Low-energy gamma rays produced in air and in lead by cosmic rays

Measurements have been made at 40.5°N geomagnetic latitude of γ rays produced in lead and in air within the energy range 25–1060 kev up to altitudes of 5.4 g/cm2 by means of balloon-borne scintillation counters. The γ rays of atmospheric origin exhibit a transition curve that peaks at 70 g/cm2 and is similar to transition curves measured with other omnidirectional detectors. The measurements give a flux of 17.3 photons/cm2/sec at the peak of the transition curve; extrapolation to the top of the atmosphere gives an albedo of 5.7 photons/cm2/sec. The spectra of the photons produced in lead and in air are very similar, and the production per gram of lead is found to be about 2.7 times that per gram of air. The power contained in γ rays up to 1 Mev is estimated to be 7 per cent of the incoming power from primary cosmic rays. Because of this large intensity the origin of the photons is difficult to explain in terms of the nucleonic component but is compatible with an explanation based on an electromagnetic origin.