Nuclear Quadrupole Resonance
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The text on this site is published with permission of RAND and taken from "Alternatives of Landmine Detection" Jacqueline MacDonald et.al, RAND report, ISBN 0-8330-3301-8, Document Number: MR-1608-OSTP, Year: 2003[1]
BULK EXPLOSIVE DETECTION TECHNIQUES
Biological and chemical methods for detecting explosive vapors currently are limited by incomplete knowledge of how explosive vapors migrate in the shallow subsurface. An additional category of explosive detection technologies overcomes this limitation by searching for the bulk explosive inside the mine. Methods being explored for this purpose include nuclear quadrupole resonance (NQR) and a variety of methods that use the interaction of neutrons with components of the explosive. These technologies emerged from interest in detecting bulk explosives in passenger baggage for the airline industry and in investigating the potential presence of explosive devices in other settings.
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Description
NQR is a radio frequency (RF) technique that can be used to interrogate and detect specific chemical compounds, including explosives. An NQR device induces an RF pulse of an appropriate frequency in the subsurface via a coil suspended above ground. This RF pulse causes the explosives’ nuclei to resonate and induces an electric potential in a receiver coil. This phenomenon is similar to that exploited by magnetic resonance imaging (MRI) used in medical testing, but NQR uses the internal electric field gradient of the crystalline material rather than an external static magnetic field to initially align the nuclei.
Strengths
NQR has a number of features that make it particularly well suited for landmine detection. The primary attraction of NQR is its specificity to landmines: In principle, it signals only in the presence of bulk quantities of specific explosives. Unlike many other technologies, its false alarm rate is not driven by ground clutter but rather by its signal-to-noise ratio (SNR). The SNR increases with the square root of the interrogation time and also increases linearly with the mass of the explosive. Thus, with sufficient interrogation time, NQR can achieve nearly perfect operating characteristics (probability of detection near one with probability of false alarm near zero). This makes NQR more attractive as a confirmation sensor used to interrogate only those locations identified by other detectors (e.g., GPR,EMI) as likely mine locations. Interrogation times of 0.5–3.0 minutes may be sufficient for performance that leads to high probability of detection (more than 0.99) and low probability of false alarm (less than 0.05). The NQR signal from cyclotrimethylenenitramine (RDX) is particularly large, implying high performance and small interrogation times (less than three seconds) for detection of mines containing RDX. Another positive feature of NQR is that it is relatively robust to diverse soil conditions; for example, because it requires bulk concentration of explosives to declare, it is not misled by trace explosive residues as can be the case with vapor-sensing techniques.
Limitations
The major weakness of NQR is the fact that, because of its nuclear properties, TNT, which comprises the explosive fill of most landmines, provides a substantially weaker signal than either RDX or tetryl, posing a formidable SNR problem. Moreover, TNT inherently requires longer interrogation times because its nuclear properties preclude interrogation more frequently than once per five to ten seconds. Another significant limitation is the susceptibility of NQR to RF interference from the environment. This is especially problematic for TNT detection because the frequencies required to induce a response from TNT (790–900 kHz) are in the AM radio band. When present, radio signals overwhelm the response from TNT. An additional weakness is that NQR cannot locate explosives that are encased in metal because the RF waves will not penetrate the case. This is not a major weakness because a large majority of antipersonnel mines have plastic cases, and EMI detection can successfully detect those with metal cases. NQR also cannot detect liquid explosives, but very few antipersonnel mines use liquid explosives. NQR is very sensitive to the distance between the detection coil and the explosive. Therefore, the detection coil must be operated very close to the ground, which can be problematic in rough or highly vegetated terrain. Moreover, current implementations require stationary detection for optimal results; detection in motion substantially degrades the SNR.
Summary Evaluation
NQR is an explosive-specific detection technology that offers considerable promise as a technique for reducing false alarms as compared with such conventional detection approaches as EMI. It offers opportunities for improvement not addressed by competing technologies—most notably that its potency derives from unique explosive signatures of mines. In principle, this specificity affords it the possibility of a zero false alarm rate against nonmetal mines. Currently, the most promising role of NQR is that of a confirmation sensor used in conjunction with a conventional scanning sensor or as part of an integrated multisensor detection system.
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References
- ↑ Jacqueline MacDonald et.al: Alternatives of Landmine Detection, RAND report, ISBN 0-8330-3301-8, Document Number: MR-1608-OSTP, 2003
