High‐energy neutrons from the Moon

Abstract

We present the first report dedicated to 0.6–8 MeV neutrons measured by the Lunar Prospector gamma‐ray spectrometer. The period of data collection extends from January 1998 to July 1999; the spacecraft altitude varied between 15 km and 140 km. Maps are at a 2° × 2° equal‐area resolution. They show high counts and high variability for the mare and, low counts and low variability for the highlands. Such variability is due to differences of Fe and Ti concentrations in the lunar regolith, plus other elements with lesser effect that have not yet been included in our calculations. Assuming that only FeO and TiO 2 concentrations are responsible for the fast neutron structures, fluxes are 2.7 times more sensitive to FeO than to TiO 2 . The statistical resolutions are 1.3% in FeO and 3.5% in TiO 2 . No discrimination of FeO is possible below 6.5%. Results are supported by numerical simulations. We also present the first lunar neutron spectrum over 16 channels between 0.6 and 8 MeV. Ninety‐five percent of the counts are below ∼4.5 MeV. In flux units the spectrum peaks at ∼1.5 MeV. Its shape agrees with numerical simulations.