A Bismuth Germanate-Shielded Mercuric Iodide X-Ray Detector for Space Applications

Mercuric iodide (HgI2) solid state X-ray detectors have many unique advantages in their application to X-ray astronomy. HgI2 detectors operate at or near room temperature so that bulky and massive cryogenic coolers are not required. HgI2 detectors also combine a high quantum efficiency wish good room temperature energy resolution. Because of the excellent quantum efficiency (due to the high effective atomic number of HgI2), background contributions that scale as the volume of the detector (e.g. those due to neutrons and gamma rays) are lower than for other hard X-ray detectors with equivalent stopping power (e.g. germanium). To measure the actual background counting rates of HgI2 that can be achieved in a space-like environment, we have constructed an instrument containing a HgI2 detector shielded by a bismuth germanate (BGO) scintillator to be flown on a high altitude balloon (40 km altitude). The first flight of this instrument is planned later this year. In June 1980 an array of 11 HgI2 detectors shielded with sodium iodide was flown in a balloon experiment to observe the black hole candidate Cygnus X-1. The results from this flight have been previously reported.