APPARATUS FOR AND METHOD OF NUCLEAR QUADRUPOLE TESTING OF A SAMPLE

Abstract: Apparatus for Nuclear Quadrupole Resonance testing a remote sample containing a given species of quadrupolar nucleus comprises at least one probe (112) having a given maximum cross-sectional dimension; means (102) for applying excitation to the probe or at least one of the probes to excite nuclear quadrupole resonance for a selected range of distance of the sample from the or one such probe, the selected range being at least one tenth of the given maximum dimension of the probe or said one such probe, the apparatus being adapted to produce a non-uniform field over the selected range; and means (106) for detecting the resonance response signal from the sample via the probe or at least one of the probes; the excitation being such as would generate non-zero resonance response signals at all distances within the selected range.

Description: APPARATUS FOR AND METHOD OF NUCLEAR QUADRUPOLE TESTING OF A SAMPLE The present invention relates to apparatus for and a method of Nuclear Quadrupole Resonance testing a sample containing a given species of quadrupolar nucleus, and to apparatus for and a method of determining a measure of the distance of a sample from a probe, the sample containing a given species of quadrupolar nucleus. The invention preferably relates to the testing of what are hereinafter termed "remote" samples. Although the following definition is not exclusive, remote samples are typically samples which lie outside the plane of the excitation/detection probe, often at a distance away from the probe which may be comparable with or greater than the largest cross-sectional dimension of the probe. With remote testing, it is often only possible to access the sample from one side, for instance if the sample is buried or concealed. Testing of remote samples which can only be accessed from one side is often termed "one-sided" testing; in such tests only the field from one side of the probe is utilised, the probe usually being shielded on the other side. More particularly, the invention relates to the detection of the presence of remote samples containing quadrupolar nuclei. As an example, the present invention has particular application to the detection of 14 N quadrupole signals in drugs, such as cocaine and heroin, concealed on or within the person, possibly using a hand-held probe. Again, in industrial processes, it can be used to detect signals from quadrupole-containing materials. Such materials might be proteins in foodstuffs, or quadrupole containing substances on conveyor belts, inside furnaces or nuclear reactors or in chemically or physically hazardous surroundings in which the probe must be located away from the remainder of the testing apparatus, possibly even on one side only of the system. The probe may be located inside the pressure vessel of the nuclear reactor, which may be at extremes of temperature and pressure. As another example, the invention has particular application to the detection of 14 N quadrupole resonance signals from explosives such as TNT, RDX, HMX, PETN and compositions of such materials, for instance Semtex, contained within objects placed on the ground surface or buried underground, or to the detection of such explosives concealed beneath or behind concealing barriers, or on the person. In the case of explosives, if provided in relatively small quantities (perhaps lOg to lOOg or 1kg) they may for example be (a) scattered on the ground surface or (b) hand laid beneath the surface at a depth of a few centimetres (say 2 to 6cm). If provided in larger quantities (perhaps 1kg to 10kg or 20kg - case (c)), they are usually laid at a depth of about 1 to 20 or 25 cm. The probe would usually be at a distance of around 5 to 20cm from the ground, and the explosive would usually be between 1 and 5cm thick. If, say the distance from the probe to the ground were a typical 10cm and the thickness of the explosive were about 3cm, then the depth that the probe would need to examine would be roughly 8.5cm for case (a), 13cm for case (b) and 10 to 35cm for case (c). Other possible uses of the invention are in exploration, for example, in mining, down boreholes (for instance for petrochemical exploration), or in structural engineering, for example, in the detection of faults in concrete or cement structures. Amongst the most noteworthy of the quadrupolar nuclei are n B, 1 N, ^Na, ^Mg, "Al, 39 K, 51 V, 55 Mn, S9 Co, ∞Cu, 75 As, 12I Sb and ^Bi. The sample would usually be in polycrystalline or powdered rather than single crystal form. As opposed to Nuclear Magnetic Resonance (NMR) techniques, Nuclear Quadrupole Resonance (NQR) techniques have the advantage that they do not require the sample to be placed in a strong static magnetic field; usually they function in the absence of any applied magnetic field. Interference from metallic or ferromagnetic objects can be eliminated. In another related application, the invention may be used to provide an estimate of the quantity of sensitive material inside the sample under examination and its distance from the probe, providing additional applications in medicine as an adjunct to magnetic resonance imaging. A method of detecting the presence of a given species of quadrupolar nucleus in a sample is known from International Patent Application No. PCT/GB94/02070 (British Technology Group Ltd.), whose disclosure is incorporated herein by reference, and which claims priority from United Kingdom Patent Application No. 9319875.2. The sample is subjected to r.f. excitation pulses of a given shape and the NQR response is observed as a free induction decay (f.i.d.) immediately following such a pulse (or pulses) and/or as echoes produced by application of two or more such pulses. The pulses are preferably shaped so as to be substantially rectangular in the frequency domain, and are subject to a phase variation which has a non-linear, preferably quadratic, variation with time over the duration of the pulse. The aforementioned patent application, together with International Patent Application No. PCT/GB92/00580 (also to British Technology Group Ltd.), describes how the excitation pulses may be manipulated so as to reduce the deleterious effects of temperature or pressure variations experienced by the sample on the performance and sensitivity of the NQR tests. Intemational Patent Application No. PCT/GB92/01004 (also to British Technology Group Ltd.) describes how a quadrupole resonance image may be obtained from samples contained within opposed coil arrays. As stated above, the present invention is mainly concemed with cases in which access to the sample is from one side only, such as is a characteristic of buried explosives. Intemational Patent Application No. PCT/US92/03116 and a closely related paper ("NQR Detection Using a Meanderline Surface Coil", by Buess, M. L. et al., J. Mag. Res., 92, 1991, 348-362) describe a meanderline surface coil which is deliberately designed to detect explosives and narcotics only to a very limited depth, typically 1 or 2cm, a very small fraction of the overall dimension of the probe coil. The limited depth to which the meanderline coil is effective is completely inadequate for many purposes of practical interest. A paper by Hirschfeld et al. entitled "Short Range Remote NQR Measurements" (J. Mol. Struct., 58, 1980, 63-77) discloses the detection of buried explosives using NQR techniques. Separate experiments are described, one in which explosive is detected at a depth of 6cm, and one in which the depth is 10cm. In each case the testing apparatus is optimised for the particular depth of interest. There is no suggestion of any technique for the detection of explosive at any other depth or even of the desirability of detection at other depths. It has now been realised pursuant to the present invention that in the testing of remote samples it is desirable to carry out tests over a substantial selected range of distance away from the excitation/detection probe or probes, rather than at just one or two fixed distances. As used herein in this context, the term "substantial" is defined in terms of the range being, say, at least one tenth of the maximum cross-sectional dimension of the excitation/detection probe. Of course, other proportions would also be possible, such as one 1/20 of the maximum dimension, or 1/5, 1/3 or 1/2. The selected range may even be the same size or larger than the maximum dimension. In putting the present invention into practice, the present inventors used as the probe a plane spiral coil of outside diameter (maximum cross-sectional dimension) roughly 25cm. The selected range was between roughly 5 and 25cm from the coil. It has furthermore been discovered that conventional excitation techniques such as that taught by Hirschfeld et al. would not operate satisfactorily over the substantial ranges referred to above. As explained in detail later, it has been discovered that, surprisingly, over any substantial range conventional techniques would be expected to give rise to a lack of uniformity of sensitivity of detection to the extent that samples at certain depths might not be detected at all. The present invention in various preferred aspects seeks not only to provide good sensitivity over a substantial distance range but also with the selected range starting at a substantial distance (remoteness) from the probe or probes. In this context "substantial" connotes, say, at least 1/20, 1/10, 1/5, 1/3 or 1/2 of, or even preferably greater than, the maximum dimension of the probe. This is to take into account that in many situations of practical interest the probe or probes may be a substantial distance away from the sample. According to the present invention, there is provided apparatus for Nuclear Quadrupole Resonance testing a remote sample containing a given species of quadrupolar nucleus, comprising: at least one probe having a given maximum cross-sectional dimension; means for applying excitation to the probe or at least one of the probes to excite (detectible) nuclear quadrupole resonance for a selected range of distance of the sample from the or oι.e such probe, the selected range being at least one tenth of the given maximum dimension of the probe or said one such probe, the apparatus being adapted to produce a non-uniform field over the selected range; and means for detecting the resonance response signal from the sample via the probe or at least one of the probes; the excitation being such as would generate non-zero resonance response signals at all distances within the selected range. The restriction that the apparatus is adapted to produce a non-uniform field over the selected range is preferably intended to be interpreted to exclude configurations where the sample is accessed from both sides (for example, coils in Helmholtz pairs) using coils which are paired to produce highly uniform B j fields. Hence "non-uniform" may imply a variation of greater than, say, 3, 5 or 10% in field over the selected range. By arranging the excitation applying means to generate non-zero resonance response signals at all distances within the selected range, the present invention can provide significantly more uniform sensitivity in the testing of remote objects over the selected range than would have been possible if conventional techniques had been employed in the testing of remote samples. As an indication of the acceptable variation in sensitivity over the selected range, preferably the excitation is such that, for a given sample, over the selected range of sample distance the minimum resonance response signal detected would be no less than one twentieth, preferably no less than 1/10, 1/5, 1/4, 1/3 or 1/2, of the maximum resonance response signal. Preferably the excitation is such as to generate a flip angle which is substantially constant over the selected range, so that the response signals would be as uniform as possible over the selected range. As taught herein, this can be achieved in a number of ways, including by the use of excitation in the form of appropriately shaped pulses. The term "substantially constant" is to be interpreted in the light of the general level of inaccuracy inherent in NQR testing. For instance, the lowest flip angle may suitably be greater than 50%, 75% or 90% of the highest flip angle. Preferably also, the excitation is such as to generate a flip angle of at least 90 o c£faaive at at least one location, preferably at at least two separate locations, in the selected range. As explained in detail later in relation to the three preferred aspects of the present invention, this feature can afford an effective way of putting the invention into practice. The probe or at least one of the probes may be in the form of a spiral coil, since this can transmit and receive excitation at a substantial distance from the probe or probes and over a substantial range of distance. In a first preferred aspect of the invention, the excitation is such as to generate a flip angle of greater than 90 o effectjve but less than 180° effective at that extreme of the selected sample distance range nearer the or said one such probe. By this feature, as explained in detail later, a particular region of the flip angle versus distance curve can be utilised where flip angle actually increases with distance away from the or said one such probe. This can give rise to an NQR testing