System and method for contraband detection using nuclear quadrupole resonance

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SYSTEM AND METHOD FOR CONTRABAND DETECTION USING NUCLEARQUADRUPOLE RESONANCE QUANTUM MAGNETICS, INC.C:24564

FAWP60\USERS\aTTY\LAMM&gt;ATENTSV2M47.PCT

SYSTEM AND METHOD -1-

?OR CONTRABAND DETECTION

USING NUCLEAR OUADRUPOLE RESONANCE

TECHNICAL FIELD

This invention relates generally jto a bulk substance detection system for detecting concealed explosives and narcotics, aid more particularly to a practical system andmethod for such contraband detection employing nuclear quadrupole resonance (NQR).

5 BACKGROUND ART

Earlier work in detecting contraband substances centered on the subject of nuclearmagnetic resonance (NMR). Work in this area is reflected;in U.S. patents 4,166,972,4,296,378 and 4,514,691. A drawback of NMR is that is requires relatively largemagnets. Magnets are relatively expersive, would likely cause personnel to be exposed10 to large static magnetic fields, and could damage magnetically recorded material.

Another attempt at explosives detection employed thermal neutron analysis(ΊΝ.Α), which can detect nitrogen in &lt;ny form. Although it could detect explosives, itwas also triggered by nitrogen-rich nj Ion and wool, and other innocuous items. These'false positives. Because it employed potentiallysystems were also required to be heavily shielded. /

As a consequence, TNA systems were very large, very expensive, and also produced ahigh rate of false positives. X-ray screening, commonly used in airports, does: not have the same overalllimitations as TNA. However, it cannot alert the operator to the presence of explosives 20 or drugs, much less identify them. X-ray screening can only "see images that theoperator must interpret quickly." Further, X-ray screening emits potentiallj hazardousionizing radiation.

With respect to explosives, plistic explosives such as C-4 and Semtex, containingRDX and PETN, have an almost infinite variety of possible shapes and uses for terrorist 25 bombing tactics. Plastic explosives are highly stable, have clay-like malleability and aredeadly in relatively small quantities. A small piece of plastic explosive, a detonator, and

shortcomings resulted in a high rate ohazardous radioactive emissions, TNA 15 F:\W?60\USERSV&lt;TTY\i^M\PATEi&lt;rS'.Q.M47.PCr -2- a trip wire inside a large mailing envelope can cause a deadly explosion. Unfortunately,without close~and potentially dangerous-visual inspection, {blastic explosives can bemade virtually untraceable. Because of the drawbacks of TNA, NMR and X-ray, asmentioned above, they have generally proven ineffective for practical bulk detection ofthese types of explosives. In particular, detection of sheet explosives, typically havinga thickness as small as one-quarter inch, has not been effectively accomplished by priortechnologies.

The wide-scale attempts to figh: the illegal drug tradie indicates that narcoticsdetection is also extremely important. The need for a simple procedure for detectingdrugs inside sealed containers, mail parcels, and other small packages, quickly andaccurately, is immeasurable. Conventional detection methods are time-consuming,costly, and have only marginal reliability at best. NQR is a branch of radio frequency spectroscopy that exploits the inherentelectrical properties of atomic nucleil Nuclei with non-spherical electric chargedistributions possess electric quadrupols moments. Quadrupole resonance arises fromthe interaction of the nuclear quadrupo e moment of the nucleus with the local appliedelectrical field gradients produced by t le surrounding atomic environment.

Any chemical element’s nucleus which has a spin quantum number greater thanone half can exhibit quadrupolar resonmce. Many substances (approximately 10,000)have been identified that exhibit quadrupolar resonance, among such nuclei being: 7Li,’Be, |4N, 17O, ^Na, 27A1,3iCl, 37C1,39K, 5SMn, 75As, ”Br, 8lBir, 127I, ”7Au, and 20?Bi. Itso happens that some of these quadrupolar nuclei are present in explosive and narcoticmaterials, among them being nitrogen f|4N), chlorine (35C1,37C1), oxygen (l7O), sodium(23Na), and potassium (39K). The most studied quadrupole nucleus for explosives andnarcotics detection is nitrogen.

In solid materials, electrons and atomic nuclei produce electric field gradients.These gradients modify the energy levels of any quadrupolar nuclei, and hence theircharacteristic transition frequencies. Measurements of these frequencies or relaxationtime constants, or both, can indicate not only which nuclei are present but also theirchemical environment.

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When an atomic quadrupolar rucleus is within an . electric field gradient,variations in the local field associated wilh the field gradient affect different parts of thenucleus in different ways. The combined forces of these fields cause the quadrupole toexperience a torque, which causes it to precess about the· electric field gradient. 5 Precessional motion generates an oscillaing nuclear magnetic moment. An externallyapplied radio frequency (RF) magnetic field in phase with the quadrupole’s precessionalfrequency can tip the orientation of the nucleus momentarily. The energy levels arebriefly not in equilibrium, and immediate ly begin to return to equilibrium. As the nucleireturn, they produce an RF signal, know i as the free induction decay (FID). A pick-up 10 coil detects the signal, which is subsequently amplified by a sensitive receiver tomeasure its characteristics.

One distinguishing feature of an NQR response is its precessional frequency.Two independent factors determine the p recessional frequency:; the quadrupolar nucleus,and its local crystalline environment. There may be one or more characteristic NQR 15 frequencies for each substance containing quadrupolar nuclei.

The second distinguishing features are the NQR relaxation times. Relaxationtimes are a measure of the nuclei’s ra:e of return to the equilibrium state followingdisturbance by an RF pulse. Relaxation times are compound-, temperature-, andpressure-specific. Relaxation times also determine the repetition rate and timing of RF 20 pulses required for exciting and detecting a specific NQR signal. Relaxation times canbe as short as a few hundred microsect nds or as long as several seconds.

