Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
2000-01-04
2001-04-24
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
C324S315000
Reexamination Certificate
active
06222364
ABSTRACT:
The present invention relates to a method of Nuclear Quadrupole Resonance (NQR) testing a sample, and to a method of configuring apparatus for NQR testing such a sample. It relates more particularly to the detection of the presence of a particular substance, such as explosives or narcotics, containing a given species of quadrupolar nucleus.
As an example, the invention has application to the detection in the field of
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. Typical explosives might be RDX, HMX, PETN and TNT, whilst typical drugs might be cocaine or heroin.
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≧1). If they are half integral, the quadrupole interaction (in zero magnetic field) produces two doubly degenerate levels for the case of I={fraction (3/2)} and one allowed transition (½→{fraction (3/2)}), three doubly degenerate levels for I={fraction (5/2)} and two strongly-allowed (½→{fraction (3/2)}, {fraction (3/2)}→{fraction (5/2)}) and one weakly-allowed (½→{fraction (5/2)}) transition, and so on. For integral spin nuclei, the most important of which is
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 one or more characteristic frequency which can be used to identify the substance under investigation. These frequencies 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 (v
o
) to excite the selected transition (at a resonance frequency v
Q
); a pulse of preset width t, rf field amplitude B
1
, and flip angle generates a decaying signal immediately following the pulse known as a free induction decay (f.i.d.).
Apparatus capable of detecting NQR response signals is disclosed in International Patent Publication No. WO 92/17794, in the name of British Technology Group Limited, whose disclosure is incorporated herein by reference.
It was reported in that document that environmental variations (particularly temperature variations) between one location and another can influence properties such as the resonance frequency of the particular NQR substance of relevance and affect the test results. Techniques for coping with such influences are disclosed in the same document.
Problems have now been encountered which are associated with temperature inhomogeneities actually within the sample. In circumstances where a sample is subjected to even modest variations in temperature over the sample (say, a variation of 1 or 2° C. over the length of an elongate sample), the temperature inhomogeneity may be such that signals from the sample can only be detected with difficulty (or not at all) by normal means. It is to be noted that substances of interest, such as the explosive RDX, can have relatively low thermal conductivities, so that sustained temperature inhomogeneities can be commonly encountered.
The present invention seeks to overcome these and other problems.
According to the present invention, there is provided a method of Nuclear Quadrupole Resonance testing a sample containing a given species of quadrupolar nucleus, the sample being subjected to an inhomogeneous distribution of an extrinsic parameter, the parameter having a variation over the sample over a particular range, the method comprising:
applying excitation to the sample at a plurality of different excitation frequencies to excite nuclear quadrupole resonance, such frequencies falling within the resonance frequency range for the nucleus corresponding to the range of the extrinsic parameter; and
detecting the resonance response signal.
In a closely related aspect, the invention provides a method of configuring apparatus for Nuclear Quadrupole Resonance testing a sample, containing a given species of quadrupolar nucleus, which can be subjected to an inhomogeneous distribution of an extrinsic parameter, the apparatus, including means for applying excitation to the sample to excite nuclear quadrupole resonance, the method comprising:
selecting a range of the extrinsic parameter over which it is expected that the parameter will vary over the sample;
determining a resonance frequency range for the nucleus corresponding to the selected range; and
arranging the excitation means to apply excitation at a plurality of different excitation frequencies, such frequencies falling within the determined resonance frequency range.
By arranging for the excitation to be applied at a plurality of excitation frequencies falling within the resonance frequency range of interest, the sensitivity of the tests can be improved.
The present invention arises from the discovery, made pursuant to the present invention, that the problems encountered with the prior art technique arise in part from a dephasing of the NQR resonance response signal due to the inhomogeneous distribution of the extrinsic parameter. For instance, if the extrinsic parameter is temperature, relatively small inhomogeneities in temperature within the sample can give rise to a significant variation of the resonance frequency of the quadrupolar nucleus. It is this variation which can in turn give rise to the dephasing of the signal.
The use of plural excitation frequencies serves to divide the sample notionally into at least two temperature zones corresponding to the respective frequency ranges covered by the different excitation frequencies. Thus, by effectively dividing up the total frequency range into smaller portions, the dephasing effects can be lessened.
For the purposes of the present invention, the relevant resonance response signal would usually be the free induction decay (f.i.d.).
As used herein, the term “excitation frequency” in the context of pulsed excitation preferably connotes the central “carrier” frequency of the relevant excitation pulse where the context so demands.
In putting the aforementioned configuration method aspect of the invention into practice, the apparatus may be configured by, say, appropriate design of or adjustments to its controlling hardware (or software). Such configuration may take place at the factory before delivery of the apparatus.
Preferably, the excitation comprises pulses at the different excitation frequencies which are arranged to have respective excitation frequency ranges which are substantially non-overlapping. By providing substantially non-overlapping frequency ranges, dephasing effects can be reduced.
For the same reason, preferably the excitation comprises pulses at the different excitation frequencies, the pulses being shaped so as to have a substantially rectangular frequency profile. By way of contrast, conventional simple “rectangular” pulses have a profile in the frequency domain which actually possesses substantial frequency sidebands, often known as “Gibbs oscillations”. Such pulses are only “rectangular” in the time domain. If such pulses were employed, either—if the pulses were substantially overlapping—very substantial dephasing effects could be caused, particularly by the sidebands, or—if there were no overlap—a substantial proportion of the potential signal could be lost. Pulse shaping as described above can make optimal use of the time available for the test.
The pulses are preferably frequency/phase swept. This can permit pulse length to be kept at a low value.
From the point of view of reducing the
Blanz Martin
Rowe Michael David
Smith John Alec Sydney
Arana Louis
BTG International Limited
Pillsbury & Winthrop LLP
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