Experimental data on the reflection and transmission spectral response of photocathodes

The spectral transmission and electron emissivity responses, measured for a series of typical photocathodes, are presented and analysed. Specifically, samples of S1, S20, S25, Bialkali and two types of solar-blind telluride photocathodes were investigated in both transmission and reflection modes of operation. The transmission mode is more convenient for imaging, night vision and for scintillation counting applications such as CT scanners and is more commonly used than the reflection mode. However, more recent work has focussed on the reflection photocathode as a source of electrons with low energy spread used for electron guns for microscopy and lithographic free electron lasers [1]. Our analysis provides a determination of the reflectivity of the substrate/cathode and cathode/vacuum interface, enabling the refractive index to be deduced. The high apparent quantum efficiency (QE) of some conventional photocathodes is shown to be due to the conversion of each photon to two or more electrons.