A mercuric iodide detector system for X-ray astronomy. Pt. 2

To establish the expected sensitivity of a new hard X-ray telescope design, described by Ricker et al. (paper I), an experiment was conducted to measure the background counting rate at balloon altitudes (40 km) of mercuric iodide, a room temperature solid state X-ray detector. The prototype detector consisted of two thin mercuric iodide (HgI2) detectors surrounded by a large bismuth germanate (Bi4Ge3O12) scintillator operated in anticoincidence. The bismuth germanate shield vetoed most of the backround counting rate induced by atmospheric gamma-rays, neutrons and cosmic rays. A balloon-borne gondola containing a prototype detector assembly was designed, constructed and flown twice in the spring of 1982 from Palestine, Texas. The second flight of this instrument established a differential background counting rate of 4.2 +- 0.7 x 10-5 counts/s cm2 keV over the energy range of 40-80 keV. This measurement was within 50% of the predicted value. The measured rate is proportional 5 times lower than previously achieved in shielded NaI/CsI or Ge systems operating in the same energy range. The prediction was based on a Monte Carlo simulation of the detector assembly in the radiation environment at float altitude. Details of the simulation can be found in paper I. (orig.)