Dm. Drake et al., EXPERIMENTAL AND NUMERICAL-SIMULATION OF MARTIAN NEUTRON DISTRIBUTIONS, Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms, 84(3), 1994, pp. 337-356
Measurements of the gamma-ray and neutron distributions produced by co
smic-ray bombardment of planetary surfaces can provide important infor
mation about water content and near-surface elemental compositions. Ac
curate interpretation of such measurements depends heavily on comparis
ons with radiation-transport calculations, which involve not only the
soil composition but also assumptions about (1) particle types and ene
rgy distributions of cosmic rays, (2) interaction mechanisms for high-
energy protons and neutrons, (3) neutron transport at low energies, an
d (4) production of prompt gamma rays and radionuclides in proton- and
neutron-induced reactions. The present experiment simulates the plane
tary situation by directing a beam of 800 MeV protons onto a large con
tainer of Martian-like sand. Measurements were made of (1) the spatial
distributions for protons and neutrons inside the container via activ
ation foils and (2) the energy spectrum for neutron leakage out of the
container via time of flight. Several configurations of the target we
re used to represent different Martian surfaces. The measured neutron-
energy spectra are in good agreement with calculations made using a co
mputer code that couples medium-energy neutron production to low-energ
y neutron transport, and the measured spatial distributions match thos
e predicted using calculated fluxes and known cross sections for radio
nuclide production. The analyses include discussions of soil water con
tent, finite-size effects, and the implications for data from possible
planetary missions.