Activation of space-borne bismuth germanate γ-ray detectors

Accurate knowledge of the sources of detector background is essential in γ-ray astronomy for correct data interpretation, and for optimising detector design and operation. In the low-to medium-energy range, a major source of background arises from radioactivity that is induced in the detector medium itself by nuclear spallation and neutron capture. Results are presented from the Shuttle Activation Monitor flown in the Space Shuttle middeck area on two missions during August 1989 and September 1991. This experiment has been valuable in the study of induced radioactivity in BGO scintillators resulting from bombardment by cosmic-rays, trapped protons, and their secondaries created by interactions in a heavy spacecraft. Data from mono-energetic proton irradiation experiments have allowed investigation of activation effects in BGO for less complicated irradiation conditions, and the results from these experiments are also presented. Detailed simulations have been performed to determine BGO detector activation and response, and these take into consideration the effects of shielding by the spacecraft structure. The results compare very well to the experimental data, allowing features in the measured spectra to be identified. They also highlight the importance of shielding both in reducing activation by trapped protons, and enhancing cosmic-ray induced activation due to secondaries