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TRANSMISSION SECONDARY EMISSION FROM THIN FILMS OF ALKALI HALIDES.

The phenomenon of secondary emission from alkali halides has been studied theoretically and experimentally in substantial detail. Transmission type measurements have been chosen, rather than reflection, because the former method gives one more parameter (thickness) for comparisons between experiments and theory, The energy loss suffered by primary electrons is obtained from published experimental results of electron scattering by thin foils. The escape of low energy electrons (. 25 to 7.5 eV) is studied in detail using time dependent perturbation theory. The resulting transport problem is solved with the exact scattering probability distribution by a Monte Carlo method. It is shown that for low energy electrons generated isotropically inside a solid, an exponential probability of escape exists. Escape parameters and energy losses are computed for CsI, KCl, NaF, and LiF, representing alkali halides from the highest to lowest Z. The experimental yield of films of those four m.aterials, for thicknesses Qf 125 to 1000x, has been measured by a pulsed method. The energy distribution of emitted electrons for the same materials has also been measured, making certain that the films remained uncharged. A Kelvin probe was used to establish that fact and also to determine a true zero of contact potential difference between collector and emitter. A mathematical model of secondary emission based on the detailed theoretical results is proposed and found to agree well with the experiments. Approxi mate internal excitation functions and composite escape functions for secondaries are computed.