Quantum Efficiency Lifetime Studies using the Photocathode Preparation Experimental Facility Developed for the ALICE Accelerator

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Abstract

This thesis describes the development of a gallium arsenide (GaAs) photocathode preparation facility (PPF) with a load-lock interface as part of an upgrade to the ALICE photo-injector electron gun. The PPF has the capacity to prepare up to six negative electron affinity (NEA) GaAs photocathodes, and permits the rapid and reliable replacement of the degraded photocathode in the gun, while maintaining the integrity of the gun vacuum system. The photocathode preparation has been studied and developed using a custum-built surface characterisation system which permitsthe preparation of NEA GaAs photocathode in conjunction with the application ofseveral surface science characterisation techniques within the same vacuum system. With GaAs/AlGaAs hetero-structure photocathodes designed and developed in collaboration with the Institute of Semiconductor Physics (ISP/Novosibirsk), quantum efficiencies (QE) of 15 - 19% at 635 nm and long lifetimes of the order of 6,800 hours are achieved in a reproducible way. Using the PPF, I have investigated the degradation in the QE of NEA GaAs photocathodes under exposures to gases typically present in the gun vacuum, namely O2, CO2, CO, H2, CH4 and N2. The effects of these gases on the NEA GaAs photocathodes have been demonstrated for the firrst time in a vacuum set-up with a base pressure in 10E-11 mbar range. It was found that H2, CH4 and N2 have no effect on the photocathodes lifetime, whilst substantial reductions of the QE have been observed during the exposures to O2, CO2 and CO. It was also demonstrated for the first time that the NEA GaAs photocathode activated with Cs and NF3 is more stable during CO2 and CO exposures than the photocathode activated with Cs and O2.

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GaAs photocathode

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