Energy and angle distributions for atmospheric and cosmic diffuse gamma rays from 2 to 25 MeV

Reported here are angle and energy distributions for gamma rays near the top and deep within the atmosphere for energies of 2 to 25 MeV and zenith angles of 0 to 500 (downward moving) and 130 to 1800 (upward moving) and the cosmic diffuse flux of gamma rays for energies from 2 to 10 MeV. Upper limits are placed on the cosmic diffuse flux from 10 to 25 MeV. These measurements were obtained with the University of California, Riverside (UCR), double Compton scatter gamma ray telescope flown on a balloon to 3.0 g/cm2 residual atmosphere from Palestine, Texas, on May 13, 1975 (geomagnetic cutoff: 4.5 GV). Growth curves from 3.4 to 100 g/cm2 of residual atmosphere are used to determine the downward moving atmospheric fluxes above the Pfotzer maximum. Results are given for six energy intervals from 2 to 25 MeV and for five angle intervals from 0 to 500. The downward fluxes are in reasonable agreement with those of Schoenfelder and Lichti (1975, 1976) at the lower energies and with those of Kinzer et al., (1974) at the higher energies, where the respective data overlap. The shape of the angular distribution is consistent with that of Lichti et al., (1975) and the model of Graser and Schoenfelder (1977) for the medium energies and Thompson (1974) at higher energies. The absolute fluxes of upward moving gamma rays from 130 to 1700 at 4.2 g/cm2 are in agreement with the semi-empirical values of Ling (1975) but increase away from the vertical contrary to his predictions. Observations were also made at 160, 330, 520 and 820 g/cm2. Azimuthal distributions are obtained for float altitude in the energy ranges 2 to 10 and 10 to 25 MeV and show positive north-south anisotropies of 3.9 +- 0.9% and 7.0 +- 2.3% respectively