the bell jar is the cryopump which pumps down from 3×10−2 to 2×10−6 torr. On the
right side beside the bell jar is the electron beam controller, pressure gauge and deposition gauge.
can then be focused to interact directly with the material in the water-cooled crucible. This causes
the material to vaporize. The atoms undergo ballistic motion and are deposited on the substrate. [12]
Before deposition of the particles begins, four variables of the electron beam must be controlled:
power, mean position, position oscillation magnitude, and position oscillation frequency. To prevent
the particles from being deposited on the substrate before the electron beam is properly adjusted, a
shutter is used to block the substrate as the material is heated. In order to determine the appropriate
beam settings, a crystal oscillator is used to measure the deposition rate of the particles.
The material used for deposition on the substrate must be taken into account when determining
the appropriate settings for the electron beam. Certain materials, such as aluminum, melt inside
the crucible before vaporizing. Since aluminum has high thermal conductivity, the entire contents
81
of the crucible will be heated even without any oscillation in the electron beam (i.e. if the electron
beam remains fixed in one location). However, when performing the deposition with materials that
are poor heat conductors (such as SiO2 ), only material that will come into contact with the beam
will vaporize. Without any oscillation, the beam would burn a small hole through the material and
hit the bottom of the crucible. As a result, the electron beam must be set to oscillate in both the x
and y directions, allowing it to interact with all the material in the crucible.
Once the desired electron beam settings are achieved and the deposition rate is stable, the shutter
is opened and particles begin to deposit on the substrate. When the desired thickness of particle
deposition is achieved, the shutter is closed and the electron beam turned off. The crucible can then
be alternated so that another material can be evaporated onto the substrate. When all the desired
depositions have been completed, the substrate is left to cool, the bell jar is vented and the sample
is removed.
5.2
Slide Deposition
Before SiO2 and Al were deposited on the germanium crystal, many test depositions were completed on glass slides. The goals of these depositions were as follows:
1. Gain familiarity with the potential challenges of the deposition experiment using inexpensive
glass slides.
2. Calibrate the deposition sensor with the actual deposition thickness; and
3. Determine the minimum thickness required for a SiO2 layer to insulate;
4. Determine how the thickness of the deposited material varies across the surface;
In order to determine the minimum required thickness for a SiO2 insulating layer, two aluminum
layers were deposited and separated by a SiO2 layer. Both aluminum layers were then connected to a
digital multimeter (DMM) and the resistance between them was measured. Four different deposition
methods were attempted in order to accomplish this.
82
5.2.1
Deposition Method 1
The first method attempted was to deposit a layer of SiO2 between two Al layers without changing
any settings in the bell jar. All three layers were 100 nm thick and equal in area, one on top of the
next one. This method was only performed once.
Since all three depositions could occur in sequence without requiring any adjustments in the
deposition chamber, there were two main advantages to this method. First, opening the bell jar
may expose the sample to dust and air which can ruin the subsequent depositions. Second, it takes
a long time to evacuate the bell jar to the required pressure. This method only requires evacuating
the bell jar once and all depositions can then be performed without exposing the sample.
When testing whether the SiO2 layer properly insulated between the two Al layers, many problems arose. It was impossible to access the bottom Al layer since it was completely covered by the
upper layers. Multiple methods were used to remove the top two layers to expose the bottom layer.
NaOH was used to etch a section of the top layer of Al, leaving an unetched part as a contact. It
was expected that the Al layer would dissolve while leaving the layer below it intact since SiO2 does
not dissolve in NaOH. However, the etch did not consistently remove the Al section, and appeared
to damage the SiO2 layer below it. The SiO2 was then scraped off to expose the bottom surface.
To ensure that there was a strong electrical contact between the lead from the DMM to the bottom surface, two places on the bottom surface were electrically connected. Once this was successful
and a proper electrical contact was ensured, a connection between the bottom and top Al layers
could be tested. In all cases, measuring across the SiO2 layer rendered a low resistance. Multiple
factors could have caused this including: damage done by the NaOH to the SiO2 layer or a touch
between the two Al layers along the perimeter. To address the problems incurred during the first
deposition, the method was modified.
5.2.2
Deposition Method 2
The second method used the shutter to control the area of the substrate covered by the deposited
material. The shutter is usually used to completely cover the sample while initially heating the
83
target material. Once a steady evaporation rate is reached, the shutter is opened and left open until
the desired thickness is reached. For this method, the shutter was operated manually, allowing a
portion of each layer to remain uncovered by the layer deposited above.
Since using the shutter avoids any requirements to open the bell jar, similar advantages were
perceived for this method as for the first method. An additional perceived advantage was that no
removal (i.e. etching or scraping) of the top two layers was required after deposition. The deposition
occurred as shown in Figure 5.2.
Multiple depositions were performed in this manner. Before deposition, the glass slides were
cleaned with soap in an ultrasonic cleaner followed by a rinse in ultrapure water and methanol.
Aluminum layers were deposited to a thickness of 100 nm and SiO2 layers were deposited to varying
thicknesses ranging from 50 to 300 nm. In all cases, not one successfully insulating layer was observed
using this method. Many different factors could have caused a contact including:
• Both aluminum surfaces contacting at the edges.
• Dirt and residue on the glass slide. Upon examination, crystal structures were seen on the
glass slides (as shown in Figure 5.8). These are believed to be caused by soap and water
residue after cleaning the slide. Small amounts of dust were also on the slide. These pieces of
dust were sometimes thicker than the SiO2 layer allowing a contact between the two aluminum
layers.
• A thin aluminum layer contact could have occurred between the two aluminum layers across
the SiO2 layer. The shutter is located about 2 cm away from the slide. If the pressure was not
low enough, aluminum particles could have collided with air molecules and diffused around the
shutter, providing a thin layer of contact between the two layers. However, this is unlikely.
Using glass slides as masks instead of the shutter ended up being a more successful approach as
demonstrated in sections 5.2.3 and 5.2.4.
5.2.3
Deposition Method 3
The next attempt was performed using glass slides as a mask. A diagram of the production steps
can be found in Figure 5.3. Many depositions were performed in this manner. Aluminum layers
84
Figure 5.2: The second method of deposition described in three steps. Light grey indicates
aluminum and blue indicates SiO2 . The shutter’s initial location is below the slide. (i) A layer of
aluminum was deposited across the entire surface of the slide. (ii) The shutter was then moved up
slightly, protecting part of the Al layer, when depositing the SiO2 layer. (iii) For the deposition of
the final (top) Al layer, the shutter was moved up even farther to prevent direct contact between
the Al layers.
were deposited to a thickness of 100 nm and SiO2 layers were deposited to varying thicknesses of 50
- 300 nm. SiO2 layers thicker than 300 nm were successfully shown to be insulating.
Different methods were employed to avoid any problems caused by performing the measurements
themselves. Two possible problems were (1) the bias voltage applied and (2) breaking the SiO2 layer,
causing a physical contact between the two Al layers.
The highest voltage output from the DMM is 2.8 V [49] which would cause an electric field of
V
over 250 nm. This can cause a breakdown in the SiO2 layer causing it to conduct. In
1.1×107 m
order to properly control the bias voltage, the resistance bridge was used to perform a four-wire
measurement with a low voltage. Initially, the 250 nm layer was connected with the maximum bias
voltage (200 mV) set on the resistance bridge. The SiO2 layer was insulating, but then small sparks
were observed on the surface after which the layer became conducting.
85
Figure 5.3: The third method of deposition described in three steps. Light grey indicates
aluminum and blue indicates SiO2 . (i) A layer of aluminum was deposited across the entire surface
of the slide.(ii) The bell jar was then opened and two glass slides were used as a mask to cover the
ends of the aluminum layer while depositing the SiO2 layer. This allowed a contact between two
points across the entire bottom aluminum layer. (iii) The bell jar was opened again and four glass
slides were used as masks on the SiO2 layer, to deposit an inner Al layer bounded on all sides by
the SiO2 layer. This prevented contact between the two aluminum layers.
Because of the geometry of methods 1, 2 and 3, touching the upper layer of aluminum could
break through the SiO2 layer and create a contact between the two aluminum layers. In order to
try to avoid this, another deposition of 50 nm SiO2 was performed and a thin piece of indium was
placed on the surface of the new sample. Then, tweezers were used to lightly contact the lead from
the resistance bridge to the surface which did not appear to have any effect on the resistance.
