Measurement of Atmospheric Neutron and Photon Energy Spectra at Aviation Altitudes using a Phoswich-Type Neutron Detector

Abstract

Neutron energy spectrum from 7 to 180 MeV and photon energy spectrum from 4 to 50 MeV were measured onboard an aircraft using a newly developed phoswich-type neutron detector at 10.8 km altitude (atmospheric depth of 249 g/cm2) and geographical latitude of 39°N (a vertical cut-off rigidity of 10.2 GV) near Japan on February 13, 2008 (at a heliocentric potential of 312 MV). Our results were compared with other measurements obtained using 3He-loaded or extended-energy multi moderator neutron spectrometers (Bonner balls) at aviation altitudes, an organic liquid scintillator on the ground, and a double-scatter neutron telescope at the top of the atmosphere and with calculations using the LUIN2000, EXPACS and RMC codes. Our measured results give a large, sharp peak around a neutron energy of 70 MeV, although Bonner balls present a broad peak around 100 MeV due to low energy resolution. Our neutron fluxes agree well with the others. The measured photon energy spectrum is between the LUIN2000- and EXPACS-calculated spectra and agrees with measured vertical photon spectra at the top of the atmosphere. This onboard study provides the first experimental neutron energy spectrum in the high-energy region (over 10 MeV) with a high energy resolution.