Detection .of NQR signals normally requires RF transmitting and receivingapparatus. To minimize noise and ratio frequency power requirements and improvereceiver sensitivities, conventional NQF systems use a narrow band (high Q) sample coil 25 in both the transmitting and receiving equipment. Even so, several factors cansignificantly degrade the effectiveness ff detecting NQR signals. Among these factorsare: (1) the presence of conductive materials inside the sample coil; (2) the presence ofmaterials with a high dielectric constant inside the sample coil; (3) temperature, whichcan affect the value of the capacitance used for tuning and matching the ,RF coil; and 30 (4) mechanical movement of the coil which respect to its surroundings. All of these factors can cause serious de-tuning of the detection apparatus, which in turn, lowers the

F:\WP6Q\USERS\ATTY\LAM\PATENT5\QM47.PCT -4- detection sensitivity of the coil. Accordingly, NQR systems have largely been limitedto small sample laboratory systems w th little or no "real-world" potential.

The NQR energy level transitions are observed primarily in the radio frequencyrange. Detection of these transitions requires an RF source to excite the transition, andan RF receiving mechanism to detect he signals returning from the nuclei. Normally,the signals appear at a pre-defined frequency. An RF coil· tuned to, or close to, thatpredefined frequency can excite and/οι detect those signals. The signals are of very lowintensity and can only be observed fbi- a short time, approximately 10^s to 10ms. Asa consequence, there is a need for an NQR receiver that can be tuned to a (usually) highQ, has very low noise, and is capable of fast recovery after a high-voltage RF pulse.

Previous work in this area is reflected in U.S. patents 4,887,034, 5,206,592, R for explosives and

5,233,300 and 5,365,171. UseofNQ narcotics detection is also discussed in Buess et al., Explosives D Section By I4N Pure NQR, Advances in Analysisand Detection of Explosives (J. Yinon (ed.)) pp. 361-368 (1993), and Shaw, NarcoticsDetection Using Nuclear Quadruple Resonance (NQR)', Contraband and CargoInspection Technology International Symposium, Washington, D.C., pp 333-341 (1992).

Detection by means of NQR is possible for both explosives and narcotics,partially because they have as a constituent element ,4N in crystalline form. Particularly with respect to narcotics, this is true obased narcotics. The hydrochloride fo cocaine base, cocaine hydrochloride and heroinrms of narcotics, such as cocaine hydrochloride,also contain quadrupolar nuclei j5Cl and 37C1. A significant factor in contrabar d detection by means of NQR is that quadrupolarpotentially readily observable, in narcotics andchlorine (”C1 and 37C1), among other possibleis it is necessary to be able to detect quadrupolarmail bags or airline baggage, including carry-on nuclei that are commonly present, andexplosives include nitrogen (14N) andnuclei. Thus, in commercial applicationuclei contained within articles of mail. and checked luggage. While the resonant frequencies of the nitrogen in these substancesdiffers for each chemical structure, these resonant frequencies are well defined andconsistent. By applying an RF signal to a container having any of these suspectedsubstances inside, and then detecting ary quadrupolar resonance thus engendered by the

F:\WP6O\USERS\ATTCLAM\PATENTS'QM«7.PCT -5- application of RF pulses, the identitydetermined. of the contraband substance can be easily

DISCLOSURE OF INVENTION

Broadly speaking, this inventibn provides a practical nuclear quadrupole resonance (NQR) detector system for improved bulk contraband detection. 'Morespecifically, the invention employs the principle of NQR to simply and relatively inexpensively, with a very low rate ofand/or narcotic materials within closed false alarms, detect the presence of explosivesor sealed packages pr within baggage having many other articles of other materials c ontained therein. It is particularly effective indetecting contraband materials in sheet form, which are as thin as one quarter inch orpossibly even thinner.

The invention is a system for detecting a target substance within a class ofexplosives and narcotics containing qradrupolar nuclei in a; specimen employing thephenomenon of nuclear quadrupole lesonance (NQR), said system comprising: asequence controller having means for providing timing and programming pulses to saidsystem; a radio frequency (RF) subsysi em comprising a variable frequency RF sourceto provide pulsed RF excitation at a free uency generally corresponding to predeterminedcharacteristic nuclear quadrupolar resonant frequency of the specimen, wherein nuclearquadrupolar resonant frequency is ihe frequency of nuclear precession due toar electric field gradients; a detection headsubsystem comprising: a single turn di:tributed RF coil sheet shaped and configured todefine a cavity of predetermined volurre therein and to receive the specimen within thecavity defined by said RF coil, said ca rity having a first end and a second end, the RFsignal from said RF source being trarsmitted within said cavity and being uniformlycoil cavity and generating a uniform field withinng as the pickup coil·for the NQR signals from applied to the specimen within said RFsaid cavity, said RF coil also functionthe specimen and providing an NQR o atput signal; apparatus for tuning said RF coil toabout the desired characteristic nuclear quadrupolar resonant frequency for'the specimenunder test; and an electrically conductive RF shield surrounding and spaced from andelectrically isolated from said RF coil, said RF shield being shaped and configured to -6- provide electromagnetic interference and radio frequency, interference (EMI/RFI)shielding from external noise and to prevent RF and magnetic; flux from escaping fromsaid RF coil cavity and RF shield combination, said RF shield Configuration being longerthan said RF coil, thereby extending beyond both said first end and said second end ofsaid coil, said RF shield being an electrically integral part of said RF coil to improve theQ and the efficiency of said RF coil and contributing to the uniformity of flux fieldapplied to the specimen within said RF c oil, said RF coil and RF shield together forminga scanner, a signal capture and data processing subsystem having a digital signalprocessor and comprising: means for rec living the NQR output signal from said RF coil;memory means storing characteristics of NQR signals from at least one target substancein the class explosives and narcotic compounds; means for processing the NQR outputsignal from said RF coil; and means for lomparing characteristics of the processed NQRoutput signal with the characteristics in memory and emitting a final output signal; and a display device receiving the final output signal from said signal capture and data 1 processing subsystem and, in response thereto, said display device selectively indicatingthe absence of the target substance, and an' the received signal from the specimen indicate the presence of the target substance,intermediate result when conditions of'that further testing is necessary.

