Buried bulk explosive threats are a major concern to the armed forces and public security agencies of many countries. Nuclear methods to detect these threats have been studied extensively since the 1950s.1 Research in this area has concentrated on neutrons and photons as probing and emitted particles because of their deep penetration. Some reactions involving these particles and the buried materials can be used to identify explosive substances, making them good candidates for confirmation sensors: slower devices that confirm the presence of an explosive following initial detection by less-specific, fast-scanning sensors. Nuclear reactions that generate characteristic radiation capable of identifying an isotope generally have relatively low cross sections (low probability of the reaction happening). To provide results within practical interrogation times, techniques using such reactions require intense sources and consequently large amounts of radiation shielding for personnel and detectors. Though personnel shielding can be significantly reduced by remote operation, this generally relegates active interrogation methods with characteristic radiation emission to either fixedposition roles, such as portals, or vehicle-mounted applications. Our research, with Bubble Technology Industries (BTI, Chalk River, Canada), has led to the development of a thermal neutron analysis (TNA) sensor as part of a tele-operated vehiclemounted multi-sensor large-landmine detector.2 The technique confirms the presence of nitrogenous explosives by measuring characteristic capture gamma rays following absorption of thermal neutrons. Four multi-sensor systems with TNA were put into service with the Canadian Forces in Afghanistan in 2002, making them the first militarily fielded TNA sensors and confirmation sensors for landmines.3 We are developing an advanced next-generation TNA using an electronic neutron generator in place of the present isotopic source, faster higher resolution scintillators, improved Figure 1. Next-generation thermal-neutron-analysis sensor head in soil pit.
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