Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1993-09-30
1996-12-10
O'Shea, Sandra L.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
G01R 3344
Patent
active
055834370
DESCRIPTION:
BRIEF SUMMARY
This application claims priority from United Kingdom Patent Application No. 9106789.2, filed on Apr. 2nd 1991, whose disclosure is incorporated herein by reference.
FIELD OF THE INVENTION
This invention relates to a method of, and apparatus for, Nuclear Quadrupole Resonance (NQR) testing a sample having at least one NQR property (particularly NQR resonance frequency or an NQR relaxation time, T.sub.1, T.sub.2, T.sub.2e and T.sub.2 *) which varies with a given environmental parameter, such as pressure, magnetic field or more particularly temperature. It also relates to a method of, and apparatus for, NQR testing for the presence of selected nuclei (particularly nuclei of integral spin quantum number, such as .sup.14 N) in a sample. It also relates to a method of determining the spatial distribution of temperature within a sample.
As an example, the invention has application to the detection in the field of .sup.14 N quadrupole resonance signals from the explosive RDX concealed in parcels or luggage or on the person, or deployed in explosive devices. As another example, it has application to the detection of concealed drugs, for instance at airports.
DESCRIPTION OF THE RELATED ART
NQR measurements have the advantage that they do not require the sample to be placed in a strong magnetic field, and therefore do not require the large, expensive and sample-size limiting magnet structures which are needed for nuclear magnetic resonance (NMR) measurements.
Quadrupolar nuclei have a nuclear spin quantum number I equal to or greater than unity (I.gtoreq.1). If they are half integral, the quadrupole interaction (in zero magnetic field) produces two doubly degenerate levels for the case of I=3/2 and one allowed transition (1/2.fwdarw.3/2), three doubly degenerate levels for I=5/2 and two strongly-allowed (1/2.fwdarw.3/2, 3/2.fwdarw.5/2) and one weakly-allowed (1/2.fwdarw.5/2) transition, and so on. For integral spin nuclei, the most important of which is .sup.14 N with I=1, there are usually three levels and three transition frequencies, dropping to one for nuclei in axially-symmetric environments. All these transitions have a characteristic frequency and relaxation time (or times) which can be used to identify the substance under investigation. These frequencies and relaxation times do not depend on the presence of other substances, provided that they do not have transitions in the same frequency range.
Nuclear quadrupole resonance response signals are conventionally detected by means of pulsed radiofrequency (rf) radiation of the correct excitation frequency (.nu..sub.o) to excite the selected transition (at a resonance frequency .nu..sub.Q); a pulse of preset width t, rf field amplitude B.sub.1, and flip angle .alpha. generates a decaying signal immediately following the pulse known as a free induction decay (f.i.d.), and two or more pulses of pre-set widths and spacings can generate echoes. The pulse width which produces the maximum f.i.d. for a given quadrupolar nucleus in a solid powder is given the symbol t.sub.m and the corresponding flip angle the symbol .alpha..sub.m (equivalent, for example, to a "90.degree." pulse in NMR).
Usually in NQR testing a repetitive pulse train is used with spacings between pulses, .tau., which depend on the relaxation times T.sub.1,T.sub.2 and T.sub.2e, and a number of f.i.d.'s and/or echoes are accumulated to provide the required sensitivity of detection. By "sensitivity of detection" is meant the number of selected nuclei (or amount of material) that can be detected at a given level of probability in a fixed volume of sample over a given test time. This sensitivity may be further improved by Fourier transformation of the accumulated signals to give an absorption spectrum, the area under which is then measured by integration between appropriate limits. A paper by N. E. Ainbinder et al. (Advances in Nuclear Quadrupole Resonance, 3, 1978, 67-130) provides information on this background state of the art.
SUMMARY OF THE ART
The present invention seeks to provide an improved
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Shaw Julian D.
Smith John A. S.
British Technology Group Limited
Mah Raymond Y.
O'Shea Sandra L.
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