Cosmic-ray-produced neutrons at ground level: Neutron production rate and flux distribution

Abstract

The absolute rates of cosmic-ray neutron production and neutron flux distribution on the ground were determined at sea level and mountain altitude at a geomagnetic latitude λ = 44°N in 1964. The thermal neutron flux was measured with a well-calibrated BF3 counter; a Maxwellian energy distribution with a shifted neutron temperature was assumed. By using two differently moderated BF3 counters, the fast-neutron flux was determined in the energy range 0.4 ev to 10 Mev. The neutron fluxes were also estimated from the measured production rate, and they are in good agreement with the measured fluxes; possible occurrence of air-ground boundary effects on the neutron flux distribution was considered. Anisotropy of the thermal neutron flux on the ground was experimentally demonstrated, and the angular distribution was well filled by the first two terms of a spherical-harmonics expansion. The air-ground boundary effects are discussed on the basis of experimental results.