Energetic neutrons and gamma rays measured on the Aryabhata satellite

An experiment to measure energetic neutrons and gamma rays in space was launched in the first Indian scientific satellite,Aryobhata, on April 19, 1975. From this experiment, the first measurements in space of the Earth’s albedo fiux of neutrons of energy between 20 and 500 MeV have been made; the values obtained for two mean geomagnetic vertical cut-off rigidities of 5.6 and 17.0 GV are (6.3±0.4)×10⁻² and (1.4±0.3)×10⁻² neutrons cm⁻² sec⁻¹ respectively. These measurements confirm that protons arising from cosmic ray albedo neutron decay, can adequately account for the protons in the inner radiation belt. Observations on gamma rays of energy between 0.2 and 24 MeV have enabled the determination of the total background gamma ray flux in space as a function of latitude. This in turn has permitted useful information on the diffuse cosmic gamma rays. We have also observed four events that showed sudden increases in the gamma ray counting rates between 0.2 and 4.0 MeV. Observational details of these events are given.