GB2057135A - Gyromagnetic detection method and apparatus - Google Patents

Abstract

An apparatus and method for the detection particularly of explosive substances by gyromagnetic means. Suspected device 12 is subjected to a constant magnetic field from magnet 32 and to a pulsed RF magnetic field orthogonal to the constant field from coil 14. The level of the constant magnetic field and the frequency of and intervals between the pulses are varied in accordance with previously obtained results to test for the presence of selected explosive substances. In particular the magnetic field strength is chosen so that an NMR frequency of one element in a desired explosive coincides with an NQR signal of a second element therein and this cross-coupling results in an enhancement of the NMR signal if the particular explosive is present. <IMAGE>

Description

SPECIFICATION Magnetic detection method and apparatus The present invention relates to methods and apparatus for the magnetic detection of unknown substances. The invention particularly relates to the detection of explosives but the apparatus and methods are suitable for the detection of some other substances.

Detection of explosives in letters and packages has recently become a considerable problem to airlines, postal authorities packing shipping clerks and many other people. Explosive devices incorporating metal are relatively easily detectable using metal detectors including for example beat frequency oscillators. More recently, however, it has been possible to make non-metallic explosive devices which are relatively lightweight and are not susceptible to conventional detection techniques for metallic bombs.

In the detection of non-metallic explosive devices it is necessary to identify the explosive constituent uniquely since the same element is often present in a slightly different composition in material surrounding or very close to the explosive substance. For example typical explosive substances contain hydrogen, nitrogen carbon and oxygen and these elements are also found in plastics material commonly used to wrap the explosive substance.

Magnetic detection techniques are now used to detect the non-metallic explosive devices. These techniques basically expose the suspected package to a constant magnetic field and to a pulse of R.F.

magnetic radiation and detect the nuclear magnetic resonance response from the element to be detected. Nuciear magnetic resonance is defined as the resonance achieved whereby energy is transferred between an R.F. magnetic field and nucleus placed in a constant magnetic field sufficiently strong to at least partially decouple the nucleus from its orbital electrons. The relationship between the frequency at which maximum energy is absorbed by the atomic nucleii of the element, the resonant frequency and the magnetic field intensity is a clue to identification of the particular element involved.

The difficulty with known N.M.R. detection techniques lies in part in the scale factors. For instance significant quantities of material must be present to concentrate the element of interest so that a sizeable response is obtained. The signals obtained by N.M.R. are ordinarily very small, thus requiring high quality detection equipment. N.M.R. signals for some elements are significantly larger than for other elements and this is particularly true for some elements where the isotope of the element of interest is present only in minute quantities. Close coupling of the elements of interest is also requied to improve the N.M.R. signal.

The present invention uses an improved method and apparatus to improve the amplitude of the N.M.R. response and to reduce the time required for a detectable response to be obtained.

The present invention is particularly concerned with the detection of a first element in the presence of a second element. The combination of first and second element is previously known to be one which would be present in a known explosive substance e.g. T.N.T.

In a preferred embodiment a method and apparatus for enhanced N.M.R. discrimination is disclosed. A sample which has a first element is placed in a magnetic field of a first intensity. A reaction between the nucleii of the first atomic element and the electromagnetic field of the apparatus produces an N.M.R. response which is tunable with field intensity. If the first atomic element of interest is intimately comingled with a second atomic element of interest as may occur in compounds and if the second element has a nuclear quadrupolar moment and if the molecular structure is proper for it to have a nuclear quadrupolar resonance (N.Q.R.) then by adjustment of the magnetic field intensity the N.M.R.

frequency of the first element is changed to substantially coincide with the N.Q.R. frequency of the second element of interest. Energy is then transferred in an enhanced fashion between the nucleii of the first element and the nucleii of the second element. This increased transfer of energy between the two nucleii reduces the N.M.R. response time of the first element and thereby considerably improves the detectability of that element. Implementation of this effect is the basis for the reduced detection time and improved discrimination achieved in the apparatus of this invention.

An improved discrimination can also be achieved by the present apparatus by variation of the elapsed time between successive observations of the N.M.R. response of the first element. This allows discrimination between the first element present in a particular compound whilst rejecting the N.M.R.

response of the same element located in different compounds.

Due to the improved responses the apparatus according to the present invention may be constructed in a compact form and may be used to detect non-metallic land mines as well as letter bombs etc.

More specifically the present invention, in a preferred embodiment, uses the transient response to yield enhanced detection and to overcome the problems of a steady state detection apparatus. The problems include a lack of sensitivity in the detector, the difficulties of obtaining adequate magnetic field strength and homogeneity at the suspected specimen, and the difficulty of separating signals from hydrogen nucleii resident in supporting materials such as wood, plastic, soil, etc. The use of transient apparatus reduces the necessity for high quality homogeneous magnetic fields. This lowers the size, cost and complexity of the apparatus.Moreover, since the coupling between nucleii or nucleii and the lattice relates to the relaxation time, the transient N.M.R. signal may be more easily analyzed to delineate hydrogen nucleii in a solid (perhaps the explosive) from hydrogen nucleii in plastic or fluid materials, typically water or pulpy materials such as wood, paper or cloth.

