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.
Electrons generated in photocathodes have a range of energies and may exit the outer layer of the photocathode with a certain distribution (possibly isotropic). In a proximity-focussed image intensifier where there is a strong electric field between the photocathode and the micro-channel plate (MCP) electrons ejected at an angle will follow a trajectory defined by the exit velocity of the electron and the strength of the field. A small spot of light projected onto the photocathode will result in a point spread function determined by the size of the gap, the field applied across it and the magnitude of the radial energy component of the electrons. By using photon counting and centroiding techniques, the events occurring on the screen of an image intensifier have been integrated and used to measure the diameter of the projected spot (~5 micron diameter) thus giving a measure of the resolution of the tube. At short (UV) wavelengths the spread of electron energies is larger and the average radial energy component is larger than at longer (visible) wavelengths. Hence the resolution is better in the visible. Resolution measurements as a function of wavelength of solar blind and S20 intensifiers show a dip in the measured spot size and hence a localised peak in the resolution in a short range of wavelengths in the UV. Combined with data obtained from measuring the electron energy distribution that shows a narrowing of the distribution in the same region, this shows evidence of multiple photoelectrons being generated within the photocathode. Such electrons would have lower energies resulting in higher measured resolution and a narrower electron energy distribution profile.
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