GRIS background reduction results using isotopically enriched Ge

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Abstract
The Gamma Ray Imaging Spectrometer (GRIS) was flown twice from Alice Springs, Australia in the spring of 1992 for a total of 32 hours at float alititude. One of the seven Ge detectors (Ge‐Lite) was isotopically enriched (≳97% 70Ge). This is the first time an enriched Ge detector has been used for astrophysical observations. Because of its thick anticoincidence shield, the GRIS instrument background is dominated by internal β‐decay in the energy range 200–1000 keV. Half of the contribution in this β‐decay ‘‘hump’’ is due to neutron activated 74Ge. GRIS observed a factor of two reduction in this energy regime in the enriched detector. In future instruments (INTEGRAL/NAE) with thicker anticoincidence shields and smaller apertures, the background reduction will be even larger. Three strong instrumental background lines (54, 67, and 139 keV) are also eliminated. The elimination of the first two is particularly important for cyclotron line observations.

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