One scale factor which presents great dlfficufties in N.M.R. techniques utilizing transient or steady state response is the extremely large values of the so-called longitudinal or spin-lattice relaxation time often observed in many compounds. These times can measure tens of minutes, sometimes hours, in solids. Detection of the N.M.R. response from such materials requires that they remain in a polarizing magnetic field, undisturbed for a time comparable to the spin-lattice relaxation time prior to testing and observation. The relaxation time is so unduly large in such materials that N.M.R. detection and measurement cannot be used other than for laboratory investigations. Practical applications are not possible because of this scale factor.

The longitudinal relaxation time (hereinafter referred to as Tt) for selected compounds may be reduced by the present invention. It has been discovered that it is possible to adjust the polarizing magnetic field applied to the specimen of interest so that two atomic elements in the speciment are intereacted. As an example, consider an explosive material which has nitrogen and hydrogen. It is possible to adjust the polarizing magnetic field so that the separation between Zeeman energy levels for the proton (hydrogen nucleii) coincides with that between the quadrupolar energy levels for the nitrogen spin system. In certain compounds, the hydrogen and nitrogen are situated relative to the lattice such that the hydrogen T1 is reduced as a result of the transfer of energy between the nitrogen nucleii and the hydrogen nucleii.This transfer is enhanced by adjusting the N.M.R. frequency of the hydrogen to .substantially coincide with the N.Q.R. frequency of nitrogen.

The present invention is further capable of discriminating the N.M.R. response of the same type nucleii in a different material. As a simple example, the N.M.R. response of hydrogen nucleii in a solid is typically different from that of hydrogen nucleii in a liquid. As another example, the N.M.R. response of hydrogen in some explosives may be discriminated from that of many nonexplosive materials. This is helpful in discriminating between different types of materials as in the detection of hidden explosives.

The N.M.R. responsive has a second time constant descriptive of it which is the transverse time response or spin-spin relaxation time constant, or T2 hereinafter. It has been found highly desirable to seek the longitudinal time response, or T1, of most elements in contrast to detection of T2. The present invention is uniquely successful in that it is able to modify and reduce T1 in selected materials to a smaller value and thereby obtain a more rapid response. This serves to distinguish the N.M.R. response of various materials from other materials. This enables prompt and rapid recognition of the unique signature of various explosive materials.

In an alternative form the magnetic field is held steady, and the time between successive N.M.R.

responses elicited from the sample is varied. Compounds having different relaxation times T1 can be discriminated by this form of the invention. For a given element in a particular compound, the response will vary dependent on the elapsed time between successive observations of the response time.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which Figure 1 shows a sample testing apparatus in accordance with the teachings of the present invention; Figure 2 is a timing chart showing how an enhanced signal can be obtained; Figure 3 is a detailed schematic block diagram circuit illustrating a means for analyzing data; Figure 4 is a graph of signal output versus time for various chemicals illustrating various relaxation times; Figure 5 is a graph of magnetic field strength versus time showing how a time-variant magnetic field tests for various explosive materials; and Figure 6 is a graph of frequency versus field intensity showing several frequencies at which coincidence will occur.

The apparatus and method of the present invention are directed to an enhanced N.M.R. detection technique. In one embodiment, first and second elements in the presence of one another are tested in a specimen which is potentially an explosive. Through the testing of the specimen having two elements in it, a magnetic field is imposed on the test specimen. If the first element has a nuclear magnetic dipole moment, then it will have a nuclear magnetic resonance at a frequency which is proportional to an externally applied magnetic field. In the apparatus of Figure 1 the magnetic field is made to be of such intensity that the N.M.R. frequency of the first element coincides with the N.Q.R. frequency of the second element thereby enhancing coupling between the species nucleii and measurably reducing Tt.

Under these conditions, energy passes more freely and rapidly between the two elements. This typically Qvill reduce T1 of one of the elements and sometimes of both.

This invention makes use of this characteristic to reduce the time required for detection of an N.M.R.

response and to provide a means for separating the N.M.R. response produced by nucleii by certain selected materials from the N.M.R. response produced by the same type nucleii in different, and usually more common materials, It should be remembered that the amplitude of the N.M.R. response is dependent upon the quantity or concentration of nucleii, the type nucleii and other scale factors. The response is also a function of the amount of time (relative to Tt) that the sample has been allowed to remain in an appropriate magnetic field prior to being tested. Some time is required to allow the nucleii to become aligned with the magnetic field such as is necessary to produce the largest N.M.R. effects.

Increased time normally aligns more of the nucleii with the polarizing field. The alternating electromagnetic field produced by the transmitter in the process of obtaining the N.M.R. response from the sample causes the nuclear alignment to be disturbed. The disturbance can be substantial.

