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 discontinuties 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 recent results of field trials of a prototype landmine detection system.

Results from 14N pure NQR of cocaine in the free base form (cocaine base) yield a nuclear quadrupole coupling constant (NQCC) e2Qq/h of5.0229 (±0.0001) MHz and an asymmetry parameter 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. The NQR peaks of a sample of cocaine base containing ~1% impurities are a factor of 3 broader than those of a recrystallized sample, but spin-lattice and spin-spin relaxation times are essentially unchanged. Both pure NQR (at 295-77 IQ and a superconducting quantum interference device (SQUID) detector (at 4.2 K) were used to measure the very low (<1 MHz) 14N 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).

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The troublesome spurious ringing phenomenon found in pulsed nuclear magnetic resonance probes is explained in terms of the electromagnetic generation and detection of ultrasonic waves. A few techniques for eliminating this problem are discussed.