Nuclear cross sections for gamma ray deexcitation line production by secondary neutrons in the Earth's atmosphere

Abstract

[1] Interactions of galactic cosmic rays and solar energetic particles with nuclei in the Earth's atmosphere produce secondary neutrons. Secondary neutrons with energies greater than a few MeV interact with atmospheric nuclei and produce gamma ray lines. The relative intensities of these lines depend on the shape of the neutron energy spectrum, and so their measurement with satellite detectors provides neutron spectral information complementary to direct observation. Reliable interpretation of such line measurements requires accurate knowledge of the production cross sections. Because the atmospheric neutron spectrum at these energies is very hard (falling by less than a factor of 10 from 10 to 100 MeV), higher-energy neutrons contribute significantly to line production. Knowledge of the cross sections at these higher energies is critical for accurate line yield predictions. In this paper we evaluate and present the most important cross sections for deexcitation line production by neutron interactions with the Earth's atmosphere, with emphasis on extending them to higher neutron energies, using both laboratory measurements and the nuclear reaction code TALYS to obtain the cross sections from a few to >200 MeV. Using these cross sections, we also demonstrate the sensitivity of gamma ray line ratios for providing information about the ∼1–200 MeV atmospheric neutron spectrum.