Realignment as necessary to obtain an appreicable N.M.R. response in subsequent tests is limited by the time constant of the nucleii. If attempts are made to repeatedly test the N.M.R. response of the sample separated by a time interval between tests that is short compared to Tut, the N.M.R. output signal is greatly reduced. The apparatus of Figure 1 varies the time constant of the nucleii in a controlled manner to reduce the time required for obtaining an N.M.R. response of a useful amplitude and yielding enhanced response from nucleii in a selected material. When the sample is placed in a magnetic field of such an intensity that the N.M.R. frequency of the type nucleii to be detected coincides with the N.Q.R.

frequency of a second type nucleii in the same compound, the T1 of the first nucleii may be reduced by an appreciable factor. In the apparatus of Figure 1 the magnetic field applied to the compound to be tested is varied in a manner to cause the material to be subjected to a field intensity such that the coincidence of N.M.R. and N.Q.R. frequencies occurs in the compound. For maximum effect, this field intensity is maintained for a period of time that is long compared to the shortened T1 of the compound.

The N.M.R. response in the selected nucleii of the compound is then tested. Following exposure of the compound to a different, second, magnetic intensity for a time period that is short compared to the T1 of the nucleii in that field intensity, the N.M.R. response in the selected nucleii is again tested. The N.M.R.

response obtained following the exposure to the first field intensity is compared to the N.M.R. response obtained following the exposure to the said second field intensity. If the compound has coincident N.M.R. and N.Q.R. frequencies at either field intensity, then this will be revealed by a difference between the first and second N.M.R. responses made apparent by comparison.

The apparatus of Figure 1 may also be used to hold the magnetic field steady and interrogate the sample with RF pulses at a varied rate. The relaxation time T1 will vary for some compounds, and the variation in time between tests will indicate the presence of a particular compound.

In Figure 1 of the drawings, the numeral 10 identifies an N.M.R. detection apparatus in accordance with the teachings of the present invention. The testing apparatus incorporates a sample holder 12 which is surrounded by a coil 14. The coil 14 is connected in a circuit communicating with a coupling network 1 6. The coil and coupling network typically handle RF signals.

A transmitter 1 8 is connected to the coupling network 1 6. A receiver 20 for the frequencies of interest is likewise connected to the coupling network. The receiver 20 forms an output signal which is communicated to a discriminator 22 which, in turn, is connected to a display 24. The timed operation of all the equipment is determined by a sequencer 26. It forms a signal which is supplied on a conductor to the transmitter 18, causing it to fire and form an output pulse. This timed event is also communicated to the discriminator 22 and the display 24. The sequencer 26 is also connected to a magnet controller 30.

It forms a suitable DC level which forms a magnetic field across the poles of a large magnet 32. The magnet 32 has a coil or winding 34 which is connected to the controller 30. Current through the winding establishes a specified magnetic field between the opposed or facing poles of the magnet.

The operation of the device is best described by reference to certain timing charts and the signals shown on them. Initially, the magnet 32 establishes a fixed magnetic field. It is adjustable to various levels, but it is a DC field. It is a low level magnetic field, typically in the range of up to a few thousand gauss. It has been observed that the amplitude of the N.M.R. signal response depends on the duration of magnetization which is inflicated on the specimen. As previously defined, N.M.R. phenomena occurs in a fixed magnetic field, and that is the field provided by the magnet 32.

N.M.R. output additionally requires an RF magnetic field at right angles to the fixed or constant magnetic field. To this end, the coil 14 has an axis approximately perpendicular to the lines of magnetic flux between the two poles of the magnet 32.

The rate at which alignment of the nucleii is achieved in the sample is indicated by the time constant Tt. Thus, after the sample is placed in the magnetic field, and the magnetic field is turned on, the amplitude of the prospective N.M.R. response increases as a function of duration. Full amplitude output is attained only after continued exposure to the magnetic field for a period in excess of several fold ofT1.

Dependent on the closeness of coupling of the element to the lattice in which it is located, there is a time-variant alignment with the field. Closely bound elements align slowly and require hundreds of seconds to obtain one time constant (63%) alignment. Moreover, each interrogation has a disrupting effect. The RF field initiates precission toward the RF lines of force from randomly achieved azimuthal positions of the element nucleii previously magnetically aligned. Thus, each RF pulse is a disturbance, upsetting alignment, and, therefore, excessive pulsing with RF pulse bursts is counterproductive.

Sampling of the N.M.R. response signal is obtained by the transmitter 1 8 supplying fairly large pulse bursts to the coil 14 via the coupling network 16. The coupling network 1 6 isolates the receiver 20 from the transmitter during the interrogation pulse and serves to receive the response signal and to couple it to the receiver 20. Each transmitted burst substantially disrupts previously achieved nuclear alignment, and, hence, realignment must thereafter be restarted to prepare for another RF pulse. This extends the time in which a full amplitude N.M.R. response (proportional to alignment) can be reached.

