Measurements of atmospheric γ rays between 10 and 200 MeV

The intensity and energy spectrum of secondary γ radiation near the top of the atmosphere have been measured at cutoff rigidities of 4.5 and 11.5 GV. These measurements were made with a balloon-borne telescope sensitive to photons between 10 and 200 MeV and consisting of a stack of nuclear emulsions used in conjunction with a wide-gap spark chamber. The rates of growth of γ rays in this energy range, as a function of atmospheric depth, are (2.54±0.56) × 10−3 and (1.17±0.32) × 10−3 γ/cm² sr g cm−2 at 4.5- and 11.5-GV cutoffs, respectively. The differential spectrum has been determined with an energy resolution of ≈(+30,-16)% and is consistent with previous measurements at 4.5 GV made at higher and lower energies. However, this is the first measurement that clearly exhibits the two-component nature of the atmospheric γ ray spectrum. Differences existing between the measured spectral shapes and calculations are discussed.