Ultraviolet quantum detection efficiency of potassium bromide as an opaque photocathode applied to microchannel plates

We have measured the quantum detection efficiency (QDE) of potassium bromide as a photocathode applied directly to the surface of a microchannel plate over the 250-1600A wavelength range. The contributions of the photocathode material in the channels and on the interchannel web to the QDE have been determined. Two broad peaks in the QDE centered at ~450 and ~1050 A are apparent, the former with ~50% peak QDE and the latter with ~40% peak QDE. The photoelectric threshold is observed at ~1600 A, and there is a narrow QDE minimum at ~750 A which correlates with 2x the band gap energy for KBr. The angular variation of the QDE from 0 to 40 degrees to the channel axis has also been examined. The stability of KBr with time is shown to be good with no significant degradation of QDE at wavelengths below 1216 A over a 15-day period in air.