An improved geometry for coupling a beam into a photomultiplier with total internal reflection sensitivity enhancement is described which allows this technique to be used with large sized beams. The equations giving the increase of the photocathode’s absorption efficiency by the use of internal reflection within the window are discussed and conditions for best performance specified. The calculations, based on modifications of experimentally verified equations employed in internal reflection spectroscopy, predict that cathodes only a few monolayers thick could be made to absorb almost all the light falling on them in very few reflections. It then is shown that this will result in high quantum efficiencies with a much reduced spectral dependence. Particularly high increases may be expected for the less efficient cathodes such as S–1. The procedure is shown to relax the requirements on electron escape depth and absorption coefficient of the cathode material so much that the range of possible materials is considerably enlarged. Also, surface and defect level photoemitters will become practical. This and the change of work function with material thickness raise the possibility of extending the region of operation of photomultipliers more into the ir. Photomultipliers so built could also show a higher frequency response to modulated light beams. Finally, means of reducing the dark current and obtaining the multiplex gain in these photomultipliers are discussed.
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