Energy spectrum and flux of fast neutrons in the atmosphere

The neutron energy spectrum and flux in the atmosphere at the Pfotzer maximum (about 100 g per cm squared) were measured at geomagnetic latitude 42 deg N, using a system capable of detecting a small flux of fast (5 to 20 MeV) neutrons in a much larger background flux of gamma rays and charged particles. The detector consisted of a cylindrical cell of organic liquid scintillator coupled to a high-resolution two-parameter multiparticle pulse shape discriminator with a two-parameter logarithmic pulse-height analyzer. The spectral parameter for the differential neutron energy spectrum was found to decrease from 4.4 plus or minus 0.8 between 3.5 and 6.0 MeV to 1.5 plus or minus 1.0 in the interval 11 to 20 MeV; a value of 0.36 plus or minus 0.10 neutrons per cm squared was calculated for neutron flux between 3.5 and 10 MeV. The importance of these results for evaluating the role of albedo neutron leakage as a source of the energetic proton flux in the radiation belts is also considered.