The invention is a also a metho 3 for detecting a target substance within a classof explosives and narcotics containing quadrupolar nuclei in a specimen, said methodemploying the phenomenon of nuclear quadrupole resonance (NQR) in a detectionsystem and comprising the steps of: forming a scanner comprised of a single turndistributed RF coil sheet shaped and configured to define a cavity of predeterminedvolume therein and to receive the specimen within the cavity defined by the RF coil, theRF coil being surrounded by an electrically conductive RF shield which is spaced fromsoil, the shield being , shaped and configured toand radio frequency interference (EMI/RFI)shielding from external noise and to prevent RF and magnetic flux from escaping fromthe scanner, the RF shield configuration being longer than theRF coil, thereby extendingbeyond the ends of the RF coil, the RF coil and RF shield being an integral combinationportion of the scanner and being designed to improve the Q and the efficiency of the RF and electrically isolated from the RFprovide electromagnetic interference

F:\WP60\U5ERS\ATTY\LAM\PaTENTS\QM47.PCT -7- coil, the RF coil and RF shield contributspecimen when it is inserted within the ing to the uniformity of flux field applied to theRF coil cavity; entering known characteristicsof NQR signals of target substances n memory in a datai signal processor in thedetection system; providing precisely programmed timing pulses to the detection system;

formed in the RF coil; then automatically tuningefficiency for RF signals transmitted within thepulses of a predetermined frequency to the RF

inserting the specimen within the cavitythe RF coil to maximum power transferRF coil cavity; providing excitation RF coil;;transmitting the RF pulses into the cavity formed by the RF coil and creating auniform flux field within the RF coil tq which the specimen is subjected; detecting bythe RF coil the NQR signals emitted by target substances within the specimen;processing the NQR signals and comparing them to signal characteristics in memory todetermine whether the detected NQR sijmals indicate the presence of a target substance;and selectively indicating whether the target substance is present in the specimen,whether the target substance is absent fiom the specimen, and; whether conditions of thereceived signal indicate that further eximination is necessary.

The invention provides a comxr ercially practical system employing the knownproperties of the substances and the kiown principles of NQR to detect and identifycontraband products which may be hidden inside airline baggage or concealed in avariety of packaging. RF pulses are applied to an RF coil in which the specimen residesfor purposes of the test. An appropriately formed RF shield prevents stray signals fromentering or leaving the cavity in the eoil so that the test results can be reliable andBecause insertion of the sample in the RF coilsystem is provided to re-tune the RF coil to provide optimum performance under A range of coil loading conditions. This tuningsystem also corrects for the possible degradation of NQR response signals caused byfactors set out above in the Background,to digital form and .is sent to a digital signal processor. The digitized signal is digitally filtered and compared with a predeterminedthreshold level. Alternatively, once tl.e signal is apodized and Fourier transformed, itoccurs as a quadrature "spike" at or close to 0 Hz in the frequency spectrum, and is thenfiltered and compared to the known signal of the material to be detected. The signal external RF radiation is insignificantcauses de-tuning, an automatic tuning temperature changes and others of theAn analog signal is converted

F.\WP60\USERS\ATT)TiAM\PATENTS\QM47.PCT -8- from a digital signal processor is appliedpackage is clean, has contraband, or no, In a practical system, the presencbeing inspected may cause acoustic rinci.system of the invention has provisions tcso that the NQR signal is isolated and i:i

BRIEF DESCRIPTION OF DRAWING io a display device which indicates whether thefurther inspection. :e of other conductive material in the packageing as a result of the applied RF pulses. Thefilter out such acoustic ringing when it occursnot hidden in the ridging signal. eds

The objects, advantages and features of this invention will be more readily appreciated from the following detailedaccompanying drawing, in which:

Fig. 1 is a block diagram of theFig. 2 is perspective view of anfor small package inspection incorporati. description, when read in conjunction with the ; basic system of the invention;actual device in accordance with the inventioning the Fig. 1 system therein;

Fig. 3 is a partially cut away perspective view of the scanner portion of the Fig.2 device;

Fig. 4 is a partially cut away perspective view of a baggage inspection deviceconstructed according to the invention;

Fig. 5 shows the auto-tune subsystem in greater schematic detail;

Fig. 6 is a flow diagram of the operation of the auto-tune subsystem of Fig. 5; and

Fig. 7 is a perspective view of the RF coil, core and cavity of the invention, showing the static tuning capacitors in he coil gap.

BEST MODE FOR CARRYING OUT

THE INVENTION NQR is a linear spectroscopy, tt|to the quantity of contraband material cfrequencies of different compounds arenot encounter false alarms from theexample, 'JN NQR absorption frequencikWhen looking for the nitrogen signal ai at is, the signal strength is· directly proportionalι mtaining quadrupolar nuclei. Because the NQRquite distinct, the system of this invention doesNQR signals of other benign materials. Fors from crystalline materials are virtually unique,the NQR frequency of RDX, for example, only

F:\WT60\LSEXS\ATTY&gt;lANfJ&gt;ATEXTS\QM47.PCT -9- nitrogen in RDX will be detected. If other compounds containing 14N axe in the sameparcel as the RDX, those other compounds would not be identified. The frequencyresulting from NQR in a target subst ince will be sharply defined, while other I4N-containing substances would not provide a sharp peak NQR response. Another factor 5 of importance is that NQR is a bulk det;ctor, that is, sheet, bulk or distributed materialsare equally detected.

The unique NQR resonance frequencies of a large number of compounds havebeen identified and recorded. The frequency information is stored in a memory in thesystem of the invention and provides a database for comparison of detected signals. For10 general reference purposes, the NQR :requencies of quadrupolax nuclei are generallywithin the range of 0.5 MHz to 5 MIL·

It is important to understand that explosives and narcotics have "fingerprints" thatare different from innocuous substances. To thwart analysis by NQR, it would requirethe impossible task of altering the chemical structure of the contraband, and the laws of15 nature cannot be altered. Thus, to cha ige a substance so that the elements of interest in the particular contraband could not be detected by means of NQR would requirechanging the chemical composition anc make it other than the contraband itself.

With reference now to the drawing, and more particularly to Fig. 1 which showsthe system, block 21 is the sequence controller subsystem. This subsystem provides20 precise timing and other control functions for all other elements and subsystems of thea microprocessor-based device which providese sequence control information to all othersubsystems, and would include appropriate data storage or memory means. It also storesinformation on the results of individual scans for future reference. As one specific25 embodiment, the microprocessor based control and storage device may be a personalcomputer (PC) with a hard disk.

