Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1994-08-26
1996-02-13
O'Shea, Sandra L.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
G01R 3320
Patent
active
054914140
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method of nuclear quadrupole resonance (NQR) testing of integral spin quantum number spin systems.
DESCRIPTION OF RELATED ART
NQR testing is used for detecting the presence or disposition of specific substances. It depends on the energy levels of quadrupolar nuclei, which have a spin quantum number greater than one-half. Quadrupolar nuclei having an integral spin quantum number (that is, I=1, 2, 3 . . . ) include .sup.14 N (I=1). .sup.14 N nuclei are present in a wide range of substances, including animal tissue, bone, food-stuffs, explosives and drugs.
In the sub-molecular environment of compounds or crystals, the nature and disposition of the electrons and atomic nuclei produce an electric field gradient which modifies the nuclear energy levels to such an extent that measurements of NQR effects can indicate not merely the nuclei which are present but also their chemical environment, thus indicating specific substances or types of substances in any tested sample.
In NQR testing a sample is irradiated with pulses or sequences of pulses of radiofrequency electromagnetic waves having a frequency which is at or very close to a resonant frequency of quadrupolar nuclei in a substance which is to be detected. If the substance is present, the irradiant energy will raise at least some of the nuclei to a higher energy level. Such nuclei will tend to return to their normal state and in doing so they will emit radiation at their resonance frequency or frequencies which can be detected as a free induction decay (f.i.d.) during a decay period after each pulse. These emissions decay at a rate which depends on two relaxation time constants, T.sub.1 and T.sub.2.
In conventional NOR testing, either a substantial part of the free induction decay is measured after each pulse or the responses are measured as echoes in relatively short sampling periods between or following a relatively rapid succession of pulses. Usually the results from a number of test pulses or test sequences are integrated to improve the signal-to-noise ratio. Various schemes of pulse sequences have been used.
In a scientific paper by Grechiskin et al. (Adv. N.Q.R., 1983, 5, 1) it has been predicted theoretically that conditions might arise which could give rise to the formation of a single echo as well as a free induction decay from nuclei of unity spin quantum number when excited by two excitation pulses at a single radiofrequency. No details are given as to how, or even whether, this can be achieved experimentally.
In a paper by Bloom et al. (Physical Review 1955, vol. 97, 1699) it has been reported that multiple echoes as well as a free induction decay have been observed in tests from nuclei of spin quantum number 3/2 in a weak magnetic field. The paper demonstrates that the magnetic field removes degeneracies which would otherwise so broaden and attenuate the echoes that no useful NOR information would be yielded. However, this technique would not be expected to work for integral spin systems, since such systems are not strongly affected by a wear applied magnetic field.
SUMMARY OF THE INVENTION
According to the present invention, a method of NQR testing includes the steps of applying to a sample in which selected nuclei have an integral spin quantum number a series of at least three excitation pulses of electromagnetic waves at a single radiofrequency to excite quadrupole resonance of the selected nuclei and detecting responses at a plurality of times when echo response signals are expected to occur, the excitation pulses being arranged to generate more echo response signals than the number of applied pulses.
We have found that, surprisingly, for samples in which the selected nuclei have an integral (e.g. unity) spin quantum number, in addition to the free induction decay which occurs immediately after a single frequency excitation pulse, there are some substantial NQR echo responses which occur after delays matching the time intervals between preceding pulses. It has been found that, i
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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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