86
5.2.4
Deposition Method 4
One possible cause of problems in previous deposition methods was the inability to contact the
top surface of Al without causing a breakthrough in the SiO2 layer and touching the bottom Al
layer. A new deposition method was designed to allow for a contact to both Al layers without a
possible breakthrough of the SiO2 layer [2]. A picture of the layout using this method can be seen
in Figure 5.4.
Figure 5.4: The fourth method of deposition described in three steps. Light grey indicates
aluminum and blue indicates SiO2 . (i) A layer of aluminum was deposited across part of the
bottom of the slide. (ii) Glass slides were then used as a mask to deposit the SiO2 over part of the
bottom Al layer and part of the glass. This allowed a contact with the bottom aluminum layer.
(iii) Then glass slides were again used as a mask to deposit a top layer of Al partially over the
initial Al layer (blocked by the SiO2 ) and partially directly onto the glass slide, which allowed for a
contact with both aluminum layers without having to risk breaking though the SiO2 .
After the first attempt with a 50 nm thick SiO2 layer, a measurement was performed with the
resistance bridge. Initially, the lowest bias voltage was set and shown to be insulating. The bias
voltage was slowly increased and was ultimately shown to insulate with a 200 mV bias (the maximum output from the resistance bridge). This was a confirmation that 50 nm of SiO2 insulates and
can be used on the detector deposition.
87
5.2.5
Deposition Method Summary
Four different deposition methods in total were used. A summary of their advantages, disadvantages and results can be found in Table 5.1.
Table 5.1: Comparison and results of various deposition methods
Method
1
Description
Al/SiO2 /Al,
one on top
of the other
Advantages
Do not need to
vent
between
deposition
Disadvantages Insulating
Difficult
No
to
access
bottom layer
2
Shutter used
to
expose
bottom layer
Do not need to
vent
between
depositions
Possible
connections
at edge of
deposition
area
No
3
Use
glass
slides
as
mask to form
concentric
rectangles
Use
glass
slides
as
a
mask
between
depositions
to
form
partially
overlapping
aluminum
surfaces
Accessible bottom layer and
no connections
at boundaries
Venting
required
between
depositions
Yes (300 nm)
but not less
Easy connection
to both aluminum layers
Venting
required
between
depositions
Yes (50 nm)
4
88
Comments
Difficult to
properly
etch upper
layers
Used
to
measure
thickness
variations
across
the
surface
Not
successful
for
thin layers of
SiO2
Worked immediately
5.3
Deposition Thickness Measurements
After performing depositions, the thickness of different layers was measured to determine:
1. Surface roughness
2. Accuracy of the deposition gauge
3. Thickness variation as a function of position
In order to accomplish this, the Dektak 8 stylus surface profiler [3] as seen in Figure 5.5, was
used.
Figure 5.5: Dektak 8 Stylus Profiler.
The sample is placed on a tray and loaded into the apparatus. The stylus is then lowered and
scans 50 µm across the surface. Variations in the surface height cause the stylus to move up and
down, giving accuracy in the height measurement on the order of 5 nm. Thickness measurements
were performed for deposition methods 2 and 3. A separate deposition was completed where the
thickest possible layer of SiO2 , constrained by the mount of SiO2 that fits in the crucible, was deposited. This deposition was used to calibrate thick SiO2 layers.
89
5.3.1
Measurement Using Deposition Method 2
When using deposition method 2, two edges going lengthwise across the surface are formed, one
from the SiO2 surface onto the bottom aluminum surface and one from the top aluminum surface
to the SiO2 surface. Because the lines span the length of the slide, it allows for thickness variation
measurements along the length. Fifteen scans of a slide were performed as indicated in Figure 5.6.
Table 5.2 contains measured thicknesses.
Figure 5.6: Scan position for layers deposited using method 2. Scans 1, 2 and 3 measured the drop
from the SiO2 to the bottom Al layer. Scans 4, 5 and 6 measured the roughness across the bottom
Al layer. Scans 7, 8 and 9 measured the roughness across the SiO2 layer. Scans 10, 11 and 12
measured the drop from the top Al layer to the SiO2 layer and scans 13, 14 and 15 measured the
roughness of the top Al layer.
Table 5.2: Measured thicknesses from deposition method 2. Error is defined by the standard
deviation about the mean.
Scan
Label
Layer
Scanned
1
2
3
4
5
6
7
8
SiO2 -Al
SiO2 -Al
SiO2 -Al
Al
Al
Al
SiO2
SiO2
Thickness/
Surface
Variation
(nm)
200
180
160
10
20
20
10
5
Error
(nm)
Scan
Label
Layer
Scanned
30
100
100
5
5
5
5
5
9
10
11
12
13
14
15
SiO2
Al-SiO2
Al-SiO2
Al-SiO2
Al
Al
Al
90
Thickness/
Surface
Variation
(nm)
10
120
160
60
5
5
5
Error
(nm)
5
20
20
20
5
5
5
In this specific deposition, both aluminum layers were 100 nm thick and the SiO2 layer was
150 nm thick, according to the deposition gauge. Two examples of scans, one performed across a
SiO2 /Al border and one performed across an Al layer, can be seen in Figure 5.7.
Figure 5.7: Two measured profiles of layers deposited on a glass slide using deposition method 2.
The graph on the left, whose position is defined in Figure 5.6, shows the boundary between the
SiO2 layer and the Al layer. This graph shows that the actual height of the SiO2 layer was
approximately 160 nm when the deposition gauge measured 150 nm. This measurement was
consistent with the accuracy determined from other scans which was approximately 5%. The
graph on the right is a scan across a flat area on the top aluminum layer.
Spikes in the height from the scan were caused by impurities on the glass slide surface. In future
depositions, more care was taken to keep the slides clean and the size of the spikes was reduced.
In some traces, massive spikes were recorded on the order of a µm as can be seen in Figure 5.8.
They were caused by crystal formation on the surface due to residue from the cleaning procedure.
The glass slides are pre-cleaned before purchase. Initially, they were cleaned again after purchase
using an ultrasonic cleaner. After observing the crystal formation that was suspected to result from
this second cleaning, depositions were performed without a second cleaning and it was found that
the initial pre-cleaning was sufficient.
91
Figure 5.8: A crystal formation and its thickness measurement. The left side shows a deposition
scan with a large spike at approximately 900 µm through the scan. The spike was caused by the
crystal formation seen in the right picture. This spike was probably due to residue from water and
soap on the surface.
5.3.2
Measurement Using Deposition Method 3
After depositing the three layers using the third method (see Figure 5.3), thickness measurements
were made using the profilometer. By scratching the surface down to the glass, an absolute measurement of the height of the bottom Al layer plus the SiO2 can be made. Measurements were also
performed by scratching a section of the glass without Al or SiO2 to ensure that the scratch in the
glass did not affect the thickness measurements. A sketch showing the deposition layout, scan and
scratch positions can be seen in Figure 5.9.
Initially, 100 nm of Al was deposited, followed by 200 nm of SiO2 and then another 100 nm of
Al. An example of a measurement from one of the scratches can be seen in Figure 5.10. Measured
thicknesses from this scan are summarized in Table 5.3.
92
Figure 5.9: Scratches and measurements performed on a glass slide using Deposition Method 3. A
deposition was performed on the glass slide using the method shown in Figure 5.3. After the
deposition was performed, a scalpel was used to make a scratch across the entire length of the
slide. A numbering system was put in place to identify the location of the scratches. Al and Si
followed by a number are located along the scratch. The number indicates the percentage of the
way along the scratch. Measurements from the second aluminum layer are labelled Al1 and are
numbered from 1 to 10 in their distance across the upper aluminum surface.
5.3.3
Maximum SiO2 Thickness Measurement
One other test that was performed was determining the maximum possible thickness attained
from the deposition. The limit on the thickness that can be attained for the SiO2 layer is given
by the amount of SiO2 that can be placed in the crucible. A test was performed to determine this
maximum thickness. Two glass slides were used as masks to be able to scan along the length of the
slide. The deposition gauge indicated a thickness of 700 nm while the scans performed on the slide
showed a thickness of approximately 900 nm (see Figure 5.11 for the scan positions and Table 5.4
for the results).
This deposition shows that without an initial aluminum layer, total thickness variation across
the surface is 5%. This variation is similar to what can be expected in the final deposition, when
the SiO2 layer is deposited directly onto the germanium crystal with no Al layer below. An example
of a thickness measurement from this set can be seen in Figure 5.12.