technique for which the sensitivity is considerably more uniform over the selected range than would have been possible if conventional techniques had been employed. Preferably, the excitation is such that that extreme of the selected sample distance range nearer the or said one such probe is nearer the location for which flip angle is 180° effective than that for which flip angle is 90° effective . This can ensure that the beneficial region of the flip angle versus distance curve is well utilised. It will be understood that a flip angle of 90° effective is one which produces a maximum free induction decay at the receiver coil/probe; a flip angle of 180° e£fcctive is one which produces a zero free induction decay at the receiver coil/probe. For an I = 1 spin system, these values correspond to 119 0 aΛar) and 257° (aΛul) respectively. For the same reason, preferably the excitation is such that that extreme of the selected sample distance range farther from the or said one such probe is farther than the location for which, between a flip angle of 0 and 180° effective , the excitation would generate the most intense resonance response signal in the sample. Again, for the same reason, preferably the excitation is such that that flip angle at that extreme of the selected sample distance range farther from the or said one such probe is less than 90° e£fective , preferably less than 75, 60 or even 30°^,^. In a second preferred aspect of the invention, preferably the excitation applying means is such as to apply, preferably at the same excitation frequency, at least a first and second kind of excitation, the first kind of excitation being arranged to generate a flip angle which differs at each and every location within the selected range from that generated by the second kind of excitation at that particular location. This is a particularly advantageous way of achieving a sensitivity of testing which is considerably more uniform over the selected range than would have been possible if conventional techniques had been employed, in that the two (or more) kinds of excitation can be individually optimised for two or more sub-ranges within the selected range. Preferably, for the first kind of excitation the sign of the resonance response signal would change over the selected range of sample distance whereas for the second kind the response signal would have no zero crossings over the selected range. This has been found to afford a particularly advantageous combination of the two kinds of excitation, especially when, as is preferred, the location within the selected range for which the second kind of excitation generates a 90° effective flip angle is a location at which the second kind of excitation would generate a resonance response signal of opposing sign to the sign of the response signal which would be generated by the first kind of excitation at that location. Preferably, the plurality of kinds of excitation are interleaved with one another to form a repeating cycle of excitation, each of the kinds of excitation being applied during each such repeating cycle. In this manner the overall duration of the test need not be increased substantially by comparison with the case where only one kind of excitation were applied. If the plurality of kinds of excitation are interleaved, preferably in each such repeating cycle the excitation generating the lower or lowest flip angle precedes the excitation generating the higher or highest flip angle. This has been found to be a feature which is particularly important for ensuring a sufficient uniformity of sensitivity. Again for uniformity of sensitivity, preferably the detection means is adapted to acquire and process the resonance signals generated in response to the respective kinds of excitation separately. The apparatus of the present invention may be for determining a measure of the distance of the sample from the or said one such probe, in which case it preferably includes means for comparing the resonance signals generated in response to the respective kinds of excitation and determining from the comparison a measure of the distance of the sample from the or said one such probe. By this feature is provided a relatively simple but effective technique for determining the distance of a given remote sample. Such information could be particularly useful if the sample is in some way concealed. This important feature is also provided independently. Hence, according to a further aspect of the present invention there is provided apparatus for determining a measure of the distance of a sample from a probe, the sample containing a given species of quadrupolar nucleus, comprising: at least one probe; means for applying excitation to the probe or at least one of the probes to excite nuclear quadrupole resonance for a selected range of distance of the sample from the or one such probe, the excitation applying means being such as to apply at least a first and second kind of excitation, the first kind of excitation being arranged to generate a flip angle which differs at each and every location within the selected range from that generated by the second kind of excitation at that particular location; means for detecting the resonance response signals from the sample via the probe or at least one of the probes; and means for comparing the resonance signals generated in response to the respective kinds of excitation and determining from the comparison a measure of the distance of the sample from the or said one such probe. Preferably, the comparing and determining means is adapted additionally to deteπnine from the comparison a measure of the quantity of the given species of quadrupolar nucleus. Preferably again, the excitation applying means is adapted to apply at least two different pairs of first and second kinds of excitation, since use of at least two different pairs of excitation can improve the sensitivity of the tests, especially where more than one sample is present in the selected range. A third preferred aspect of the invention is now introduced. Adiabatically applied excitation is known in the separate field of Nuclear Magnetic Resonance imaging to have the ability to produce a constant flip angle over a given region, as described for example by M. Garwood and K. Ugurbil, NMR Basic Principles and Progress, Vol. 26, Springer-Verlag, 1992, pp. 109-147. However, adiabatically applied excitation would not have been expected to function in Nuclear Quadrupole Resonance, and especially would not have been expected to be able to produce a constant flip angle over a given region. In NQR, where the initial macroscopic magnetisations of the constituent crystals/crystallites are distributed in all directions (so that each experiences a different flip angle), it would have been supposed that such an omni-directional distribution would have prevented the proper functioning of adiabatic pulses. It has now been discovered pursuant to the present invention that, suφrisingly, adiabatically applied excitation can actually function satisfactorily in the field of NQR. Therefore, according to the third preferred aspect of the present invention, the excitation applying means is adapted to apply the excitation adiabatically, and to generate a substantially constant flip angle over the selected range. This can ensure a particularly uniform sensitivity of test over the selected range. This important feature of the invention is provided independently. Hence, according to the third preferred aspect of the present invention, there is provided apparatus for Nuclear Quadmpole Resonance testing a sample containing a given species of quadrupolar nucleus, comprising: means for applying excitation to the sample adiabatically to excite nuclear quadmpole resonance; and means for detecting the resonance response signal from the sample; the excitation being such as to generate a substantially constant flip angle over a given region. The given region may be defined in terms of a range of distances away from the probe of the NQR apparatus, as discussed above. The term "substantially constant" is meant to be interpreted in the context of the inaccuracies inherent in NQR testing, and may preferably connote a variation of less than 50%, preferably less than 20, 10 or 5% between the maximum value of flip angle and the minimum value. In order to ensure the most sensitive tests, it may be preferable that the excitation is arranged to produce a substantially constant flip angle for a variation of the field of at least a factor of one to five, preferably one to ten, twenty or even thirty. Preferably, the excitation is in the form of an adiabatic fast passage. Suitable excitation sequences involving an adiabatic fast passage are the Adiabatic Half Passage (AHP), the Adiabatic Full Passage (which involves two half-passages), and the variable-angle plane rotation pulse BIR-4, which includes a number of segments of AHP and AFP. Preferably, the amplitude or frequency or phase of the excitation is arranged to be modulated according to hyperbolic tan or inverse tan functions. This has been found a particularly advantageous way of putting the invention into effect. Preferably again, the excitation includes an adiabatic plane rotation pulse. An adiabatic plane rotation pulse would not have been expected to operate successfully in NQR, since it would have been believed to require the setting of particular values of flip angle in order to function successfully. This is not possible in NQR where the crystallites in a sample have random orientations. An adiabatic plane rotation pulse, it has now been found, can in NQR enable the generation of echo resonance response signals, due to the fact that it can conserve phase between one pulse and the next. Indeed, the invention preferably provides that the excitation is arranged to generate echo response signals. The present invention extends to a method of Nuclear Quadmpole Resonance testing a remote sample containing a given species of quadrupolar nucleus, comprising: providing at least one probe having a given maximum cross-sectional dimension; applying excitation to the probe or at least one of the probes to excite nuclear quadmpole resonance for a selected range of distance of the sample from the or one such probe, the selected range being at least one tenth of the given maximum dimension of the probe or said one such probe, a non-uniform field being produced over the selected range; and detecting the resonance response signal from the sample via the probe or at least one of the probes; the excitation being such as would generate non-zero resonance response signals at all distances within the selected range. The probe or the probes generally may be positioned (at any one time) on one side of the sample only. Preferably, the excitation is such that, for a given sample, over the selected range of sample distance the minimum resonance response signal detected would be no less than one twentieth, preferably no less than 1/10, 1/5, 1/4, 1/3 or 1/2, of the maximum resonance response signal. Preferably, the excitation is such as to generate a flip angle which is substantially constant over the selected range. Preferably, the excitation is pulsed excitation repeated at a pulse repetition time τ, the sample has a spin-lattice relaxation time T 1} and the value of τ/Ti is less than 5, preferably less than 2, 1, 0.5, 0.1 or even 0.05. By this feature, it has been found that the sensitivity of detection can be more nearly uniform over the selected range. Preferably, the excitation is such as to generate a flip angle of at least 90° effective at at least one location, preferably at at least two separate locations, in the selected range. In a first preferred aspect of the invention, the excitation is such as to generate a flip angle of greater than 90° effective but less than 180° eHeαive at that extreme of the selected sample distance range nearer the or said one such probe. This important feature is provided independently. Hence, according to the first preferred aspect of the present invention, there is provided a method of detecting the presence of a remote sample containing a given species of quadrupolar nucleus, comprising: providing at least one probe; applying excitation to the probe or at least one of the probes to excite nuclear quadmpole resonance for a selected range of distance of the sample from the or one such probe; and detecting the resonance response signal from the sample via the probe or at least one of the probes; the excitation being such as to generate a flip angle of greater than 90° effαaive but less than 180° effective at that extreme of the selected sample distance range nearer the or said one such probe. Preferably, the excitation is such that that extreme of the selected sample distance range nearer the or said one such probe is nearer the location for which flip angle is 180° e£fective than that for which flip angle is 90° effective . Preferably, the excitation is such that that extreme of the selected sample distance range farther from the or said one such probe is farther than the location for which, between a flip angle of 0 and 180° eβective , the excitation would generate the most intense resonance response signal in the sample. Preferably, the excitation is such that that flip angle at that extreme of the selected sample distance range farther from the or said one such probe is less than 90 ° e - f - ct i v - > preferably less than 75, 60 or even 30 o effeclive . In a second preferred aspect of the invention, at least a first and second kind of excitation are applied, preferably at the same excitation frequency, the first kind of excitation being arranged to generate a flip angle which differs at each and every location within the selected range from that generated by the second kind of excitation at that particular location. This important feature is provided independently. Hence, according to the second preferred aspect of the present invention, there is provided a method of Nuclear Quadmpole Resonance testing a remote sample containing a given species of quadrupolar nucleus, comprising: providing at least one probe; applying excitation to the probe or at least one of the probes to excite nuclear quadmpole resonance for a selected range of distance of the sample from the or one such probe; and detecting the resonance response signal from the sample via the probe or at least one of the probes; applying excitation to the probe or at least one of the probes to excite nuclear quadmpole resonance for a selected range of distance of the sample from the or one such probe; and detecting the resonance response signal from the sample via the probe or at least one of the probes; at least a first and second kind of excitation being applied, the first kind of excitation being arranged to generate a flip angle which differs at each and every location within the selected range from that generated by the second kind of excitation at that particular location. Preferably, for the first kind of excitation the sign of the resonance response signal would change over the selected range of sample distance whereas for the second kind the response signal would have no zero crossings over the selected range. Preferably, the location within the selected range for which the second kind of excitation generates a 90° e£fectjve flip angle is a location at which the second kind of excitation would generate a resonance response signal of opposing sign to the sign of the response signal which would be generated by the first kind of excitation at that location. Preferably the plurality of kinds of excitation are interleaved with one another to form a repeating cycle of excitation, each of the kinds of excitation being applied during each such repeating cycle. Preferably, in each such repeating cycle the excitation generating the lower or lowest flip angle precedes the excitation generating the higher or highest flip angle. Preferably, the resonance signals generated in response to the respective kinds of excitation are acquired and processed separately. The method may be a method of determining a measure of the distance of the sample from the or said one such probe, in which case preferably the resonance signals generated in response to the respective kinds of excitation are compared and a measure of the distance of the sample from the or said one such probe is determined from the comparison. This important feature is also provided independently. Hence, according to a further aspect of the present invention there is provided a method of determining a measure of the distance of a sample from a probe, the sample containing a given species of quadrupolar nucleus, comprising: providing at least one probe; applying excitation to the probe or at least one of the probes to excite nuclear quadmpole resonance for a selected range of distance of the sample from the or one such probe, at least a first and second kind of excitation being applied, the first kind of excitation generating a flip angle which differs at each and every location within the selected range from that generated by the second kind of excitation at that particular location; detecting the resonance response signals from the sample via the probe or at least one of the probes; and comparing the resonance signals generated in response to the respective kinds of excitation and determining from the comparison a measure of the distance of the sample from the or said one such probe. Preferably, additionally determined from the comparison is a measure of the quantity of the given species of quadrupolar nucleus. Preferably, at least two different pairs of first and second kinds of excitation are applied. In a third preferred aspect of the present invention, the excitation is applied adiabatically, and generates a substantially constant flip angle over the selected range. This important feature of the invention is provided independently. Hence, according to the third preferred aspect of the present invention, there is provided a method of Nuclear Quadmpole Resonance testing a sample containing a given species of quadmpolar nucleus, comprising: applying excitation to the sample adiabatically to excite nuclear quadmpole resonance; and detecting the resonance response signal from the sample; the excitation generating a substantially constant flip angle over a given region. Preferably, the excitation is arranged to generate a substantially constant flip angle for a variation of the field of at least a factor of one to five, preferably one to ten, twenty or thirty. The excitation may be in the form of an adiabatic fast passage. The amplitude or frequency or phase of the excitation may be modulated according to hyperbolic tan or inverse tan functions. The excitation may include an adiabatic plane rotation pulse. The excitation may generate echo response signals. As a general example of the efficacy of the present invention, the example of detecting the presence of explosives (in this particular case, the explosive RDX) in a given area is considered. An acceptable signal-to-noise ratio (defined as signal r.m.s noise voltage) is commonly considered to be 5:1. Experiments conducted pursuant to the present invention show that a signal-to-noise ratio of 5:1 can be attained from a moderately sized sample of RDX at a realistic distance from a probe (some tens of centimetres) in a few seconds. Similar acquisition times would apply alike to smaller samples at lesser depths and larger samples at greater depths. Indeed, within limits, simultaneous detection of explosives over a range of depths could be achieved within a similar acquisition time. It has been estimated that the technique of the present invention could be employed to clear an area of explosives in an acceptably short time. Preferred features of the present invention as well as examples of its operation and the theory underlying the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a series of plots of the signal received by a probe as sample distance, Z, is varied; Figure 2 is a block diagram of a preferred embodiment of testing apparatus according to the present invention; Figure 3 is a block diagram of a pulse shaping unit forming part of the testing apparatus; Figure 4 shows a series of plots of signal intensity versus flip angle for the various T/T J values shown in the inset, τ being the pulse repetition time, T, being the spin-lattice relaxation time; Figure 5 shows a series of plots of signal intensity versus distance, Z, for the various values of the ratio τ/T, shown in the inset; Figures 6(a) and 6(b) are spectra for samples of the explosive RDX at two different distances from an r.f. probe of the testing apparatus; Figure 7 is a spectmm similar to those of Figures 6 but for a different distance, Z; Figure 8 is a pulse sequence for use with an embodiment of the present invention; Figure 9 is a contour plot of signal intensity versus B j field and frequency offset for an adiabatic half passage pulse sequence; Figure 10 is a similar plot for a BIR-4 pulse sequence; Figure 11 is a similar plot for a simple rectangular pulse sequence; and Figure 12 is a plot of signal intensity versus time for a BIR-4 pulse sequence. The theory underlying the invention is first described, with reference to Figure 1. Known techniques for detecting the presence of a particular substance in a sample conventionally do so whilst the sample is on a conveyor belt or the like, and do so using a pair of Helmholtz r.f. coils, the sample being placed between, and close to, the coils. In this fashion, it is possible to maintain a relatively uniform r.f. field throughout the sample, and hence maintain relatively uniform sensitivity of detection. In the detection of remote samples such as buried or concealed objects, on the other hand, the distance from the sample to the probe would usually be significant, greater for instance than some measure of the size of the probe such as its maximum dimension (usually the outer diameter of the r.f. coil). Also, it is generally possible to place a probe or probes on one side of the sample only. In such circumstances, with the use of conventional excitation, the r.f. coil is not generally capable of generating a uniform r.f. field Bj within the total field of view; this can yield a sensitivity of detection which may be highly non-uniform. This is explained in detail in the following analysis. An excitation pulse at the resonance frequency ω Q of the relevant quadrupolar nuclei may be regarded as generating a nominal flip angle α which depends on the r.f. field B j and the pulse width t-,. Unless the context otherwise demands, the term "flip angle" as used herein connotes the actual rather than "effective" value. For example, for spin-1 systems, 119° is the flip angle which gives the maximum signal, thus corresponding to an effective 90° flip. For spin-1 nuclei α = 2 γ 5. t w (1) The signal S x generated in the coil by a polycrystalline sample depends on α according to the expression _ sinα -αcosα ,~ s x " ω <? • : (2) a which has a maximum at α=119° (0.66π radians), a zero at 257° (1.43π rad.) and the first negative maximum at 340° (1.90π). Equation 2 has been derived for the fully relaxed case where τ is greater than or equal to 5T τ is the pulse repetition time, the time between successive repeated pulses or pulse sequences. Tj is the spin-lattice relaxation time for the relevant nuclear species. Reference is now made to Figure 1. Figure 1 shows a series of plots of the signal received by a tme spiral r.f. coil as sample distance from the plane of the coil, Z, is varied; each individual plot is for a pulse whose width (duration) (termed "^(119°)") has been chosen to produce the optimum flip angle of 119° at a particular sample distance, Z. The t-,(119 0 ) pulse width is given alongside the sample distance in the inset to the figure. The signal intensity scale has been normalised to 1 at Z = 0. Each plot therefore represents the signal received by the coil as the sample is moved from its 119° position. In Figure 1 the full line, termed ("coil function") represents, for a tme spiral coil, the signal received by the coil from the response of a sample at Z to a t p (119°) pulse appropriate to the particular value of Z. Since a t p (119°) pulse has been chosen, the magnetisation induced in the sample will be the same for each Z value. By the reciprocity law for transmission and reception of magnetic fields for a coil, this plot, with suitable scaling, would be identical to a plot of the distribution of B j field over distance Z. The variation of B, with Z for a tme spiral coil is governed by Equation 4, given below. Aside from the coil function plot, the remaining plots are theoretical plots, being plots of the relevant Bessel function response curve. However, the particular pulse widths t p (119°) at given values of Z have been derived experimentally for the particular apparatus employed to test experimentally the theories underlying the present invention. In more detail, the plots of Figure 1 other than the "coil function" plot have been derived by a simultaneous solution of Equations 1, 2 and 4. Given the variation for a particular pulse of the B x field with Z from Equation 4, B can then be converted into flip angle using Equation 1 to yield the dependency of flip angle with Z; it will be apparent that for a particular pulse the flip angle generated in the sample reduces as Z is increased. The signal intensity generated at the sample for given Z can then be deduced given the variation of flip angle with Z from Equation 2. Finally, the signal received at the (receiver) coil can be obtained from the variation of the signal intensity at the sample with Z by a further application of Equation 4. Whilst the plots of Figure 1 are specific to the particular type of spiral coil employed, analogous plots could of course be derived for other coil configurations by use of a different relationship in Equation 4. It is be noted that the curves presented in Figure 1 have been derived for the situation where τy j is greater than or equal to 5. In other words, it is assumed that the response signal is fully relaxed such that there is no saturation of the signal. Suppose now that it is desired to detect buried samples up to maximum distances of, say, 25 to 30 cm from the plane of the coil. Suppose further that, to produce the maximum signal from a sample at a depth of (say) 30cm, the B t field and/or pulse width t,, is adjusted so as to give the optimum flip angle (for example 119°), at this distance. This would be achieved by setting the pulse width at 1400μs for a root mean square pulse power of 675W using the particular testing apparatus developed pursuant to the present invention. Figure 1 then shows the signal received by the r.f. probe as the sample distance