The sequence controller subsysi em also includes a pulse programmer which isa high-precision, high-resolution device that runs off the standard computer bus. Thepulse programmer provides the precise sequence control required for correct operation 30 of all other major components in the NQR scanner of the invention. In combinationwith the personal computer, it also provides the precisely defined pulses and triggers to invention. Il generally would comprisemeans to download and initialize tl

F;\WP6O\USeKS\ATTYUAM\PATENTSMJM47.PCT -10- signal source 23, RF power amplifieramplifier 26 and detectors 27 and 28 activate the subsystems to which it is connected and which jwill be discussed in detail below.

Radio frequency (RF) subsysten i 22 has several functional elements including RF24, receiver RF preamplifiers 25, receiver RFThe detectors are here shown as phase-sensitive detectors. A 90° degree phase shift ger erator 31 is also part of the RF subsystem. Thisis one embodiment of the invention and is used when detectors 27 and 28 are phase shiftdetectors. Other types of detectors cculd be employed and the phase shift generatorwould not be required. Conventional amplifier protection devices 29 are also part of the10 RF subsystem. They are typical Rf amplifier-related elements and need not bedescribed in detail here. RF signal source 23 provides hither continuous or pulsed RF excitation at afrequency corresponding to the resonan: frequency of the sample material. For example,RDX-based plastic explosives have a resonant frequency of approximately 3.410 MHzhave a resonant frequency of approximately 890 KHz. The excitation source is fed into amplifier 24 of sufficient power rating togenerate about 1 gauss of RF magnetic field within the coil. The excitation frequencyneed not be exactly the same as the target substance NQR frequency but it should bewithin about 500-1000 Hz. The RF excitation for NQR detection could be a single pulse 20 of 10/iS-500/iS duration, depending or. the substance beingtested for. Such a singlepulse could cause an NQR return, but ihe nuclei may not have reached a steady state of ?e sufficiently strong 'to be detectable or useful.

For a letter bomb scanner, approximate ly three seconds of RF pulses at a repetition rate i of 667 pulses per second, meaning a tr iin of 2000 pulses having a pulse width of 200μδ25 each, would preferably be applied. Th ϊ pulse repetition rate can range between 300 Hzand 2 KHz. This would result in a series of NQR signals which are added and averaged in digital signal processor 44. Thiswhere target signals are added linearl) is an application of the conventional techniquewhile noise adds randomly, thereby building aclearly definable pulse by improving he signal-to-noise ratio (SNR). Any method toimprove SNR might advantageously be used. 30

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The power requirements ofdetection coil volume. An explosivescoil volume might have an RF power he invention are generally proportional to thescanner for mail packages with a 25 liter detectoramplifier rated at aboiat 25 Watts, peak value, for example. The amplifier produces a uniform RF field of about 1 gauss over the entire25-liter volume. In other applications, such as in narcotics detection, the RF field maybe greater than this value. For airline s baggage, an explosives detection head of about300 liters (10 ft3) volume within the coil requires a 1 to 2 KW RF power amplifier.These parameters are provided for reference purposes and are not meant to define orlimit the actual characteristics of a practical NQR system.

The RF excitation pulses are f :d from amplifier 24 into detection head 33, theoperation of which will be discussed below. After the sample in the detection head hasbeen excited by the RF pulse, a short &lt;F coil "ring-down" or dead time occurs, duringwhich the receiver is "deaf," before sensing occurs. This ring-down time could, forexample, be 500gs. Then RF coil 3 4 detects the NQR signals and the response isamplified by low-noise, high-gain pre: unplifiers 25 having a gain of 20 to 30 dB, anda noise figure of 1 to 2 dB. Examples of such preamplifiers are Anzac Model AM-110and Mini-Circuits Model ZFL-500 L?&gt; S.

In the package or letter scanner size configuration of the invention, after thereceived signal has been sufficiently amplified by RF amplifiers 25 which, together withamplifier protection components 29, include appropriate conventional filter functions,the received signal is fed into two phas ϊ sensitive detectors 27 and 28, having referencesignals shifted 90° from each other by means of phase shift element 31. Note thatreference RF signal from RF source 23 is applied to phase sensitive detector 27 while j the Reference signal to phase sensitive detector 28 passes through phase shift element 31.The two mutually phase-shifted analog signals are then fed into signal-capture and dataprocessing subsystem 41, which will be discussed below.

Detection head subsystem 33 is comprised of four main components. These are RF coil 34, an RF probe circuit which is RF tuning and matching network 35, auto-tune subsystem 36 and RF shield 37. The detection head serves two primary purposes. One is to produce a homogeneous RF field in the RF coil. The other is to receive the raw NQR signal, if present, from the iterr under investigation. The manner in which a

F;\WP60\US£RS\ATTY\LAM\PATENTS\QM47,PCT -12- depth of copper at 3.4 MHz is about900 KHz is about 0.002 inches. Dii homogeneous field is ensured within the RF coil cavity to achieve uniform tip angles inthe nuclei of the target substances wil t be described with respect to Fig. 7. RF coil 34, which may also referred to as an antenna, is made of a highlyconductive material, such as copper. The conductor should have a thickness in the orderof at least five times the skin depth oi the material of the conductor at the operationalfrequency. This ensures a minimal am junt of resistance to the flow of current when thecoil is energized with RF. A 25 liter detection volume (for a mail scanning device) hasa single turn, high-Q, 0.010 inch-thick copper coil made of a single sheet The skin0.001 inches and the skin depth of copper atect coil tuning results in an increased overall efficiency for the mail scanning embociment of the invention. The single-turn, high-Qcoil, when no sample is present, tha is, the coil is empty, requires approximately30,000 pF of capacitance for tuning at ί.bout 3.4 MHz in order to detect the 14N resonantfrequency of RDX explosives. Using a series of switches to add or remove capacitancein order to re-tune the coil under differing load conditions, it has been determined thatit would be useful for the system ta be re-tunable for a 10% change in tuningcapacitance. In this particular applicat on, the coarse tuning,increments in capacitancewere selected to be approximately 80 f F, and in the fine tuning mode, 10 pF. The RFsignal source and amplifier (23,24) of RF sub-system 22 used to exercise the auto-tunesubsystem are the same as those used to excite the RF coil for substance detectionpurposes. Details of the auto-tune sub system are set out hereinbelow.

