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Session 70 - Cosmic Backgrounds and the Distant Universe.
Oral session, Thursday, June 11
Presidio,
We report on the Cosmic Diffuse Gamma Ray (CDG) spectrum from 800 keV to 30 MeV obtained with the Imaging Compton telescope COMPTEL, on-board the Compton Gamma Ray Observatory. The spectrum is constructed by measuring the count rate of gamma rays from high galactic latitudes during periods when the Earth is outside the COMPTEL field-of-view. Special data selections are applied to suppress the prompt and delayed background contributions. Above 4.2 MeV, in the absence of long-lived background, the count rates are extrapolated to zero cosmic-ray intensity to eliminate the prompt background and arrive at the CDG count rates. The prompt background is produced primarily by the interactions of neutrons and protons with the spacecraft. The delayed emission from long-lived radioactivity, present only below 4.2 MeV, is determined by fitting the energy spectrum. Their contributions are first subtracted before the extrapolation to zero cosmic-ray flux to determine the CDG contributions. These COMPTEL CDG flux values in the 1-30 MeV range are in good agreement with the our results reported earlier. The COMPTEL flux in the 1-10 MeV range is about 5-10 times lower than the pre-COMPTEL reports. The 1-30 MeV flux is consistent with power-law extrapolations from lower and higher energies. The CDG measurements show no evidence of a 'MeV bump' in the 1-10 MeV range.
