Negative Affinity X-Ray Photocathodes

True image-forming telescopes for use in X-ray astronomy are available now with arc-second resolution for A ≥3A. Such telescopes have been very successfully applied on rockets and on the recent Skylab mission for solar observations. However, their application to observations of extra-solar X-ray sources has been severely limited due to a lack of sensitive X-ray imaging detectors. We describe here a new type of detector using a GaAs photocathode with quantum efficiency approaching 100%. This detector allows us to detect sources 10 to 20 times fainter than previously possible, while preserving spatial resolution on the several micron scale. We present data on the X-ray photoelectron yield for GaAs (Cs-O2) negative affinity cathodes both in reflection mode using a thick cathode and in the transmission mode using a 3.7 micron wafer. A theoretical model is developed and compared with the data to determine the electron diffusion length, the front surface electron reflection coefficient and the back surface electron escape probability for the transmission cathode. There is good agreement between the crystal parameters obtained from X-ray data and from optical data. Application to X-ray telescope imaging in a practical device is discussed.