Evaluation of x-ray image intensifiers as detectors for x-ray astronomy

Proposed experiments in X-ray astronomy require a detector capable of forming a high resolution image of a weak source. An X-ray image intensifier incorporating a microchannel plate (MCP) has been studied as a candidate for this application. Experiments have shown that the device is sensitive to single X-ray photons and has adequate quantum efficiency. Consideration has been given to the possibility of improving the quantum efficiency by deposition on the input surface of the MCP of a material with high photoelectric yield in the wavelength region of interest. Photographs of the output light from the image intensifier show than an X-ray photon detected at the MCP results in a spot on the film of about 60 μ M diameter. Since the position of the centroid of the spot could be determined to better accuracy, the device h as sufficient resolution for the contemplated experiment. Detector noise is found to be so low that it would no limitation on the experiment's sensitivity. The image can be retrieved and transmitted to the ground by focussing a television camera on the face of the fiber optic output of the X-ray image intensifier; the data show that a Secondary Electron Conduction vidicon is sensitive enough to se e the output resulting from a single X-ray photon. An alternative technique would sense charge pulses from the MCP on a fine wire grid, avoiding the need for a phosphor and providing superior time resolution and simpler data compression. Since the detector is to be used in a satellite-borne experiment, its space qualification must be established. Two launches on sounding rockets have failed to impair the usefulness of one unit that has been studied over a period of more than a year. A long term test of continuous performance in high vacuum has so far shown encouraging results. A count life test has shown that an MCP can last through detection of orders of magnitude more counts than could be expected in the course of a mission. X-ray image intensifiers appear entirely suitable for high resolution X-ray astronomy.