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

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

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

Cherenkov photons produced (15mm C6F14 , 5.7-7.8 eV): <485> Photon losses Cherenkov photons produced (15mm C6F14 , 5.7-7.8 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>

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

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

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

QE Measurement Quality control: VUV scanner

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

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

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 0.54 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 1.05-1.01 0.4-1 1.5-3.5 3-7 18o – 8o 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 Air 1.00029 6 20 39 1.4o Helium 1.000033 60 115 0.5o

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

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

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