Abstract
The poor quantum efficiency of alkali photocathodes in the near infrared can be improved by multiple total internal reflection. A limit for the improvement attainable by this method is calculated and the maximum ratio of the enhanced over the conventional quantum efficiency is found to be the inverse of the absorptance at normal incidence. A novel, simple way of measuring the absorptance of standard tubes is described. Experimental results for the quantum-efficiency enhancement and the absorptance are presented and agree with the theoretical considerations. The measured ratio of the enhanced over the conventional quantum efficiency for the extended S20 photocathode ranges from 2 at 400 nm to more than 12 at 980 nm. The authors believe that, by combining multiple total internal reflection techniques with the use of dedicated photocathodes, the quantum efficiency in the near infrared can be increased by factors of 20-30.
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