Diffuse cosmic gamma rays observed at an equatorial balloon altitude

A 3×3 NaI(T1) crystal-photomultiplier assembly with a 4 charged particle anticoincidence shield is used to determine the gamma ray spectrum in the energy region of about 100 keV to 8.5 MeV at a balloon altitude of 4.7 g cm^–2 over Hyderabad, India (geomagnetic latitude 8°N). The atmospheric growth curves are used to obtain the contribution of the diffuse cosmic gamma ray flux in the above energy range. The cosmic photon flux thus deduced up to about 8.5 MeV is consistent with a continuation of the power law spectrum observed at energies between 20 and 200 keV with an index of –2.3, and hence contradicts the flattening trend observed in ERS-18 at an energy of a few MeV.Results presented at the IAU/COSPAR symposium No. 55 on X-and Gamma-Ray Astronomy Madrid, 11–13th May, 1972.