The basic tuning of the coil to :reate a uniform field; within the RF coil cavityfor optimum operation and sensitivity of thebecause it is highly desirable to cause uniformexpected measurement volume. Of course, the measurement volume of interest is the target substance (contraband) within the specimenor sample in the coil cavity. Uniform se nsitivity in the receive mode, due to reciprocity,to the nuclear precession-generated fielc s, is equally important. A "hole" in the field cantarget substance by reducing the effect of thegenerated signal (less than complete arjd uniform nuclei tipping) and at the same timeresulting in a reduced received signal. during the transmit mode is necessarysystem. This homogeneity is importanitip angles of the nuclei throughout the

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Further details of these conceIt is desirable for a volumethroughout the detection volume. WiRF flux field in that detection volumein sensitivity due to reduced tippingtransmitted pulse. By reciprocity,induction signals in those regions of nof changes in tip angle and the changes25% reduction in the effect of the exqireceiver effect, or overall sensitivity.

With reference now to Fig. 7,greater than the height X or the wiregion 52 provided that the static tuniuniformly along gap 101 of the coil. I:capacitance in the central region can ccoil with a length not substantially gnfield near the ends will be less if the cagap. The field at the ends can be mad:of the total capacitance placed near thecapacitors 102 are shown in gap 101of Simplicity, and because their placeipthejdetection system is constructed,the total capacitance for an empty RF3.41 MHz, the 14N resonant frequencysubstances have different crystallinethe distributed static tuning capacitanccapacitors 102 are determined by theBy way of example, the ideal t:frequency is 3.410 MHz. The totalaccording to the equation: follow. detection system to have a uniform sensitivityNQR this can be assured by having a uniformA region of reduced field may cause a reductionthe nuclei during the time of the excitation ore antenna will be less sensitive to the nuclearejduced field during the receive mode. The effectsin receiver sensitivity are cumulative. Thus, aitation pulse results in about a 50% reduction in f ts of til for a single turn coil 34 with a length L muchY, a uniform RF;field will exist in centralag capacitance is distributed or spaced generallyinproper placement of the distributed static tuningause variations in the i field in that region. For aeater than the lesser of the length or width, theaacitance is uniformly; distributed along the entiremore uniform by increasing the relative portionends of the gap. A multiplicity of static tuning

They are shown evenly spaced here for purposesent and respective values are determined whenreference purposes, it was previously stated thatcoil is about 30,000 pf for static tuning at aboutof RDX explosives. Of course, different targetctures and different resonant frequencies, soe would be different. The’ sizes and spacings ofDrocedure which follows. p angle for RDX is 117° and its nuclear resonantcapacitance necessary in the coil is determined c.th

Fcr

: SUU actual factual

3.4ioJ

Eq. 1 \2 Λ

F:\WP60\US£RS\aTIY\LAMU&gt;ATENTS\QM47.PCT 10 15

The procedure for determiningprobe connected to an inductance meterBy approximation, a multiplicity of cajdistributively connected across the gapmeter, the resonant frequency of the cojlis plugged into Eq. 1, the divisionmultiplied by 30,000 (C„), giving a closis repeated until the resonant frequencycapacitor distribution is adjusted tocavity, as set out below.

To determine if the capacitorRF cbil is connected to a signal generatecoil for the specified target substanc;conventional impedance meter is used t&lt;of the loop is parallel to the axis of thealong the axis, some capacitance is remareas of low field. The process of mea£continued until the desired field homog 20 25 -14- -Toui&gt; the desired capacitance, is to connect aicross coil gap 101 and measure the inductance. i pacitors, totalling approximately 30,000 pf, areshown in Fig. 7. Then with an impedanceis measured to provide f^. That frequencyade, the result squared, and that number is;er approximation for This static tuning of the coil is about 3.410 MHz. Then theieve homogeneity of the field within the coil as acti pla; ement is correct for the empty cavity coil, ther tuned to about the resonant frequency of theand a small pickup loop connected to amap the RF flux field inside the coil (the axiscoil). If the field isi not sufficiently uniformcjved from areas of high field and transferred touring the field and transferring capacitance iseneity is achieved.

iF

AUTO-TUNE SUBSYSTEM

Apparatus for automatic fine adverse conditions is shown in Fig. 5.control programming 91 for auto-tunepreferably incorporated within RF shieldInput/output line 92 connects the tuned connects the coil as the receiver of theThe system consists of a series onumber of vacuum relays 94. The anl&lt;circuit is determined by measuring theamplifier 24 to RF detector coil 34 (oi"reflected" power.) The means to mear tuning of the NQR detection coil/head underWithin sequence controller 21 is software orsubsystem 36. The; auto-tune subsystem is37, as are RF coil 34 and matching network 35.coil to the amplified RF excitation signal and NQR signals to lA wave line 38 (Fig. 1). 1 fixed value capacitors 93 switched by an equalount of capacitance switched into the tuningamount of power being transferred from RF, more precisely, the: amount of "forward" to;ure this power transfer efficiency consist of a 30 F:\WP60\USERS\ATrY\LAM\?ATENTS\QM»7.PCr 15- 10 15 20 25 variety ,of common RF techniques. For one application, a directional watt meter is usedto measure the amount of "forward" to "re fleeted" power. Based on the power transferefficiency, capacitors are switched in or out of the circuit to maximize power transferefficiency from the RF amplifier to the Rl’ coil. The system is thus re-tuned to providethe most efficient and most sensitive RF c oil. Once the state of tune of the RF coil hasbeen determined by the values of the forward and reflected power, the coil is re-tunedby switching in capacitance according to the algorithm described below.

Tuning of the RF coil consists of two stages: coarse tuning and fine tuning. Aflow diagram for the sequence is shown jn Fig. 6. The value of "C" in Fig. 5 has beenchosen to be lOpf, so each capacitor i| a multiple of "C.“ Other values could beassigned as desired.

