Energy spectrum and flux of fast neutrons in the atomsphere

Abstract

The energy spectrum of fast neutrons (5-20 MeV) in the atmosphere at the Pfotzer maximum (\ensuremath{\sim} 100 ${\mathrm{g}/\mathrm{c}\mathrm{m}}^{2}$) was measured at geomagnetic latitude 42\ifmmode^\circ\else\textdegree\fi{}N (${P}_{c}=4.4$ GeV). The $n$-$\ensuremath{\gamma}$ detector was a 4.6 \ifmmode\times\else\texttimes\fi{} 4.65-cm cylindrical cell of organic liquid scintillator (NE213) coupled to a high-resolution, two-parameter, multiparticle pulse-shape discriminator with a two-parameter (64 \ifmmode\times\else\texttimes\fi{} 64) logarithmic pulse-height analyzer. An anticoincidence charged-particle shield completely enclosed the $n$-$\ensuremath{\gamma}$ detector. The spectral parameter $\ensuremath{\beta}(E)$ of the differential neutron energy spectrum decreased from 4.4 \ifmmode\pm\else\textpm\fi{} 0.8 in the range 3.5-6.0 MeV to 1.5 \ifmmode\pm\else\textpm\fi{} 1.0 in the range 11-20 MeV. The integrated neutron flux at Pfotzer maximum between 3.5 and 10.0 MeV was 0.36 \ifmmode\pm\else\textpm\fi{} 0.10 ${\mathrm{n}\mathrm{e}\mathrm{u}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^{2}$ sec. The implications of these results for the high-energy albedo neutron leakage as a source of the energetic proton flux in the radiation belts are discussed.