The stability of quantum efficiency and visible light rejection of alkali halide photocathodes

We have studied the UV sensitivity degradation of CsI and KBr thin films under UV illumination and the stability of their visible light rejection for CsI and KBr opaque photocathodes evaporated on microchannel plates. For both materials the greatest degradation of the relative quantum detection efficiency (QDE) was observed near the photocathode sensitivity cut off, while there was almost no change in their EUV response. The ageing of the photocathodes is likely to be independent of the angle of radiation incidence. Of the two materials the CsI films appeared to be more solar blind and less subject to visible sensitivity activation by UV exposure. The QDE of KBr-coated MCP at 5500 Å increased by more than 7 orders of magnitude, from 9x10 to 4x10, after exposure to ~10 photons cm at 1849 Å, while in case of CsI photocathode the activation was only 4 orders of magnitude, from 2x10 to 3x10. The observed activation was stable in vacuum of 10 Torr over a period of at least 15 hours. Initial restoration of solar blindness by illumination with visible light is a very fast process, although the complete deactivation may require large doses. The photocathode activation in the interchannel web area was observed to be much more pronounced than that inside a microchannel pore, suggesting that the angle of radiation incidence is a crucial parameter for the activation dynamics.