Electricity: measuring and testing – Particle precession resonance
Patent
1996-03-25
1998-09-29
Arana, Louis M.
Electricity: measuring and testing
Particle precession resonance
324301, G01V 300
Patent
active
058149872
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of International application PCT/GB94/02070, filed Sep. 23, 1994.
This invention relates to apparatus for and a method of Nuclear Resonance testing a sample. Such testing may be Nuclear Magnetic Resonance (NMR) or Nuclear Quadrupole Resonance (NQR) testing.
In particular, the invention relates to NQR testing for the presence in a sample of certain substances which exhibit NQR effects. Such substances include drugs such as heroin and cocaine, and explosives such as TNT, RDX, HMX and PETN. Typically, the apparatus and method might be employed to test for the presence of such substances in airport luggage.
Methods of NQR testing for the presence of certain substances are known from, for example, United Kingdom Patent Applications Nos. GB-2254923, GB-2255414 and GB-2255830 (all of which are in the name of British Technology Group Ltd.). Such methods include the steps of applying excitation to the sample to excite NQR resonance, and detecting the NQR resonance response signal. Since the substances of interest (such as heroin, cocaine, TNT, RDX, HMX and PETN) have well characterised and distinct resonance response characteristics, the NQR resonance response signal can be analysed to yield information as to whether or not a particular substance is present in a sample. If the substance is determined to be present, an alarm may be sounded.
In the known techniques, the excitation takes the form of pulses of radio-frequency irradiation. In the time domain the waveform of these pulses is rectangular. Such rectangular pulses appear in the frequency domain as a spectrum having a sharp central peak and side bands which decrease in amplitude away from the central peak. It will be appreciated that the frequency domain bandwidth of a rectangular pulse is inversely related to its time domain duration (or width), so that the shorter the pulse, the greater the bandwidth. Since the exact NQR frequency is dependent on the temperature of the sample, for testing for the presence of the various substances of interest when the sample temperature is not exactly known, a large bandwidth is required (typically 20 kHz). This requires a short pulse (say 30 .mu.s). In order to excite the substance sufficiently for detectable response signals to be derived, the peak radio-frequency power of the pulse may need to be, for example, 2 kW or more, depending on the volume of the probe. Such a level of peak power may be undesirably high.
The present invention seeks to provide apparatus and a method of NQR testing which can afford similar performance to that afforded in the known techniques, but using a lower peak radio-frequency power and preferably avoiding the use of rectangular pulses.
Further prior art of relevance is described in U.S. Pat. No. 5,153,515, WO-A-92/21989, Magn.Res.Med 5 (1987) 217-237, EP-A-O 394 504 and EP-A-O 242 911. None of these documents describe Nuclear Quadrupole Resonance techniques.
According to the present invention, there is provided apparatus for Nuclear Quadrupole Resonance (NQR) testing a sample, comprising: excitation to the sample to excite NQR resonance, the excitation comprising at least one excitation pulse, the or each excitation pulse covering a selected excitation frequency range, the phase of the or each excitation pulse varying generally non-linearly with the excitation frequency over the selected range, the control means further being adapted to control the a detecting means to detect the NQR response signal.
Suitably, the excitation is pulsed excitation at a carrier frequency .nu..sub.0 and the phase of the excitation varies generally non-linearly with the frequency off-set (.DELTA..nu.=.nu.-.nu..sub.0).
This invention arises from the surprising discovery in the field of Nuclear Quadruple Resonance, made pursuant to the present invention, that by applying excitation whose phase varies non-linearly with the excitation frequency, the peak power of the excitation can be reduced very significantly. In one example, peak power has been reduced by more than an order of magnitu
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Blanz Martin
Smith John Alec Sydney
Arana Louis M.
British Technology Group Limited
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