Extreme ultraviolet quantum detection efficiency of rubidium bromide opaque photocathodes

We present measurements of the quantum detection efficiency (QDE) of rubidium bromide opaque photo-cathodes over the 44-1560-A wavelength range. We achieved QDEs of >60% at lambda = 68 A, and>40% at lambda approximately 920 A, for RbBr photocathode layers applied to the surface of microchannel plates (MCPs). The photoelectric threshold is observed at lambda approximately 1560 A, and there is a broad ( approximately 100-A) QDE minimum centered at lambda approximately 775 A which correlates with 2x the band gap energy for RbBr. The QDE is characterized by four peaks centered at lambda approximately 68 A, lambda approximately 400 A, lambda approximately 600 A, and approximately 1050 A. The QDE peaks at lambda approximately 400 A, approximately 600 A, and approximately 1050 A correspond with emission of 3, 2, and 1 photoelectrons, respectively. The QDE at the lambda approximately 68-A peak is associated with a d-f resonant absorption feature of RbBr. QDE contributions of the photocathode material inside the channels, and on the interchannel web, have been determined. Measurements of the angular variation of the QDE from 0 degrees to 35 degrees to the channel axis are also presented. We describe a simple QDE model and show that its predictions are in accord with the QDE measurements. Preliminary assessment of the stability of RbBr indicates that no QDE degradation occurs after limited exposure (20 h) to air at low humidity (<30%). Examination of the photocathode structure with an electron microscope reveals a rough surface with a scale of the order of 0.5 microm.