Accordingly, excessive sampling is self-defeating in that the time to obtain a high degree of alignment is extended. The alignment process must start all over again as a result of any disturbance to alignment caused by the transmitted energy burst applied to the coil 14.

There is a relationship between the magnetic field of the magnet 32 and the frequency of the field formed by the coil 14. This is given by the relationship of equation 1.

Freq=kx H where Freq = transmitter frequency k = a constant H = the static magnetic field strength.

By choosing a value of magnetic field strength, a particular frequency for the N.M.R. excited element is achieved. The magnetic field strength is adjustable to vary the N.M.R. frequency. The adjustment will benefit the test provided the field intensity adjustment is made with a view to finding and matching the frequency of the second sample element in the N.Q.R. mode.

Under the assumption that the first element is present with a second element subject to N.Q.R., the two frequencies are matched at a common frequency. The N.Q.R. mode of excitation is not universal to all elements. It is limited to those having a nucleii spin, number greater than 1/2, and includes isotopes of chlorine, iodine, nitrogen and others. It is eventually a fixed frequency phenomena. The N.O.R. frequency can be varied slightly by external magnetic fields, but it cannot be widely tuned by external means as can the N.M.R. frequency. It is pre-existent, and the frequency is dependent upon internal electric fields in the molecular structure of the material. Therefore, the magnetic field is varied to vary the N.M.R. frequency.The N.Q.R. frequency of the second element, present in close proximity to the first element in the lattice, is fixed, and the N.M.R. frequency is tuned to achieve a match-up.

Coupling between the first and second elements is achieved such that energy is exchanged between elements to accelerate alignment of the first element. The frequency match-up need not be perfect, but the rate of alignment is improved as the match-up is improved. The N.Q.R. is intrinsic to the material of the lattice and is primarily independent of external stimulation. When the N.M.R. mode of excitation in the first element is achieved, there is an interchange between the two elements, thereby transferring energy between them and modifying the longitudinal relaxation time of the first element. This time will be represented hereinafter as T3. T3 is thus the modified longitudinal relaxation time.

Consider an example of the two element relationship. For a sample of the explosive RDX, the nitrogen 14 has three frequency groupings where N.Q.R. occurs, one being in the range of 1.830 to 1,733 megahertz, a second frequency of about 3.359 to 3.410 megahertz, and a third of about 5.192 to 5.240 megahertz. The N.M.R. frequency of hydrogen in the explosive RDX corresponding to these three N.Q.R. frequency ranges was achieved at magnetic field intensities of about 400, 800 and 1200 gauss, respectively. This data has been obtained for hydrogen and nitrogen in the presence of one another in the explosive RDX, using the nitrogen isotope having a molecular weight of 14. As will be observed, in the explosive RDX, each frequency is not a single resonant frequency, but it is a collection of several closely grouped frequencies.As an example, the frequencies mentioned above are ranges, there being at least two frequencies or more in each grouping. While higher frequencies may exist at which the N.M.R. of one element matches the N.Q.R. of another element, it may be easier to use the lower frequencies listed above, but higher crossover frequencies provide an improved N.M.R. response.

As will be observed from the foregoing data, multiple frequencies exist in the explosive RDX at which the hydrogen-nitrogen energy transfer occurs. The relationship between the N.M.R. frequency of hydrogen in RDX and magnetic field strength is thus shown in Figure 6, along with the crossover regions where coincidence occurs with the N.Q.R. frequencies of nitrogen 14. Spreading of the N.Q.R. lines is a result of the Zeeman effect caused by the magnetic field intensity.

Attention is next directed to Figure 2 of the drawings. In Figure 2 of the drawings, several timed events are shown. Figure 2 is a timing chart. The numeral 40 identifies a first magnetic level applied to the specimen from the magnet 32. Preferably, a constant magnetic field is achieved for the moment.

The transmitter 1 8 is operated to form a first RF burst 42 of a specified length. After a pause, another burst 44 is applied from the transmitter. Typically, the RF burst lengths may be on the order of 10 microseconds and the pause between bursts of similar duration. After the application of the two bursts, the receiver 20 forms an output pulse 46 which occurs after the second pulse. This pulse 46 is indicative of the N.M.R. echo signal from a single element of the material present in the field. To this juncture, the N.O.R. effect of the second element has not come into play.

It is presumed that the pulses 42 and 44 have a fixed and common frequency duration and amplitude. Thereafter, the following excitation is applied to the specimen. The level 48 identifies a different magnetic field intensity level. This different, fixed field acts on the sample which has first and second elements in it which are intimately comingled with one another. This is a magnetic field level which brings the N.M.R. frequency of the first element to a frequency matching the N.Q.R. frequency of the second element.

The field intensity is again returned to the level 40 and the N.M.R. echo is obtained by the transmitted interrogation pulses identified by the burnt 50 and a second pulse burst identified at 52. The pulses 50 and 52 are the same as the pulses 42 and 44 in frequency, power level spacing and length.

