Wavelength-dependent resolution and electron energy distribution measurements of image intensifies

摘要

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 intensifies 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.