93
Figure 5.10: Thickness profile of a scratch through a 100 nm Al surface. The entire curve is
plotted in blue. Red sections of the curve indicate places before and after the scratch and the top
of the surface. Green sections of the curve indicate sections at the bottom of the scratch. From the
scratch, extra material was built up on either side causing spikes in the scan. To measure the
depth of the scratch, the average value of the red section was subtracted from the median value of
the green section. The yellow line indicates the median value of the scratch section and the black
line indicates the minimum value.
5.4
5.4.1
Germanium Crystal Deposition
Deposition Preparations
The glass slide deposition experiments proved that a 50 nm SiO2 layer was sufficient to insulate.
The thickness measurements proved that the calibration of the deposition gauge was sufficient for
the accuracies that were required.
94
Table 5.3: Measured thicknesses from deposition method 3
Scan Label
Layer
Scanned
Error
(nm)
Scan Label
Layer
Scanned
SiO2 -Al
Thickness/
Surface
Variation
(nm)
210
Al1 0
20
Si 2
Al1 5
SiO2 -Al
190
20
Si 25
Al1 9
SiO2 -Al
210
20
Si 5
Al 0
Al- SiO2
195
20
Si 75
Al 5
Al- SiO2
205
20
Si 9
Al1 9
Al- SiO2
200
20
Scratch
Al - Al
80
10
Scratch
Al - Al
100
10
Al Scratch
95
Al Scratch
99
Glass
Scratch
SiO2 SiO2
SiO2 SiO2
SiO2 SiO2
SiO2 SiO2
SiO2 SiO2
Al - Al
Scratch
Al - Al
90
10
Al
00
Al
01
Al
05
Thickness/
Surface
Variation
(nm)
230
Error
(nm)
245
20
235
20
230
20
225
20
110
10
Al - Al
100
10
GlassGlass
5
5
20
Figure 5.11: Deposition performed to measure the maximum thickness of SiO2 . Ten scans were
performed on the deposited layer and are labelled in the figure. These correspond to measurements
found in Table 5.4.
95
Table 5.4: Measurement of the SiO2 layer with maximum thickness
Scan
Label
1
2
3
4
5
Thickness
(nm)/Surface
Variation
950
940
925
910
900
Error
(nm)
Scan
Label
50
50
50
50
50
6
7
8
9
10
Thickness
(nm)/Surface
Variation
910
920
940
945
940
Error
(nm)
50
50
50
50
50
Figure 5.12: Sample scan of the maximum thickness measurement (scan 3). This measurement had
much less fluctuation in the height relative to measurements using deposition methods 2 and 3.
This scan demonstrated that the SiO2 layer was much more uniform than the aluminum layer and
that the maximum thickness is approximately 1000 nm.
Four different thickness layers of 0 nm, 50 nm, 300 nm and 1000 nm were chosen to be deposited
on the crystal:
• A layer without any SiO2 as a control;
• A thin insulating layer of 50 nm as a minimum thickness layer;
• A layer of 300 nm as a moderate thickness layer; and
• A layer of 1000 nm as a maximum depositable thickness layer.
96
A substrate holder was created to control the surface area where each layer would be deposited.
This holder prevents any deposition on the rim of the detector and any contact between the separate
aluminum electrodes on the detector surface. Each deposition was designed to cover just less than
half of one surface with a 5 mm gap between the different areas. A picture of the holder can be seen
in Figure 5.13. A schematic of the deposition geometry can be seen in Figure 5.14.
Figure 5.13: The holder used for deposition on the germanium crystal. The semicircle hole is the
opening for the materials to be deposited on the detector surface. A 2 mm rim was placed around
the edge to ensure that no aluminum would be deposited on the sides of the detector.
5.4.2
Test Depositions
Before performing the final deposition on the germanium crystal, a glass plate was obtained that
had the same shape as the germanium crystal. Four layers were deposited on the glass plate as a
test run. While the deposition was successful, after the deposition the 1000 nm thick SiO2 layer
began to flake. Figure 5.15 shows a picture of the glass plate with a flaking surface.
97
Figure 5.14: Thickness of SiO2 layers deposited on the crystal. A SiO2 layer with the thicknesses
stated above, followed by a 100 nm aluminum layer was deposited on each section. The aluminum
layer is deposited to act as an electrode. 5 mm was left in between the two electrodes to ensure
that no connection occurred and to provide a place to glue the TES sensors (see section 6.5). A 2
mm rim ensured that no aluminum was deposited along the sides of the detector.
This effect had not been seen before. A second test deposition was performed with a glass slide
instead of a glass plate. All four depositions were successful with the glass slide and no flaking
occurred.
5.4.3
Final Deposition
Immediately before the final deposition could be performed, the detector was immersed in a
solution of hydrofluoric acid (HF). This isotropically etched the germanium crystal, removing any
layers of oxidation on the crystal which would interfere with the deposition.
98
Figure 5.15: The glass plate with the layer of SiO2 that flaked off the surface.. Top surface is 1000
nm SiO2 . The layer at the bottom of the picture is the deposition without SiO2 . Possible reasons
for flaking are discussed in section 5.4.3.
Two SiO2 layers of 0 and 1000 nm were deposited on the top side of the detector, while SiO2
layers of 50 nm and 300 nm were deposited on the bottom side. A picture of the top side of the
detector after deposition, mounted inside the detector housing, can be seen in Figure 5.16.
A full description of the procedure can be found in Appendix A.
5.4.3.1
Corroding Surfaces
After the deposition finished, the two thicker surfaces began experiencing flaking. Pictures of the
flaking surfaces can be seen in Figure 5.17.
To attach the electrodes to the DIB board, the Westbond 7476E wire bonder was used. Using an
ultrasonic pulse, the wire bonder melts an aluminum wire onto the aluminum electrode. This method
is also used on CDMS detectors to electrically connect the sensors to the DIB board while causing
minimal damage to the sensor. The wirebonds did not properly stay connected and interactions
at flaking areas would not register a signal. Thinner layers of SiO2 proved to be more stable and
effective. Possible reasons for this degrading and flaking of the surfaces include:
99
• Tension caused by a difference in crystal structure between the SiO2 and the Ge;
• Air moisture attacking the barrier between the SiO2 and the Ge; and/or
• Residue from the HF etching of the germanium crystal.
Polishing the crystal in order to deposit the thicker SiO2 layers again would be very time consuming and would not ensure a successful second deposition. It was decided to continue with the
experiment and attempt to measure the thicker layers despite their flaking off.
Figure 5.16: The top side of the detector placed in the detector housing after the deposition. The
close side has a SiO2 thickness of 0 nm and the far side has a SiO2 thickness of 1000 nm. This
picture was taken before connecting the electrodes to DIB board.
100
Figure 5.17: The deposition surfaces after some aluminum has flaked off. Flaking did not occur on
the 0 nm and 50 nm surfaces. The 300 nm surface (left side) and the 1000 nm surface (right side)
experienced significant flaking, did not properly wirebond, and could not be used as electrodes.
101
Chapter 6
Composite Phonon Detector
Parallel to the SiO2 deposition experiments, a second set of experiments was performed focusing
on the phonon sensors used with the detector. The goal of this set of experiments was to provide
an independent measurement of the interaction energy to study the details of the charge energy. If
these sensors work, this simpler phonon sensor production method could also replace the QETs used
in the CDMS experiment (described in section 2.3.1).
The composite phonon detector used at the QTF consists of a tungsten layer on a silicon substrate glued onto a germanium crystal. Such a phonon sensor would greatly simplify the detector
production process compared to the current CDMS phonon sensors. Before tungsten samples were
glued to the detector, they were tested, characterized and proven to be capable of measuring phonon
signals.
6.1
Composite Phonon Sensors in the CRESST Experiment
Currently, CDMS uses a method in which the phonon sensor is deposited directly onto the crystal.
Due to the complicated geometry of the sensor, this method allows for accurate measurements of
phonons before they thermalize. The timing of these signals is used in order to discriminate against
surface events, as they have a faster rise time than bulk events. However, if it is possible to control
surface events (for example, through an insulating layer deposited on the detector as described in
102
section 5, or through the new iZIP technology discussed in section 2.8.1 or other means), complicated
phonon sensors would not be required and it would be possible to use a simpler sensor glued onto
the detector. Using a simple TES sensor would have many advantages over the current method
including:
• Easily testing sensors before coupling them onto the crystal;
• Easier to mass produce detectors;
• Ability to avoid problems of having varying transition temperatures across the sensor; and
• No possible damage to detectors from deposition experiments
Other experiments including CRESST have used composite phonon sensors for cryogenic dark
matter detection [33]. CRESST (see section 1.4.4.1) uses scintillation and phonon signals to detect
a particle interaction and discriminate between nuclear and electron recoils. The deposition of a
phonon sensor directly onto the scintillating CaWO4 crystal requires heating the crystal which reduces the scintillation efficiency. Therefore, the CRESST collaboration has developed a composite
detector where tungsten is deposited onto a small substrate which is glued to the scintillating crystal
[34].
