Lunar fast neutron leakage flux calculation and its elemental abundance dependence

A numerical code has been developed to simulate fast neutron leakage flux f rom planetary surfaces. These neutrons are produced in copious number by in teractions between galactic cosmic rays and the nuclei of surface material. Their leakage flux spectrum depends on the composition of the upper layers of planetary bodies. Measurement of these neutrons allows the mapping of m ajor compositional units. This technique is successfully used on Lunar Pros pector data to obtain global geochemical maps of the lunar surface. With th e help of simulations, relations between soil composition and neutron fluxe s in the fast energy range [500 keV, 10 MeV] are established. The numerical code reproduces correctly the measurements of fast neutrons over the Moon. Variation in the neutron flux between highland and mare soils is calculate d. A simple formula is suggested to estimate quickly the integrated fast ne utron leakage flux for a given soil composition.