Extreme Ultraviolet Quantum Efficiency Of Opaque Alkali Halide Photocathodes On Microchannel Plates

Abstract

In recent years there has been considerable interest in the use of optically opaque layers of alkali halides on microchannel plates (MCP's) to improve quantum detection efficiency (QDE). However, only a few materials have been studied, in particular CsI and MgF2 in the soft X-ray regime (<200Å). We present comprehensive measurements of the quantum detec-tion efficiency for a number of materials, CsI, KBr, KC1, and MgF2, over a wide wavelength range (44Å to ~1800Å). These results show that high (>40%) quantum detection efficiency may be achieved for many materials, at certain wavelength regions in the extreme ultraviolet (EUV). We also observe structure in the wavelength dependence of the quantum detection efficiency that is directly related to the valence band to conduction band gap energy, and the onset of atomic-like resonant transitions. We show that a simple photocathode model allows interpretation of these features, and the variation of the quantum detection efficiency as a function of illumination angle. The structure of photocathode layers has been examined with an electron microscope, and shows that typical cathodes are granular. We also demonstrate that the cathode layer structure changes when degradation of the QDE occurs for hygroscopic materials.