Soft x-ray and extreme ultraviolet quantum detection efficiency of potassium bromide photocathode layers on microchannel plates.

The quantum detection efficiency (QDE) of potassium bromide, applied directly to the surface of a microchannel plate (MCP), has been measured over the wavelength range from 44 to 1216 A. We present the first measurements for the QDE of KBr between 44 and 256 A. These show that there is a high QDE peak (~70%) centered at ~70 A. The results at wavelengths above 256 A agree with our previous study. Investigation of the angular dependence of the QDE indicates that maximum efficiencies are achieved for graze angles </=10 degrees at wavelengths below 171 A. Examination of the photocathode surface with a scanning electron microscope shows that the KBr layer is rough, with granularity at the 0.5-microm level.