6.2
TES Chip Tc Measurements
Before gluing any wafers onto the custom detector, extensive studies were required. The wafers
required characterization to ensure reasonable transition temperatures, transition ranges, and to
demonstrate that they would work effectively. As discussed in section 4.3.1, four wafer samples
were sent to the QTF with different transition temperatures (see Table 4.3). These samples were
too large to glue onto the detector. Samples L23#3 and L23#4 each had two small chips cut out
using a glass cutter. For simplicity, the four chips intially labelled L23#4-a, L23#4-b, L23#3-a and
L23#3-b were renamed as chips 1, 2, 3 and 4, respectively. Transition curves of all four sensors were
measured using a four-wire measurement and can be seen in Figure 6.1.
All four sensors were deemed appropriate to be glued onto the germanium crystal.
103
Figure 6.1: Transition curves of all four chips. Measurements were taken over a long period of time
so Tc s while increasing and decreasing in temperature were equal within the uncertainty ( 0.5 mK).
6.3
Silicon Chip Detector
In order to show that the silicon chips can be operated as detectors, chips 1 and 2, with a Tc of
approximately 55 mK were mounted inside a standard CDMS detector housing and connected like
normal CDMS phonon sensors, which can be seen in Figure 6.2.
The mount for chips 1 and 2 consisted of two small copper rods glued onto a ground plate, fitting
the CDMS detector housing. To reduce the heat link, only two corners of the chip were glued to
their respective mounts. Directly on top of each chip, a collimator with an 241 Am source was placed.
For chip 1, the collimator was left uncovered to observe alpha interactions in the chip. The
241
Am
source above chip 2 was covered with a thin piece of tape to absorb the alpha particles from the
source so only the gammas would interact with the silicon chip.
104
Figure 6.2: Two TES chips placed inside a detector housing to be used as detectors. The copper
plate (attached to the hexagonal housing) and copper rods were created to mount the chips in the
housing. The chips were glued to the rods on the plate using GE varnish. The chips were then
wirebonded to the DIB board.
6.3.1
IbIs Measurements and Analysis
Full IbIs profiles were measured on both chips at a range of different temperatures. Combined
IbIs profiles for chip 2 (gammas) can be found in Figure 6.3. An example of an IbIs trace for chip 1
can be found in Figure 6.4
The transition temperatures measured for chip 1 and chip 2 were 50 mK and 43 mK, respectively. This behaviour is in contrast to a transition temperature of 55 mK which was measured from
the silicon wafer from which they were cut. When the chips were removed from the apparatus, it
was noticed that they did not stick to their holder. A weak heat link between the chips and the
copper plate may have been responsible for the observed discrepancy between this and the previous
Tc measurements. Another possible contribution to the discrepancy may be the prominent noise
observed on the sensor as seen in Figure 6.3. A repeat test was performed on the sensors in a later
run and the transition temperatures were found to be 57 mK and 56 mK. These measurements are
very close to the initial temperature measured; the difference from the initial measurement of 55
105
Figure 6.3: Combined IbIs plot for chip 2. An unusual bump features appeared at ±50 µA. The
transition temperature was found to be 43 mK. The noise is likely to have been caused by the
DCRC board.
Figure 6.4: IbIs trace for chip 1 with an alpha source. As soon as the TES enters the transition
region, there are frequent jumps in Is . This is due to the sensor becoming normal conducting from
alpha interactions.
106
mK can be attributed to two possible effects: either systematic uncertainties in the thermometry
or the fact that the initial chip with a Tc of 55 mK may have small variations in Tc across the surface. When a chip is cut from a substrate, it may end up with a slightly different Tc than the original.
6.3.2
Trace Data Measurement and Analysis
6.3.2.1
Noise
For measurements of both chip 1 and chip 2, various types of noise affected the measurement:
• Large spikes were observed at regular intervals clearly generated by the readout equipment;
• A low frequency sine wave was observed; and
• Bit noise.
A combination of these three effects can be seen in Figure 6.5.
Figure 6.5: Three types of noise observed in measurements of chip detectors. Approximately every
25 µs, a large negative spike occurred. A 5 kHz sine wave was also evident. The large amplitude
alpha pulses were not affected by this noise, but it did affect the lower amplitude gamma pulses.
107
By plotting the derivative of the curve as a function of time, the spikes could be identified as
a large negative derivative immediately followed by a large positive derivative. The bins in which
spikes occurred were set to the values of adjacent points to remove the spikes. The sine noise and
bit noise did not affect the measurements of chip 2 as the noise was very small in magnitude relative
to the signal from the alpha interaction. Both types of noise had to be removed for chip 1 as the
gamma interactions were of much lower energy. The bit noise was minimized by using an average
filter on the trace.
6.3.2.2
Alpha Source Measurements
The typical energy deposited from an alpha interaction was large enough to make it go fully into
the normal region. Once the sensor is completely normal, an increase in temperature does not cause
an increase in resistance and a constant resistance is read until the TES cools back down below the
transition temperature. Consequently, for any interaction generating a pulse with an amplitude that
was roughly 0.4 V or greater, the TES railed and did not produce a signal greater than 0.4 V. A
typical trace showing a pulse caused by an alpha interaction can be seen in Figure 6.6.
To analyze the traces from this detector, first the spikes were removed as described above. Pulses
were then tagged by a large increase in the derivative as a function of time. Multiple pulses close
together (pileup) were removed. Some pulses began before the trace, while others were very close to
the end and as a result not completely within the trace. Both of these types of pulses were removed
as well. The amount of time for which the pulse railed was used as a measure of the interaction
energy. The histogram of the energies can be found in Figure 6.7.
6.3.2.3
Gamma Source Measurements
The analysis of chip 2 with the gamma rays was more challenging. A typical raw gamma pulse
with the different sources of noise can be seen in Figure 6.8.
Spikes were removed as described above. Then a box averaging filter was applied to the dataset
to remove the bit noise. The derivative array was found of the averaged dataset and large changes
108
Figure 6.6: A typical pulse caused by an alpha interaction with chip 1. The spikes have been
removed from this trace. Once the amplitude of the pulse exceeds 0.4 V, the TES is fully in the
normal region and the pulse rails.
in the derivative were used to tag pulses. As with the analysis for chip 1, pileup and incomplete
pulses were removed. Once good pulses were tagged and passed previous selection criteria, they
were temporarily removed from the trace. A sine function was then fit to the remaining part of the
trace. Once this fit was found, the sin function fit was subtracted from the full trace with the pulse.
The remaining pulse was then analyzed by determining its area and height. Histograms of both the
area and height were generated and can be seen in Figure 6.9.
Calibrations for both the alpha and gamma rays were not very accurate. However, the goal of
this experiment was not to accurately calibrate the particle interaction with the TES. The experiment proved that the silicon chips could be used as sensors with the CDMS hardware.
109
Figure 6.7: A histogram of the alpha energies based on the rail length of the trace. The energy was
measured by the length of time that the pulse was railing. A calibration was performed by setting
the top of the peak of the histogram to the energy of the alpha particles from 241 Am which is 5.49
MeV.
6.4
Cryogenic Glues
In order to couple the chips to the crystal, three different glues were tested: GE varnish, Hardman epoxy, and Araldite epoxy. GE varnish and Hardman epoxy were already used at the QTF for
cryogenic purposes. For example, GE varnish was used for many applications including gluing TES
samples to sample holders (section 4.3.1) or gluing wiring to heat sinks (section 3.5.2.3).
When the striplines first arrived at the QTF, the cold ends had to be reinforced as seen in Figure 2.11. Without this reinforcement, the striplines were weak and prone to breaking at the cold
end. Hardman epoxy was used in order to reinforce the striplines as seen in Figure 6.10. Both GE
varnish and Hardman epoxy have been shown to be effective at cryogenic temperatures. Therefore,
they were chosen as the first two glues. The third glue tested was an Araldite epoxy, which is used
in other cryogenic experiments (including CRESST, CUORE and EDELWEISS) to couple thermal
sensors to a crystal.
