Download HMPID Detector

Production of Cesium Iodide Photocathodes for
the ALICE/High Momentum Particle IDentification
RICH Detector
Erwin Rossen
Student Session
11th August 2005
Contents
• Why ALICE?
• ALICE detectors
• ALICE‘s HMPID RICH detector
• HMPID‘s CsI Photocathodes
Quark-Gluon Plasma
Collision of two heavy ions  QGP:
Deconfinement !
Particle Identification
ALICE uses all known
methods for particle ID
HMPID Detector
Concept: RICH (Ring Imaging CHerenkov)
• A charged particle travelling in a medium faster than the
speed of light in that medium with  Cherenkov radiation
• The angle is a function of the velocity of incoming the
particle
vc = speed of light in
refractive medium
vp = b c = speed of
incoming particle
q = angle of
Cherenkov photon
1
cos q 
nb
HMPID Detector
Radiator: 15 mm of C6F14
Photon converter:
300 nm layer of CsI
(QE = 23% @ 170 nm)
MultiWire Proportional
Chamber (MWPC): filled
with CH4 at atmospheric
pressure
Photon losses
Loss mechanism
Cherenkov photons
produced (15mm C6F14 , Absorb C6F14
5.7-7.8 eV): <485>
# lost photons
<107>
Absorb quartz
<18>
Absorb CH4
<2>
Absorb wire planes
<38>
Reflected CsI
<22>
CsI QE
<261>
Chamber + FEE
<5>
Detected photon
<31>
CsI Photocathodes
• 300 nm CsI on a substrate
• Size: 40x64 cm
• Segmented into 8x8 mm pads
CsI
gold front
surface
(0.4 mm)
nickel barrier
layer (7 mm)
Substrate
multilayer pcb with
metalized holes
CsI Photocathodes
• 7 Modules, each containing 6
photocathodes
• 11 square meter of active area
Module with
six padplanes
Backside with
space for frontend electronics
CERN Evaporation Chamber
substrate
protective
box
• Evaporation of CsI in
vacuum
• CsI dissolves in water
 you need protection
against humidity (air)
• Air tight protective
box after evaporation
QE Measurement
Quality control: VUV scanner
I norm 
I CsI  I CsI _ noise
I PM  I PM _ noise
Actions
• Mount substrate onto rail
• Install boats with CsI (0.75 gr. per boat)
• Create vacuum (~10-6 mbar)
• Residual Gas Analysis
• Evaporation
• Measurements
• Data analysis
• Extraction of photocathode
3.4
3.1
3.3
3.0
Normalized current
Normalized current
Data
3.2
3.1
3.0
2.9
2.8
2.9
2.8
2.7
2.6
2.5
2.7
0
1
2
3
4
5
Time (hours)
Good photocathode
6
0
1
2
3
4
5
6
Time (hours)
Bad photocathode
Quality depends on temperature, pressure, water
concentration and perhaps things we don’t know yet
7
A Large Ion Collider Experiment
• Collision of Pb nuclei, energies up to 5,5 TeV per nucleon
 >1,000 TeV per collision
• Expected temperature phase transition: 175 MeV
• Expected temperature in ALICE: 600 MeV
• Up to 8,000 particles per unit of rapidity produced in
central collision
• Rapidity range HMPID: -1 < h < 1
RICH Configurations
focusing
proximity focusing
Material
n
Πthr(GeV/c) Kthr(GeV/c)
Pthr(GeV/c)
θmax (β=1)
Diamond
2.417
0.06
0.25
0.42
65o
Plexiglas
1.488
0.13
0.45
0.85
48o
Vodka
1.363
0.15
0.53
1.01
43o
Beer
1.345
0.15
0.54
1.03
42o
Water
1.332
0.16
0.56
1.07
41o
C6F14
1.29
0.17
0.60
1.13
39o
CF4 (liquid)
1.226
0.19
0.7
1.32
35o
1.5-3.5
3-7
18o – 8o
Aerogel
1.05-1.01
0.4-1
C4F10
1.00140
2.6
9
17
3o
Isobutane
1.00127
3
10
18
2.9o
Argon
1.00059
4
14
27
2o
CF4 (gas)
1.00050
5
16
30
1.8o
Methane
1.00051
5
16
30
1.8o
Air
1.00029
6
20
39
1.4o
1.000033
17
60
115
0.5o
Helium
Photocurrent and pads
I [nA]
X = Y = 0.6 mm
One 8,0 x 8,0 mm pad
Several pads
Series production of CsI PCs
For each PC: overview scan of 280 points on the PC
min( Ipc)  1.5
min ( Ipc)  1.5
Example PC 46:
Inorm
Inorm
Mean value: <Inorm> = 3.71
min-max variation 6%
other PCs: inhomogeneities up to
10% - 12% min – max
Test Beam results (with single
photon counting) confirm these
inhomogeneities.
x [mm]
x [mm]
y [mm]
y [mm]
Mean values used to compare PC‘s
stdev ( Ipc)
mean ( Ipc)
 0.01
mean ( Ipc)  3.71
Layer thickness
h
e-
Active zone ~ 60 nm
CsI layer 300 nm
Substrate
Thickness >> Active zone:
• Provide additional stability against moisture, which
can destroy a thin layer much faster than a thick one
• Minimize any influence of the substrate