Cosmic-ray neutron demography

Abstract

The equilibrium spatial and energy distribution is calculated for neutrons made in the earth's atmosphere by cosmic rays. The neutron current leaking into space is found, and the density of neutron decays in the vicinity of the earth is computed for a future determination of importance as a source for the Van Allen belts. The spectrum and the leakage current below 10 Mev are determined from multigroup diffusion theory; the leakage above 10 Mev (<1 per cent of total) is approximated from geometrical arguments. An integrated source over each square centimeter of earth surface (at geomagnetic latitude 44°N) of 6.2 neutrons/sec (1.2 from ‘knock-on’ processes above 10 Mev and 5.0 from evaporation processes below 10 Mev) gives an equilibrium spectrum in agreement with the ±25 per cent accuracy of measured values. This source corresponds to a global average of 4.6 neutrons/cm2/sec, of which 0.8 leaks into space, 2.9 form C14, and 0.9 is absorbed in other processes. Gravitationally trapped neutrons (<0.66 ev) amount to less than 1 per cent of the total leakage, but they contribute substantially to the decay density near the earth (e.g., 40 per cent of total decays at 1/2 earth radius).