Detection of TNT and RDX landmines by standoff nuclear quadrupole resonance

ABSTRACT Nuclear Quadrupole Resonance (NQR) combines the compound specific detection capability offered by chemical detectiontechniques with the spatial localization capability and convenience of an induction coil metal detector. In this paper wepresent the first results of the detection of TNT by NQR with sufficient sensitivity for detection of many antipersonnel minesand essentially all antitank mines. In addition, we review the results of a blind in-field demonstration of the system indetecting RDX in which 28 out of 31 RDX-only targets were found with I false alarm in a 110 m test lane, and a second test in which 2 1 out of 2 1 RDX mines were found with zero false alarms at a clearance rate of 1 . 1 m2 per minute. Keywords: landmines, chemical detection, NQR, magnetic resonance 1. INTRODUCTION NQR exploits the electromagnetic interaction between atomic nuclei having a nonspherical electric charge distribution andthe internal electric fields due to the crystal structure in the condensed phase. For example, nitrogen nuclei experience a nettorque leading them to precess about the local crystalline electric field gradient present in many explosive compounds. Anexternally applied radio frequency magnetic field pulse at the precession frequency generates a coherent, oscillating nuclearmagnetic moment, which can be detected with a tuned antenna and very sensitive receiver. In essence, NQR is a variant ofnuclear magnetic resonance (NMR) in which the internal electric field gradient replaces the external dc magnetic field whichmust be applied in NMR.The NQR resonant frequency is specific to individual compounds, resulting in very low incidence of false alarms in thepresence of other materials. The volumes that can now be quickly inspected are appropriate for a number of practicalapplications ranging from inspecting airline luggage, mail and parcels, to screening individuals in airports and high securityareas. In recent years the principal focus has been on the detection of RDX and PETN, which are the active components ofplastic explosives, with a secondary effort in detection cocaine and heroin'.There have been various attempts to apply nuclear quadrupole resonance to landmine detection dating from the Vietnamconflict2. The programs reported to date have generally suffered from a lack of sensitivity, particularly for TNT, thepredominant explosive used in landmines, and an unacceptable false alarm rate mostly due to piezoelectric ringing artifactsfrom ground materials. In 1997 a new initiative that included NQR was begun by the U. S. Defense Advanced ResearchProjects Agency (DARPA) to develop chemical-based sensing for landmine detection. This program is now co-funded bythe U. S. Marine Corps and the U. S. Army Night Vision and Electronic Sensors Directorate.