Satellite observation of atmospheric nuclear gamma radiation

Abstract

We present a satellite observation of the spectrum of gamma radiation from the Earth's atmosphere in the energy interval from 300 keV to 8.5 MeV. The data were accumulated by the gamma ray spectrometer on the Solar Maximum Mission over 3½ years, from 1980 to 1983. The excellent statistical accuracy of the data allows 20 atmospheric line features to be identified. The features are superimposed on a continuum background which is modeled using a power law with index −1.16. Many of these features contain a blend of more than one nuclear line. All of these lines (with the exception of the 511‐keV annihilation line) are Doppler broadened. Line energies and intensities are consistent with production by secondary neutrons interacting with atmospheric 14N and 16O. Although we find no evidence for other production mechanisms, we cannot rule out significant contributions from direct excitation or spallation by primary cosmic ray protons. The relative intensities of the observed line features are in fair agreement with theoretical models; however, existing models are limited by the availability of neutron cross sections, especially at high energies. The intensity and spectrum of photons at energies below the 511‐keV line, in excess of a power law continuum, can be explained by Compton scattering of the annihilation line photons in traversing an average of ∼21g cm−2 of atmosphere.

Atmospheric gamma rays from solar energetic particles and cosmic rays penetrating the magnetosphere

“…We have compared spectra of gamma ray lines and continua produced in the Earth's atmosphere by cosmic ray irradiation and by proton bombardment from an intense solarenergetic particle event. Our study of the quiescent spectrum from cosmic ray impact, accumulated over a 9-year period, is an incremental improvement over a detailed study performed by Letaw et al [1989] on a more limited set of data. We list the energies and widths of 20 resolved gamma ray lines observed and identify nuclei that may be responsible for the emission.…”

“…Willett and Mahoney [1992] characterized the energies and widths of seven strong lines using the high-resolution capabilities of the germanium detectors and accumulations up to 253 days. In contrast, Letaw et al [1989] identified about 20 lines using atmospheric spectra integrated over a 3.5-year period. In this paper we describe observations of atmospheric gamma rays produced by cosmic ray interactions; these were made by the SMM spectrometer over its full 9-year lifetime.…”

“…These neutrons also excite nuclear states directly and through products of nuclear reactions. Letaw et al [1989] listed the most intense gamma ray lines from these processes based on earlier compilations by Ling [1975], Ramaty et al [1979], and nuclear data tables.…”

Upper limit on the steady emission of the 2.223 MeV neutron capture ?-ray line from the Sun

“…We fitted each 3-day 'Earth minus Sun view' spectrum between 1.85-2.65 MeV by a model spectrum consisting of a power law plus three lines at 2.135, 2.310, and 2.223 MeV. As Figure 1 shows, the power law and lines at 2.135 and 2.310 MeV are the dominant features of the Earth-albedo spectrum in this energy interval (Letaw et al, 1989). Any 2.223 MeV line emission expected from the Sun ought to appear as a * A small error (-~ 5~o) was introduced into this negative solar spectrum by the small exposure to the Sun which was included in the Earth-viewing angle bins, as noted above, partially cancelling the Sun-viewing spectrum.…”

Simulation of HEAO 3 Background

“…SIMULATION The cross-sections for spallation from neutrons and protons incident on the detectors, shields and support structures were taken from Alice91 [6]. These cross-sections and the neutron and proton flux were evaluated over the [7]. This spectrum was folded through both the open collimator response and the response to shield leakage.…”