The receiver output is an enhanced or enlarged N.M.R. signal 54, if material of characteristics described in the immediately preceding paragraph is present in the sample under test. It is enhanced by the coupling between the first and second element at a field level 48 which reduces T, toT3, and thus allows greater nuclear alignment or polarization of the first element to occur within the time period separating the burst pair 42 and 44 from the pair 50 and 52 than occurred during the time period between the first application of the field 40 and the first burst pair 42 and 44. The larger amplitude is indicative of the enhanced N.M.R. echo amplitude.

The timing chart of Figure 2 thus shows an enhanced received signal. The enhancement is the result of the greater polarization achieved in the first element within the available time as a result of shortened relaxation time which results from the match-up of the N.M.R.-N.O.R. frequencies. The N.Q.R. frequency of the second element and the N.M.R. frequency of the first element are matched, and energy then easily transfers between the two elements. It should be noted that the N.M.R. frequency is variable dependent on field intensity. By and large, the N.Q.R. frequency is only slightly variable by external stimuli and is fixed by the molecular structure of the element.

The detectable N.M.R. amplitude is quite small at the beginning of the magnetic field because there is very little intial alignment among the nucleii within the field. The rate at which alignment occurs relates to the definition of the spin-lattice relaxation time T1. Because the initial amplitude is small, an N.M.R. signal at this time may be difficult to detect.

In Figure 2, the magnetic field is dropped back to the level 40. Again, two more transmitter bursts are applied to the coil 14. There are the pulse bursts 56 and 58 in Figure 2. The receiver will again provide an output pulse 60. It is shown to have reduced amplitude. This is the result of the small nuclear realignment achieved in the short time interval compared to the relaxation time, elapsed since the last distrubance, the pulse pair 50 and 52. It should be noted that during the time period between pair 50 and 52 and pair 56 and 58 the magnetic field intensity is such that coincidence of the N.M.R.-N.Q.R.

frequencies does not occur, and the relaxation time is not reduced.

It should be noted that the time (t,) between the pulse pair 42 and 44 and pulse pair 50 and 52 may be the same as the time period, t2, between pulse pair 50 and 52 and pulse pair 56 and 58. During the time period, t,, the nucleii attain greater alignment or polarization because the time constant T1 is reduced to T3. This reduction is achieved by the enhanced coupling that occurs between the nucleii of the first element and the nucleii of the second element when the magnetic field intensity is such that the N. M. R. frequency of the first element coincides with the N.Q.R. frequency of the second element as previously described. T3 may be much shorter than T1, and nuclear alignment will then occur at a much faster rate with the shorter time constant than is the case with the longer time constant.By choosing the times oft1 and t2 to be short compared toT1 but long compared toT3, the nuclear alignment that occurs during the interval t1 will be much greater than that which occurs during the time t2. This causes the N.M.R.

echo 54 to be larger than the N.M.R. echo 60 when the material contains a compound wherein field intensity 48 causes a reduction of T1 as previously described. The two N.M.R. signals 54 and 60 obtained from materials which do not contain a compound with these characteristics will be of very nearly equal amplitudes. A comparison of the amplitudes of these two signals yields information on the presence of the compound of interest in the material under test.

Attention is next directed to Figure 3 of the drawings where the discriminator is shown in greater detail. It is triggered by the sequencer 26. It has an input signal from the receiver 20 which is connected to three similar, even identical, sample and hold amplifiers. Each amplifier is turned on by a pulse generator. The pulse generator 62, the generator 64 and generator 66 are respectively connected to amplifiers 72, 74 and 76. The first and second amplifiers are connected to a first comparator 68. A second comparator 70 is connected to the second and third amplifiers. They measure the difference in the signals from the sample and hold amplifiers and provide outputs to first and second signal shapers 78 and 80. They, in turn, drive indicators 82 and 84.Returning now to Figure 2 of the drawings, the sequencer 26 triggers the sample pulse generators to take samples in the timed sequence indicated by the timed wave forms 86, 88 and 90 of Figure 2. These signals are the input signals for the comparators. From the timed operation of the sample and hold amplifiers, the signals are delivered for use in comparing with known criteria to identify the presence of a particular compound in the test specimen.

For various and sundry explosives, it will be appreciated that the signal provided is dependent on the chemical and crystalline makeup of the explosives. The relaxation time of several explosives is rather long. This is illustrated in Figure 4 of the drawings. Figure 4 thus illustrates how the response will differ.

The ordinate of the plot is the peak amplitude of the hydrogen free induction decay nuclear magnetic response which follows a single burst of appropriate RF energy from the transmitter. A similar plot would be applicable to the N.M.R. echo following a doubie pulse burst as previously described. Figure 4 thus shows the manner in which the hydrogen N.M.R. response increases as a function of time. Time is the time after first exposure of the sample to the magnetic field or the elapsed time following the prior disorienting transmitter burst. For the explosive material RDX, it is also plotted on a tenfold scale in Figure 4. As will be appreciated, its response is so siow that time will not ordinarily permit the use of the N.M.R. detection techniques without the enhanced response taught by the present invention. In other words, the enhancement taught herein is aimost essential to detect RDX in any reasonably short period of time.

