The Feasibility of Neutron Moderation Imaging for Land Mine Detection

Abstract

Neutron moderation land mine detection involves irradiating the ground with fast neutrons and subsequently detecting the thermalized neutrons which return. This technique has been studied since the 1950s, but only using non-imaging detectors. Without imaging, natural variations in moisture content, surface irregularities, and sensor height variations produce sufficient false alarms to render the method impractical in all but the driest conditions. This paper describes research to design and build a prototype land mine detector based on neutron moderation imaging. After reviewing various neutron detector technologies, a design concept was developed. It consists of a novel thermal neutron imaging system, a unique neutron source to uniformly irradiate the underlying ground, and hardware and software for image generation and enhancement. A proof-of-principle imager has been built, but with a point source offset from the detector to roughly approximate a very weak uniform source at the detector plane. Imagery from the detector of mine surrogates is presented. Realistic Monte Carlo simulations were performed using the same two dimensional neutron imaging geometry as the detector in order to assess its performance. The target-to-background contrast was calculated for various soil types and moisture contents, explosive types and sizes, burial depths, detector standoffs, and ground height variations. The simulations showed that the neutron moderation imager is feasible as a land mine detector in a slow scanning or confirmation role and that image quality should be sufficient to significantly improve detector performance and reduce false alarm rates compared to non-imaging albedo detection, particularly in moist soils, where surface irregularities exist and when the sensor height is uncertain. Performance capability, including spatial resolution and detection times, was estimated.