Fast neutron latitude variations in the atmosphere at solar minimum

The fast-neutron flux in the atmosphere has been measured during solar minimum. Data from ground level to about 4 g/cm2 were obtained in a series of seven high-altitude balloon flights. The flights were conducted between September 1964 and August 1965 at four locations between λ = 8°N and λ = 69°N, conventional geomagnetic latitude. The detector, a phoswich-type scintillator, was sensitive to neutrons in the range 1–10 Mev. A seven-channel pulse-height analyzer permitted the evaluation of the energy spectrum in this range. The neutron flux at the transition maximum increased from 0.17±0.02 cm−2 sec−1 at λ = 8°N to 1.9±0.1 cm−2 sec−1 at λ = 69°N. Neutron leakage from the top of the atmosphere was estimated by extrapolation. The magnitudes of the leakage fluxes, as well as the pole-to-equator ratio of 16, were in substantial agreement with the results of the diffusion calculation of Lingenfelter. The best fit to an inverse power law of the differential neutron energy spectrum was found to be independent of latitude within the limits of our experimental precision. Averaged over all flights, the best fit was for a special index of n = 1.05 ± .15. This spectrum is harder than that calculated by Lingenfelter. Differences in the shapes of the neutron profiles of this experiment and of the calculation are consistent with the difference in spectrum.