A Satellite Imager for Atmospheric X-Rays

A high-sensitivity X-Ray Imaging Spectrometer (XRIS) was developed for measurements of atmospheric bremsstrahlung X-rays. The XRIS instrument flown on a 3-axis stabilized polar orbiting satellite (S81-1) employed a one-dimensional pinhole camera to acquire a 2-dimensional X-ray image as the satellite passed over an auroral scene. Using a position sensitive gas proportional counter, with an active area of 1200 cm2 divided into sixteen cross-track pixels, the instrument had a geomitric factor of about 0.4 cm -steradian per pixel (6 cm2-sr total) for X-rays of 4 to 40 keV. At an orbital altitude of 250 km, it provided a spatial resolution of 30 km and the temporal resolution was one-eighth of a second. Designed primarily to detect artificial electron precipitation at lower latitudes, the instrument also produced the first satellite X-ray images of the aurora during May and June, 1982. Special features of the instrument included a quadrupole broom magnet to reject energetic electrons, a multilayer plastic-on-tantalum shielding to suppress the bremsstrahlung X-rays generated from electrons which impact the instrument surface, and a new technique for position sensing within the detector, using signal division in a resistor array.