Diffuse Cosmic Gamma Radiation Measured by HEAO 1

The High Energy Astronomical Observatory 1 (HEAO 1) contained as one of 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, had a geometrical area of 43 cm², and were collimated to a circular field of view of 17° FWHM. During a period of operation starting in 1978 April, these detectors were operated in a manner designed to provide a precise measure of the diffuse cosmic background gamma-ray flux. Previous measurements of the spectrum in this range were not of sufficient precision to distinguish between various models explaining the hard X-ray background either as an ensemble of discrete sources or due to a global effect at cosmological distances. The detectors could be alternately blocked or unblocked with a moveable shutter of CsI, which could be operated in an active anticoincidence mode or as a passive shield. Data taken in these various modes and analyzed with respect to the varying radiation environment of the 23° inclination 400 km circular orbit permitted separation of various background effects that have plagued previous measurements of this radiation using scintillation spectrometers. Over the ~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) × 10^-3(E/100 keV)^-(2.75±0.08 photons cm^-2 s^-1 keV^-1 sr^-1. This fit, in general agreement with previous results in this energy range, joins smoothly with measurements at lower energies, and when extrapolated to higher energies, it agrees with the recently reported COMPTEL results at energies above 2 MeV obtained on the Compton Gamma Ray Observatory. The accompanying demonstration of the experimental difficulties associated with scintillation spectrometer measurements of the diffuse cosmic background provides significant clarification of the observational status of this important measurement. When account is taken of measurements with possible local background estimation uncertainties, the spectrum between ~200 keV and ~50 MeV appears more simplified than previous estimates.