Z is varied, using the pulse duration (1400μs) which is optimised for the 25 to 30cm depth (see the short-dashed line). There is indeed a maximum in the signal intensity between 25 and 30cm, but at intermediate distances the signal oscillates in magnitude and sign, and, at some distances, for example 13.5 and 17.5cm, the signal is actually zero, so that samples centred near these distances would generate very weak or even zero responses. There are in fact no less than seven null points in the region from Z = 10cm to the region of interest such as could give rise to a serious lack of sensitivity. Moreover, if there were two samples to be detected within the field of view, with one adjacent a positive peak and one adjacent a negative peak, their signals could have nearly or exactly opposite phase at resonance. Hence the signals could nearly or even exactly cancel, thus generating a false negative response from the testing apparatus. In the example described above, if one sample were at a distance of 26cm and the other at a distance 14cm, it can be seen from Figure 1 that signal cancellation could occur. A similar effect could also occur with a single elongate sample. Furthermore, if the temperatures of the two samples and therefore their resonance frequencies were different, for example due to thermal gradients in the ground or within the sample, partial or complete cancellation could occur at any distance. The problem could not be overcome by improving the received signal intensity at shorter distances, for instance by using a shorter pulse of, say, 140 μs duration at the same power. Figure 1 shows that this would produce acceptable signals at distances of between 8 to 14cm, but at (say) 30cm the received signal would be unacceptably weak. It will be understood that the strength of a signal relative to noise can be improved according to the number of signal accumulations. Hence by "unacceptably weak" is meant that an adequate signal-to-noise ratio cannot be achieved within a given acquisition time. Such a time is usually defined by physical restraints such as the speed of a conveyor belt transporting the sample past the testing apparatus, or the need to clear a given area of explosives in as short a time as possible (for instance 1 or 2 minutes per square metre). The present invention seeks to provide, in three preferred aspects, solutions to the above described problems concemed with a non-uniform B λ field and hence non- uniform sensitivity of detection. These aspects are described shortly; next however a preferred embodiment of testing apparatus for putting the invention into effect is described. The preferred embodiment is described with reference to Figure 2. The embodiment is particularly suited to the detection of objects or samples underground or beneath or behind concealing barriers, where it is important to be able to move the r.f. probe during the measurement of NQR responses. The apparatus is suitable for the application of simple excitation pulses (that is, those which are rectangular in the time domain). It is however particularly suited to the application of shaped adiabatic pulses or pulses which have a substantially rectangular shape in the frequency domain. This is by virtue of its capacity to modulate simultaneously the amplitude and phase/frequency of the pulse. The apparatus comprises in general terms a control computer 100, means 102 for applying one or more radio-frequency excitation pulses to the sample covering a selected excitation frequency range and for a given duration, means 104 for generating composite or shaped pulses, that is frequency/phase and/or amplitude modulated pulses for passing to the excitation applying means 102, means 106 for detecting the NQR response signal, and means 108 for generating an alarm signal in dependence upon whether the presence of a given substance is detected above a predetermined threshold. This may trigger an audio or visual alarm which alerts the operator to the presence of the sample to the detected. In more detail, the excitation pulse application means 102 includes a radio¬ frequency power amplifier 110 whose output is connected to an r.f. excitation and detection probe 112. The probe contains one or more spiral antenna coils 114 whose dimensions depend on the field of view required. In many applications, the probe, together with its attendant preamplifiers and isolating circuitry, may need to be portable. The remainder of the apparatus would not normally be portable, but would be connected to the probe by a length of cable. The probe may have one open unshielded end to apply to the sample under examination, and one closed end, perhaps shielded within a suitable screening enclosure. Sufficient clearance should be left between the coil and the box. . In certain circumstances, a Faraday shield or cage on the open side of the probe may be employed to reduce E-field effects; typically this would be a thin sheet of metal (Al, Cu), with a slit down the centre to reduce eddy current effects. The width of the slit would be optimised according to geometry and frequency of operation. E-field effects occur with piezoelectric samples such as sand. The probe also forms part of the detection means 106, which includes an r.f. preamplifier, receiver and detection circuitry 120. The frequency/phase and amplitude modulated pulses are generated by the shaped pulses generating means 104 under the control of the control computer 100. The means 104 for generating shaped pulses, (that is, frequency/phase, and/or amplitude modulated pulses) includes a pulse programmer 130, and a spectrometer 132, manufactured by SMIS, United Kingdom, for generating the r.f. carrier signal at a known carrier reference frequency and fixed amplitude, the signals being gated by the trigger signals from the pulse programmer 130, and a signal function generator 134, manufactured by Farnell, U.K (model SFG 25). The r.f. carrier passes to a pulse shaping unit 136. The function generator 134 is operated by a TTL logic signal from the pulse programmer 130. The pulse shaping unit 136 is now described in more detail with reference to Figure 3. A 0-90 degrees 5MHz two-way combiner 140, manufactured by Mini Circuits (U.K.) under model no. PSCQ-2-5, itself fed with the r.f. signal from the spectrometer 132, supplies two mixer circuits 142 and 144. Each such mixer circuit is also fed with a modulation signal via a respective Channel A and B from the function generator 134. The actual mixer circuits employed were constituted as Hatfield (U.K.) double- balanced mixer units (model 1754). However, other suitable circuits are also available such as a Burr-Brown model MPY 634 precision analog multiplier based radio¬ frequency mixer. The main criterion is that the modulation of the r.f. input by the function generator is linear over the largest possible range of voltage; an additional low power amplifier may be required to achieve this. If a precision analog multiplier is employed, it may be necessary to incorporate capacitors between the power supplies and earth and to incorporate trimming capacitors to remove low frequency voltage offsets from the mixing circuit which might otherwise cause pulse distortion. Two other potential sources of non-linearity must also be eliminated, namely distortion of the pulse due to the limited band width of the r.f. probe and distortion due to non¬ linearity in the r.f. power amplifier. The output from the mixer circuits 142 and 144 is passed via a combiner 146 to the amplifier 110. The combiner is a Hatfield (U.K.) component model no. DR102 or 3320. The present invention is preferably capable of detecting samples at considerable range from the plane of the coil or coils 114 and over a significant range of distance. A typical detection range is from 8 to 35cm. To achieve this detection range, in the preferred embodiment a planar Archimedean spiral coil is used, with an outside diameter and number of turns determined by the depth which it is intended to probe. Although in the preferred embodiment a planar spiral coil is employed, any form of surface coil would be appropriate. Because of the requirement of remote detection, coils of the solenoid, saddle or Helmholtz form are not generally appropriate. For samples which are likely to be found at distances not greater than, say, 30 to 50cm from thε coil plane, it has been discovered pursuant to the present invention that an Archimedean spiral of the form r = a — (θ in radians) (3) 2π with a (the distance between conductor centres) = 1.5cm has a satisfactory performance when used with a tubular conductor of diameter 0.8cm. A total of 7.75 turns can produce a spiral of outside diameter about 25cm and overall length 3.4m. Supposing that the wavelength, λ, of the r.f. carrier frequency is 5.2MHz, the overall length is λ/17. This is close to the rule-of-thumb limit of λ/20 usually believed necessary to avoid phase effects. A planar Archimedean spiral generates an on-axis r.f. field B, (- 1 -) given by the expression B,ω - - b lnfVθ where for the present embodiment θ varies between 1.5 π (θ j ) to 17 π (Q^) radians, b=a/2π, a=1.5cm and I is the r.f. current. The effective field of view at a given distance Z, meaning the dimensions of the circle in a plane parallel to that of the plane of the spiral for which the total B 1 field has fallen off by 25% relative to its value at the centre, depends on the distance apart, Z, of the two planes; it is 14cm for Z = 3.2cm and 20cm for Z = 28.2cm, which it has been found pursuant to the present invention give acceptable performance for detection of explosives in the range of, say, 8 to 35cm from the plane of the r.f. probe. The function of Equation 4 is shown plotted in Figure 1 as the "coil function" curve. The present invention has the aim, amongst other aims, of solving the problems encountered in the prior art and discussed previously. These problems are associated with the detection of samples which are located in a non-uniform r.f. field within the total field of view. Solutions to these problems are sought to be provided in three preferred aspects of the invention. These three aspects are now described in order of the increasing r.f. pulse power required to implement them. For all of the aspects, examples have been provided of the operation of the invention with reference to one particular resonance frequency (5.2 MHz) of the explosive RDX. The invention would, of course, also function satisfactorily at other resonance frequencies and for other NQR substances. (1) First Aspect In the first preferred aspect of the present invention, based on minimal r.f. power requirements, usually a pulse sequence involving a single r.f. pulse is employed, repeated in a time of the order of or less than Tj, the spin-lattice relaxation time of the nuclear species being excited. Strict control is exercised over the pulse width and power. By way of background to this aspect of the invention, it should be explained that, if the pulse repetition time, τ, is of the order of T j (typically less than 5T,), then the response will not be completely relaxed. (A sequence of repeated pulses for which τ is of the order of T j is herein termed a "T- limited cycle".) In this case, Equation 2 no longer holds tme. In fact, it has been shown by Vega (J. Chem. Phys. 61 1093 (1974)) that peak signal decreases in intensity and moves to lower flip angle as τ/T i decreases from a value of 5. The various equations presented by Vega have been solved numerically pursuant to the present mvention for the case of quadmpole resonance of an I = 1 powdered sample. The results are shown in Figure 4. This figure shows a series of plots of signal intensity versus flip angle for the x/T j values shown in the inset; signal intensity is normalised to a maximum signal intensity of unity obtained at a flip angle α of 119° for the τ/T^S case (assumed to be fully relaxed). It can be seen that peak signal decreases in intensity and moves to lower flip angle as τ/T j decreases from a value of 5. For all the plots, zero signal intensity is reached at α=257°. It should be noted that Figure 4 shows the signal intensity at the sample rather than at the (receiver) coil. It will be apparent from the tme spiral "coil function" plot of Figure 1 that, all other factors being equal, a signal received at a receiver coil from a more distant sample will be less than that received from a nearer sample. On the other hand, it will be apparent from Figure 4 when taken in combination with Equations 1 and 4 that over a range of flip angle (extending up to 257°), and hence over a range of distance, the signal generated at the sample for a given pulse can actually increase the more distant the sample is. The present invention in the first preferred aspect seeks to balance these two effects to produce a received signal intensity which is as invariant with distance as possible over a specified range of distance (which range is itself as large as possible). It has been found pursuant to the present invention that these effects can better be balanced the lower the value of the ratio τ/T„ in other words that for lower values of the ratio t/T j the observed signal shows a reduced variation with sample distance. In more detail concerning the first aspect of the invention, the (usually) single pulse in the pulse sequence is of such a width and root mean square power as to produce a zero in the