110
Figure 6.8: A typical pulse from a gamma interaction can be seen at approximately 100 µs. All
three types of noise (spikes, sine wave and bit noise) significantly affected the signal measurements.
Figure 6.9: Two histograms of gamma pulse energies based on pulse area and pulse height. Both
were calibrated by setting the maximum height of the peak to 59.5 keV, the gamma ray released
from 241 Am.
111
Figure 6.10: The stripline cold ends with Hardman epoxy used as a reinforcement. The Hardman
epoxy is the grey substance seen beneath the copper plate on the stripline.
6.5
Crystal Geometry
After the deposition of SiO2 and Al on the Ge crystal (section 5.4.3), the four TES samples were
glued onto the bottom surface of the detector, close to the DIB board. A summary of the sensors,
positions and glues can be found in Table 6.1. The sensors were then wirebonded to the DIB board
as found in Figure 6.11. Sensors A and B required very long wirebonds. To avoid any contacts between the wirebonds and the aluminum electrode, the electrode was partially covered with Kapton
tape. This was the final step required before mounting the crystal inside the cryostat and testing it
as a detector.
112
Table 6.1: Summary of chip properties
Chip
Label
1
Name
2
3
4
Silicon Chip
Detector
Alpha source
Tc
(mK)
57
L23#4b
Gamma
source
56
L23#3a
L23#3b
N/A
61
Phonon Glue
Sensor
B
Dot
of
Araldite
epoxy
A
Smear of
Araldite
epoxy
C
GE varnish
N/A
58
D
L23#4a
Hardman
epoxy
Figure 6.11: Bottom side of the detector after phonon sensors were glued. The right side electrode
is 1000 nm of SiO2 with Al deposited on top. In the top right corner, some of the electrode is
beginning to flake off. The left side electrode is just an Al layer without SiO2 . In the middle of the
detector between the electrodes, four TES sensors were glued and wirebonded to the DIB board,
seen at the bottom of the picture. Kapton tape was placed on the right side of the electrode to
prevent an electrical contact between the wirebonds from the TES and the electrode.
113
Chapter 7
Custom Detector Experiment
Once the preliminary experiments described in chapters 5 and 6 were completed, the germanium
detector could be tested in the cryostat, which involved taking both charge and phonon measurements. Some initial difficulties were experienced and modifications to the hardware were required.
7.1
Initial Difficulties
After depositing the SiO2 layers and gluing the four TES samples, the detector was placed in
the housing, wirebonded to the DIB board, assembled in the tower and mounted in the cryostat.
Two
241
Am sources were placed on the top side of the detector, facing the 0 nm and 1000 nm SiO2
layer electrodes. They were placed in collimators and separated from the detector by a thin piece
of aluminum tape to prevent alpha particles from interacting with the detector. Initial attempts to
cool the cryostat and take reliable measurements were unsuccessful. Problems found included:
• Rapid deneutralization possibly caused by infrared radiation entering the setup. For the second
cooldown, all small holes in the tower were carefully covered to prevent any light leakage and
the problem disappeared.
• A new DCRC board that was capable of measuring charge channels was used. When this
board was connected a high amplitude noise was observed that heated the tower from 45 mK
114
to approximately 250 mK. The high amplitude noise also prevented most charge pulses from
being observed. Eventually, the power supply for the DCRC board was identified as the source
of this noise and replaced in the next cooldown, resolving this problem.
• During the second run, both positive and negative charge pulses appeared with a single bias
polarity. This was caused by a disconnect in the wirebond between the DIB board and the
electrode which made the electrode float relative to ground.
• During the first run, the TES sensors did not transition even though the base temperature
was lower than the transition temperature. A possible origin of this problem may have been
the broken LEDs located in the detector housing which are used for neutralizing the detectors.
A noise-induced current going through the broken LEDs may have heated the sensors. After
disconnecting the wirebonds of the LEDs, the TESs transitioned properly in the next run and
were capable of measuring phonon signals.
As explained in section 2.6.2, ensuring that the detectors are neutralized is essential for taking
charge data. Because the LEDs were disconnected, the detector could not be neutralized in the standard manner. In order to neutralize the detector, it was grounded and a 137 Cs source was placed near
the detector. The high interaction rate from this source would create many electron-hole pairs that
would combine with the charge buildup to neutralize the detector. Both CDMS and EDELWEISS
have used this method in the past.
7.2
7.2.1
Charge Measurements
Detector Configuration
During the first attempt to take data with the detector, the wirebond connecting to the half of
the detector with a 0 nm and 50 nm SiO2 layer was broken, making it impossible to measure a
charge signal from this half. A charge signal was measured on the other half of the detector with the
300 nm and 1000 nm layers of SiO2 . The
241
Am sources were mounted facing the 0 nm and 1000
nm SiO2 layers, but the power supply had not yet been identified as the source of the noise and
low energy charge pulses were unobservable. However, charge data was taken with a
137
Cs source,
demonstrating that the detector can perform adequately when taking charge measurements with
115
thicker layers of SiO2 .
Once the noise from the power supply was identified and removed in a subsequent cooldown, low
noise traces were obtained and a more advanced analysis was performed. During this cooldown, the
241
Am sources were mounted facing the 50 nm and 300 nm SiO2 layers, but a wirebond broke on
the half of the detector with 300 nm and 1000 nm layers of SiO2 . Therefore, all charge data was
obtained from the side of the detector with 0 nm and 50 nm.
7.2.2
Initial Measurements
A 137 Cs source was mounted outside the cryostat and 662 keV gamma pulses were observed in the
charge readout. These pulses exceeded the noise level and triggering on these pulses was possible.
Using a Butterworth filter, the noise was reduced and the pulses could be analyzed. An example of
pulses before and after being filtered can be found in Figure 7.1.
Figure 7.1: Charge pulse before and after being filtered. A periodic noise from the DCRC board
can be seen in the pulse before the Butterworth filter was applied. The filtered pulse that contains
a highly reduced noise signal was analyzed.
116
A quick analysis was performed by recording the maximum height of each filtered pulse. A
histogram of these results can be seen in Figure 7.2. The pulse height plotted as a function of time
can be seen in Figure 7.3.
Figure 7.2: Histogram of pulse heights from 137 Cs. The energy was calibrated by setting the peak
at the end of the curve to 662 keV. This data set demonstrated that the layers with 300 nm and
1000 nm of SiO2 could be used to measure a charge signal. The source of a large peak around 200
keV is unknown.
7.2.3
Charge Analysis
In a later run, low noise charge data was taken with the half of the detector with 0 nm and 1000
nm SiO2 layers. The readout electronics board was not capable of triggering on low energy pulses.
Therefore, instead of taking 0.3 ms traces (approximate length of a charge pulse), 100 ms long traces
with random triggers were taken and an algorithm was developed to identify and isolate individual
pulses from these traces for further analysis. The first step in the analysis was to create two data
arrays of the initial trace after being passed through averaging filters of different lengths. Of the
two averaged arrays, the one with the shorter filter size had observable pulses that were smaller in
magnitude than the original pulse. The array with the longer filter size had very tiny pulses because
it was averaged by a much larger baseline signal. The difference between these two arrays only
became significant where a pulse existed. This difference was used to tag pulses.
117
Figure 7.3: Pulse height decay over 8 minutes for the half of the detector with 300 nm and 1000
nm of SiO2 . Lines at just over 200 keV and below 700 keV can be seen to remain relatively
constant over the 8 minute period. This shows that over approximately 10 minutes, the detector
remains neutral when there are thick SiO2 layers deposited between the germanium crystal and the
electrode.
The baseline offset of the trace was then found by taking a section of the trace and ensuring that
it did not contain a pulse by measuring the standard deviation of that section. The mean value of
this section was then subtracted from the entire trace.
Pulses that were incomplete or had pileup were removed. A template pulse, as can be seen in
Figure 7.4, was created by adding hundreds of pulses together. This template was then used to fit
the pulses in the original unfiltered trace. Examples of a 60 keV and 8 keV pulse with the template
fit can be seen in Figure 7.5.
7.2.4
Energy Calibration
Two energy calibrations were performed. The first calibration measured the pulse spectrum using
a
137
Cs source. The spectrum obtained from the
137
118
Cs source can be seen in Figure 7.6.
Figure 7.4: The template charge pulse used in this analysis. This pulse was created by taking
hundreds of charge pulses, aligning them, adding them together and normalizing them. Pulses
were selected manually to avoid pulses with a very low signal to noise ratio and pulses with pileup.
Figure 7.5: A 60 keV and 8 keV pulse fit with a template.
