Atmospheric gamma ray angle and energy distributions from sea level to 3.5 g/cm² and 2 to 25 MeV

Abstract

Angle and energy distributions are reported here for gamma rays from ground level to 3.5 g/cm² residual atmosphere at energies of 2 to 25 MeV and zenith angles of 0° to 50° and 180° to 130°. They complement the previous results of Ryan et al. (1977) for altitudes above 100 g/cm². The observations were made with the University of California, Riverside (UCR), double Compton scatter telescope from a balloon launched from Palestine, Texas, geomagnetic cut‐off of 4.5 GV, on May 13, 1975. The ground level measurements were made at Riverside, California, at 1000‐g/cm² residual atmosphere, geomagnetic cut‐off of 5.4 GV, on July 21, 1977. Growth curves for downward‐moving gamma rays have pronounced maxima at 160±30 g/cm² residual atmosphere, while those for upward‐moving gamma rays are essentially constant at altitudes above the Pfotzer maximum. Below about 300 g/cm² both the downward and the upward fluxes decrease with average e‐folding depths of 192±15 and 179±12 g/cm², respectively. The ratio of the downward‐ to upward‐moving gamma rays near the vertical is 17±5, independent of depth below the Pfotzer maximum. The energy variation, E−1.1, of the downward‐moving gamma rays is flatter than the E−1.8 of the upward moving ones at all depths below 100 g/cm². All zenith angle distributions show peaks toward the horizon. Above the Pfotzer maximum the downward flux decreases more rapidly away from the horizon than the upward flux, while below the converse is true. The azimuth angle distributions at 4.0 g/cm² show northward anisotropies that increase with energy from 3.9±0.9% at 2–10 MeV to 7.0±2.3% at 10–25 MeV.