The dependence of quantum efficiency of alkali halide photocathodes on the radiation incidence angle

We studied variation of quantum efficiencies of CsI, KBr and KI-evaporated reflective planar photocathodes with the angle of radiation incidence (±55 degrees) in the spectral range of 25-115 nm. The photocurrent increases with the photon incidence angle for short wavelengths (by as much as ~60% at 55 for 25-30 nm illumination), while it changes only ~5% at wavelengths ~90-115 nm. The theoretical calculations of the photocathode angular response based on the absorption length of the photons and the escape length of the photoelectrons are in a relatively good agreement with the measured data. A detailed study of the detection efficiency angular variation for the microchannel plates with CsI, KI and KBr photocathodes in the spectral range of 25-191 nm is also presented. Heat annealing of the planar photocathodes did not result in any significant variation in their angular response.