Theoretical zenith angle distribution of atmospheric MeV gamma radiation at high balloon altitudes

A simple model is developed in order to interpret the recently measured zenith angle distribution of MeV γ rays in the upper atmosphere, which shows a maximum of intensity at angles around 120° (upward moving direction). Because γ ray production by electron bremsstrahlung is by far the most important process in this energy range, an attempt has been made to derive the γ ray source functions within the atmosphere from measured electron fluxes. The transport of γ rays from their source position to the detector altitude is simulated in a Monte Carlo calculation. The calculation shows that the observed maximum around zenith angles of 120° is caused by those γ rays which were originally ejected preferentially in the horizontal direction but which were Compton-scattered on their way to the detector. Because the mean Compton scattering angle and the scattering probability depend on energy, the maximum in the zenith angle distribution varies with energy: with increasing energy it is more and more shifted toward 90°.