A New Component of Cosmic Gamma Rays Near 1 MeV Observed by the ERS-18

A Nal omnidirectional scintillation counter 7.65 cm long and of 6.35 cm diameter with a 1-cm plastic scintillation anticoincidence shield aboard the Environmental Research Satellite 18 has measured counting rates due to rays in space. These rates, uncorrected for efficiency, are 0.94 counts (cm2 sec MeV)-1 in the energy4oss range 0.250.6 MeV; 0.29 from 0.6 to 1.0 MeV; 0.14 from 1.0 to 2.0 MeV; 0.053 from 2.0 to 3.7 MeV; and 0.036 over the range 3.7-6.0 MeV. The actual photon flux in these energy ranges will be a factor of 1.53 higher, depending on the energy and the assumed photon spectrum at higher energies. Evidence based on known mechanisms of ray production in the 8 g cm of satellite material, comparison of the spectra with the shield anticoincidence on and off, and the lack of correlation with solar modulation of cosmic rays all indicate that the measured -rays were indeed cosmic. The spectrum above 1 MeV lies considerably above the extension of the diffuse cosmic background X-ray spectrum as determined by Ranger III and other experiments. This suggests an additional galactic or cosmic -ray component above that usually accounted for by intergalactic electrons scattering on the 3 K radiation.