Quality-evaluation of Cesium Iodide photocathodes for the ALICE/High Momentum Particle Identification detector by means of a VUV-Scanner system Herbert Hoedlmoser on behalf of the ALICE/HMPID collaboration 5 th International Workshop on Ring Imaging Cherenkov Counters RICH 2004

RICH 2004 Herbert Hoedlmoser – CERN 2 Contents CsI RICH concept: detection of Cherenkov light with CsI photo-cathodes (PCs) CsI photocathodes: substrate & CsI deposition facility In-situ quality control by the VUV Scanner: 2D scan of photocurrent on PCs Series production of PCs: status and comparison of measurements in the scanner with test-beam results Studies of CsI properties: - post deposition heat enhancement - ageing due to exposure to humid air - recovery effect due to annealing Ageing of CsI PCs due to ion bombardement inside the detector Conclusions and outlook

RICH 2004 Herbert Hoedlmoser – CERN 3 Concept: proximity focussing CsI RICH liquid C 6 F 14 radiator MWPC cathode pads coated with CsI 3840 pads per PC with individual analogue readout The ALICE/HMPID Detector 7 modules: total area 11 m 2 largest application so far 42 PCs (64 cm x 40 cm) in total (6 per module)

RICH 2004 Herbert Hoedlmoser – CERN 4 grounding plate (double sided Cu pcb) 2 Photo Cathode pcb’s 40 cm 60 cm Alu frame double layer Cu clad PCB coated with Ni, Au Substrate for the CsI photocathode (PC)

RICH 2004 Herbert Hoedlmoser – CERN 5 Thickness monitor PC Remote controlled enclosure box Evaporation of pre-melted CsI powder from 4 sources by resistive heating inside a 2000 l vacuum chamber Deposition of 300 nm CsI controlled with thickness monitors Labview online monitoring & control Protection box to close the PC under Argon atmosphere after deposition and quality control 4 CsI sources + shutters CERN CsI PC deposition plant protective box pcb substr ate

RICH 2004 Herbert Hoedlmoser – CERN 6 diameter of UV spot: 16mm photo-e- extracted from PC by anode bias reference measurement on a photomultiplier by means of rotative mirror Quality control: VUV scanner Quality control: VUV-Scanner Measurement of the photocurrent on the cathode: 2D scan of the photo-current across the PC surface with a beam of UV light from a D 2 source Reference CsI PM PC current reading D 2 light source + 100V PM Translation stage

RICH 2004 Herbert Hoedlmoser – CERN 7 X-motion VUV-Scanner: 2D scan of the PCs X-motion Y-motion 2D movement of the UV-beam on the PC PC moves in x-direction, optical system in y-direction, movement controlled by Labview

RICH 2004 Herbert Hoedlmoser – CERN 8 Normalized current and measurement uncertainties normalization of the photocurrent from the PC to the reference on the PM repeated measurements of a single position on the PC:  = 2 %

RICH 2004 Herbert Hoedlmoser – CERN 9 Series production of CsI PCs For each PC: overview scan of 280 points on the PC Example PC 46: Mean value: = 3.71 min-max variation 6% other PCs: inhomogeneities up to 10% - 12% min – max TB results (with single photon counting) confirm these inhomogeneities. Mean values used to compare PCs

RICH 2004 Herbert Hoedlmoser – CERN 10 Series production of CsI PCs 17 PCs produced since May 2004 variation of final level of mean photocurrent: 33 % max-min

RICH 2004 Herbert Hoedlmoser – CERN 11 Comparison of test beam and scanner results Number of resolved clusters per track compared with mean photocurrent at the position of the Cherenkov ring 15 clusters minimum requirement Possible reason for discrepancy: slow long term increase in QE between scanner and TB measurement  re-scan e.g. PC 55 in the scanner

RICH 2004 Herbert Hoedlmoser – CERN 12 Study of CsI PC properties with a test PC Test PC to study CsI properties: Post deposition heat enhancement Ageing due to exposure to humid air Recovery effect due to annealing

RICH 2004 Herbert Hoedlmoser – CERN 13 Heat enhancement, exposure, recovery Example for a CsI deposition on the test-PC: Evaporation at room temperature (23 °C) increase in temperature to trigger enhancement effect Continuous scan of the photocurrent on the PC during enhancement phase Plot of an average current from 50 points in time to visualize enhancement and recovery effects

RICH 2004 Herbert Hoedlmoser – CERN 14 Heat enhancement, exposure, recovery

RICH 2004 Herbert Hoedlmoser – CERN 15 Heat enhancement in the series production Conclusion: difference between good and bad PCs largely due to different efficiency of the heat enhancement!

RICH 2004 Herbert Hoedlmoser – CERN 16 Ageing due to ion bombardement Study of the ageing effects due to radiation causing bombardement of the PC with avalanche ions Irradiation of the PC inside a detector with Sr 90 sources Irradiation of 3 positions located on a Cherenkov ring for subsequent test beam analyses Only 8 wires across the irradiated positions on HV

RICH 2004 Herbert Hoedlmoser – CERN 17 Profile of the irradiated zones: narrow, sharp across the wires wide along the wires  effect caused by avalanche ions Results obtained with the VUV-scanner for 3 irradiation with different doses of accumulated charge anode wires Decreased photo-current in aged zones

RICH 2004 Herbert Hoedlmoser – CERN 18 First tests: irradiation with very high doses and doserates! For which doses / doserates does this effect start? Time development: continued degradation without further irradiation

RICH 2004 Herbert Hoedlmoser – CERN 19 Dose-effect relation from 3 different irradiated zones Different doses on 3 irradiated positions Dose-response relation for 49 days and 129 days after irradiation (time effect taken into account by interpolation) expected dose for 10 y ALICE (0.5 mC/cm 2 ) Pos 3 Pos 1 Pos 2

RICH 2004 Herbert Hoedlmoser – CERN 20 Dose-effect relation from dose profile inside a single zone Dose variation within position 1 2 nd method to calculate a dose-effect relation: use the variation of the dose within one irradiated spot (charge profile) and compare to the measured profile of the decrease measurement 129 days after irradiation on position 1 Dose-effect relation from dose profile inside a single zone Dose-effect relation from irradiations in 3 zones with different dose  good agreement

RICH 2004 Herbert Hoedlmoser – CERN 21 Conclusions & outlook Series Production of PCs - Continuation of the series production of PCs after some difficulties with the setup after repairs of turbo pump. - Re-measure one of the PCs in the scanner, for which TB and scanner did not agree Ageing Test - New irradiations with lower doses and lower dose-rates - Attempt to understand the process behind the degradation (deposition of a layer of ions on the PC?) - Try a different analysis (small sample, electron microscope?)