Explosives Detection with Nuclear Quadrupole Resonance

An emerging technology will help to uncover land mines and terrorist bombs

Joel Miller, Geoffrey Barrall

Beyond Explosives

Although the ubiquity of ¹⁴N in explosives makes NQR well suited for detecting them, revealing hidden bombs is by no means the only application of this technique. Narcotics, too, frequently contain ¹⁴N, which opens the possibility of detecting smuggled drugs of abuse. We have demonstrated detection of heroin and cocaine in reasonable quantities with good sensitivity. However, the great specificity of NQR, useful in differentiating explosives and narcotics from other materials, can sometimes be a liability. In particular, the detection of illicit drugs becomes rather complicated because they exist in more than one form and because their purity varies widely, causing NQR resonance frequencies to shift and to broaden.

Although such changes are problematic for the detection of narcotics, this phenomenon suggests another potential application of NQR: for quality control in the chemical and pharmaceutical industries. Work at the Naval Research Laboratory has shown, for example, that the width of the NQR resonance lines in the explosive RDX correlates with its sensitivity to detonation, an important parameter in formulating explosives that are safe to handle.

In many crystalline substances, defects in the orderly packing of atoms introduce strain at a microscopic scale, which in turn influences the frequencies (and frequency ranges) of the NQR resonance. Strain also can be induced by outside forces, and where and how it builds up in structural materials can have especially important consequences—namely mechanical failures. Not surprisingly, a large sub-field in engineering is devoted to the nondestructive evaluation of strain, an area in which NQR holds great promise. For example, NQR may be especially valuable for testing fiber-reinforced composite materials, which are found in everything from tennis rackets to aerospace components. These materials are not highly crystalline and usually do not contain a significant number of quadrupolar nuclei, so they would not typically provide an NQR signal.

This problem can be overcome in two ways: by embedding a small amount of a crystalline substance containing quadrupolar nuclei during manufacture of the composite material, or by later applying a coating of such a substance to the finished structure. Tests on fiberglass composites with embedded strain-sensing crystals, performed last year at Quantum Magnetics, showed that NQR indeed provides a very sensitive method of nondestructive evaluation.

The phenomenon of NQR allows, in principle, for even more ambitious applications. For example, a number of research groups, including those of Daniel J. Pusiol (National University of Córdoba in Argentina), Rainer Kimmich (Ulm University) and Bryan H. Suits (Michigan Technological University), have demonstrated the potential for NQR imaging and for the spatial localization of strain. These early efforts suggest that it may one day be possible to obtain high-resolution images showing the distribution of everything from temperature and strain state to chemical composition and purity. Given the rapidity with which MRI moved out of the laboratory and into hospitals, it seems fair to wonder: Will the benefits of NQR prove great enough to spur similarly dramatic advances?

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