Figure 5 shows a timed and shaped pulse for the magnetic field (level 48 in Figure 2) which assures N.M.R. frequency level crossing between the relatively fixed N.Q.R. responses of various explosives which are comprised of at least hydrogen and nitrogen in compounds. As shown in Figure 5, the magnetic field is measured in gauss, and it is stepped or varied to the indicated levels. As it is varied, it passes through various intensities indicated on the decay curve where the hydrogen N.M.R. frequency is equal to the nitrogen N.Q.R. frequency for the indicated explosive compounds. The curve thus shows how the N.M.R. frequency of the hydrogen nucleii is made equal to the N.Q.R. frequencies of the coupled nitrogen nucleii in the compound and where within a short interval the nucleii become aligned to enable detection.

Returning now to Figure 2 of the drawings for explanation of another means of discrimination where the relaxation time of the element of interest is not altered, attention is directed to the response of the receiver shown on the chart. Assume that the sample includes an element in it which is to be detected. The element has a specified relaxation time which is relatively long compared to that of interfering materials which may be present. The time between the first doublet 42 and 44 and the second doublet 50 and 52 is made long in comparison to the relaxation time. The time between the second doublet burst and the third doublet burst is shorter than the first time and preferably shorter than the T1 of the material to be tested.The amplitude of the N.M.R. response following the second doublet burst is maximum while the amplitude following the third doublet burst may be relatively small. The two differing received responses provide a basis for discrimination.

The apparatus shown in Figure 3 is used for obtaining this measure. The frequency selected for the doublet burst from the transmitter is selected such that the nucleii to be detected is resonant when the magnetic field is at the level 40 shown in Figure 2. The different magnet field intensity 48 is not required for this discrimination technique.

The described apparatus and method of the present invention finds application primarily in the detection of explosives, but it can be used to detect the presence of elements in other types of compounds. It functions quite nicely with inorganic materials. Organic materials present no difficulty either. An example of a detectable nonexplosive material exemplifying another hydrogen-nitrogen coupling is hexamethylenetetramine.

The present invention provides output data which can be compared with the signature of selected chemical compounds. While there might be some ambiguity, in the sense of detecting explosives, the ambiguity presents no problem. Thus, the explosive RDX may have a signature similar to a nonexplosive compound. When used in inspection for bombs and the like, it is wise to treat the nonexplosive compound as an explosive. This occurs out of an abundance of precaution, and, to that extent, the ambiguity might be inconvenient but certainly not dangerous. More importantly, this ambiguity is highly unlikely in inspecting packages, letters and other mail. Therefore, the existence of possible ambiguities in the data is not meaningful.What is meaningful is that RDX has a characteristic signature in parameters of the N.M.R.-N.Q.R. cross couplingbwttthehydrogen and nitrogen in the explosive.

Needless to say, other elements of the materials can also be excited and tested. It is not necessary to test only for hydrogen and nitrogen. The tests can be run for hydrogen and nitrogen, subsequently rerun for hydrogen-chlorine interaction and so on. In each instance, a different signature can be developed and compared with standards obtained from laboratory measurements.

Representative test data for the hydrogen N.M.R. frequency equal to the nitrogen 14 N.Q.R.

frequency for several materials is as follows: Frequency Chemical Field in Gauss in Megahertz RDX explosive 1220 5.2 RDX explosive 790 3.4 RDX explosive 420 1.8 PETN 210 0.9 PETN 120 0.5 PETN 104 0.4 TNT 204 0.87 HMT 185 0.79 The explosives listed above can be scanned by the time shaped magnetic pulse of Figure 5 which is representative of the range of variations or intensity levels. The variations of field intensity interrogates for the listed explosives. HMT (or hexamethylenetetramine) is not an explosive, and it is included to show the response of a nonexplosive. Indeed, the signature of a two element (one isotope N.Q.R.

responsive element) compound or mixture can be analyzed. The signature is quickly obtained, and it is readily compared to the expected data. In these tests, the N.M.R. of hydrogen at 587 gauss is a frequency of about 2.5 megahertz. The frequency is not critical for scanning for other coupled N.Q.R.

elements, and, hence, the frequency can be any value, say 2.0 to 5.0 megahertz. For best discrimination, it should not be selected to coincide with the N.C.R. frequency of a material to be detected. Where a reduction of the relaxation time only is desired, it may be selected to coincide with the N.C.R. frequency.

Claims (48)

1. Magnetic detection apparatus for the detection of the presence of a compound including first magnet means for producing a constant magnetic field to act on a sample containing the suspected compound; RF coil means for producing an RF magnetic field acting substantially at right angles to the constant magnetic field, transmitter means connected to the RF coil means to produce a transmitted pulse burst of specified frequency amplitude and duration acting on the sample, a receiver connected to the RF coil means to form an output voltage proportional to the nuclear magnetic resonance response of the nucleii of a first element in the compound and including control means for controlling the magnetic field intensity of the first magnet means.

