Search for 100 TeV gamma-ray emission from the Galactic disk

Upper limits have been obtained to the diffuse gamma-ray emission from the Galactic disk. The limits I(gamma)/I(p) less than 0.2 percent, i.e., I(gamma)(greater than 130 TeV) less than 3.2 x 10 exp -13/q cm/s/rad provide the first measurement obtained in the angular window corresponding to the distribution of the ISM. It is still, however, a factor of about 30 higher than the flux calculated from the cosmic ray interactions by Berezinsky and Kudryavtsev (1990). A lower limit of gamma greater than 2.3 is obtained to the power-law index of the differential energy spectrum of Galactic gamma rays from 100 MeV to 100 TeV, setting a limit to the possibility of a hard extrapolation of the Galactic gamma-ray spectrum to the 100 TeV region.