A photocathode with a high quantum efficiency (Q.E.) and long lifetime can be fabricated by evaporating a protective layer onto a high Q.E. semiconductor photocathode. In this study we tried to improve the stability and lifetime of a Cs-Te photocathode by coating a CsBr protective layer of 20 nm in thickness over the surface. After the protective layer was applied, we measured the Q.E. of the photocathode and the lifetime in the evaporation chamber. The Q.E. of the coated Cs-Te was confirmed to be about 0.80% and to remain unchanged for a period of longer than 25 d. The effectiveness of the protective layer against exposure of the Cs-Te photocathode to oxygen gas was also confirmed for the first time. Our experiments were the first to show a lifetime improvement of a photocathode installed inside an rf gun compared to the same type of photocathode with no protective coating.

A: At Waseda University, we have been studying for high quality electron beam generation using 1.6 cell Cesium Telluride (Cs-Te) photocathode rf-gun. We use photocathode as the electron source, which can generate high-quality electron beam such as low emittance, and short bunch. The performance of photocathode is evaluated mainly in terms of quantum efficiency (Q.E.) and lifetime. Cs-Te photocathode is known for high Q.E. about 10% with UV light and relatively longer lifetime among semiconductor photocathodes. Since it is a hard environment for photocathode inside the gun, it is necessary to replace the photocathode every several months. In other words, in order to achieve long-term operation of rf-gun, it is necessary to establish highly durable photocathode recipe. In this report, we report the evaluation results of Cs-Te photocathode by co-evaporation, CsBr coated photocathode performance in an rf-gun and future prospects. K: Ion sources (positive ions, negative ions, electron cyclotron resonance (ECR), electron beam (EBIS)

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