2. Apparatus as claimed in Claim 1 in which the receiver is connected to an amplitude comparator means.

3. Apparatus as claimed in Claim 2 in which the comparator means is connected to an indicator.

4. Apparatus as claimed in Claim 2 or Claim 3 in which the comparator means includes sample and hold amplifiers.

5. Apparatus as claimed in Claim 1 wherein said receiver means is connected to first, second and third sample and hold amplifier means; and further including first, second and third timing generators for switching on said sample and hold amplifiers in a timed sequence; and further including comparator means connected to said amplifier means for receiving the outputs thereof in timed sequence controlled by said timing generator means to form output signals resulting from said comparisons indicative of the nuclear magnetic resonance signals received from the first element experienced at differing magnetic intensities from said magnet means.

6. Apparatus as claimed in Claim 1 including means responsive to the output of the receiver for indicating a reduction in the relaxation time of the first element.

7. Apparatus as claimed in any one of Claims 1 to 6 wherein said magnetic field control means is operable to cause the field intensity to change between two or more selected levels and to remain at each level for a selected period of time.

8. Apparatus as claimed in any one of claims 2 to 5 wherein said comparator means is adjustable to be responsive to the difference between the amplitude of the nuclear magnetic resonance signal from the first element at a first magnetic field intensity and the amplitude of the nuclear magnetic resonance signal from the first element at a second magnetic field intensity.

9. Apparatus as claimed in any one of Claims 2 to 5 wherein said comparator means is adjustable to be responsive to the difference between the amplitude of the nuclear magnetic resonance response following a first transmitted pulse burst and the amplitude of the nuclear magnetic resonance response following a second transmitte pulse burst.

10. Apparatus as claimed in any one of Claims 2 to 5 wherein the magnetic field intensity controlled by said magnetic field control means during the first transmitter pulse burst is at a first intensity level, the magnetic field intensity is then changed to a second intensity level and remains at said level for a selected period of time, and then the magnetic field intensity is returned to the first intensity level and the second transmitted pulse burst is then generated by said transmitter means.

11. Apparatus as claimed in Claim 10 wherein the first magnetic field intensity is such as to cause the nuclear magnetic resonance frequency of a first element in a sample to be approximately equal to the frequency of said transmitter means and within the frequency range of said receiver means and the second magnetic field intensity causes the nuclear magnetic resonance frequency of a first element to be approximately equal to the nuclear quadrupole resonance frequency of a second element comingled with the first element in the sample.

12. Apparatus as claimed in Claim 10 wherein the first magnetic field intensity is such as to cause the nuclear magnetic resonance frequency of the first element in the sample to be approximately equal to the frequency of said transmitter means and within the frequency range of said receiver means and approximately equal to the frequency of the nuclear quadrupole resonance of the second element comingled with the first element in the sample and wherein the second magnetic field intensity level is such as to cause the nuclear magnetic resonance frequency of the first element to be different from the nuclear quadrupole resonance frequency of the second element.

1 3. Apparatus as claimed in Claim 10 wherein the said second magnetic field level is varied by said magnetic field control means over a range of specified intensities.

14. Apparatus as claimed in Claim 13 wherein the said second magnetic field level is varied over a range of intensities and remains at selected intensity levels for selected periods of time.

15. Apparatus as claimed in Claim 1 or Claim 10 wherein the range of field intensity variations causes the range of the nuclear magnetic resonance frequency of the first element to correspond to the nuclear quadrupole resonance frequency of the second element in the sample.

16. Apparatus as claimed in Claim 14 wherein the selected field intensities are such as to cause the nuclear magnetic resonance frequency of the first element to be approximately equal to the nuclear quadrupole resonance frequency of one or more other elements in the sample.

17. Apparatus as claimed in any one of Claims 1 to 6 wherein the said transmitter means is adapted ot produce two or more pulse bursts of specified frequency, amplitude and duration acting on the sample.

1 8. Apparatus as claimed in Claim 1 7 wherein said transmitter means forms pulse bursts as doublets.

1 9. Apparatus as claimed in Claim 1 7 wherein the first transmitted pulse burst is followed by a second transmitted pulse burst and wherein the period of time separating the first pulse burst from the second pulse burst is selected to cause the amplitudes of the nuclear magnetic resonance response following each burst to differ.

20. Apparatus as claimed in Claim 1 7 wherein the time period between a first burst and and a second burst is different from the time period between the second burst and a third burst to vary the amplitude of the nuclear magnetic resonance response following the third burst compared to that following the second burst

21. Apparatus as claimed in any one of Claims 17 to 20 wherein the pulse bursts are repeated in a sequence such that the time periods between consecutive bursts is alternated between selected values.

