DIFFUSE COSMIC GAMMA-RADIATION MEASURED BY HEAO 1

The High Energy Astronomical Observatory 1 (HEAO 1) contained as one o f its instruments the UCSD/MIT hard X-ray and gamma-ray instrument, a complex of actively shielded scintillation counters that operated over the nominal 10 keV to 10 MeV range. The two medium-energy detectors ( MEDs) employed in this investigation covered the range 80 keV-2 MeV, h ad a geometrical area of 43 cm(2), and were collimated to a circular h eld of view of 17 degrees FWHM. During a period of operation starting in 1978 April, these detectors were operated in a manner designed to p rovide a precise measure of the diffuse cosmic background gamma-ray fl ux. Previous measurements of the spectrum in this range were not of su fficient precision to distinguish between various models explaining th e hard X-ray background either as an ensemble of discrete sources or d ue to a global effect at cosmological distances. The detectors could b e alternately blocked or unblocked with a moveable shutter of CsI, whi ch could be operated in an active anticoincidence mode or as a passive shield. Data taken in these various modes and analyzed with respect t o the varying radiation environment of the 23 degrees inclination 400 km circular orbit permitted separation of various background effects t hat have plagued previous measurements of this radiation using scintil lation spectrometers. Over the similar to 80-400 keV band, systematic errors were small and correctable, with a resulting spectrum fitted by a power law of dN/dE = (2.62 +/- 0.05) x 10(-3)(E/100 keV)(-(27.75+/- 0.08) photons cm(-2) s(-1) keV(-1) sr(-1). This fit, in general agreem ent with previous results in this energy range, joins smoothly with me asurements at lower energies, and when extrapolated to higher energies , it agrees with the recently reported COMPTEL results at energies abo ve 2 MeV obtained on the Compton Gamma Ray Observatory. The accompanyi ng demonstration of the experimental difficulties associated with scin tillation spectrometer measurements of the diffuse cosmic background p rovides significant clarification of the observational status of this important measurement. When account is taken of measurements with poss ible local background estimation uncertainties, the spectrum between s imilar to 200 keV and similar to 50 MeV appears more simplified than p revious estimates.