119
Figure 7.6: Charge signal calibration from a 137 Cs source. A distinct peak can be seen at the end
of the histogram. This was calibrated to be at 662 keV [11]. A significant spike can be seen around
60 keV which comes from the 241 Am source. The Compton edge can be seen at 477 keV.
To perform a second energy calibration, the
137
Cs source was removed and a
mounted outside the cryostat. The spectrum from the
In both histograms, the peak from the
241
57
57
Co source was
Co source can be seen in Figure 7.7.
Am source was evident. Calibration factors between
pulse height in Volts and the gamma energy are summarized in Table 7.1. The calibration factors
along with uncertainties can be found in Figure 7.8. Using a weighted mean, the calibration was
calculated to be (3410 ± 30) keV/V.
120
Figure 7.7: Charge signal calibration from a 57 Co source. Higher energy peaks correspond to
energies of 122 keV and 136 keV. The distinct peak around 60 keV is from the 241 Am which was
located inside the cryostat.
7.2.5
Deneutralization
After remaining at a constant bias, detectors eventually deneutralize as explained in section 2.6.2.
Studies were completed to test how the detector lost its neutralization over time. To take data, a
random trigger was activated manually and operated over approximately half an hour. This was
done with just the
241
Am source over 35 minutes with a 3V bias. A graph of the pulse heights as a
function of time can be seen in Figure 7.9.
Tests were also performed to study how the detectors deneutralize as a function of both bias
and time. The detectors were neutralized for 10 minutes before starting the tests. One of four
different bias voltages (+3, -3, +6, -6) was turned on and left for 30 minutes. Every 7.5 minutes, a
set of data was taken to study the pulse spectrum. The results can be found in Figures 7.10 and 7.11.
121
Table 7.1: Calibrations obtained from histograms of charge signal
Source
137
Cs
Am
137
Cs
241
Am
57
Co&
241
Am
57
Co&
241
Am
57
Co&
241
Am
241
Am
Calibration Factor (keV/V)
&
Peak
Energy
(keV)
662
3450
Standard
Deviation
(keV)
3.0
Calibration
Uncertainty
(keV/V)
16
&
59.5
3440
1.4
81
122
3380
1.6
44
136
3390
1.3
32
59.5
3360
1.6
90
59.5
3420
1.1
63
241
Figure 7.8: Calibration of voltage readout to charge signal energy. Dots indicate calibration values
and their uncertainties as found in Table 7.1. The solid line indicates the final calibration value as
determined by a weighted average of the calibration points. Dotted lines show one standard
deviation from the mean of the calibration.
122
Figure 7.9: Pulse height decay over 35 minutes. As time passes, the 60 keV pulses start rendering
a reduced signal. At approximately 15 minutes, the rate increased due to a faster manual data
taking rate.
For all plots, various trends existed including:
• Higher bias voltage allowed detectors to have full charge collection for longer. This behaviour
occurs because the charge signal is measured based on electron-hole pairs drifting through the
crystal. As the detector deneutralizes, the charges build a counter-field and the net electric
field becomes reduced. With a lower bias voltage, more electron-hole pairs can recombine and
a lower charge signal will be measured.
• Negative bias deneutralizes at a faster rate than positive bias. This behaviour could be due to
either the SiO2 layer transmitting electrons worse than holes or the aluminum layer transmitting holes worse than electrons.
• Both the -3V and -6V bias demonstrate an interesting effect where at 7 min and 15 min,
respectively, the 60 keV peak splits into two different energy ranges. This behaviour may
be due to the range at which the pulses are able to penetrate and cause electron-hole pairs.
Immediately adjacent to the electrode, most gamma rays interact, create electron-hole pairs,
create a charge buildup and deneutralize that area. Some gamma rays are able to penetrate
farther and cause electron-hole pairs in areas without loss of neutralization. These pulses will
show a higher charge readout as the net electric field in those areas is stronger.
123
Figure 7.10: Deneutralization of detectors over 30 minutes with ± 3V. The histograms shows a
clear trend that the +3 V bias stays neutral for longer than -3 V bias.
124
Figure 7.11: Deneutralization of detectors over 30 minutes with ± 6V. This is similar to the trend
observed with ±3V but ±6V stays neutral for longer times.
125
7.2.6
241
Am Spectrum
When studying the charge channel, the ultimate goal was to study the spectrum from
The main gammas from
241
241
Am.
Am are 60 keV and are within the same range as an expected nuclear
recoil from a WIMP. Also, the 60 keV gamma cannot penetrate deeply through germanium. Consequently, when the source was placed on one side of the detector (ie. below the 50 nm SiO2 layer),
all interactions would occur near the 50 nm SiO2 layer. By performing studies with 241 Am on either
side of the detector, interactions that are close to the respective layers (many of them surface events)
can be studied. Consequently, much time was spent studying the spectrum from the
The bias voltage used to study the
241
241
Am source.
Am spectrum was +6 V (as determined to be the optimal
voltage from the deneutralization study). Multiple data sets were combined into a histogram which
can be seen in Figure 7.12.
Figure 7.12: A histogram of the pulse heights from a 241 Am source. The 60 keV peak is evident. A
small hint of lower energy peaks at 14 keV and 18 keV can be observed. Unexpected peaks at
approximately 3 keV and 8 keV are present. The 3 keV peak was caused by noise. The 8 keV peak
could be due to poor charge collection at the edge of the electrode.
14 keV and 18 keV peaks were expected, but not clearly observed although there is a slight hint
of an 18 keV peak. The tape placed over the
241
Am source to absorb the alpha particles layer could
be responsible for attenuating the low energy gamma rays, preventing most of them from interacting
with the detector. When gammas interact with the germanium crystal, electrons are released from
126
the valence band. When these electrons relax, an x-ray is released. X-ray escape is possibly the
reason for the small peak at 50 keV.
Peaks are found at approximately 3 and 8 keV. The 3 keV peak was caused by noise, but the 8
keV peak was possibly due to poor charge collection around the edge of the electrode. In another experiment performed within CDMS, where detector T3Z2 was studied, a similar result was observed.
It was found that these peaks were caused by events that occur at the edge of the detector where
the electric field is not uniform. The low energy peaks from this study can be seen in Figure 7.13.
Figure 7.13: Low energy charge peaks in CDMS detector T3Z2.[60] After detailed investigation,
events in the 5 keV peak were found to have occurred around the periphery of the detector where
poor charge collection exists. In standard CDMS detectors, the QO channel can be used to
discriminate against this background. The custom detector at the QTF did not have such
capabilities.
In the standard CDMS analysis such events are discarded by means of the Qouter electrode, a
feature that does not exist in the custom detector at the QTF.
127
7.2.7
Charge Results Summary
It has been suspected that the accumulation of charges underneath rapidly deneutralizing the
detector. The results from the charge measurements prove that even a detector with thick SiO2
layers can be operated and deneutralize on a time scale similar to standard CDMS detectors. A
linear dependence of the pulse height on the interaction energy was confirmed through calibrations.
Future improvements in the deposition can possibly lead to stable layers of SiO2 thicker than 50 nm.
A similar measurement with the 241 Am source on the side of the detector with no insulating layer
was planned to determine if the SiO2 is responsible for attenuating the lower energy gamma rays
and to see if there is a reduction in surface events. Unfortunately, a failure in the cryostat prevented
this measurement.
7.3
7.3.1
Phonon measurements
IbIs Measurements
IbIs curves were taken of all four phonon sensors. To perform an IbIs measurement with the new
DCRC board, a triangle wave was used as a bias current and the Is was read. A sample IbIs curve
taken from the custom detector can be seen in Figure 7.14.
This IbIs curve is significantly different from the previous IbIs curves for multiple reasons. The
bias current was obtained by sending a triangle wave down the QET bias line. Due to the voltage
range of the amplifiers, Is railed before Ib was large enough to make the sensor completely normal.
Consequently, the entire range of the IbIs curve was not obtained. While the initial measurement
was Ib vs time, the Ib vs Is curve was recovered based on the properties of the triangle wave.
The second difference is the shape of the transition region. Previously, there was a very sharp
change in Is when changing from the superconducting region to the transition region, but here, the
curve is rounded. Previously with these sensors, IbIs curves appeared normal (see Figure 6.3). A
similar effect would be observed if the sensor had a very large transition range. To demonstrate
128
Figure 7.14: Sample IbIs curve from sensor A of the custom detector taken at 63 mK. This curve
did not have the same range that previous IbIs curves had. Also, there is a much slower transition
between the superconducting and normal regions.
the very slow transition, a simulation was performed where the transition of a sensor occurred over
300 µA instead of 20-50 µA. The simulation can be seen in Figure 7.15. However, a slow transition
curve for these sensors is in contradiction to IbIs curves previously found (see Figure 6.3).
