Gamma rays from the solar-cosmic-ray-produced nuclear reactions in the Earth's atmosphere and lower limit on the energy of solar protons observed at Minneapolis

Gamma rays from the solar-cosmic-ray-produced nuclear reactions in the earth's atmosphere have been observed by balloon-borne NaI(T1) scintillation detectors. An attempt has been made to understand the processes that lead to γ-ray production, and from that the total cross section for the γ-ray production in air by protons of 30 to 300 Mev is calculated. It is found to be σγ ≈ 940 × 10−27 cm2. Assuming that the information about the solar proton spectrum obtained from the directly measured protons of energy 110 Mev and more could be applied to the protons of energy less than 110 Mev, we have also tried to estimate Emin, the minimum energy with which the protons were arriving over Minneapolis during the storm. Such estimations during the May 12, 1959, event show that solar protons of energy as low as 30 to 40 Mev were allowed at the middle-latitude station of Minneapolis.