Detection of explosives has the flavor of those mathematical problems that are not invertible. It is easier to hide explosives than to find them. Many approaches have been proposed and executed for the remote detection of explosives, contraband materials, weapons of mass destruction, currency, etc. Most detection technologies suffer from a common problem: the features they look for, such as discontinuities in electrical conductivity, are not unique properties of the target but are contained, to some degree, in the more benign surroundings. Such a degeneracy leads to "clutter" in the response. For example, resolving the false alarms generated by this clutter can determine the rate of advance of a conventional electromagnetic metal detector employed as a landmine detector. One approach that provides a "unique" signature is nuclear quadrupole resonance (NQR) (the technique is also called QR, to avoid confusion with strictly nuclear techniques). This paper outlines the important physical principles behind the use of NQR for remote detection, indicates areas of applicability, and presents results of field trials of a prototype landmine detection system.

Remote sensing by nuclear quadrupole resonance

Results from {sup 14}N pure NQR of cocaine in the free base form (cocaine base) yield a nuclear quadrupole coupling constant (NQCC) e{sup 2}Qq/h of 5.0229 ({+-}0.0001) MHz and an asymmetry parameter {eta} of 0.0395 ({+-}0.0001) at 295 K, with corresponding values of 5.0460 ({+-}0.0013) MHz and 0.0353 ({+-}0.0008) at 77 K. Both pure NQR (at 295-77 K) and a superconducting quantum interference device (SQUID) detector (at 4.2 K) were used to measure the very low (<1 MHz) {sup 14}N transition frequencies in cocaine hydrochloride; at 295 K the NQCC is 1.1780 ({+-}0.0014) MHz and the asymmetry parameter is 0.2632 ({+-}0.0034). Stepping the carrier frequency enables one to obtain a powder pattern without the severe intensity distortions that otherwise arise from finite pulse power. A powder pattern simulation using an NQCC value of 5.027 MHz and an asymmetry parameter {eta} of 0.2 agrees reasonably well with the experimental stepped-frequency spectrum. The use of pure NQR for providing nondestructive, quantitative, and highly specific detection of crystalline compounds is discussed, as are experimental strategies. 31 refs., 8 figs., 1 tab.

Detection of 14N and 35Cl in Cocaine Base and Hydrochloride Using NQR, NMR, and SQUID Techniques

https://escholarship.org/content/qt48f4q18j/qt48f4q18j.pdf?t=p0t1ly

Short range remote NQR measurements

A method and system for canceling extraneous signals in a nuclear quadrupole resonance (NQR) system used for the detection of explosives and drugs. A specimen is irradiated with a specific sequence of electromagnetic pulses, referred to as a spin-locked inversion mid-echo (SLIME) pulse sequence with each pulse having a specified phase and duration. The pulses are separated by specified time intervals. The SLIME pulse sequence has a single excitation pulse. As a result of the characteristics of the SLIME pulse sequence, if the target substance to be detected is present in the specimen, sets of oppositely-phased NQR echo signals are generated from the target substance. NQR echo signals of one phase are subtracted from NQR echo signals having the opposite phase, rendering a cumulative echo signal, and simultaneously subtracting out the same-phase extraneous signal.

Method and system for cancellation of extraneous signals in nuclear quadrupole resonance spectroscopy

RF probe recovery time reduction with a novel active ringing suppression circuit.

FT-NQR (Fourier Transform Nuclear Quadrupole Resonance) spectroscopy is one of the newer analytical tools. Although nuclear quadrupole coupling constants were first observed in atoms by Schmidt and Schuler1 in 1935 and in molecules by Kellog2 and coworkers in 1936, it was the discovery of pure quadrupole coupling transitions in solids by Dehmelt and Kruger3,4 in 1950 and 1951 that initiated the understanding of the NQR phenomenon as presently utilized.

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Fourier Transform Nuclear Quadrupole Resonance Spectroscopy

: The results of this research have established that the nuclear quadrupole resonance (NQR) spectrometric technique is ideally suited for the characterization of explosives such as TNT. NQR data yields information on chemical structure, crystalline states and morphology. For the explosive TNT it has been shown that: a. TNT exists in two morphological states, b. TNT undergoes a phase transition at approximately 200K. In addition a new mechanism was discovered for obtaining NQR signals at high data rates thus enhancing system efficiency by a factor of up to 100. Finally, initial research was performed using soft ferrite cores in place of the standard sample coils in an effort to reduce the amount of sample required for an NQR measurement. (Author)

Stanley M. Klainer

Papers

Fourier Transform Nuclear Quadrupole Resonance Spectroscopy

An NQR Study of TNT Characteristics.