Download No-light events in Run 31

CRESST
Cryogenic Rare Event Search with Superconducting
Thermometers
Jens Schmaler for the CRESST group at MPI
MPI Project Review
December 14, 2009
1
Outline
1. The CRESST experiment
2. Status and progress of CRESST
3. New CRESST test facility
4. Outlook: Search for new scintillators
2
Outline
1. The CRESST experiment
2. Status and progress of CRESST
3. New CRESST test facility
4. Outlook: Search for new scintillators
3
Dark Matter Detection
c
• goal: detection of WIMPs via elastic
scattering off nuclei
 ~ A2
• challenges:
– low recoil energies: O(10 keV)
– low event rate: < O(10 kg-1 yr-1)

• needs:
–
–
–
–
low energy threshold
excellent background rejection
large target mass
heavy target nucleus
• CRESST target: scintillating CaWO4
crystals
– up to 33 crystals (10 kg target mass)
4
Ø 40 mm, h=40 mm, 300 g
thermal
coupling
W film
crystal
film resistance [mW]
Detector Principle
350
300
250
200
150
∆RΔR
∆T
100
50
ΔT
0
15
16
17
18
19
20
21
temperature [mK]
• detectors operated at ~ 10 mK
• evaporated tungsten film as thermometer:
– stabilized in transition to superconducting state
– particle interaction  temporarily warms up crystal  resistance change
– SQUID-based readout electronics
• energy threshold: ~ 1 keV
5
Passive Background Suppression
•
dealing with very low signal
event rates < O(10 kg-1 yr-1) :
1.
3.
Gran Sasso underground lab
below 1400 m of rock
several layers of passive
shielding against ambient
radioactivity
only very radio-pure materials
•
O(104
2.
kg-1
yr-1)
background
events still remain
1m
cryostat
PE (10 t)
muon veto
radon box
lead (24 t)
detectors
copper (10 t)
active background
discrimination needed!
6
Active Background Discrimination
•
→ “detector module”
•
simultaneous measurement of
–
–
•
reflective
housing
crystals + separate light detector
Light
absorber
CaWO4
target
energy in crystal: E
scintillation light: L
define:
W film
light yield := L / E
W film
β+γ
(LY=1 by def.)
•
event-by-event discrimination of
particle type
•
higher light output of crystal
 better discrimination
α (LY=1/5)
O-recoils (neutrons, LY=1/10)
W-recoils (WIMPs, LY=1/40)
7
CRESST Detectors
phonon detector
light detector
8
Composite Detectors
• motivation:
evaporation of W film on crystal degrades light output
 reduces background discrimination power
• solution:
– first evaporate film on small carrier crystal
– glue carrier to large crystal
glue
W film
carrier crystal
crystal
• light output can be increased by ~ 50%
9
Outline
1. The CRESST experiment
2. Status and progress of CRESST
3. New CRESST test facility
4. Outlook: Search for new scintillators
10
Reminder: 2008 Data Taking (1)
• 9 modules operated between August and December 2008
• observation: varying number of no-light events in signal region
Rita/Steven, ~ 7 kgd
phonon detector
no light
90% of W-recoils
below this line
with
light
• properties of no-light events:
– rate decreases with time
– partly: different pulse shape in phonon detector
→ can be rejected by shape cut
detector effect
rather than
particle interactions
11
2008 Data Taking (2)
•
most likely origin of no-light events:
clamp
stress relaxation events
(at contact surface between clamp and crystal)
•
two possibilities:
1. in crystal („micro cracks“)
2. in clamps
•
clamps were mostly covered with plastic layer
 more no-light events than with pure metal clamps
12
CRESST – Current Status (1)
• consequence of 2008 run: new clamps for all crystals
1. thinner material → softer (less pressure on crystals)
2. material: only metal, no plastic coverage
• new cooldown started mid of May 2009
– delayed by earthquake in L’Aquila region
– fast cool-down (stronger thermal coupling of detectors installed)
– 10 detector modules operational and taking data since June
• two composite detectors
• one ZnWO4 crystal (alternative target material)
13
CRESST - Current Status (2)
standard
90% of all W-recoils
below this line
•
•
number of no-light events strongly reduced
no-light events with different pulse shape
disappeared
•
composite detectors work well
–
–
high light-output
together with good light detector:
significantly lower discrimination threshold achievable
acceptance region
prelim.
discrimination threshold
10 keV threshold
composite
5 keV threshold
preliminary
acceptance region
prelim.
