The Cosmic Diffuse Gamma-Ray (CDG) spectrum between 800 keV and 30 MeV has been measured with the Imaging Compton telescope COMPTEL, aboard the Compton Gamma Ray Observatory. Although COMPTEL is primarily an imaging detector, it is well suited to measure the CDG flux mainly because of its large detection area, wide field-of-view (~1.5 sr), low background and long exposure times. The major difficulty in measuring the CDG radiation at MeV energies is the intense instrumental background. The instrumental background in COMPTEL is created mainly in the surrounding material. The striking feature of the pre-COMPTEL CDG spectrum was an apparent flattening between 1 and 10 MeV. A simple power law extrapolation from the X-ray regime showed the presence of an excess, referred to as the MeV bump, in the 1 to 10 MeV range.

These CDG flux measurements in the 1 to 10 MeV range are about 5 to 10 times lower than the pre-COMPTEL estimates. They show no evidence of a MeV bump in the 1 to 10 MeV range. The measured CDG emission between 0.8 and 30 MeV is well described by a power-law photon spectrum with an index of -2.4 ± 0.2 and a flux normalization of (1.05 ± 0.2) x 10-4 photons/cm^2-s-sr-MeV at 5 MeV. This work represents the first significant detection of the CDG emission in the 9 to 30 MeV range. No statistically significant deviations from isotropy is observed in the 4.2 to 30 MeV CDG emission when comparing the spectrum from the Virgo and the South Galactic Pole directions.

The CDG spectrum was measured using COMPTEL data by first measuring the count rate of gamma rays from high galactic latitudes, during those periods when the Earth was outside the COMPTEL field-of-view. Special data selections were applied to suppress the prompt and delayed background components. Above 4.2 MeV, in the absence of long-lived background, the count rates were extrapolated to zero cosmic-ray intensity to eliminate the prompt background and arrive at the CDG count rates. The delayed emission from long-lived radioactivity, present only below 4.2 MeV, was determined by fitting the energy spectrum. Below 4.2 MeV, their contributions were subtracted prior to the extrapolation to zero cosmic-ray flux to determine the CDG count spectrum. The CDG flux was determined by deconvolving the resultant count spectrum with the computed instrument response for an isotropic diffuse source having a power-law distribution in energy.

In the light of this work, possible contributions from all processes that explain the MeV bump, such as primordial matter-antimatter annihilations, are significantly lower than previously believed. Our measurements below 3 MeV within errors are consistent with the level predicted by the model where the CDG emission arise from the decay of elements produced in supernovae. A blazar origin for the CDG emission seems likely above ~100 MeV. However, the situation is less certain in the 1 to 30 MeV range because the blazar emission at these energies is not well known. A simple continuation of the blazar contribution from higher energies is allowed by the COMPTEL measurements.