Performance of bismuth germanate active shielding on a balloon flight over Antarctica

The GRAD (Gamma-Ray Advanced Detector) gamma-ray spectrometer was flown on a high-altitude balloon at an altitude of 36.6 km over Antarctica on January 8-10, 1988 where it was used to make observations of Supernova 1987A. The performance of the bismuth germanate (BGO) active shielding in the near-space environment over Antarctica is examined. The promised effectiveness of this shielding in the suppression of unwanted background has been demonstrated. The BGO-shielded GRAS spectrometer detected gamma-ray lines with fluxes of 2×10^-3 cm^-2 s^-1 from SN 1987A in a radiation background approximately a factor of 4 more intense that over Alice Springs, Australia. This level of sensitivity indicates that BGO is at least as effective as CsI when used as active shielding. Isomerism is common, both in the bismuth and germanium regions of the nuclear chart, but is found to be less of a problem for background suppression in the latter region than in the former