Optimum thickness of polycrystalline semiconductor multialkali photocathodes is studied by the exponential attenuation rate of the radiation intensity and formulas for the quantum yield spectra and optical absorption coefficient of multialkali photocathodes. It has been found that if I(alpha )/IO > 0.4, photocathode optimum thickness D will be in excess of around 1000 angstroms and if I(alpha )/IO increases further, D will tend to increase too, while photoelectrons excited by energetic photons will appear in the inner surface of the photocathode. Furthermore, D should be in the vicinity of 300 angstroms for the first kind of photocathode (S-20, S-20R, and S-25) and 900 angstroms for the second kind of photocathodes (new S-25, Varo, and LEP). It is suggested that the optimum photocathode thickness D be studied by the spectral response peaks of multialkali photocathodes. We came to the conclusion that photoelectron escape depth and incident wavelength tends to increase and along with it the optimum photocathode thickness. It has been calculated that the optimum thickness of multialkali photocathodes possessing good response in visible and infrared light must be in the vicinity of 1200 angstroms.
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