Neutron‐induced gamma ray spectroscopy: Simulations for chemical mapping of planetary surfaces

Abstract

Cosmic rays interact with the surface of a planetary body and produce a cascade of secondary particles, such as neutrons. Neutron‐induced scattering and capture reactions play an important role in the production of discrete gamma ray lines that can be measured by a gamma ray spectrometer on board an orbiting spacecraft. These data can be used to determine the concentration of many elements in the surface of a planetary body, which enables us to recognize individual geological units and provides clues to the bulk composition and in turn the origin and evolution of the body. To investigate the gamma ray fluxes induced by accelerator neutrons, experiments were carried out by irradiating thin targets with neutrons of energies from 14 MeV to 0.025 eV. The neutron fluxes at target position were measured by foil activation techniques. The ratio of the epithermal to thermal neutron flux was determined to be 2.0, a value that is similar to that in the moon. Gamma rays in the energy range of 0.1 to 8 MeV emitted by the targets and the surrounding material were measured by a high‐resolution germanium detector. Most of the gamma ray lines that are expected to be used for planetary gamma ray Spectroscopy were found in the recorded spectra. These spectra were unfolded, background was subtracted, and gamma ray attenuation corrections were made to obtain the corresponding gamma ray fluxes from the targets. The majority of gamma ray lines were narrow without noticeable Doppler broadening except for the very broad 4.4‐MeV line of carbon and five asymmetric germanium lines produced by the detector itself. The agreement of measured gamma ray flux ratios with calculated flux ratios for neutron‐capture reactions showed that thermal neutron data can be used for theoretical calculations of low‐energy neutron‐induced gamma ray fluxes. This study was a first step toward a more realistic simulation of cosmic‐ray‐induced gamma‐ray production and it indicates the importance of accelerator irradiation experiments to future planetary missions.