Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-11-05
2003-06-10
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06577128
ABSTRACT:
BACKGROUND OF THE INVENTION
Substances which have relatively low NQR frequencies (perhaps 1 or 2 MHz or less) and relatively long values of T
1
(perhaps 500 ms, 5 or 10 s or more) include the explosives PETN and TNT, Potassium Nitrate (KNO
3
) and
27
Al in alumina. For example, PETN has resonance frequencies around 0.9 MHZ, a T
1
of roughly 30 s at room temperature, as well as a spin-spin relaxation time (T
2
) of approximately 20 ms. It is noted in passing that T
2
is preferably defined herein as the exponential constant measured by means of a Hahn echo or similar pulse sequence.
NQR testing is used for detecting the presence or disposition of specific substances, and in particular polycrystalline substances. It depends on the energy levels of quadrupolar nuclei, which have a spin quantum number I greater than ½, of which
14
N is an example (I=1).
14
N nuclei are present in a wide range of substances, including animal tissue, bone, food stuffs, explosives and drugs. One particular use of the technique described herein is in the detection of the presence of substances such as explosives or narcotics. The detection may be of baggage at airports, or of explosives or drugs concealed on the person or buried underground or elsewhere. Other nuclei of interest are
27
Al(I=5/2) and
63
Cu(I=3/2).
27
Al is present in minerals, cement and concrete, whilst
63
Cu is present in ores and many high Tc superconducting materials.
In conventional Nuclear Quadrupole Resonance testing a sample is placed within or near to a radio-frequency (rf) coil and is irradiated with pulses or sequences of pulses of electro-magnetic radiation having a frequency which is at or very close to a resonance frequency of the quadrupolar nuclei in a substance which is to be detected. If the substance is present, the irradiant energy will generate a precessing magnetization which can induce voltage signals in a coil surrounding the sample at the resonance frequency or frequencies and which can hence be detected as a free induction decay (f.i.d.) during a decay period after each pulse or as an echo after two or more pulses. These signals decay at a rate which depends on the time constants T
2
* for the f.i.d., T
2
and T
2e
for the echo amplitude as a function of pulse separation, and T
1
for the recovery of the original signal after the conclusion of the pulse or pulse sequence.
As described in International Patent Application No. WO 96/26453 in the name of British Technology Group Limited, the subject matter of which is incorporated herein by reference, spurious interfering signals (also termed “ringing”) which are not associated directly with or due to the nuclear resonance may sometimes arise from a sample during NQR tests.
For example, one group of materials which can cause interference problems includes metallic conductors. Such materials may be commonly found in many types of objects in baggage. It has been discovered that the interference may be particularly pronounced when a sample includes metallic or ferromagnetic material as a layer of plating on another material, especially, it has been found, when the plating layer comprises Nickel. Objects which are particularly prone to such problems include screws or key-rings. The cause of this type of interference has not been proven, but it is believed to emanate from ferromagnetic or like resonance effects in the B
1
field of the sample coil, and be due to a form of magneto-acoustic ringing. It should be emphasised that this interference is not an artefact of the particular detection apparatus used, but a feature of the material itself. Also it will be understood that, in the context of the detection of the presence of a particular substance in a sample, it would usually not be the particular nuclear species to be detected but the remainder of the sample which would give rise to the interfering signals.
The spurious interfering signals (or “artefacts”) commonly have decay characteristics very similar to those of true NQR signals, and, furthermore, are often many times stronger; they can last for several milliseconds. The phase of those inter fering signals and that of the resonance response signal following a single radio-frequency excitation pulse are entirely determined by the rf phase within the pulse. There is, however, one important distinction. When two or more pulses are used, the phase of the NQR response signal, whether it be a free induction decay (f.i.d.) or an echo, depends on the relative phases of the two preceding pulses, unlike that of the interfering signal, which is determined almost entirely by that of the immediately preceding pulse.
This distinction has been exploited in WO 96/26453 in an attempt to remove the interfering signal from an NQR response signal. The proposed solution involves the use of at least one pair of excitation pulse sequences (or blocks) in which the phase of the pulses is controlled in such a way that when the response signals from the two member sequences of the pair are compared the spurious signals can be largely eliminated whilst the genuine NQR signals can be retained.
SUMMARY OF THE INVENTION
It has been discovered pursuant to the present invention that, when applying a multiple pulse sequence such as one of those described in WO 96/26453, the response off-resonance varies with frequency in a periodic fashion. An example of a typical off-resonance response to a multiple pulse sequence for a typical substance is shown in FIG.
1
. The response has been found to have narrow peaks and wide troughs which are believed to be due to the pulsed nature of the excitation; the separation of the peaks is believed to be related to the pulse repetition rate. Furthermore, the resonance frequency of the peaks varies with temperature and other such environmental parameters. Unless the excitation is exactly at the resonance frequency, or exactly at the frequency of one of the other peaks, there will not reliably be any response signal. Therefore, for example, in a typical situation (such as airport security checking) where the exact temperature of the sample is not known, the usefulness of such multiple pulse sequences may be reduced.
Further, it has been found pursuant to the present invention that the off-resonance behaviour of multiple pulse sequences can cause particular problems when they are used in pairs as described above to reduce spurious signals, particularly if the substance under test has a relatively low resonance frequency and/or long spin-lattice relaxation time.
The present invention seeks to maintain or improve upon the level of spurious signal suppression achieved using the technique described in WO 96/26453, but to improve the off-resonance response, especially for long T
1
substances. The invention also seeks to improve the sensitivity of NQR tests. The invention is based in part upon the discovery, pursuant to the invention, that an improvement in the off-resonance response and the sensitivity of the NQR test may result if the delay time between the two excitation pulse blocks mentioned above is carefully controlled.
Prior to the present invention, it was considered that sufficient time must be left between the two excitation blocks to allow the NQR magnetization generated during the first block to recover. However, it has now been discovered pursuant to the present invention that, by having a delay between the two blocks which is insufficient for the magnetization to recover, the off-resonant response behaviour and the sensitivity of the NQR test may be considerably improved.
According to the present invention there is provided a method of Nuclear Quadrupole Resonance testing a sample containing quadrupolar nuclei exhibiting a given value of spin-lattice relaxation time, T
1
, the method comprising:
applying two (or possibly more) excitation blocks to excite nuclear quadrupole resonance, there being a given delay time between the two blocks;
detecting resonance response signals; and
comparing the response signals from respective blocks;
wherein the delay time is less than
Barras James
Peirson Neil Francis
Smith John Alec Sydney
BTG International Limited
Lefkowitz Edward
Nixon & Vanderhye
Vargas Dixomara
LandOfFree
NQR method and apparatus for testing a sample by applying... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with NQR method and apparatus for testing a sample by applying..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and NQR method and apparatus for testing a sample by applying... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3107350