Fast neutrons in the Earth's atmosphere: 1. Variation with depth

A series of balloon flights at lambda = 41 deg N to atmospheric depths of about 3.6 g/cm/sup 2/ were conducted in 1963. A phoswich type of scintillation counter, which has a high efficiency for neutrons in the 1- to 14- Mev range, was employed. Gamma rays and charged particles are distinguished through pulse-shape discrimination and are separately telemetered. the experiments revealed that the mean free path for absorption in the lower atmosphere was 169 plus or minus 13 g/cm/sup 2/, and that the neutron flux at the 90g/cm/sup 2/ transition maximum was 1.1 neutrons/cm/sup 2/ sec. The flux at the highest altitudes attained was 0.39 n/cm/sup 2/ sec, and an extrapolation to the top of the atmosphere that follows Newkirk's theory yields 0.24 plus or minus 0.02 n/cm/sup 2/ sec for the neutron flux at the boundary. the absolute values are accurite to plus or minus 20 per cent. The flux as a function of depth is in good agreement with the calculations of Newkirk and of Lingenfelter. It is in disagreement with the original calculations of Hess, Canfield, and Lingenfelter. The extrapolated flux is lower than the fluxes measured by others in satellite experiments. The experimental discrepancy maymore » be caused by local production effects in the satellites. A featureless differential energy spectrum with an E/sup -1.3 plus or minus .1/ dependence was measured. (auth)« less