All four sensors were fully characterized and the transition temperature for sensors A, B, C and
D were found to be 68 mK, 65 mK, 67 mK and 75 mK, respectively. Discrepancies from previously
measured temperatures (see section 6.3.1) could be due to glues and improved heat sinking affecting
the apparent transitioning properties.
7.3.2
Pulse Shapes
Standard CDMS phonon sensors absorb pulses before they thermalize. Based on the shape of the
fast phonon pulse, surface events can be identified. In this experiment, phonon sensors that were
smaller and had weaker couplings were used. Consequently, thermal phonons would be absorbed,
with roughly identical pulse shapes dependent on the glue.
129
Figure 7.15: Simulation of an IbIs curve for sensors with a very wide transition region. The shape
is much more rounded in the transition region than what can be seen in Figure 4.3. This can
explain the rounded shapes of the transition region in Figure 7.14.
No phonon pulses were observed from the 60 keV interactions caused by
actions with the detector caused observable phonon pulses. Gammas from a
241
137
Am. Muon inter-
Cs source that was
mounted outside the detector also occasionally went directly through the TES and caused a very
quick pulse at the beginning of the curve followed by a slow pulse due to the interaction with the
germanium crystal. Figure 7.16 shows pulses of the four sensors.
Sensors A and B seemed to operate the best and render the most distinct pulse shapes. Sensor
C had the longest pulses of the four different sensors and had a much lower amplitude. Because of
the low frequency noise, sensor C was the least effective as the noise has a stronger effect on slower
pulses. Sensor D had a quick pulse, but an even smaller amplitude than sensor C. A very strong
60 Hz noise can be seen in the pulse from sensor D. Of all the pulses with gamma rays interacting
with the TES, sensor D had the largest difference between initial pulse magnitude and slow pulse
magnitude, demonstrating poor phonon transmission through the Hardman epoxy.
130
Figure 7.16: Typical pulse shapes observed from the four phonon sensors. Phonon sensors with
gamma rays going through the TES exhibit two pulses: a fast pulse caused by the gamma ray in
the TES and a slow pulse caused by energy entering the sensor from the Ge substrate.
131
7.3.3
Phonon-Charge Calibration
The phonon pulses for sensors C and D had much lower pulse heights for a given interaction energy
than sensors A and B. The low frequency noise greatly affected sensors C and D and analytic fits
were not successful on those phonon sensors. Pulse shapes were also more distinct for sensors A and
B; therefore they were used to calibrate phonon height with the charge calibration that was found
as described in section 7.2.4.
In order to measure the heights of phonon signals from sensors A and B, an analytic function
was fit to the pulses. This function assumed two separate exponential components of the function
for the rise time and fall time of the pulse:
f (t) = A(1 − e−
t−st
rt
)e−
t−st
ft
(7.1)
where A is the amplitude, st is the start time, rt is the rise time and f t is the fall time. The start
time, rise time and fall time were all estimated from studying phonon pulses and were not variables
in the fit.
Once the amplitudes of phonon pulses for sensors A and B were found, corresponding charge
pulses were fit. Most interactions that had enough energy to produce phonon signals also caused a
railing charge signal. The railing signal was caused by the amplifiers when output voltages exceeded
approximately 2 V, which corresponds to a 6.8 MeV pulse. Because most phonon pulses were
observed with interactions at energies greater than 6.8 MeV, a function was fit to the section of the
pulse that did not rail. A function with multiple exponential terms was used to fit the charge pulse:
y = At [A1 (1 − e−
t−st
rt
) ∗ e−
t−st
f t1
+ A2 e −
t−st
f t2
]
(7.2)
where At is the total amplitude of the pulse, A1 is the amplitude of the first term, st is the start
time, rt is the rise time and f t1 is the fall time of the first term, A2 is the amplitude of the second
term and f t2 is the fall time of the second term. All parameters except At were determined from
a fit to the charge template seen in Figure 7.4. An example of a railing charge pulse fit with the
analytic fit can be seen in Figure 7.17.
132
Figure 7.17: Analytic fit to a railing charge pulse. The fit was only performed on the part of the
pulse which did not rail and was used to obtain a value for the energy of the interaction as
measured by the charge signal.
Once the magnitude of the analytic function (At ) was known, the charge calibration (see section
7.2.4), was used to determine the interaction energy. Phonon pulse heights were then plotted as a
function of measured charge energies which can be seen in Figure 7.18.
7.3.4
Phonon Sensor Comparison
Phonon sensor A was found to have a larger magnitude voltage output than the other three sensors
for all energies. Sensitivities of the other three sensors relative to that of A were calculated. Voltage
outputs for other sensors were divided by the voltage output of A to obtain a relative sensitivity
and were then plotted as a function of energy.
133
Figure 7.18: Calibration of phonon energies for sensors A and B using charge energy. The charge
signal was assumed to scale linearly up to energies on the order of 30 MeV. Linear relationships
were observed with both sensors A and B. Sensor A produces a larger voltage than sensor B from
an interaction.
7.3.4.1
Phonon Sensor B
Using the data obtained from the analytic fits of events in sensors A and B, the two sensors were
compared. The output voltage obtained from sensor B was divided by the energy of sensor A and
was plotted as a function of charge energy, as can be seen in Figure 7.19.
The output from sensor B was on average 20% lower than that of sensor A. This difference could
be due to poorer phonon transmission through a dot of Araldite epoxy, as opposed to a smear of
Araldite epoxy. Sensor A is slightly smaller than sensor B and a larger sensor requires more energy
to increase in temperature, so that larger sensors would be less sensitive. This size difference could
cause the difference in output voltages between sensors A and B.
134
Figure 7.19: Comparison of sensors A and B. Sensor B is approximately 20% lower in magnitude
than sensor A. Correlation of pulse heights at energies under 10 MeV are much more scattered
than higher energy calibrations.
7.3.4.2
Phonon Sensors C and D
Due to low frequency noise and low amplitude pulses, analytic functions could not be fit to sensors
C and D. In order to estimate the sensitivities of these phonon sensors, 20 coincident pulses between
all four sensors were manually inspected. Pulse heights of the four sensors were determined from
the difference between the mean values of the high frequency noise at the maximum height and the
start of the pulse. All pulse magnitudes were divided by the magnitude of sensor A. The charge
signal was used as a measurement of the interaction energy. The magnitude of sensors C and D
relative to A are plotted as a function of interaction energy and can be seen in Figures 7.20 and
7.21, respectively.
Both sensors C and D have a very poor relationship at low energies. For sensor C, the relationship
to sensor A appears to stabilize around 10 MeV at a value of approximately 0.3. The relationship
between sensor D and sensor A appears to settle at 0.2 for pulses greater than 12 MeV.
135
Figure 7.20: Comparison of sensors A and C. Sensor C has low sensitivity relative to sensor A. GE
varnish is less effective than Araldite epoxy for coupling phonon sensors to crystals.
Figure 7.21: Comparison of sensors A and D. Sensor D exhibited the poorest sensitivity of all
different glues.
136
7.3.5
Phonon Pulse Summary
A summary of the properties observed from the 4 different phonon sensors can be found in Table
7.2.
Table 7.2: A table of typical pulse shapes observed from the four different sensors.
Phonon Sensor
Glue
Rise
(ms)
A
Smear
of
Araladite
epoxy
Dot of Araladite epoxy
GE Varnish
Hardman
Epoxy
5
B
C
D
Time
Fall
(ms)
Time
60
Energy
Threshold
(MeV)
4
Voltage Output Relative
to Sensor A
1
5
60
4
0.8
30
7
120
100
10
12
0.3
0.2
Rise times and fall times were found by manually inspecting pulses from the different sensors.
Thresholds for sensors A and B were calculated by finding the standard deviation of the baseline
and multiplying it by a factor of 5. Thresholds for sensors C and D were determined from plots 7.20
and 7.21, when a linear relationship with sensor A was clear.
Because the amount of energy required to heat a sensor is dependent on the size, smaller phonon
sensors could be used to improve thresholds and sensitivity. Alternative gluing methods could be
implemented that allow for more efficient phonon propagation. Thinner glue layers could be more
effective at coupling the phonon sensor to the crystal. As discussed in section 1.4.3, the specific heat
of materials generally decreases as a function of temperature. By using phonon sensors with lower
Tc s, an interaction at a lower temperature would cause a greater temperature increase in the sensor,
increasing its sensitivity.
