Time dependent worldwide distribution of atmospheric neutrons and of their products: 1. Fast neutron observations

Abstract

The time dependent worldwide distribution of atmospheric fast neutrons has been determined in balloon and aircraft measurements from 1964 to 1971. The 1- to 10-Mev neutron spectrum was measured with a phoswich detector employing seven channels of pulse height analysis. Solar modulation effects were greatest near the high-latitude transition maximum, where the flux varied by more than a factor of 2 from solar minimum to the deepest Forbush decrease. Near solar maximum and during Forbush decreases the relation between the neutron flux in the upper atmosphere and the counting rate of the Deep River neutron monitor deviated from a single-valued function. The differential neutron spectrum between 1 and 10 Mev can be represented, within the resolution of the detector, by a power law N(E) = AE−n, where n = 1.17−0.20+0.12 near the transition maximum, n = 1.08−0.20+0.13 at 3- to 5-g/cm² atmospheric depth, and n at sea level is larger than these values and dependent on terrain. The spectral index remains the same to ±0.1 over the solar cycle at fast neutron energies. The fast neutron data are self-consistent to ±7% from 2 to 300 g/cm² over the range of cutoff rigidity and solar cycle variations. The characteristics of the fast neutrons as outlined here serve as a basis for checking a Monte Carlo calculation of the entire neutron distribution and its products.