Detection of ammonium nitrate inside vehicles by Nuclear Quadrupole Resonance
Nuclear quadrupole resonance (NQR) has been demonstrated for the detection of explosives in a number of potential applications, including baggage screening, mail screening and landmine detection. A further highly desirable application concerns the detection of explosives concealed within a vehicle, which from the point of view of RF penetration can be considered as a large metal box with holes in it. A perceived problem of the technology is its reduced effectiveness in detecting materials that are enclosed by metal due to the poor penetration of radio-frequency (RF) magnetic fields. Thus it has generally been thought that such vehicle bodies would make NQR detection extremely difficult.
However, initial research carried out at King 's College London showed that there is appreciable penetration ofRF magnetic fields inside certain metal boxes. This finding prompted a research programme to determine whether sufficient magnetic field penetration could be obtained within a vehicle to allow NQR to be used to detect materials concealed inside . This was shown to be feas ible at the resonance frequencies ofammonium nitrate (AN), which was chosen because it has been used as the major constituent in many vehicle bombs throughout the world. A full scale technical demonstrator was designed, built and successfully demonstrated, using novel pulse sequences to generate and detect NQR sign als from AN concealed within the trunk of a car and in the loading bay of a (metal-sided) van.
Among the key technical advances that made possible the effective operation of this system was the development of pulse sequences that generate detectable NQR responses for magnetic fields that are both very weak and very inhomogeneous. Further advances resulted from studies into how magnetic field s penetrate vehicle bodies. This information was critical in Ihe design of the novel high-Q resonant probe, which was shaped not only for optimal penetration of fields into vehicles but also for optimal rejection of RF interference and ease of shielding. This programme has demonstrated that NQR is a suitable technology for development into a large vehicle screening equipment, in which a number of materials could potentially be detected, including other explosives and drugs . In addition the radiological safety issues are less onerous than those that are associated with ionising radiation techniques.