The energy spectrum of cosmic-ray induced neutrons measured on an airplane over a wide range of altitude and latitude

Abstract

Crews of high-altitude aircraft are exposed to radiation from galactic cosmic rays (GCRs). To help determine such exposures, the Atmospheric Ionizing Radiation Project, an international collaboration of 15 laboratories, made simultaneous radiation measurements with 14 instruments on a NASA ER-2 high-altitude airplane. The primary instrument was a sensitive extended-energy multisphere neutron spectrometer. Its detector responses were calculated for energies up to 100 GeV using the radiation transport code MCNPX 2.5.d with improved nuclear models and including the effects of the airplane structure. New calculations of GCR-induced particle spectra in the atmosphere were used to correct for spectrometer counts produced by protons, pions and light nuclear ions. Neutron spectra were unfolded from the corrected measured count rates using the deconvolution code MAXED 3.1. The results for the measured cosmic-ray neutron spectrum (thermal to >10 GeV), total neutron fluence rate, and neutron dose equivalent and effective dose rates, and their dependence on altitude and geomagnetic cut-off agree well with results from recent calculations of GCR-induced neutron spectra.

Radius of influence for a cosmic-ray soil moisture probe : theory and Monte Carlo simulations.

“…Our results, shown in Figure 2, indicate that above 10 4 eV the differential spectrum is not in equilibrium with slowing down and therefore varies significantly from the 1/E equilibrium predicted by slowing down theory. The importance of this process in governing the fast neutron spectrum has been verified by simulation and measurement of the secondary cosmic-ray neutron flux (Goldhagen et al 2004). The implication is that the proportion of fast neutrons to slow neutrons is significantly greater than would be predicted by an equilibrium slowing down spectrum.…”

“…To simulate the attenuation of neutron-producing cosmic ray flux as it penetrates the subsurface, the source intensity in our model decreases exponentially with depth according to an e-folding length of 160 g cm -2 . The initial neutron energy along this line is randomly sampled from an evaporation energy spectrum (Pelowitz 2005) with a central energy of 2x10 6 eV, which corresponds approximately to the evaporation peak measured with a multisphere neutron spectrometer (Goldhagen et al 2004). Because neutrons are first emitted from the ground within a lateral radius of only 0.5 m on average from where they originated on the line source, on the scale of hundreds of meters this can be considered approximately as a point source on the land surface.…”

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

“…Yajima et al 13) also measured the neutron energy spectrum using a Bonner ball (red line) on the same flight as our measurement, but their maximum energy was 20 MeV because their Bonner ball had low sensitivity to neutrons at energies greater than 20 MeV. Goldhagen et al 26) measured the neutron energy spectrum using an extended Bonner ball (Ext. B. ball) incorporating heavy metal layers to improve the sensitivity to neutrons at energies greater than 20 MeV at an atmospheric depth of 201 g/cm 2 in the USA (4.4 GV vertical cut-off rigidity).…”

Measurements of Cosmic-Ray Neutron Energy Spectra from Thermal to 15 MeV with Bonner Ball Neutron Detector in Aircraft

“…[2][3][4][5][6][7][8][9][10][11] However, only one neutron spectrum measurement on board was performed in the low cutoff rigidity region over Japan on Feb. 27,1985, by Nakamura et al 12) A multimoderator spectrometer, the so-called Bonner ball detector, is the standard neutron spectrometer used for radiological protection and environmental measurements, where the neutron energy range is below 15 MeV. In most aircraft measurements of the cosmic neutron energy spectrum, [2][3][4]6) a specially constructed Bonner ball spectrometer, to which a metal shell converter was added, was used to extend the energy range over 1 GeV. However, the accuracy of this instrument is insufficient for measuring the cosmic neutron spectrum and evaluating the exposure dose at aviation altitude in the energy region above 15 MeV with the Bonner ball, due to the absence of verification by other neutron spectrometers.…”