signal response (corresponding to α=257°) at a distance slightly less than the minimum value at which the sample or samples are to be detected. This ensures that the desired range corresponds to the appropriate range of flip angle as determined from Figure 4 and that there are no null points or negative going signals proceeding outwardly from the minimum distance. Further, the value of the ratio x/T l is set relatively low (less than 5, preferably less than 2, 1 or even 0.5), so that the maximum signal response at the sample is achieved at a sufficiently low value of flip angle. At values of flip angle lower than the value corresponding to this maximum, signal intensity decreases rapidly. Hence the useable flip angle range (and thus the desirable range of distance from the plane of the coil) is bounded approximately on the one side by the flip angle corresponding to the maximum signal and on the other side by the zero crossing flip angle value of 257°. As can be seen from Figure 4 the useable flip angle range is greater at lower values of x/T x . Now suppose, for example, that the pulse width \ , is set to generate a 257° pulse at an on-axis distance of 2.05cm from the plane of the probe and a 119° pulse at 5.2cm, and that τ/Ti is set at 0.5. At distances greater than 2.05cm, the flip angle diminishes and the free induction decay signal intensity generated in a probe notionally located at the sample would increase. On the other hand, at the actual, distant r.f. probe, all other factors being equal the received signal would decrease according to Equation 2. The net effect, it has been discovered, is a partial or complete cancellation of the two conflicting effects over a limited range, such that over a distance of, say, 2.5 to 13cm the received signal varies very little. Reference is now made to Figure 5 to illustrate this point. Figure 5 shows the variation of signal intensity (as received by the probe) with distance Z from the plane of the probe along the coil axis for various values of the ratio τ/T^ As stated above the r.f. pulse is selected to give a 257° flip angle at Z = 2.05cm and a 119° flip angle at Z = 5.2cm. In Figure 5, thε various curves are theoretical predictions, whilst the data points are experimentally derived values, as shown in the inset to the figure. The theoretical plots were derived from the plots of Figure 4 in a fashion analogous to that described for the derivation of the plots of Figure 1, except that Equation 2 was replaced by the Vega equations as discussed above (or their graphical solution as shown in Figure 4). The experimental results were derived using a 38g sample of "PE-4" (based on the explosive RDX) at two different values of the ratio τ/T namely 4 and 0.5, corresponding to the triangular and circular symbols respectively. In Figure 5, the theoretical plots were scaled to the peak of the fully relaxed experimental data (T T J = 4). It can be seen that the experimental results agree well with the theoretical predictions. It is noticeable in Figure 5 that for the maximum signal intensity does not occur at the α=119° position of Z, but at a somewhat nearer Z value. This is on account of the attenuation at greater distances of the signal from the sample as received by the probe, in accordance with Equation 4. It can be deduced from the experimental data in Figure 5 that, between Z = 3 and Z = 13cm, the τ/T 1 = 4 response varies by a factor of 10, whilst the τ/T j = 0.5 response varies by a factor of only 2.4. More generally, it can be seen from the various theoretical curves that, for lower values of τ/T„ for example, 0.5 or less, the observed signal obtained by Fourier transformation of the f.i.d. does show a considerably reduced variation with distance above 3cm than pertains for higher values of the ratio. Hence it is preferable to employ these lower values of the ratio x/T However, for certain substances, for example, the explosive RDX, a practical lower limit on the value of the ratio x/Ti can be imposed by the free induction decay time, T 2 *. For the free induction decay to be detected properly, it is usually thought that the pulse repetition time x should not be much less than 5T 2 *. For RDX at room temperature T 2 * is roughly 1.5ms whilst Tj is roughly 15ms. Hence the lowest satisfactory value of x for RDX at room temperature may be 7.5ms, corresponding to a value of τ/Tj of 0.5. In the example described above, typically a positive signal of substantial signal strength could be obtained over a range of 2.5 to 13cm. As another example, for a pulse set to produce a flip angle of 119° at 14cm and an appropriate value for the ratio τ/Ti, a positive detectible signal could be derived over a range of roughly 10 to 20cm with the received signal intensity varying by a factor of about five. It is estimated that r.m.s. pulse powers of less than 100 W would be required to achieve a signal-to-noise ratio of 5:1 for a sample of RDX of about 40 gm between 9 and 16cm from the plane of the probe in a time of less than 10 s. For the same power, longer times would be required to detect a 1kg sample at a distance of 30cm. Whilst the reduction in signal intensity as x/Tγ is reduced can be partly offset by the use of more pulse cycles within a given acquisition time, a Tj -limited cycle as described in the first preferred aspect of the present invention may be relatively slow in operation. However, it does require only low r.f. powers. It would be particularly appropriate in circumstances where it was required to detect the presence of remote objects over a relatively small range of distance. (2) Second Aspect In a second prefeπed aspect of the present invention, two or more pulses (or pulse sequences) are used at the same excitation carrier frequency but are of differing powers and/or have differing pulse widths. The powers and/or widths are selected to generate the optimum flip angle of 119° at two or more different distances from the plane of the r.f. probe, a particular power/width coπesponding to a particular distance. The f.i.d. responses from each of the pulses are acquired and processed independently, and adjusted to produce the same phase after Fourier transformation. The resulting absoφtion spectra are then added, or preferably the area under each resulting absoφtion spectmm is added. A particularly fast sequence uses just two differing pulse types and is termed herein a two pulse sequence. The principle of the technique of the second prefeπed aspect of the present invention is described with reference first to Figure 1 in relation to an exemplary two pulse sequence, although of course the description could equally be applied to sequences with a greater number of differing pulse types. The pulse sequences could be T j -limited cycles as described in relation to the first prefeπed aspect of the invention discussed above. However, it may be desirable not to employ fast pulsing sequences; the pulse separation, τ, may in this embodiment preferably be greater than 3 or even 5 times T j , so that a fully relaxed state is reached before each new pulse. In the exemplary sequence, pulse 1 in the two pulse sequence is selected for the apparatus developed pursuant to the present invention to have a width t^ of between 100 and 140 μs and an r.m.s. power of about 675W; it can be deduced from Figure 1 that such a pulse, given sufficient quantities of the substance to be detected, could generate satisfactory signal responses at distances of between, say, 8 and 15cm from the plane of the r.f. probe (which in the prefeπed embodiment is the spiral coil to which reference has already been made). Pulse 2 has a width of between 200 and 450 μs at the same r.m.s. power, or altematively the same width as pulse 1 but with the r.f. power adjusted to increase the B j field proportionally. It can be deduced from Figure 1 that pulse 2 would generate satisfactory responses at distances of between 15 and 30 or 35cm from the plane of the r.f. probe. Hence the two differing pulses in combination (if suitably arranged) can generate satisfactory responses at distances of between, say, 8 and 35cm from the plane of the r.f. probe, without there being any null points or negative going signals in this range. The exemplary two-pulse sequence is therefore particularly suited to the detection of buried or concealed explosives. Experimental spectra obtained using the above-described exemplary two-pulse sequence to detect differing quantities of the explosive RDX at different distances from the plane of the r.f. probe are shown in Figures 6 and 7. In Figures 6(a) and (b) are shown the f.i.d.'s of the 14 N response from the 5.19MHz line of RDX with a lOO s pulse of r.m.s. power 675W. For Figure 6(a) the RDX was in a quantity of a few grammes at a distance of 9cm from the plane of the r.f. probe, whilst for Figure 6(b) the coπesponding values were a four tens of grammes at a distance of 13.65cm. An acceptable signal-to-noise ratio was achieved for both cases. Equally, Figure 7 shows that an acceptable signal-to-noise ratio was achieved with RDX in a substantially larger quantity at a distance of 30cm, with a 200μs pulse of the same r.m.s. power and frequency. 200 ts is the value of t p (119°) for 13.65cm. In fact it has been found pursuant to the present invention that with the two- pulse sequence described above (specifically one in which one pulse has a width of lOOμs and the other a width of 450 s), good results can be obtained over a range of Z from 8 to 35cm. It will of course be understood that other pulse combinations than the exemplary two-pulse sequence described above are possible according to the nature of the particular problem to be solved. The pulse widths can be adjusted to match the requirements of the particular search, that is, the quantities of material to be detected and the distances at which they are likely to be found. It may be necessary to use more than two differing pulses if the samples to be detected are located over a wide range of distance. One possible combination of pulses employs a pulse which is shaped so as to have a substantially rectangular shape in the frequency domain at least as the longer or longest of the pulses. Such a pulse would have a better bandwidth performance than a simple rectangular pulse, which would be of value at greater distances. For simplicity one or more simple rectangular pulses could be employed for the other pulses. It is one of the important advantages of the second prefeπed aspect of the invention that the use of two or more pulse sequences of differing widths does not necessarily increase the acquisition time proportionally. This is achieved by interleaving of the two or more pulse sequences. If the pulse sequences are so interleaved, it is important that they are applied in the conect order, if more than one sample is likely to lie in the field of view. - In a two pulse sequence, for example, because T 2 *, the f.i.d. time, is invariably much shorter than T„ the spin-lattice relaxation time, the shorter pulse should precede the longer. This point is illustrated first with reference to Figure 8, which shows a prefeπed interleaved pulse sequence for the puφoses of this aspect of the invention. The first pulse is a short pulse of width t-, following which the first f.i.d. S j is captured and digitised. There is then a time interval of about t- = 2T 2 *, during which the nuclei excited by this pulse, which are located predominantly closer to the probe, dephase. x i may suitably be as low as roughly T 2 * to as high as roughly 3 or 5T 2 *. The dephasing may be partial or complete (so that in the latter case no transverse magnetisation exists), dependent upon the distance of the sample from the plane of the r.f. probe and hence the flip angle (which may be the maximum 119° or some lesser value). Also, since T 2 * « T v the longitudinal magnetisation will not have recovered to any significant extent. The second, longer pulse, of width ^ (or the r.f. power may be varied, and the width kept constant) is now applied and its f.i.d. S 2 captured and digitised. The signal from this pulse would come predominantly from a sample or samples further away from the plane of the r.f. probe. The two-pulse sequence is repeated after an interval after the second pulse x 2 which is of the order of T j to 2T, (say, between 0.5 and 3T j ) at the temperature of the sample or samples. The pulse repetition rate, x, is thus x + x 2 . After N such repetitions, the summed signals S j and S 2 are separately acquired, processed and examined by the computer, the separately Fourier transformed data finally being added if appropriate. Separate processing is important to avoid the cancellation of one positive signal by a negative signal. If two or more samples are present at different depths, as mentioned above it is important that the former shorter pulse is applied before the latter longer pulse. In this way there need be no risk that the (say) two samples interfere with each other so that only a negligible response signal is produced. This is now explained further with reference to a 140/450 μs combination of pulses (see Figure 1). The case is first considered in which the nearer sample is in the region where the latter, longer 450 μs pulse would produce a negative signal at the probe (between Z=10 and 13cm), whilst the farther sample is at a significantly greater depth. The former, shorter 140μs pulse would