Remote sensing by nuclear quadrupole resonance

Abstract

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.

Optical magnetometry

“…Detection of NQR is a promising technique for identification of explosives, since most explosive materials contain the 14 N nuclei, possessing a large quadrupole moment. However, widespread use of NQR for this purpose has been limited due to the weakness of the NQR signals, which are typically detected with an rf coil only after substantial signal averaging [97]. Detection of weak radio-frequency signals requires modification of usual atomic-magnetometer arrangements that are designed for detection of quasi-DC magnetic fields.…”

An easily constructed, tuning free, ultra-broadband probe for NMR

“…for explosives and narcotics, where the signals intrinsically cover a broad range of frequencies [13] and fast transient recovery time can increase the signal to noise for fast decaying-signals [3,13].…”

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

“…In high frequency MRI studies, losses in the samples generally dominate, and the Q factor of sampleloaded coils cannot be increased beyond a certain value. On the other hand, in low frequency investigations, such as laser-polarized noble gas MRI studies [1], industrial process and quality control by permanent magnet NMR [2] or 14 N-based illicit substance detection by NQR [3], the sample losses are insignificant compared to the losses in the coils themselves and, therefore, high sample-loaded coil Q factors can be obtained by alterations in the design. This approach to improving the SNR, unfortunately, is associated with some serious difficulties.…”