Nuclear Quadrupole Resonance, NQR
Nuclear Quadrupole Resonance (NQR) is a radio frequency (RF) technique offering an unequivocal method of detecting the presence of the nitrogen isotope N-14, a common component in many forms of high explosives, narcotics, and drugs. Recently, the technique has received increasing attention as an important method for detecting land mines; this as the NQR signal offers a unique signature, differentiating it from most other mine detection techniques that suffer from trying to detect non-unique features. Further, NQR is also of particular interest due to the possibility of using the technique to detect explosives at airports and other public places, as well as for detecting narcotics. Current methods for detecting land mines have serious disadvantages. Metal detectors, for example, have difficulties in magnetic soils and with mines of low metal content, and ground penetrating radar has problems in clay or wet and conducting soils, and with mines very close to the ground surface. Although mine detection using NQR faces problems with interference and with very low signal-to-noise ratio (SNR), recent reports indicate that the unique NQR signature offers, in principle, exceptionally high probability of detection for a given false alarm rate.
The observed NQR frequencies depend on the interaction between the electric quadrupole moment of the nucleus and the electric field gradient generated at the nuclear site by external charges. All common high explosives, such as TNT, RDX, and PETN, contain N-14, a quadrupolar nucleus generating three sets of resonance frequencies, providing a reliable technique for detecting and identifying an explosive, as well as estimating its quantity and depth. Because of its high specificity there is little or no interference from other nitrogen-containing material that may be present, such as the mine casing or fertilizer in the soil. As with a metal detector, a specifically designed planar RF antenna is placed close to ground level and fed with a sequence of RF pulses at or close to the NQR frequency of the explosive to be detected. The same (or second) antenna is then used to detect the weak signals emitted by the explosive following the excitation. These signals are of two types, free induction decays immediately following the pulse, and echoes observed midway between a string of pulses, the latter having the advantage that a large number of signals can be averaged in a short time to improve sensitivity. The important difference from metal detection is that it is the explosive that is detected and not any feature of the land mine. As a result, the false alarm rate will likely be low. However, the NQR signals can be very weak, particularly from the common explosive TNT, and are prone to strong RF interference.