generate nearly a 119° flip angle at the nearer sample, so that no significant longitudinal magnetisation would remain to be excited by the latter pulse. Hence the nearer sample would generate no response to the latter pulse, and there would thus be no interference between the nearer and farther sample, the farther sample being detected by the latter pulse. The case is next considered where the nearer sample is in the positive response region of the latter, longer pulse (Z>13cm, beyond the zero crossing point of the latter pulse), but is either within the excitation range of the shorter pulse (say, Z<20cm) or nearer than the maximum for the longer pulse (say, Z<18cm). The farther sample is again at a significantly greater depth. If the nearer sample were within the excitation range of the former, shorter pulse at all, the former pulse would generate less than a 119° flip, and some longitudinal magnetisation would therefore remain to be excited by the latter pulse. However, since signals received by the probe from the two samples would both be positive, the resultant effect would be additive. Again, there is therefore no interference, either partial or complete, between the responses generated by the two pulses if two or more samples are present. Considering now finally the case where the nearer sample is either outside the excitation range of the shorter pulse (say, Z>20cm) or farther than the maximum for the longer pulse (say, Z>18cm), it is clear that the response from the latter, longer pulse would predominate in respect of both samples, so that there would again be satisfactory detection of both samples. Suppose, on the other hand, contrary to the prefeπed manner of putting the invention into practice, that the two pulses were applied in the reverse order, with the longer coming first, and again that there werε two or more samplεs within thε field of view. In the first place, as already described in relation to Figure 1, the response signal from the nεarer sample could partially or completely cancel that from the farther. The signal S 2 (in respect of the longer but now former pulse) could therεforε bε wεak or even undetεctablε, despite the presence of material to be detεcted in both the near and the far region. Furthεrmore, by the time of arrival of the second, shorter pulse, after an interval of about 2T 2 *, nuclei in the near region already perturbεd by thε first pulse could have dephasεd, so that S j (in rεspεct of thε shorter but now latter pulse) could also be weak or evεn undetεctablε, dεspitε the presence of material to be detεctεd in the near region. Hεncc whεre two (or more) samplεs are presεnt at diffεrεnt dεpths, a situation might arise in which no signal can be detεctεd at εithεr dεpth with εithεr pulsε. In short, thεn, applying thε two pulsεs in rεvεrse order leads to problems because of the possibility that the longer pulse may give rise, at inteπnεdiatε dεpths, to signals which arε nεgative and which hence may cancel signals from the shorter pulse. It will be apparent from the foregoing discussion that it is important not only that the shorter and longer pulses are applied in the conect order, but also that they are adjustεd to create the maximum 119° flip angle at the conect relative distances. Specifically, it is prefεrable that the 119° flip angle position of the shorter pulse falls within the first range of distance, approaching from infinity, for which the longεr pulse generatεs an opposite signed signal intensity. If the 119° position of the shortεr pulsε wεrε εithεr furthεr away from or nεarer to the plane of the probe, this pulsε would bε incapable of making the necessary compensation for thε nεgativε going rangε of thε longer pulse. If more than two different pulse types (say, threε or four) wεrε usεd, εach pulsε bεing optimised for a different depth, then the sensitivity of detεction might bε more uniform ovεr depth. On the other hand, thε cancεllation εffεcts refened to previously might be more significant, and it may bε particularly important to ensure that the shortεst pulse is applied first in the sεquεncε, and thε longεst last, but with pulsεs optimisεd for adjacent distances bεing sεparatεd from εach othεr. If, say, at lεast four pulses were used, then it might be advantageous to apply the pulses fo the order first, third, second, fourth (first denoting the shortest) providεd that thε first and third, and second and fourth, pulses wεrε not pronε to mutual cancεllation. As an εxamplε of thε application of an interleavεd two pulsε sεquεncε, thε εxplosivε RDX at 295K is considεred, whose 14 N signal at 5.19 MHz has T 2 * = 1.4ms and T j = 12ms. Without interlεaving of thε two pulsεs t. and t^ εach sεquεncε would occupy 1.5s for 100 accumulations, setting the pulse repεtitioπ timε to 15ms. Hεncε thε total duration of both pulsε sεquences would be 3s (neglεcting thε pulse widths). On the other hand, with interlεaving of the pulses, thε total time with τ, = 3ms and τ 2 = 15ms is 1.8s. It is a further feature of a two (or more) pulse sequence as previously dεscribεd that it can bε εmployεd to providε an εstimatε of thε distance of a sample from thε planε of thε r.f. probε and/or of thε approximatε quantity of thε material of intεrεst. This is a consεquεnce of the fact that at constant pulse r.f. power, and hence constant r.f. cunent to the probe, both the Bj field and the r.f. signal induced in the probe depend only on the distance of the sample from the plane of the probe. This conclusion is strictly tme only for on-axis locations, but the modifications needed to allow for samples off-axis are not large and are well-known from publications such as that of CB. Bosch and J.J.H. Ackεrman, "NMR Basic Principles and Progress", Vol. 27, Springer- Vεrlag, 1992, pp. 3-44 (see in particular Figures 3a to 3d on pages 8 and 9). With this feature of the presεnt invεntion, it is important that thε signal is captured aftεr εach pulse and processed in an identical fashion. For instance, the same receiver, the same computer, the same phase gain, and so on, should be used, failing which the accuracy of the distance or quantity εstimation could be prejudiced. In putting this feature into practice, two different sequεnces arε used, both sequεncεs being fully relaxεd (that is, with τ≥ 3 or 5 T,). One such sequence contains pulses of width t j and the other pulsεs of width t 2Ϊ εach pulsε has the same r.m.s. power. Each sequεncε on its own providεs strong signals from diffεrεnt spatially localised regions. For examplε, as is apparent from Figure 1, a pulse of width I40μs produces strong signals from samples at distances of betwεen 8 and 14cm, whilst the conesponding distance range for a pulsε of width 450μs is between 14 and 35cm. Distance to the sample is determined by comparing the accumulated signal after (say) 100 pulses of width t v which is tεrmεd S with that aftεr thε samε numbεr of pulsεs of width t 2 , say S 2 . Thε ratio S^ is a sεnsitivε function of distance over a considerablε rangε. Rεfεning to Figure 1, it can bε sεεn that S j =S 2 at a distance (z) of 15.6cm from the plane of the r.f. probe. At other distances, its value can be both positivε or negative, taking into account the phases of the accumulated signals. The following tablε providεs valuεs of thε ratio Sj/S 2 for various valuεs of Z. Z(cm) 9 12 15 18 21 s,/s 2 3.6 -2.7 1.4 0.55 0.38 The particular pair of pulse types dεscribed above provides reasonablε spatial localization ovεr distancεs from 8 to about 20cm from thε planε of thε r.f. probε; othεr combinations will havε diffεrεnt rangεs of distance. Oncε thε dεpth of thε sample is known, the stronger signal can be used to provide an estimatε of quantity. This can, for example, be derived from the theoretical plots in Figure 1 and a prior calibration of the signal to be expected from the testing apparatus from a known quantity of the material to be detected at a known distance from the r.f. probe. It will bε undεrstood that thε accuracy of the quantity estimatε will bε affεctεd by thε size and shape of the sample, the uncertainty being greatεr thε largεr the extension of the sample along the B, gradient. If there is more than one sample in the field of view, a minimum of two different pairs of pulsε sεquεncεs arε nεeded to providε an εstimatε of thεir distancεs and quantitiεs. A computer programme or the like, which is based on the graphical data in Figure 1, determines from two different values of the signal ratios (S j /S^ and (S J /S^ J from each pair of sequεncεs which combination of quantitiεs and distancεs bεst fits thε εxpεrimεntal ratios. Howεvεr, whεrε thεrε arε two (or more) samples there will be a coπespondingly greatεr uncertainty and poorer resolution than in the case of a single sample. If it is required to pεrform thε spatial localization in thε minimum timε, thε pair of pulsε sεquεncεs may be interleaved, under the same conditions as have previously been discussed in relation to the second prefεπεd aspεct of the presεnt invεntion. Howεvεr, it is now nεcessary to allow for the effεct of the different flip angles induced by pulse 1 on thε signal gεnεratεd by pulsε 2. For example, if pulse 1 has a width of 140μs, Figure 1 shows that at distances greatεr than (say) 13cm, there will still be a finite longitudinal magnetisation presεnt in any samplε located in this region, which will affect the response following pulse 2, as has already bεεn discussed. If the spin-lattice relaxation timε T, of thε quadrupolar nuclei is known, this effεct can be allowed for by means of the usual relaxation equations, and the ratio S j /S. 2 can still provide an estimate of distance, but with a greatεr uncertainty than when the two pulse sequεncεs are conducted separatεly undεr fully-relaxed conditions. (3) Third Aspect As discussed above, for typical "onε-sided" coils, the Bj field decrεasεs shaφly as distancε away from thε coil incrεasεs. With thε typεs of εxcitation dεscribed so far, this dεcrεasε in B j causes a coπesponding shaφ dεcrεase in the flip- anglε α, as dεfinεd by Equation (1) abovε. In a third prεfεπεd aspεct of thε present invention, so-callεd "adiabatic fast passagε" pulses are employεd which arε shapεd so as to providε (in a givεn rεgion) a substantially constant flip angle over a wide rangε of B v As a consεquεnce, all nuclei within thε fiεld of viεw of the r.f. probe (definεd here as the region over which the adiabatic criterion is satisfied) are perturbed to the same extent, and the signals received by the r.f. probe depεnd only on their distance from the probe. The term "adiabatic fast passage" connotes a pulse for which the rate of change of the magnetic field is sufficiently fast to avoid T, effects, but sufficiently slow to enable the crystallite magnetisation to follow the effectivε field, although this definition is not intended to limit the scope of the presεnt invεntion. As discussεd earlier, adiabatic pulses are known in thε field of Nuclear Magnetic Resonancε imaging, but would not havε been εxpεctεd to work in thε field of Nuclear Quadmpole Resonance, particularly Nuclear Quadmpole Resonancε testing of polycrystalline samples. Indeed, it is not fully understood why they do. Notwithstanding that adiabatic pulses have beεn found pursuant to thε present invεntion to function succεssfully in NQR, the adiabatic pulses which have been tested exhibit poorer off-resonant pεrformancε than in NMR. Howεvεr, as mentioned later, it may be possible to make significant improvemεnts to thε pεrformancε of the basic adiabatic pulses presεntεd herein by theoretical optimisation considering the spin transformations occurring in polycrystalline samples, so that thε dεficit in pεrformancε may not bε particularly significant. In the context of the present invention, an important feature of adiabatic pulsεs is that thεy can bε tailored to produce the optimum flip angle of 119° over a range of depths, and can hencε provide a genεral method of obtaining optimum sensitivity with a complεtε absεncε of interferencε bεtwεεn signals from samplεs at diffεrεnt dεpths, since there is no variation of flip angle with depth. Two possible disadvantages of adiabatic pulses in the prεsεnt context are firstly that to use them to best effεct requires higher r.f. powεrs than for comparable rectangular pulses, and sεcondly that thεy havε a reduced bandwidth performance, and are therefore more sensitivε to εffεcts such as tεmpεraturε which may shift thε NQR frεquεncy. In ordεr to dεfinε thε adiabatic conditions, refεrence is made to a rotating coordinate frame at a frequεncy ω Q , in which thε starting frεquεncy at thε bεginning of the adiabatic pulse is off-set by Δω m „; the effectivε frequency in the rotating frame is then coπesponding to an effectivε fiεld B £f Thε εxcitation carrier frequency is then swept from the starting frequency (at the frequεncy offsεt Δω m „) rapidly to rεsonancε, as B, is simultanεously varied. B x is maintained very much grεatεr than B loc , thε local field in the solid. For the whole procεss to be adiabatic, the magnetisation must always follow B le£f and the rate must be fast compared to T, and T 2 *. At resonancε, Δω = 0 and, for NMR, thε magnεtisation would bε lεft pointing along B, and would bε rotatεd through 90° (εffεctivε) inespective of the initial value of B,. The effεct is thε samε as that of a 90° (effectivε) pulsε, but thε flip anglε is indεpεndεnt of the Bj r.f. field. Such a