Abstract
The results of two experiments carried out with a large volume balloon-borne omnidirectional plastic scintillator sensitive to high-energy gamma-radiation are reported here. The two flights were carried out on 18 November 1990 and 24 February 1992, at places of 11.1 and 11.5 GV geomagnetic cut-off rigidities, respectively. For an omnidirectional detector and for energies greater than several MeV, most of the detected photons are secondary γ-rays produced through interactions of the charged cosmic rays with the atmosphere. To unfold the observed pulse height spectrum and obtain the incident γ-ray spectrum, the detector response function is calculated numerically with some simplifying assumptions. By using this response function, a differential flux dJ/dE = E-1.2±0.2 photons cm-2 s-1 MeV-1, averaged over all zenith angles, is estimated for the atmospheric γ-radiation, at an atmospheric depth of 5 g cm-2, in the 10-100 MeV energy range. This flux turns out to be compatible with other observations made with different types of detectors and at different geomagnetic cut-offs.
