Cosmic-ray production of low-energy gamma rays

An attempt has been made to detect and measure any vertically incident flux of low-energy (0.25 to 10 Mev) gamma rays that might be present at high altitudes. The experiment consisted of a series of balloon flights carrying a phoswitch gamma spectrometer with pulse height ecording to altitudes of about 5.5 g/cm2 atmosphere depth. Directional sensitivity was obtained by placing a lead collimating shield around the detector and by periodically opening and closing a lead shutter above the opening of the shield. At altitudes between 5.4 and 6.0 g/cm2 atmospheric depth the experiment yielded a value of 0.000±0.034 photons sec−1 cm−2 sterad−1 for the vertical flux of gamma rays in the vicinity of 0.5 Mev energy. This is a null result; however, it places a new upper limit on the vertical gamma-ray flux that is lower than any previously reported for this energy region. At lower altitudes it was observed that gamma rays are generated by cosmic rays in the atmosphere and in the collimating lead shield. At an atmospheric depth of 300 g/cm2 the flux of gamma rays from the atmosphere was about 0.3 photons sec−1 cm−2 sterad−1. Convincing evidence was found that the gamma rays of atmospheric origin were generated by the secondary nucleonic component, and that they are not genetically related to the electromagnetic or ‘soft’ component of the secondary cosmic rays.