process is tεrmεd Adiabatic Half Passage. If the frequency sweεp is continuεd to a frεquεncy offsεt of -Δω maJt the magnetisation is inverted and the equivalent of a 180° (effεctivε) pulsε is obtainεd. This lattεr procεss is tεrmed Adiabatic Full Passage. It will be undεrstood that whilst thε flip anglε is independεnt of B-, rεcεivεd signal strength will vary with distance. With Adiabatic Half Passage (AHP), two major requirεmεnts must bε fulfilled. Firstly, as for all adiabatic pulses, the adiabatic condition |(dα/dt)|«l |γ3 leff | must be met throughout the duration of the pulse. Secondly, B, eff must rotate through π/2 radians (or through π radians for adiabatic full passage). Within these constraints adiabatic passage may be performεd using any suitable pair of amplitude and phase/frεquεncy modulation functions. The ability of the particular modulation functions chosεn to satisfy thε adiabatic condition governs the magnitude of the Bj range and the absolute B j range ovεr which thε pulsε will pεrform satisfactorily. Hence, all adiabatic pulses have upper and lowεr pεak r.f. powεr limits bεtween which thεy will opεrate adequatεly at a given frequεncy swεεp and pulsε duration. It has been discoverεd pursuant to thε present invention that the adiabatic conditions can be achievεd for thε majority of crystallitεs in a polycrystalline sample by the use of hyperbolic tan and inverse tan modulation functions for amplitude and phase-modulated half-passage: ω ι( > = ω iιx-« taήh t- (7) P φ t ) = (8) wherε ω lπux and Δω mtI arε modulation amplitudεs, ω,(t) is thε r.f. amplitudε at ti ε t, φ(t) is thε phasε at timε t (in radians), t_, is thε pulsε duration, and ζ, K arε constants, sεt in thε prεfεπεd εmbodiment to 10 and 1.52 radians respεctivεly. Aω mα was sεt to 2.83 x 10 5 radians. Although thε tanh and 1/tan modulation functions havε bεεn dεscribεd abovε, it may be advantageous in certain circumstances to derivε tailored optimised modulation functions to operate over the required B j rangε. A numεrical optimisation procedure could be employed to reduce the unknown variables. The BIR-4 pulse sequεncε is now dεscribed. BIR-4 is prefεnεd to AHP (or AFP), bεcausε, unlike AHP, it is capable of performing plane or spherical rotations of the magnεtisation. π or π/2 radian planε rotation pulses would usually be considered necεssary if it is desired to genεratε εchoes. BIR-4 has also beεn found to providε good compensation for B x inhomogenεity and rεsonancε offsεt, marginally outpεrforming adiabatic half passage at comparable powers. As already stated, BIR-4 is a type of adiabatic plane rotation pulse. Such pulsεs may in general be regardεd as being composed of a number of segments each of which can be adiabatic half-passage, adiabatic half passage time rεversed, or adiabatic full passage. With the BIR-4 type of pulse sequencε employed in the prefened embodiment, these may be regardεd as consisting of four sεgmεnts, εach consisting of a half-passagε or timε-rεvεrsed half passage. It has bεεn found that adiabatic conditions can bε achieved using the following hypεrbolic tan and invεrsε tan modulation functions for the four segmεnts: Sεgmεnt 1 (0 < t < 0.25 t ): Sεgmεnt 2 (0.25 t p < t < 0.5 t p ): ω x (t) = ω.(0.5t p - t) (11) φ(t) = φ(0.5t p -t) Δφ. (12) Sεgmεnt 3 (0.5 t p < t <0.75 t„): φ(t) = φ(t - 0.5tp + Δφ. (14) and Sεgmεnt 4 (0.75 t p < t < t p ): whεrε Δφ j and Δφ 2 arε discontinuous phase jumps set by the required final flip anglε, α, of thε BIR-4 pulsε: Δφ, = π + α/2 (17) Δφ 2 = -(π + α/2) (18) The efficacy of both AHP and BIR-4 pulses has been proven experimentally pursuant to the presεnt invεntion. Thε experimental conditions were as follows. An RDX sample was contained within a solenoidal coil of Q-factor 70 and volumε 130ml; pulsεs of 1ms width wεrε usεd, since this was assumed to be thε maximum appropriate pulse width for the detection of thε 5.19 MHz linε of RDX (at around 25°C) whεrε thε frεε induction dεcay constant T 2 * is approximatεly 1.4ms. In the particular expεrimεnts, Bj was varied by changing the power delivered to the coil. For AHP, Figure 9 shows a contour map of the relative (normalised) signal intensity received at thε r.f. probε following an AHP pulsε as a function of B j (vertical scale, peak value). The intensity scale is shown at the top of the figure, and the horizontal scale is the frequency off-set Δf in kHz of the excitation frequεncy from the resonance frequεncy of 5.19 MHz. Also shown is thε distance Z(cm) from the plane of the r.f. probe along the coil axis conesponding to the Bj scale for the r.f. powεr usεd. Δω-,.-, was 45,000 rad s "1 , εquivalεnt to 7kHz for a 1ms pulsε. ζ = 10 and 1.52 radians. In thε experimεnt, AHP was found to function well over a Bj range of 0.09 to at least 2.2mT, a minimum rangε of 2.1mT, conesponding to sample distancεs of 20 to 2 cm from thε spiral r.f. probε coil. Bandwidth was naπow at lowεr Bj valuεs, but incrεasεd considerably at higher B x . At least 80% of the maximum obtainable signal was obtained over a rangε of 0.30 to 2.17mT ovεr a bandwidth of 0.6kHz or more. Furthεr εxperiments conducted with a value of Δω m „ = 283,000 rad s" 1 (equivalent to a frequency sweεp of 45kHz for a 1 ms pulsε) εxhibitεd a somεwhat broadεr bandwidth at low valuεs of B l5 but this was at the expensε of somεwhat worsε performance at higher B-. The value of Δω mM = 45,000 rad s -1 (as used in the derivation of Figure 9) may be best suited for the detεction of εxplosives since the high signal region extεnds over a broad range of B j field and has a large bandwidth of over 3kHz at high fields, coπesponding to a tempεraturε variation of ±3°C in RDX. This valuε of Δω,. ^ impliεs a frequency sweεp of approximately 7kHz, and could thus allow a relatively high probe Q with resulting lower detector power requiremεnts and highεr sεnsitivity. In furthεr εxpεriments, the effect of changing the constants ζ and was investigated. It was found that within limits thesε constants do not influence the performance of the pulse. For BIR-4, Figure 10 shows the comparable performance of a BIR-4 pulse with Δω.^ = 283,000 rad s "1 , equivalent to 44 kHz for a 1 ms pulse, and Δφ. = Δφ 2 = 225°, coπesponding to a flip angle of α=90° effcctive . ζ = 10 and = 1.52 rads. BIR-4 was found to function well over a Bj range of 0.20 to at least 2.4 mT, coπesponding to distances of 0 to 10 cm at a frequεncy offsεt of around 0.5kHz. Thε bandwidth of thε 80% signal rεgion at this offsεt is approximately IkHz. However, there is an additional high signal region, at an offset of around -2kHz. This region could be used to increasε signal acquisition rate, but is of restricted bandwidth below a Bj field of about 0.7mT. Further εxpεrimεnts εstablishεd that the value of Δω m „ = 283,000 rad s" 1 was optimal. In yet further expεrimεnts, the effεct of changing thε constants ζ, K, Δφ j and Δφ 2 was invεstigatεd. It was found that within limits thε pεrformancε of thε pulsε is insεnsitive to the value of ζ, but somewhat sensitive to thε valuε of K. K = 1.52 was found to bε an optimal valuε. Sεnsitivity was also exhibited to the constants Δφ j and Δφj j , but Δφ j = Δφ 2 = 225° was found to be optimum. The sεnsitivity to constants Δφ j and Δφ 2 possibly suggests that BIR-4 acts as a variable flip angle pulse in NQR. This sensitivity might possibly be exploited by stepping the values of Δφj and Δφ 2 during the pulse or between pulses. For thε sakε of comparison, Figure 11 shows the comparable pεrformancε of a simplε rectangular pulsε of 1 ms duration. Thε εxpεctεd sidε-band bεhaviour is exhibited. For the main signal, the pulse givεs 80% or more of maximum magnεtisation signal at pεak B x fields between 0.054mT and 0.12mT, a range of 0.064mT, with a bandwidth of roughly 0.6kHz. By comparison with the simple rectangular pulsε, AHP improvεs thε insεnsitivity to the peak Bj field for the detεction of RDX by morε than 20-fold, with improved excitation bandwidth. The AHP pulsε is insensitive over a roughly 20-fold variation in the B, field. Thesε benefits are obtained at the expεnsε of an increase in minimum B. field for spin excitation by a factor of at least 2, and hεncε an increase in the r.f. power by a factor of at least 4. The pεrformancε of thε BIR-4 pulse is similar to that of the AHP pulse, although BIR-4 is thought to be marginally bettεr in that it has a better response to frequency off-set than AHP at comparable powεr lεvels. Another important feature of the third, adiabatic pulses, aspect of the mvention concerns the use of echoes. Signal to noise ratio for a given acquisition time is improved by the use of a BIR-4 sequεnce containing two or more suitable pulses aπanged to genεratε echo response signals. In various expεrimεnts with RDX, two 1 ms BIR-4 for adiabatic half range (with an effεctive flip angle of 90°) pulses, specified by ω aa = 283,000 rad s~\ ζ=10, =1.52, and φ : = - φ 2 = 225° for Adiabatic Half Passage (with an effectivε flip anglε of 90°) and φ. = - φ 2 = 270° for Adiabatic Full Passage (with an effectivε flip anglε of 180°), conesponding to a JI/2 - τ - π - echo sequencε, with a pεak B j fiεld of 0.9 mT and a valuε of τ of 6.5ms, gavε thε εcho signal shown in Figure 12 after 50 accumulations. Echoes werε also genεratεd using a Pulsεd Spin Locking εcho sεquence of the form (π/2 - x- ) n - echo, with φj = φ 2 = 225°. In similar experiments carried out using half passage pulses, no echo response signals were genεrated. It has therefore been demonstratεd for the first time in the fiεld of Nuclear Quadrupole Rεsonancε that BIR-4 pulsεs can be used to genεrate trains of echoes. However, it would appear on the basis of the expεriments conducted so far that a plane rotation adiabatic pulse is necessary if echoεs arε to bε gεnεrated, and that therefore AHP and AFP could not genεrate echoes. The utility of the technique of the third prefened aspect of the present invεntion may be less than that of the other two aspects due to the relatively high power requiremεnts and limitεd dεtεction bandwidth of the AHP and BIR-4 pulses. Thεsε factors may offset the advantage in speed of inspection derived from the B j insensitivity of adiabatic pulses. Whether an adiabatic pulse could favourably be used would dεpεnd on thε tεmpεrature variation, and consequεnt resonant frεquεncy variation, of thε sample over time in any particular detεction scεnario and on whεther the local temperature in the sεarch area can readily bε dεtεrminεd with sufficient accuracy by other methods. This would often need to be detεrminεd by fiεld trial. In the technique of any of the prefeπεd aspεcts of thε present invention, any uncεrtainty in thε temperature of the samplε can be allowed for as taught, for examplε, in Intemational Patent Application No. PCT/GB92/00580 (British Technology Group Limited). In the casε of thε first and sεcond prefened aspects, it is particularly prefεπed to use εxcitation pulsεs which arε shapεd to havε a quadratic phasε " or frεquεncy variation and a nεar-rεctangular amplitudε variation in the frequεncy domain. Such pulses can have comparatively low power consumption whilst having comparatively high bandwidth. They have beεn discussed in detail in Intemational Patent Application No. PCT/GB94/02070 (British Technology Group Ltd.), whose disclosure is incoφorated herein by reference, and which claims priority from United Kingdom Patent Application No. 9319875.2. These shapεd pulsεs would not howevεr gεnεrally bε usεd in conjunction with thε adiabatic pulsεs of the third prefeπεd aspεct of thε present invention. If the substance to be detεctεd is buriεd undεrground, its tεmpεraturε is likely to vary in a relatively predictable fashion according to its depth underground. The invention may in those circumstances employ one or more tempεraturε sεnsors in conjunction with a look-up tablε to predict thε resonant frequency of the substance. In the case of the sεcond prεfεπed aspect of thε invεntion, if temperature does vary with the distance of thε samplε from the probe in a predictable manner, compensation for any temperature differεncεs may be made by optimising the first kind of excitation to a first resonancε frequency and value of T x relevant to a first temperature (and hence sample distance), and by optimising the second kind of excitation to a second resonance frequεncy and value of T x relevant to a second such tempεraturε, conεsponding, say to a greater sample distance. A similar technique could be used with the third prefeπεd aspεct of thε invention, by replacing the rectangular or shaped pulses with two different kinds of adiabatic pulsεs tailored to diffεrεnt tεmpεraturεs (and dεpths). Furthεrmorε, in thε case of the detεction of buriεd objεcts, any temperature uncertaintiεs arε likely to become less substantial as depth increases, since temperature conditions underground tεnd to be more stable at greatεr dεpth. As against this, duε to thε requirεmεnts of the testing apparatus, excitation bandwidth is likely to bε morε restricted at greatεr dεpths, and hεncε such tεmpεrature induced frequency shifts as may exist may bεcome more difficult to cope with. It will be undεrstood that the presεnt invεntion has bεεn dεscribed abovε purely by way of example, and modifications of detail can be madε within thε scopε of thε invεntion. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided indepεndεntly or in any appropriatε combination.

