Planetary neutron leakage fluxes provide a measure of surface composition. However to be used in geological studies, a quantitative relationship between measured fluxes and surface composition is needed. The present work shows that neutron production is expected to be a function of the atomic mass, and that the fast leakage flux in the energy range between 0.6 and 8 MeV is linearly related to the average soil atomic mass. This result is consistent with laboratory measurements, and with Lunar Prospector observations of the Moon. When calibrated with returned lunar samples, this relationship is used to construct a map of the average atomic mass of lunar soils.