Coarse Tuning ·' Both the forward and reflected pokver are measured. If reflected power is greaterthan a predefined percentage of forward power, then the system self-adjusts to coarsetuning by making the large jumps mentioned above (by increasing capacitance untilreflected power drops below the maximi m value of reflected power that the fine tuningmode can handle). When that condition is reached, then the system goes into finetuning. The upper limit of the size cf the capacitance jump is determined by thecapacitance range of the fine tuning subsystem. When the reflected power drops belowa preset upper size limit, then the systei i will begin fine tuning. This is the "start finetuning mode" point

Fine' Tuning

After taking a step (either increasing or decreasing capacitance) the reflectedpower is measured again. If reflectei power has increased and the direction (i.e.,increase or decrease of capacitance) ha&gt; been reversed from the previous step, then thesystem goes back one step to the "start fine tuning mode" point. Fine tuning beginsagain, only this time in the opposite direction (that is, adding capacitance instead ofsubtracting it). If, however, the reflected power did not increase, then another step is taken in the same direction (adding oruntil another reversal of the direction i: subtracting capacitance). This process continuesencountered. At this stage the system goes back one step and the fine tuning is compl :te. The reflected power is now at a minimum. 30

F;WP60\USERS\ATTY\LAM\PATENTS\QM47.PCT -16- coil to provide optimum performanceSecondly, it determines the state of the

The forward power is measured and compared to a pre-defined value, to ensure correctfunctioning of the RF transmitter. : Auto-tune subsystem 36 perform^ two major functions. One is to re-tune the RFunder a range of toil loading conditions,tune by comparing it to a pre-defined "zero" setting. The system consists of a radio frequency power source, a directional watt-meterand switched capacitors to vary tuning re actance. Control unit 21 operates the RF powersource, measures the reflected power and then varies the tuning reactance until aminimum in reflected power is reached. The system’s ability to tune the sample coildirectly results in increased overall effic .ency. Antenna tuning systems commonly used • ;ϊ' . . : in radio electronics are unnecessarily co nplex for coil fine tuning in NQR applications.They also have certain inefficiencies for NQR applications: ;they cannot tune the coildirectly, and they experience higher ft ed line losses, which can contribute to noise.Furthermore, antenna tuning systems tend to be too general in terms of what is beingmatched (for example, tuning range). RF probe 35 is a matching network and Baiun which provides tuning andpreamplifiers 25 from the high voltages in thematches RF coil 34 to a 50Q unbalanced input.

This, makes the coil look like a 50Q tritnsmitter/receiver and.'is conventional matchingtechnology. The function of ’/« wave line 38 is to isolate the receiver from thetransmitter. Transmitter isolation diodes 39 and 40 have a related function. The auto-tune. subsystem determines the state of the tune of RF coil 34 in the detector head bymatching the RF coil to its load in the detection volume. It measures the amount ofpower transferred directly to the RF coil (the "forward" power), and the amount ofpower reflected back due to losses in th s circuit and mis-tuning (the "reflected” power).Once the tuning state is determined by comparing the values of the forward and reflected:nng capacitance according to a predetermined matching of the coil, and also protectscoil during RF excitation. RF probe 35 powers, the coil is re-tuned by switc'sequencing as has been discussed abo\When coil 34 is loaded with 8 e. package of unknown contents, it becomes de- tuned. In one application of this invention, to re-tune the· coil, eight vacuum relays switch the capacitors arranged in pF 3 alues of powers of two, that is, 10, 20, 40, 80.

F:\WP60\USERS\aTTY\LaM\PAIENTS\QM47.PCT -17-

This particular arrangement is capabletuning the system, with a maximumsystem with one relay for each value ofthe number of relays needed to produce20 + 80 = 100, etc.), and affords very f;same algorithm can be used with a conti amotor could be employed and the actualdescribed for discrete, direct capacitorarrangement described above is prefem:

Using capacitors switched by vji6 ms or less to allow the relays to opsteady-state value. The benefit in overallsmall size due to the fixed switch capa::in precision tuning which might havevariable capacitors. However, becauseby a computer operated sequence contramount of system de-tuning.

This tuning sub-system offers imautomatic fine tuning of the sample coituning required manual tuning of the syundesirable for field use. This systemon fixed capacitors switched by vacuunthan bulkier and slower variable capacicoil loading, a feature not available oneasier to use than a manually tuned s;state-of-tune of the RF coil which cansample). The system also gives the conRF amplifier.

Physical configurations of therespect to Figs. 2, 3 and 4. RF coilconductive material, as previously ast of producing 256 values of capacitance for re-of 3000 pF. Rather than overloading thecapacitance, this power arrangement minimizesa given value of capacitance (eg. 10 + 20 = 30;operational speed. It should be noted that theuously-variable capacitance system. A steppertuning sequence would be very similar to thattuning. The direct coil tuning capacitance d for this invention,cuum relays requires a "settling time" of aboutirate and for the reflected power to achieve asystem ruggedness, efficiency, reliability, andritor scheme overcome any possible advantagebeen achieved using the more conventional' he system uses switching commands controlleddling device, it can get exact information on the proved sensitivity for NQR systems by optimum (RF coil). Previous developments in coil fineitem, which is acceptable for the laboratory butoffers the advantage of automatic tuning basedrelays (designed for high RF switching) ratherdors. The proposed system measures changes ino ther detection systems' The system is faster andample coil, and provides information about theve an indication of the contents of the coil (theitrol unit an indication of the performance of the gji' scanner of the system will be described with34 is a hollow rectangular tube of thin sheetdescribed, formed on thin-walled rectangular F:\WP60\USERS\ATmLAWA1tNTS\QM47.PCr -18- insulator 51 (See Fig. 3). Shield 37 is a conductor in the shape of a rectangular copper(or other highly conductive material) sleeve enclosing the coil and spaced from it by adistance of about one half the length of the shortest side of the coil. The shortest sideof the coil is represented by distance "X" in Fig. 3 and the spacing is preferably X/2.As an example of actual size, X is five t&lt; six inches, so the spaicing between coil 34 andshield 37 would be about 2.5 to 3.0 inches. Another significant measurement is thedistance between the edge of coil 34 and opening 52 through which the item to be testedis inserted. That is the same X/2 distance, or about 2.5-3.0 inches. The RF shield provides the coil and probe units, thatnecessary EMI/RFI (electromagnetic intefrom: external noise. At the same time. s, the structure within the RF shield, with therference/radio frequency interference) shieldingthe structure inhibits: RFI from escaping fromthe specimen testing cavity. This configuration has been optimized to provide the bestbalance between noise isolation of the c )il, loading of the coil, and minimization of the total system volume.

