Energy spectrum and flux of 3- to 20-Mev neutrons and 1- to 10-Mev gamma rays in the atmosphere

The flux and energy spectrum of fast neutrons (3 < E < 20 Mev) and γ rays (1 ≤ E ≤ 10 Mev) near the top of the atmosphere have been measured with an organic liquid scintillator. For a power law description of the omnidirectional neutron energy spectrum N(E) = AE−β(E) from 3 to 20 Mev at 3.5 g/cm² over Palestine, Texas (Pc = 4.4 Gv), the spectral index β depends strongly upon E. It changes from 1.8 ± 0.2 in the range 3 ≤ E ≤ 10 Mev to 0.3−0.4+0.1 for 15 ≤ E ≤ 20 Mev. From 20 to 50 Mev the neutron energy spectrum must be at least as flat as from 15–20 Mev to yield the observed proton recoil spectrum from 10 to 18 Mev, assuming that the neutron spectrum above 50 Mev varies as E−2. The neutron energy spectrum had essentially the same shape at Ft. Churchill, Canada (Pc = 0.3 Gv). The neutron leakage current at Pc = 4.4 Gv from 3 to 10 Mev at solar minimum was 0.089 neutron/cm² sec and from 3 to 20 Mev was 0.12 neutron/cm² sec. The measurements reported here agree with recent calculations by Armstrong and Chandler and by Merker but exceed Lingenfelter's predictions by factors ranging from 2 to ∼10. Assuming a power law description of the 1- to 10-Mev atmospheric γ ray energy spectrum, we find β = 2.2 ± 0.3 for 1 ≤ E ≤ 4 Mev and β = 1.6 ± 0.3 for 4 ≤ E ≤ 10 Mev for a partially unfolded count rate spectrum with a total flux of 0.66 photon/cm² sec at 3.5 g/cm² at λ = 42°N. Essentially the same spectral shape was found at Ft. Churchill, Canada.