137
Chapter 8
Conclusion & Recommendations
8.1
Conclusion
To aid with future detector development and characterization for the SuperCDMS collaboration, a
test facility with a dry dilution refrigerator was installed at Queen’s University. The refrigerator was
commissioned and the thermometry was calibrated. Wiring was installed for four-wire measurements
and hardware was installed to mount CDMS detectors in the cryostat. Multiple detectors were studied and characterized. Transition temperatures were measured on a set of different tungsten samples.
Two sets of experiments were performed to improve ionization signal measurements and simplify
phonon sensor production.
An electron beam evaporator was used to deposit SiO2 as an insulting layer onto a substrate. A
SiO2 layer between two aluminum electrodes was deposited on glass slides to determine the thickness
required for an insulating layer. Multiple deposition methods were attempted. Ultimately, a 50 nm
thick layer of SiO2 was shown to insulate between two aluminum electrodes with an applied voltage
of 200 mV and an electric field of 4 × 106 V /m. Four different SiO2 layers of different thicknesses (0,
50, 300, and 1000 nm respectively) were deposited on the germanium crystal detector. Thin layers
of SiO2 were found to be more stable as thicker layers deposited on germanium flaked off of the
detector surface.
138
The ionization signal from the detector was carefully calibrated and was found to be sensitive to
2 keV. Ionization signal measurements were found to be possible with insulating SiO2 layers. The
detector deneutralized after approximately 10 minutes, similar to standard CDMS detectors. A +6
V bias was shown to have a lower rate of deneutralization than +3, -3 and -6 V biases. Due to
equipment problems, an
241
Am spectrum was only measured for interactions on a SiO2 layer of 50
nm. To determine the effectiveness of this layer at removing surface events a control measurement
would be required.
Two tungsten samples on silicon substrates were mounted into the CDMS detector housing and
operated as detectors. Complete IbIs traces were obtained for both sensors. Calibrations were performed on the phonon signals for both alpha and gamma particle interactions.
Composite phonon sensors were successfully used on a germanium detector. An approximate
phonon energy scale was determined based on the charge signal for phonon sensors glued with
Araldite epoxy. Araldite epoxy was shown to be more effective than GE varnish and Hardman
epoxy at coupling phonon sensors to a germanium crystal. The amplitude of a phonon sensor glued
with a small dot of Araldite epoxy was systematically lower by 20% compared to the sensor with a
smear of Araldite epoxy. This discrepency could be due to the relative sizes of sensors A and B. The
output voltage from the sensor glued with GE varnish was approximately 30% of the output voltage
of the sensor glued with Araldite epoxy. The output voltage of the sensor glued with Hardman epoxy
was approximately 20% of the output voltage of sensor glued with Araldite epoxy. The threshold energy for phonon sensors glued with Araldite epoxy was approximately 4 MeV, while CDMS phonon
sensors must be sensitive to approximately 10 keV. Better gluing methods, and smaller sensors with
lower Tc s could be used to increase sensitivity.
8.2
Recommendations
Further work is required to determine the effectiveness of an insulating layer at reducing surface
events. Pulse spectra must be obtained using an
241
Am source on a germanium crystal without
SiO2 to compare with results obtained for a 50 nm layer of SiO2 .
139
Experiments with smaller phonon sensors and different glue thicknesses can be performed to
increase the sensitivity of TES samples glued onto the germanium crystal. Because the specific
heat of tungsten decreases with temperature, TES samples with lower Tc s can also be used to create
composite phonon sensors that are capable of measurements at low energies. Glues need to be tested
again to ensure that results obtained are repeatable.
140
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Appendices
146
Appendix A
Deposition Procedure
A.1
HF Etch
• Ensure that all material used is not soluble in HF
• Place 200 mL of ultra pure water in a container
• Slowly pour 40 mL of 49% HF into the container (ratio must be 5:1)
• Dip germanium crystal in the solution for 7 minutes
• Remove the crystal and dunk in multiple baths of ultra pure water to remove all HF acid
residue
• Carefully dispose of all acid and clean materials used
• Crystal is ready for deposition
A.2
Germanium Crystal Deposition
• Open the bell jar and fill crucibles with Al and SiO2
• Place the germanium crystal with the holder in the bell jar with the bottom side down
• Evacuate the bell jar and deposit 50 nm of SiO2 followed by 100 nm of Al
147
• Vent the bell jar and rotate the crystal by 180 degrees to perform a second deposition on the
bottom side of the crystal
• Evacuate the bell jar and deposit 300 nm of SiO2 followed by 100 nm of Al
• Vent the bell jar, refill the crucibles and flip the crystal to perform depositions on the top side
• Evacuate the bell jar and deposit 100 nm of Al
• Vent the bell jar and rotate the crystal by 180 degrees to perform a second deposition on the
top side of the crystal
• Evacuate the bell jar and deposit 1000 nm of SiO2 followed by 100 nm of Al
• Vent the bell jar, place the germanium crystal in the detector housing and wirebond electrodes
to the DIB board
148
Appendix B
Custom Designed Hardware
Figure B.1: Heat sink for the custom wiring connector on the 4 K plate. The cutout in the center
prevents the heat sink from contacting the leads on the connector.
149
Figure B.2: Heat sink for the custom wiring connector on the base plate. The cutout in the center
prevents the heat sink from contacting the leads on the connector.
150
Figure B.3: Heat sink used for the custom wiring.
151
Figure B.4: Plate for mounting chip detectors. Dimensions are designed so that it fits within the
detector housing.
152
Figure B.5: Holder for the germanium crystal deposition. The empty hole allows the material to
deposit on the exposed surface. The rest of the crystal is covered.
153
Appendix C
IbIs Measurements
C.1
G31
Figure C.1: IbIs curves for G31 sensor A
154
Figure C.2: IbIs curves for G31 sensor B
Figure C.3: IbIs curves for G31 sensor C
155
Figure C.4: IbIs curves for G31 sensor D
156
C.2
S7
Figure C.5: IbIs curves for S7 sensor A
Figure C.6: IbIs curves for S7 sensor B
157
Figure C.7: IbIs curves for S7 sensor C
Figure C.8: IbIs curves for S7 sensor D
158
Appendix D
CDMS Detector Production
Process
Before beginning the process of depositing materials on the detector surface, the detectors are
throughly cleaned using xylenes, acetone and isopropyl alcohol. They are then etched in a HF acid
solution.
The five steps of the sensor production process are explained in Figure D.1 - D.5. All figures are
taken from [37].
Figure D.1: First stage of the sensor production process. The cleaned germanium crystal (green) is
loaded in to a deposition chamber sputterer. Three layers of Si (40 nm), Al (300 nm) and W (30
nm) are then deposited on the top side (to become the phonon sensor). On the bottom side
(charge sensor) the same three materials are deposited, but with different thicknesses: Si (40 nm),
Al (20 nm) and W (20 nm). Photoresist layers are then applied to the bottom and top side. The
bottom side has a 7 µm thick layer of photoresist to protect the films. The top side has a 1.8 µm
layer to start the steps to make the fin-layer patterns required in QETs.
159
Figure D.2: Second stage of the sensor production process. The top side is exposed to a mask
(used for making the aluminum fins in the QET) and developed. The remaining exposed W and Al
films are wet etched with H2 O2 . Both photoresist layers are then chemically stripped. The
photoresist is then reapplied. The top side now has the patterned aluminum-fins protected with
the photoresist protecting it. The bottom side is now prepared for the electrode to be patterned.
Figure D.3: Third stage of the sensor production process. The thin photoresist layer deposited on
the bottom side of the detector is exposed to the electrode grid masked and developed. The thin
Al and W layers are then etched, but the Si layer is still left covering the Ge crystal. Both
photoresist layers are chemically stripped again. Then the top side of the detector has another W
film deposited on it which will later become the TES.
Figure D.4: Fourth stage of the sensor production process. Again, photoresist layers are applied
both on top and on bottom. The top side is then exposed to the TES masked and developed. The
exposed W is wet etched and the photoresist layers are chemically stripped again. The photoresist
is then reapplied to both sides.
160
Figure D.5: Fifth stage of the sensor production process. The bottom layer is exposed to a mask
which is used to separate between the inner and outer electrode channels. The bottom layer is then
etched with CHF3 O2 . Then both photoresist layers are stripped and the production process is
complete.
161