INSTRUMENTAL BACKGROUND IN GAMMA-RAY SPECTROMETERS FLOWN IN LOW EARTH ORBIT

Techniques are presented for calculating the instrumental continuum background in gamma-ray spectrometers flown in low Earth orbit. Simple methods are developed for scaling from the better-understood measurements and calculations of background in balloon-borne instruments. Secondary radiations are found to scale with the parameter E0.7 defined as the integral of the particle energy raised to the 0.7 power times the incident proton spectrum. The new scaling methods give background results for the HEAO 3 gamma-ray spectrometer that compare well with measurements. The key results are predictions of the background, sensitivity and orbit effects for future spaceborne high-resolution gamma-ray spectrometers. The background for these instruments will be highly dominated from 0.1 to 2 MeV by the beta-decay component. Detector segmentation and pulse-shape discrimination beta-rejection techniques will therefore be critically important. Loss of observing time and sensitivity due to activation by trapped particles in the South Atlantic Anomaly (SAA) is strongly dependent on orbit inclination and altitude. The largest losses occur between 0.2 and 1.0 MeV. For the scientifically important energy range near 0.6 MeV, a 28-degrees, 500 km orbit has up to 70% loss of observing time. For a 28-degrees inclination orbit, acceptable time loss of less than 50% can only be achieved by flying under the SAA in a 300-400 km orbit. To reduce data loss to less than 20%, the > 250 MeV trapped proton daily fluence must be less than 2 x 10(4) proton cm-2, which occurs for orbits with inclinations less than 20-degrees for 400 km altitude and less than 15-degrees for 500 km altitude.