To complete the dimensions of the scanner of Fig. 3 for purposes of example, the long dimension of the rectangular cavity at the opening may be 16 inches, and the length of the cavity within the coil may be 24 inches. Surrounding shield 37 may have a depth of 10-11.5 inches, a width of about 20-22 inches, and a front-to-back length ofat least about 27 inches. The volume o7 the cavity would be about 2000 in3 (26 liters).The scanner described above may be referred to as a box with a cavity therein, havingexternal access opening 52 to the cavity

Different arrangements are nece ;sary for the front andhack of the coil. The bestRFI shielding is normally an electrical y connected and grounded box that completelyencloses the RF coil, such that exterml noise cannot reach the RF coil directly. Formost real-world applications of this ted nology, this arrangement is not always possible.An RFI trap or cut-off device is needec to permit access to one or both ends of the coilfor movement of the sample item in and out of the coil. In an application of thisinvention, a portable hand-fed mail or package scanning device, only one end is openand this end, door 85 (Fig. 2), is closed after the package is inserted and before the testis commenced. This closed configurai ion completes the RFI trap.

F:\WP60\USERS\ATTinLAM\PAraJTS\QM47.PCT 19-

For a conveyor system to scan aiiline baggage, both ends of the RF shield are, provide the necessary RFI shielding, a tunnel,commonly known as a "wave guide belo1 v cut-off' of about the same maximum cross-sectional dimension as the coil, is required. Ends 66 and 71 provide the wave guidebelow cut-off for this configuration of th ϊ invention. While the overall dimensions aregreater, the coil, shield and opening relati jnships remain substantially consistent. In this of necessity, open, as shown in Fig. 4. Tc case the "X" dimension between coil 6 and shield 62 is X/2, the same as between edge 63 of the coil and end 64 of the rrain part of baggage-size scanner structure 67.

As an example, the X dimension may bebe around 28 inches. Opening 65 is thetunnel end extension 66, the main tunnelout through wave guide or tunnel end e: 18 inches and the width, dimension "Y," couldsame size all the way through wave guide or(not shown) through test apparatus box 67, andctension 71.

Some additional exemplary dimensions are given: here for purposes ofih completeness. The front-to-back leng36 inthes and the cavity volume would of the cavity, in scanner box 67, is aboutbe about 10.5 ft3 (305 liters).

While X/2 is the preferred spacir g discussed above, it heed not have exactly thatrelationship to the short dimension of t le coil cavity. The shield spacings may rangebetween X/3 and X, with X/2 being pre feired at the present time.

In addition to the coil and shield, some typical materials for facing 53 and forinner rectangular frame 51 are wood an i plastic. They should be relatively light, rigid,and be an electrical insulator. In the larger, double open-ended version of Fig. 4,external surfaces 72 and 73 of tunnel ends 66 and 71 would likely be copper oraluminum, while the inside and facing would be plastic or wood. A practical smaller size, or por:able, mail scanner 81 is shown in Fig. 2. Theelectronics and control functional elem ants can be contained: in box 82. The scanningdevice itself is box 83 mounted on tap of box 82 by stand-offs 84. The front ofscanner 83 is normally closed by from door or lid 85 which is hinged to the top box.A package 86 is shown in opening 52, within the RF coil, ready for test.

To reiterate, in the portable mail scanner of Figs. 2 and 3, only one end of thecoil is required to be open for access to the coil compartment or cavity. The shieldentirely surrounds the coil, except for an opening of the same cross-sectional area as the

F;\WP60\USERS\A iTjUAM\PATENTS\QM47.PCT -20- coil. This opening forms a slot througin the shield is positioned in such a wayend of the coil) that the magnetic fluxthe shield itself. Thus, little fluxenter the cavity. In order to further mi:in the form of a grounded, aluminumoverlaps the coaming may besurrounding shielding 37 and does notoverhang 68 if desired (see Fig. 3)provides improved RFI shielding forpackage 86 in the cavity. Suitable rubbercompletes the EMI/RFI shielding.

Once the auto-tuning procedurebegins. The scanning procedure is sta-detection applications. In one applicationcombination of RF pulses, commonlyand NPAPS (non-phase-alternated puls^free precession) pulse sequence.5,365,171, which is incorporated herei|iexplanation. However, there are otherin NQR procedures which are also appliand readily useable by those of ordinaijyWhen test button 87 is pushed,accomplished and at least one of thetuning and testing are being completed,contraband being tested for is present,target substance has been found in slight 94 is illuminated, it means thereat or further tested. It could mean therjsyellow and green lights illuminated memetal or other conductive material p escapes emplc yi which packages can foe passed. The opening(approximately two 'to three inches from theom the coil is "forced" to be contained withinfrom the shielded opening and little flux caninimize EMI/RFI noise entry, a secondary shieldenclosure, with EMI/RFI gaskets and a lid whiched. This is an aluminum casing closelyadd to the overall dimensions, except to makeis overhang is both aesthetically pleasing andexternal RF. Lid 85 covers opening 52 withgaskets impregnated! with conductive material

Tliis has been completed, the scanning procedure.idard for detecting NQR signals in real-worldof this invention, the procedure consists of awn as PAPS (phase-alternated pulse sequence)sequence) versions of the SSFP (steady statese sequences are described in U.S. patentby reference to the extent necessary for fullsequences of RF pulses which are commonly usedable for use in this invention. These are known skill in this technical field.

