Low-energy gamma radiation in the atmosphere at midlatitudes

Balloon-borne experiments with directional scintillators that measured the spectrum of gamma radiation with energies between 30 and 570 kev in the atmosphere up to 130,000 feet were conducted during 1967 and 1968. These data are of interest in connection with understanding the origin of atmospheric photons and with estimating planetary and stellar gamma albedos, as well as effecting improvements in gamma-ray astronomy. The measurements, conducted in the United States and in Australia, showed that a continuum and structure are present at depths X greater than 90 g cm−2; the intensity of the continuum depends exponentially on depth for X ≥ 102 g cm−2, but the shape is relatively depth-independent. The spectrum softens considerably at lesser depths. Structure is observed in the spectrum at 130,000 feet, superimposed on a continuum approximated by the power law 27.8E−1.87 photons cm−2 sec−1 ster−1 kev−1. The latitude, altitude, and energy variations of the spectrum are discussed in terms of the various factors responsible at various depths for the spectrum; it is shown that several factors are responsible for the continuum and that the observed structure is mainly due to ( n, γ) effects from cosmic-ray neutrons, and an upper limit is established for a line flux at 511 kev. The 95% confidence level upper limit of 0.2 photon cm−2 sec−1 for line radiation at the positron annihilation energy is slightly below the fluxes previously reported by others.