1. Production of Cesium Iodide Photocathodes for
the ALICE/High Momentum Particle IDentification
RICH Detector
Erwin Rossen
Student Session
11th August 2005

2. Contents Why ALICE? ALICE detectors ALICE‘s HMPID RICH detector
HMPID‘s CsI Photocathodes

3. Quark-Gluon Plasma
Collision of two heavy ions  QGP:
Deconfinement !

4. Particle Identification
ALICE uses all known
methods for particle ID

5. Concept: RICH (Ring Imaging CHerenkov)
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

7. HMPID Detector Radiator: 15 mm of C6F14 Photon converter:
300 nm layer of CsI
(QE = 170 nm)
MultiWire Proportional Chamber (MWPC): filled with CH4 at atmospheric pressure

9. Cherenkov photons produced (15mm C6F14 , 5.7-7.8 eV): <485>
Photon losses
Cherenkov photons produced (15mm C6F14 , eV): <485>
Loss mechanism
# lost photons
Absorb C6F14
<107>
Absorb quartz
<18>
Absorb CH4
<2>
Absorb wire planes
<38>
Reflected CsI
<22>
CsI QE
<261>
Chamber + FEE
<5>
Detected photon
<31>

10. gold front surface (0.4 mm)
CsI Photocathodes
300 nm CsI on a substrate
Size: 40x64 cm
Segmented into 8x8 mm pads
gold front surface (0.4 mm)
CsI
nickel barrier layer (7 mm)
Substrate
multilayer pcb with metalized holes

11. CsI Photocathodes 7 Modules, each containing 6 photocathodes
11 square meter of active area
Module with six padplanes
Backside with space for front-end electronics

13. CERN Evaporation Chamber
protective box
substrate
Evaporation of CsI in vacuum
CsI dissolves in water  you need protection against humidity (air)
Air tight protective box after evaporation

14. QE Measurement
Quality control: VUV scanner

15. 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

16. Data Good photocathode Bad photocathode
Quality depends on temperature, pressure, water concentration and perhaps things we don’t know yet

17. 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

18. RICH Configurations
focusing proximity focusing

19. 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
0.54
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
Aerogel
0.4-1
3-7
18o – 8o
C4F10
2.6 9 17 3o
Isobutane 3 10 18
2.9o
Argon 4 14 27 2o
CF4 (gas) 5 16 30
1.8o
Methane
Air 6 20 39
1.4o
Helium 60
115
0.5o

20. Photocurrent and pads One 8,0 x 8,0 mm pad Several pads
X = Y = 0.6 mm
I [nA]
Several pads
One 8,0 x 8,0 mm pad

21. Series production of CsI PCs
For each PC: overview scan of 280 points on the PC
Example PC 46:
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.
Mean values used to compare PC‘s

22. Thickness >> Active zone:
Layer thickness e- h
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