Abstraet. The development of a system for the detection of ammonium nitrate (AN) in vehicles by nuclear quadrupole resonance (NQR) is described. The results from studies of the penetration of radiofrequency (RF) magnetic fields inside certain metal enclosures, including full-scale vehicles, were critical in the design of a novel high-Q resonant probe. The probe was shaped not only for optimal penetration of RF magnetic fields into vehicles,-but also for optimal rejection of RF interference and ease of shietding. A full-scale technical demonstrator was designed, built and successfully demonstrated, using novel pulse sequences to generate and detect NQR signals from AN concealed within the boot (trunk) of acar 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 RF magnetic fields that are both very weak and ver'/ inhomogeneous.

A comparison of the NQR parameters of the monoclinic and orthorhombic phases of TNT and their relation to the twist or dihedral angle between the plane of the NO 2 substituents and that of the benzene ring as determined in the X-Ray crystal structure analysis enables an assignment of different frequencies to specific sites in the two independent molecules in the unit cell of both forms to be made. The slow transformation of the metastable orthorhombic phase to monoclinic can then be followed by monitoring the NQR spectrum in which specific lines can be assigned to molecular sites in the two phases. NQR spectra of TNT referred to in the literature often differ; this could be due partly to the TNT often being a mixture of monoclinic and orthorhombic phases and partly to changes in the spectral line width, factors which must be taken into account when NQR is used to detect landmines.

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