[he coil is tuned and scanning of the package is s is illuminated. White light 91 flashes while

Illumination of green light 92 indicates that noIllumination of red light 93 indicates that thea quantity as to be significant. If yellowbe something present which should be lookedis a significant amount of metal present. Boththere was no clear NQR signal and there wasBoth red and! yellow lights illuminated lig ht uch it esent.

F:\V/P60\USERS\ATrYV.AM\PATENTS\Qto47,PCT -21- indicates that the target substance may fc e present, but it is at lieast partially obscured bymetal. Those are indeterminate resuts. Not shown is an ON/OFF button on anon-visible side of unit 81.

One challenge which must be overcome in proceeding from the laboratory to apractical NQR detection system for seaming airline baggage is that of acoustic ringing.A standing wave can be set up in a conductor placed in a pulsed RF field. Thisacoustical wave is picked up by the RF coil. The signal produced is often close to thesame magnitude and sufficiently close in characteristics to ari NQR signal to possiblycause a false alarm. The acoustical signal is often coherent with the exciting RF pulse,and hence can potentially be mistaken f m an NQR signal, which is also coherent withthe exciting RF pulse. Moreover, commc n methods for reducing spurious ringing effectsin . laboratory NQR systems, such as signiil averaging and/or reversing the RF phase, willem. Certain types of commonly-occurringmaterials, such as spring steel, are particularly prone to acoustic ringing.

In the preferred embodiment of t lis invention, a simple but effective method ofreducing the effects of acoustic ringing it i NQR detection applications is employed. Theprimary differing characteristic of an N|QR signal compared with an acoustic ringingat pre-defined frequencies. Acoustic ringinged by any frequency of an RF excitation pulse. signal is that NQR signals occur onlysignals, on the other hand, can be genera1Thus, by operating the NQR scanning system at a frequency outside the range of theNQR sample frequency, using a standard or modified RF pulse sequence, no signal willbe generated by or be detected from an; &lt; target material. Under these conditions, if asignal is seen, it is from acoustic rnging. Implementation of this method is straightforward. The "ring detect" sequence can be implemented before or after the main 1 sample detect sequence and is part of th&lt; programming and RF signal generation. Thisfrequency excursion can easily be provided by the auto-tune aspect of this invention.

As an alternative for detection of acoustic ringing, the standard target substancedetection scanning cycle can be employed. It is a principal of acoustic ringing that theringing signal decays with time. Withir a limited time period, between respective RFpulses, the NQR signal increases with time. This feature can be used to determine thenature of the signal response. This procedure can be used in some instances, and is

F:\W?60\USERS\ATTY\LAM\I&gt;ATENTS\QM47.PCT -22- limited at the highest sensitivity lev :1s, where the noise-, level of the system iscomparable to the signal level.

In the package or mail scanner donfiguration of this invention, when employinganalog detectors, signal capture and data processing subsystem 41 comprises two analogand digital signal processor 44. The receivedsignals from phase sensitive detectors Z 7 and 28 are fed to A/D converters 42 and 43respectively. All signals produced by the sample scan and ring detect sequences are fedid by the digital signal processor. Through theadded or subtracted, according to the algorithm outlined in patent 5,365,171. The addiljion/subtraction algorithm reduces the effects ofRF coil ring-down and magnetoacoustic ringing.

In a practical configuration of t lis portion of the invention, signal capture andout on a plug-in PC A/D converter card. Theand a 2 MHz sampling rate. Subsystem 41 also into the A/D converters and are processsample scan sequence, signals are either 10 most of the signal processing is carriedcard has two channels, 14-bit resolution, 15 performs on-board digital signal processing functions, such as addition or subtraction of consecutive data sets as required. Once processing the output signal is completed, it is digitally filtered and compared to a predefined threshold level. Alternatively, once the ed, it occurs as a quadrature "spike" at or closeis then filtered and compared to the "known" 20 25 signal is apodized and Fourier-transfomto 0 Hz in the frequency spectrum, ancsignal of the material to be detected.

In the frequency domain, the bignal capture and data processing subsystemcompares other signal factors to the expected signal factors. For example, it maycompare the signal shape (Lorentzian or Gaussian) to the line-width at half height. Acombination of the above signal factors may be used to determine the presence orabsence of the target substance. The oi ltput of the digital signal processor is then sentto display device 46.

The NQR detected signal is com] &gt;ared with a predetermined threshold level storedin memory in digital signal processor *4. If the detected signal is equal to or greaterthan the predetermined threshold, red lij^it 93 flashes on the operator’s panel on displaydevice 46, indicating the presence of the target substance. If the signal is less than thepredetermined threshold, green light 92 flashes, indicating the absence of the target 30

F:\WP60\LfSERS\ATWLAM\?ATENTS\QM47.PCT -23- substance. If the auto-tune algorithmthe coil is necessary, compared to an detects that an excessive amount of re-tuning ofaverage investigation or predefined threshold, or an acoustic ringing signal is detected, the condition is flagged and yellow warninglight 94 illuminates. The yellow warning light indicates that; (1) there is an abnormallyhigh amount of metal in the coil, (2) a high quantity of high dielectric material isdetected, or (3) a spurious acoustic sigial has been detected.. Further alternative testingor visual inspection can be used to re: olve inconclusive results of the NQR test.

In addition to the illumination indications mentioned above, the display devicecan optionally provide graphical display 95 of the signal showing both the in-phase andquadrature signals, as well as other signal and system characteristics. Also optionally,printed output 96, including the time, late, signal amplitude and frequency, as well ascoil tuning parameters, and other info rmation such as acoustic signal responses fromspeaker 97, can be provided.

The factors which have degraded the effectiveness: of previous NQR signaldetectors are reduced or eliminated by this system. If conductive or high dielectricmaterials are present in the sample, the auto-tune sub-system will be employed in anattempt to neutralize the effect of the : ‘oreign material. Then visual inspection can beaccomplished if there is reason to do so. The auto-tune capability can quickly accountfor changes in temperature which affe :ts tuning capacitance, as well as movement ordistortion of the coil which might occi cr when samples are put into the cavity.

Examples of two embodiments of the invention have: been described above. Itis likely that modifications and improvements will occur to those skilled in this technical field which are within the scope of the appended claims.