The characterization of x-ray photocathodes in the 0.1–10-keV photon energy region

Abstract

A method and an instrument are described for the measurement of the absolute quantum yield for front‐surface and transmission photocathodes in the 0.1–10‐keV photon energy region. The total and the secondary electron photoemission yields have been measured for the Al, Au, CuI, and CsI photocathodes as required for the absolute calibration of the x‐ray diode detectors and for the x‐ray streak cameras. The relative secondary electron yields have also been measured for the same photocathodes by high resolution electron spectroscopy of the secondary electron energy distributions, which are in good agreement with the absolute yield measurements. The secondary electron yield of CsI is ten to one‐hundred times higher than that for Au in the 0.1–10‐keV region and with a secondary energy distribution that is appreciably sharper. For these reasons, CsI should be an effective photocathode for sensitive, time‐resolved spectroscopy into the picosecond region. It is verified experimentally that the secondary electron quantum yield varies approximately as Em(E), with E as the photon energy and m(E) as the photoionization cross section, and that the primary (fast) electron quantum yield is a small fraction of the total yield and varies approximately as E²m(E). A simple model for x‐ray photoemission is described which leads to semiempirical equations for front‐ and back‐surface secondary electron photoemission as based upon an escape depth parameter that may be obtained from yield‐versus‐photocathode thickness data. The model predictions are in good agreement with experiment.