Claims: CLAIMS 1. Apparatus for Nuclear Quadmpole Resonance testing a remote sample containing a given speciεs of quadrupolar nucleus, comprising: at least one probe having a given maximum cross-sectional dimension; means for applying excitation to the probe or at least one of the probes to excite nuclear quadmpole resonancε for a selected range of distance of the sample from the or one such probe, the selεcted range being at least one tenth of the given maximum dimεnsion of thε probε or said one such probε, the apparatus being adaptεd to produce a non-uniform field ovεr thε selected range; and means for detεcting the resonancε response signal from the sample via thε probε or at lεast one of the probes; the excitation being such as would gεnεratε non-zεro resonance response signals at all distances within the selεcted range. 2. Apparatus according to Claim 1 wherein the excitation is such that, for a given sample, over the selεcted range of sample distance the minimum resonance responsε signal dεtεctεd would bε no less than one twentieth, prefεrably no lεss than 1/10, 1/5, 1/4, 1/3 or 1/2, of thε maximum resonance response signal. 3. Apparatus according to Claim 1 or 2 wherein the excitation is such as to genεratε a flip anglε which is substantially constant over thε selected range. 4. Apparatus according to Claim 1, 2 or 3 wherein the excitation is such as to generate a flip angle of at least 90° effective at at least one location, preferably at at least two separate locations, in the selected range. 5. Apparatus according to any of the preceding claims wherein the probe or at least one of the probes is in the form of a spiral coil. 6. Apparatus according to any of the preceding claims wherein the excitation is such as to genεratε a flip angle of greatεr than 90° effective but lεss than 180° eSec4ive at that εxtremε of thε sεlεcted sample distance range nearer the or said one such probe. 7. Apparatus according to any of the preceding claims, wherein the excitation applying means is such as to apply, preferably at the same excitation frequency, at least a first and second kind of excitation, the first kind of excitation being arranged to genεratε a flip angle which differs at εach and every location within the selεctεd range from that genεrated by the second kind of εxcitation at that particular location. 8. Apparatus according to Claim 7, for determining a measure of the distance of the sample from the or said one such probe, including means for comparing the resonance signals generated in response to the respective kinds of excitation and detεπnining from thε comparison a measure of the distance of thε samplε from the or said onε such probε. 9. Apparatus for dεtermining a measure of the distance of a sample from a probe, the sample containing a given species of quadrupolar nucleus, comprising: at least one probe; mεans for applying εxcitation to thε probe or at least one of the probes to εxcitε nuclεar quadmpole resonancε for a sεlεctεd range of distance of the sample from the or one such probe, the excitation applying means being such as to apply at lεast a first and second kind of excitation, the first kind of excitation being arranged to genεrate a flip angle which differs at each and evεry location within thε selected range from that gεnεrated by the second kind of excitation at that particular location; means for detecting the resonance response signals from the sample via the probe or at least onε of the probes; and mεans for comparing thε resonance signals generatεd in responsε to thε respective kinds of εxcitation and dεtermining from the comparison a measure of the distance of the sample from the or said one such probe. 10. Apparatus according to Claim 8 or 9 wherein thε comparing and determining means is adapted additionally to detεrminε from thε comparison a measure of the quantity of the given speciεs of quadrupolar nuclεus. 11. Apparatus according to Claim 8, 9 or 10 whεrεin thε εxcitation applying means is adapted to apply at least two different pairs of first and second kinds of excitation. 12. Apparatus according to any of thε preceding claims wherein the excitation applying means is adapted to apply the εxcitation adiabatically, and is adaptεd to gεnεratε a substantially constant flip anglε over the selεctεd rangε. 13. Apparatus for Nuclear Quadmpole Resonance testing a sample containing a givεn spεciεs of quadrupolar nucleus, comprising: means for applying excitation to the sample adiabatically to excite nuclear quadmpolε resonance; and means for detecting the resonance response signal from the samplε; thε εxcitation being such as to genεratε a substantially constant flip anglε ovεr a given region. 14. Apparatus according to Claim 12 or 13 wherein the excitation is arranged to produce a substantially constant flip angle for a variation of the field of at least a factor of one to five, preferably one to ten, twenty or thirty. 15. Apparatus according to Claim 12, 13 or 14 wherein the excitation is in the form of an adiabatic fast passage. 16. Apparatus according to any of Claims 12 to 15 wherein the amplitude or frequency or phase of the excitation is arranged to bε modulated according to hyperbolic tan or inverse tan functions. 17. Apparatus according to any of Claims 12 to 16 wherein the excitation includes an adiabatic plane rotation pulse. 18. Apparatus according to any of Claims 12 to 17 wherein thε εxcitation is aπangεd to generate echo responsε signals. 19. A mεthod of Nuclεar Quadrupolε Rεsonancε tεsting a remotε sample containing a given species of quadrupolar nucleus, comprising: providing at lεast onε probe having a given maximum cross-sectional dimension; applying excitation to the probe or at least one of the probεs to excite nuclear quadmpole resonancε for a selected range of distance of the sample from the or one such probe, the selected range being at least onε tenth of the given maximum dimension of the probe or said one such probe, a non-uniform field being producεd over the selεctεd rangε; and dεtecting the resonancε responsε signal from thε samplε via thε probe or at least one of thε probεs; thε εxcitation bεing such as would gεnerate non-zero resonancε responsε signals at all distancεs within thε sεlεcted range. 20. A method according to Claim 19 wherein the excitation is such that, for a given sample, over the selected range of sample distance the minimum resonancε response signal detected would be no lεss than onε twεntiεth, prefεrably no lεss than 1/10, 1/5, 1/4, 1/3 or 1/2, of thε maximum rεsoπancε response signai. 21. A method according to Claim 19 or 20 wherein the excitation is such as to genεratε a flip angle which is substantially constant over the selected range. 22. A method according to Claim 19, 20 or 21 wherein the excitation is pulsed excitation repεatεd at a pulse repetition time τ, the sample has a spin-lattice relaxation timε T j , and thε value of xfY is less than 5, preferably less than 2, 1, 0.5, 0.1 or. even 0.05. 23. A method according to any of Claims 19 to 22 whεrein the excitation is such as to genεratε a flip angle of at least 90° effecl i Ve at at least one location, prefεrably at at least two separatε locations, in thε sεlεctεd rangε. 24. A mεthod according to any of Claims 19 to 23 whεrein thε excitation is such as to generate a flip angle of greater than 90° rffeetive but less than 180°^^,. at that extreme of the selectεd sample distance range nearer the or said one such probe. 25. A method of detecting the presence of a remote sample containing a given species of quadrupolar nucleus, comprising: providing at lεast onε probε; applying εxcitation to thε probε or at lεast onε of thε probεs to excite nuclear quadmpole resonance for a selected range of distance of the sample from the or one such probe; and dεtecting the resonance responsε signal from thε samplε via the probe or at least one of the probes; the excitation being such as to genεrate a flip angle of greater than 90° effβctive but less than lδO^,^,. at that extreme of the selεcted sample distance range nearer the or said one such probe. 26. A mεthod according to Claim 24 or 25 whεrεin thε excitation is such that that extreme of the selεcted sample distance rangε nεarεr thε or said onε such probε is nearer the location for which flip anglε is 180° effective than that for which flip anglε is 90°.^,. 27. A mεthod according to Claim 24, 25 or 26 whεrεin the excitation is such that that extreme of the sεlεctεd samplε distance range farther from the or said one such probe is farther than the location for which, betwεεn a flip anglε of 0 and 180° efiective , the excitation would generate the most intense resonancε rεsponsε signal in thε samplε. 28. A mεthod according to Claim 24, 25, 26 or 27 whεrein the excitation is such that that flip angle at that extremε of thε sεlected samplε distance range farther from the or said onε such probε is lεss than 90° e£fective , prεfεrably lεss than 75, 60 or even 30°-^-. 29. A method according to any of Claims 19 to 28 wherein, prefεrably at thε samε excitation frequεncy, at lεast a first and second kind of excitation are appliεd, thε first kind of εxcitation bεing arrangεd to gεnerate a flip angle which differs at each and every location within the selεcted range from that genεratεd by thε sεcond kind of excitation at that particular location. 30. A method of Nuclear Quadmpole Rεsonance testing a remote sample containing a given speciεs of quadrupolar nucleus, comprising: providing at least one probe; applying excitation to the probe or at least one of the probes to excite nuclear quadmpole resonance for a selεctεd range of distance of the sample from the or one such probe; and detecting the resonance response signal from the sample via the probe or at least one of the probes; at least a first and second kind of excitation being appliεd, thε first kind of excitation being aπanged to generate a flip angle which differs at each and εvεry location within thε sεlεctεd range from that generated by the second kind of εxcitation at that particular location. 31. A mεthod according to Claim 29 or 30 whεrεin for thε first kind of εxcitation thε sign of thε resonance responsε signal would changε over the selected range of sample distance whereas for thε second kind the responsε signal would havε no zεro crossings ovεr thε sεlected range. 32. A method according to Claim 29, 30 or 31 wherein thε location within thε selected range for which the second kind of excitation generatεs a 90 o efiective flip anglε is a location at which thε sεcon kind of excitation would genεratε a resonancε response signal of opposing sign to the sign of the responsε signal which would be generated by the first kind of excitation at that location. 33. A method according to Claim 29, 30, 31 or 32 wherein the plurality of kinds of excitation are interlεavεd with onε another to form a repeating cycle of excitation, εach of thε kinds of εxcitation being applied during each such repeating cycle. 34. A method according to Claim 33 wherein in each such repeating cycle the excitation genεrating thε lowεr or lowεst flip anglε precedεs the excitation genεrating thε highεr or highεst flip angle. 35. A method according to any of Qaims 29 to 34 wherein the resonancε signals generatεd in responsε to the respective kinds of excitation are acquired and processεd sεparatεly. 36. A method according to any of Claims 29 to 35, of detεrmining a measure of the distance of the sample from the or said one such probe, wherein the resonance signals generatεd in response to the respεctivε kinds of excitation arε compared and a measure of the distance of the sample from the or said one such probe is determined from the comparison. 37. A method of detεrmining a measure of the distancε of a samplε from a probε, thε samplε containing a givεn species of quadrupolar nucleus, comprising: providing at lεast onε probε; applying εxcitation to thε probe or at least one of the probes to excite nuclear quadmpole rεsonance for a sεlεctεd rangε of distance of the sample from the or one such probε, at lεast a first and sεcond kind of εxcitation bεing appliεd, thε first kind of εxcitation gεnεrating a flip anglε which differs at each and every location within the selected range from that generatεd by thε sεcond kind of εxcitation at that particular location; dεtεcting the resonance responsε signals from the sample via the probe or at least one of the probεs; and comparing thε resonance signals genεratεd in response to the respective kinds of excitation and determining from the comparison a measure of the distance of the sample from the or said one such probe. 38. A method according to Claim 36 or 37 wherein additionally determined from the comparison is a measure of the quantity of the given species of quadrupolar nucleus. 39. A method according to Claim 36, 37 or 38 wherein at lεast two different pairs of first and second kinds of excitation are appliεd. 40. A mεthod according to any of Claims 19 to 39 whεrεin the excitation is applied adiabatically, and generates a substantially constant flip angle over the selected range. 41. A mεthod of Nuclεar Quadmpolε Resonance testing a sample containing a given spεciεs of quadrupolar nucleus, comprising: applying excitation to the sample adiabatically to εxcite nuclear quadmpole resonance; and detecting the resonance response signal from the sample; thε excitation generating a substantially constant flip angle over a given region. 42. A method according to Claim 40 or 41 wherein thε εxcitation producεs a substantially constant flip angle for a variation of the field of at least a factor of one to five, prefεrably one to ten, twenty or thirty. 43. A method according to Claim 40, 41 or 42 wherein the excitation is in the form of an adiabatic fast passage. 44. A method according to any of Claims 40 to 43 wherein the amplitude or frequency or phase of the excitation is modulated according to hyperbolic tan or inverse tan functions. 45. A mεthod according to any of Claims 40 to 44 whεrεin the excitation includes an adiabatic plane rotation pulse. 46. A method according to any of Claims 40 to 45 wherein the excitation genεrates echo response signals. 47. Apparatus for Nuclear Quadmpole Resonancε tεsting a samplε containing a given speciεs of quadrupolar nucleus, substantially as herein dεscribed with refεrεncε to and as illustratεd in Figure 1, Figures 2 and 3, Figures 4 and 5, Figures 6 to 8, or Figures 9 to 12 of the accompanying drawings. 48. A method of Nuclear Quadmpole Resonancε testing a sample containing a given species of quadrupolar nucleus, substantially as hεrεin dεscribed. 49. Apparatus for determining a measure of the distance of a sample from a probe, the sample containing a given speciεs of quadrupolar nucleus, substantially as herein describεd with rεfεrεnce to and as illustrated in Figures 1 to 8 of the accompanying drawings. 50. A method of detεrmining a mεasurε of thε distancε of s sample from a probe, the sample containing a given speciεs of quadrupolar nucleus, substantially as herein describεd.

Inventors: BEEVOR SIMON PETER, ROWE MICHAEL DAVID, SMITH JOHN ALEC SYDNEY, BEEVOR, SIMON, PETER, ROWE, MICHAEL, DAVID, SMITH, JOHN, ALEC, SYDNEY
Applicants: BEEVOR SIMON PETER, BRITISH TECH GROUP, ROWE MICHAEL DAVID, SMITH JOHN ALEC SYDNEY, BEEVOR, SIMON, PETER, BRITISH TECHNOLOGY GROUP LIMITED, ROWE, MICHAEL, DAVID, SMITH, JOHN, ALEC, SYDNEY
Application year: 1996
Main classification: G01R33
IPC codes: G01N24, G01R33