EXPERIMENTAL AND NUMERICAL-SIMULATION OF MARTIAN NEUTRON DISTRIBUTIONS

Measurements of the gamma-ray and neutron distributions produced by cosmic-ray bombardment of planetary surfaces can provide important information about water content and near-surface elemental compositions. Accurate interpretation of such measurements depends heavily on comparisons with radiation-transport calculations, which involve not only the soil composition but also assumptions about (1) particle types and energy distributions of cosmic rays, (2) interaction mechanisms for high-energy protons and neutrons, (3) neutron transport at low energies, and (4) production of prompt gamma rays and radionuclides in proton- and neutron-induced reactions. The present experiment simulates the planetary situation by directing a beam of 800 MeV protons onto a large container of Martian-like sand. Measurements were made of (1) the spatial distributions for protons and neutrons inside the container via activation foils and (2) the energy spectrum for neutron leakage out of the container via time of flight. Several configurations of the target were used to represent different Martian surfaces. The measured neutron-energy spectra are in good agreement with calculations made using a computer code that couples medium-energy neutron production to low-energy neutron transport, and the measured spatial distributions match those predicted using calculated fluxes and known cross sections for radionuclide production. The analyses include discussions of soil water content, finite-size effects, and the implications for data from possible planetary missions.