Characterization of subsurface water ice by active neutron counting with variable source-to-detector distances

Identification of subsurface water ice on planetary bodies is essential for understanding the volatile transport processes and enabling future in-situ resource utilization. Although passive neutron spectroscopy has contributed to global hydrogen mapping on planetary bodies such as the Moon and Mars, characterizing the depth profile of water ice and determining its local distributions remains challenging. In this study, we propose a novel active neutron counting system weighing only a few hundred grams, based on a measurement protocol in which the distance between a fixed scintillator and a movable neutron source is systematically changed. We perform Monte Carlo simulations under lunar surface conditions, modeling subsurface regolith layers with different water contents (0.1–10 wt.%) buried at depths of 10–60 cm. The neutron counts decrease with increasing source-to-detector distance, and the rate of decrease depends sensitively on both water content and burial depth, thereby enabling the estimation of both parameters. These findings demonstrate that, despite its compact size, low mass, and minimal power requirements, the proposed technique enables effective characterization of subsurface water ice and represents a promising approach for future planetary exploration missions.