14
CRESST – Current Status (3)
• preliminary analysis:
a few candidate events observed in the signal region
• origin of these events?
– still detector effect (cracks,...)?
– signal (e.g. inelastic Dark Matter)?
• more statistics needed to investigate properties
aim:
continue this run at least until mid 2010
15
Inelastic Dark Matter
•
assumes inelastic scattering
nucleus  c  nucleus  c *
with   mc *  mc  (100 keV)
•
•
can reconcile claimed DAMA
signal with other experiments
δ=130 keV
CRESST:
–
–
tightest limits of all experiments
(due to heavy target nucleus)
already excludes most of parameter
space
current data should bring
decision on this scenario
CRESST (2007 data only)
CDMS
16
[Schmidt-Hoberg et al. arXiv:0907.3940]
Outline
1. The CRESST experiment
2. Status and progress of CRESST
3. New CRESST test facility
4. Outlook: Search for new scintillators
17
CRESST Test Facility (1)
• motivation:
full-sized CRESST detector modules cannot be operated over
ground
(too high count rate)
• up to now: many tests of new detector developments only in
CRESST cryostat itself
– long cooling cycle (typically one run per year)
– risk to reduce outcome of CRESST data taking
• aim: decouple R&D and science runs
small test cryostat installed in Gran Sasso
underground lab by MPI group in 2009
18
CRESST Test Facility (2)
•
small cryostat
•
•
short cooling cycle (3 days)
before: used at MPI
(~300 cooldowns)
•
moderate Pb shielding
•
readout for one complete
detector module
(including new data taking
system)
•
ready to be cooled down for
testing
expected to be useable
early 2010
•
19
Outline
1. The CRESST experiment
2. Status and progress of CRESST
3. New CRESST test facility
4. Outlook: Search for new scintillators
20
Outlook: New Scintillators
• amount of scintillation light is limiting factor:
1. discrimination threshold:
CaWO
effect of 2x more
light4
2. W-recoils: light yield 1/40
not safely distinguished from
no-light events at low energy
CaWO
effect of 2x more
light4
W-recoils
β,γ
no-light events
W-recoils
find new scintillating target materials
• higher light output
and/or
• higher light yield for heavy elements
candidates?
• CdWO4
• CsI
21
Summary
• status of CRESST:
– data taking with 10 detector modules ongoing
– new holding clamps: strongly reduced no-light events
– important role of CRESST: constrain inelastic Dark Matter model
• new CRESST test facility at Gran Sasso
– allows for efficient detector R&D under low-background
conditions
– ready for operation
• plans for 2010:
– continue data-taking to clarify nature of remaining events in the
signal region
– search for new scintillators as target
22
Backup Slides
23
CRESST Detectors
The phonon detector:
300 g cylindrical CaWO4
crystal. Evaporated
tungsten thermometer with
attached heater.
The light detector:
Ø=40 mm silicon on sapphire wafer.
Tungsten thermometer with attached
aluminum phonon collectors and thermal link.
Part of thermal link used as heater.
24
Detector Readout
• SQUID-based readout of W-thermometer:
Ibias
Rreference
Rfilm(T)
x
x
SQUID
Rreference
25
Background from Surface α-Decays
• in particular from decay chain after 222Rn:
210Po → 206Pb (104 keV) + α (5.4 MeV)
• Po can be implanted in surface → Pb-recoil in the
relevant energy range (below 40 keV)
reflective
housing
crystal
a
Pb
Po
• scintillating surrounding as veto when hit by α-particle
26
Commissioning Run – DM Limit
4.8x10-7 pb
for
50 GeV/c2
WIMPs
[G.Angloher et al., Astropart. Phys. 31 (2009), 270-276]
27
Clamp Discrimination Detector
• modified phonon detector:
– normal crystal + 1cm slice
of CaWO4
– parts glued together
– thermometer and clamps:
on small part
glue
thermometer
clamps
• signals from the small part should have different pulse shape
 possibility to discriminate clamp-induced events
28
Comp. Detectors – Pulse Shape
target crystal
thermometer carrier
29