Development of Large Area CsI Photocathodes for the ALICE/HMPID RICH Detector

The work carried out within the framework of this PhD deals with the measurement of the photoelectric properties of large area thin film Cesium Iodide (CsI) photocathodes (PCs) which are to be used as a photon converter in a proximity focusing RICH detector for High Momentum Particle Identification (HMPID) in the ALICE experiment at the LHC. The objective was to commission a VUV-scanner setup for in-situ measurements of the photoelectric response of the CsI PCs immediately after the thin film coating process and the use of this system to investigate the properties of these photon detectors. Prior to this work and prior to the finalization of the ALICE/HMPID detector design, R&D work investigating the properties of CsI PCs had been performed at CERN and at other laboratories in order to determine possible substrates and optimized thin film coating procedures. These R&D studies were usually carried out with small samples on different substrates and with various procedures with sometimes ambiguous results. Consequently it was necessary to further analyze various properties and procedures using the final large area substrates and production facilities. Among the investigated phenomena the most important ones were: • Post deposition treatment: from the R&D studies it was known, that the PC response can be increased by heating the PC after the coating process. Within this thesis this effect was investigated both with large area test PCs and with the final PCs for the detector. It could be shown that the enhancement effect is mandatory to achieve the photon conversion efficiency required by the detector design. Furthermore it could be shown that any difference in PC quality is due to differences in this enhancement effect. The results of these measurements were and are continuously used to refine and optimize the production procedure. • Ageing effects: CsI PCs age under exposure to humidity due to the hygroscopicity of CsI and under high photon flux and ion bombardement inside the Multi Wire Proportional Chamber (MWPC) of the detector. All three effects have been investigated with the VUV scanner. The first effect requires a careful treatment of the CsI PCs to avoid exposure to humid air. Furthermore this effect was found to be reversible if the PC is heated. High photon fluxes are irrelevant in a Cherenkov detector dealing with single photons, however the problem needed to be investigated to verify that the measurement process itself does not damage the PCs. The third mechanism is very important as it occurs during normal detector operation and depends only on the radiation environment of the experiment. An accelerated test was carried out by irradiating a detector with a radioactive source. For high doses corresponding to 20 years of operation inside ALICE (and much higher dose rates) a clear degradation of up to 40 % of the PC response was observed. The results show that low dose and low dose rate measurements are still required to draw a conclusion on the projected lifetime of the PCs inside ALICE. With the results of these studies the production procedure for the 42 PCs for the ALICE/HMPID was optimized and the first 17 PCs have been produced and analyzed by means of the VUV-scanner. The results were compared with an evaluation of the PCs inside the final detector modules using charged particle beams. The comparison of scanner and test beam results showed the reliability of the VUV-scanner system and the comparability of a high photon flux photocurrent measurement under vacuum with the single photon counting process in the detector. This thesis is structured in the following way: Chapter 1 provides an introduction to the ALICE experiment at LHC. Chapter 2 outlines the concept of particle identification with Cherenkov detectors and describes the ALICE/HMPID detector. In the Chapter 3 the properties of CsI photocathodes are summarized and the thin film coating setup for production and VUV-scanner setup for evaluation are introduced. Chapter 4 elaborates on the various commissioning tests carried out to clarify the influence of various measurement parameters and setup conditions on the measurement process. In Chapter 5 the results of the investigation of the post deposition heat enhancement phase of CsI PCs are given. Chapter 6 treats ageing of CsI by exposure to humidity. Chapter 7 summarizes the results obtained during the production of the first 17 PCs for the detector and compares the VUV scanner measurements and the beam tests. Finally, in Chapter 8 the ageing of CsI under ion bombardement is discussed, followed by the final conclusions in Chapter 9.