Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1997-09-03
1999-11-16
Oda, Christine
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324322, 324307, G01V 300
Patent
active
059864559
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates generally to a bulk substance detection systems for detecting concealed explosives and narcotics employing either nuclear quadrupole resonance or nuclear magnetic resonance, and more particularly to a practical system and method for tuning such detection systems.
BACKGROUND ART
Certain atomic nuclei, typically having a spin quantum number of 1/2, exhibit magnetic signatures when they are within an externally applied magnetic field. This magnetic resonance effect is most commonly observed in .sup.1 H, and is known as nuclear magnetic resonance (NMR). Atomic nuclei with a spin quantum number of >1/2 can also show another magnetic signature associated with the interaction of the nuclei with the local electric field. This phenomenon is known as nuclear quadrupole resonance (NQR).
For both of these phenomena, the energy level transitions are observed primarily in the radio frequency range. Detection of these transitions thus requires a radio frequency source to excite the transition, and a radio frequency receiving mechanism to detect the signal. Normally, the signals appear at a pre-defined frequency. An RF coil tuned to, or close to, that predefined frequency can excite or detect those signals. The signals are of very low intensity and can only be observed for a short time. approximately 10 .mu.s to 2 ms. As a consequence, there is a need for an NQR or NMR receiver that can be tuned to (usually) high Q, has very low noise, and is capable of fast recovery after a high voltage RF pulse. In most conventional magnetic resonance (NMR and NQR) experiments, small and fairly homogeneous samples are investigated.
Over the past few years there has been considerable interest in the larger-scale "real world" applications of both NQR and NMR. These applications do not benefit from the luxury of small-scale laboratory investigations. They usually require investigation of large volumes filled with materials of vastly differing physical and chemical composition. Investigation of the contents of mail or baggage for the presence of explosives or narcotics is an example.
With respect to explosives, plastic explosives such as C-4 and Semtex, containing RDX and PETN, have an almost infinite variety of possible shapes and uses for terrorist bombing tactics. Plastic explosives are highly stable, have clay-like malleability and are deadly in relatively small quantities. A small piece of plastic explosive, a detonator, and a trip wire inside a large mailing envelope can cause a deadly explosion. Unfortunately, without close--and potentially dangerous--visual inspection, plastic explosives can be made virtually untraceable. In particular, detection of sheet explosives, typically having a thickness as small as one-quarter inch, has not been effectively accomplished by prior technologies.
The wide-scale attempts to fight the illegal drug trade indicates that narcotics detection is also extremely important. The need for a simple procedure for detecting drugs inside sealed containers, mail parcels, and other small packages, quickly and accurately, is immeasurable. Conventional detection methods are time-consuming, costly, and have only marginal reliability at best.
Detection by means of NQR or NMR is possible for both explosives and narcotics, partially because they have as a constituent element .sup.14 N in crystalline form. Particularly with respect to narcotics, this is true of cocaine base, cocaine hydrochloride and heroine based narcotics. The hydrochloride forms of narcotics, such as cocaine hydrochloride, also contain quadrupolar nuclei .sup.35 Cl and .sup.37 Cl.
A significant factor in contraband detection by means of NQR in particular is that quadrupolar nuclei that are commonly present, and potentially readily observable, in narcotics and explosives include nitrogen (.sup.14 N) and chlorine (.sup.35 Cl and .sup.37 Cl), among possible other nuclei. Thus, in commercial applications it is necessary to be able to detect quadrupolar nuclei contained within articles of mail, mail bags or a
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Butler et al., High Power Radio Frequency Irradiation System with Automatic Tuning, Rev. Sci. Instrum., 53 (7), pp. 984-988 (Jul. 1982).
Oda Christine
Quantum Magnetics Inc.
Shrivastav Brij B.
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