22. Apparatus as claimed in Claim 1 9 wherein the magnetic field is constant at the intensity required to make the nuclear magnetic resonance frequency of the first element to be approximately equal to the frequency of said transmitter means and within the frequency range of said receiver means to cause the differences in the amplitude of the nuclear magnetic resonance responses of the first element following each transmitter burst to be enhanced when the relaxation time of the first element nucleii is within specified ranges.

23. Apparatus as claimed in Claim 22 wherein the magnetic field intensity is changed between bursts.

24. Apparatus as claimed in Claim 9 wherein said comparator means is responsive to the amplitudes of the nuclear magnetic resonance responses at selected times separated by a selected time interval following the transmitter pulse burst.

25. Apparatus as claimed in any one of Caims 3 to 6 in which the compound includes as the first element is hydrogen and a second element which is nitrogen, and in which the two elements are in an explosive material placed in said magnetic field and wherein said indicator means forms an output indication indicative of the signatures of known explosives where each signature is obtained by adjustment of said magnet means to one or more specified magnetic intensities.

26. Apparatus as claimed in Claim 9 wherein said magnetic means is operated at a first magnetic level for a specified time, and in which the magnetic field intensity is changed to alternate between first and second levels during a specified interval.

27. Apparatus as claimed in Claim 1 0 wherein the variations in the magnetic field intensity vary the nuclear magnetic resonance frequency of the first element proportionally to magnetic field intensity variations, and the signal from said receiver means is enhanced by shortening the time interval required by the first element to achieve nuclear polarization.

28. A method of detecting a first element in the presence of a second element in a sample of interest characterised in that it comprises the steps of placing the sample suspected of having the elements therein in a magnetic field of suitable intensity; varying the magnetic field intensity to a level selected such that the magnetic field interacts with the first element in a nuclear magnetic resonant mode which resonant interaction has a frequency approximating the nuclear quadrupole resonant frequency of the second element and the frequencies are sufficiently close to permit the interchange of energy between the two elements so that the transferred energy shortens the nuclear magnetic resonant response time of the first element; interrogating the sample by means of at least one transmitted RF pulse burst approximately orthogonal to the magnetic field which pulse burst has a selected frequency, duration and magnitude; and detecting after interrogation the nuclear magnetic resonance signal of the first element as a measure of its presence and concentration.

29. The method as claimed in Claim 28 including the step of exposing the sample to a first magnetic field intensity level for a specified interval and thereafter altering the magnetic field intensity to another level to vary the first element to an alternate nuclear magnetic resonance frequency approximating the nuclear quadrupole resonance frequency.

30. The method as claimed in Claim 28 or 29 wherein the step of interrogating the sample utilizes two pulse bursts separated by a specific time.

31. The method as claimed in any one of Claims 28 to 30 wherein the transmitted pulse burst has a frequency which is independent of the nuclear quadrupole resonance frequency.

32. The method as claimed in any one of Claim 28 to 31 wherein the second element is selected from those isotopes having a spin number greater than 1/2.

33. The method of Claim 32 wherein the first element is hydrogen.

34. The method of Claim 33 including the step of varying the magnetic field to about 1220 gauss in testing for RDX explosive.

35. The method of Claim 33 including the step of varying the magnetic field to about 790 gauss in testing for RDX explosive.

36. The method of Claim 33 including the step of varying the magnetic field to about 420 gauss in testing for RDX explosive.

37. The method of Claim 33 including the step of varying the magnetic field to about 210 gauss in testing for PETN explosive.

38. The method of Claim 33 including the step of varying the magnetic field to about 120 gauss in testing for PETN explosive.

39. The method of Claim 33 including the step of varying the magnetic field to about 104 gauss in testing for PETN explosive.

40. The method of Claim 33 including the step of varying the magnetic field to about 204 gauss in testing for TNT explosive.

41. The method of Claim 28 wherein the magnetic field is set at a first level and thereafter varied to a second level over a specified interval to sweep past at least two suspected nuclear quadrupole resonance frequencies which frequencies differ for differing explosive samples.

42. The method of Claim 28 wherein the magnetic field is established for an interval and thereafter varied toward zero gauss to vary the nuclear magnetic resonance of the first element to match the nuclear quadrupole resonance of the second element.

43. The method of Claim 28 for detecting the presence of dynamite explosives.

44. The method of Claim 33 for detecting TNT explosive.

45. The method of Claim 33 for detecting PETN explosive.

46. A method of detecting the presence of an element in a sample characterised in that it comprises the steps of placing the sample in a magnetic field of specified intensity; transmitting an interrogation signal comprising an RF magnetic field into the sample in a direction orthogonal to the magnetic field which signal comprises a pair of pulses separated in time by a specified interval and which pulses have a specified frequency and duration; detecting a nuclear magnetic resonance response from the sample and periodically repeating the transmitted pair of pulses at intervals which are varied over a specific range to obtain an enhanced detected signal.

47. The method of Claim 46 wherein the relaxation time of the element is relatively short compared to spacing of the pairs of pulses.

48. The method of Claim 47 including the step of comparing the output relative to varied spacing of the pairs of pulses.