WO2006088544A1

WO2006088544A1 - Reduction of man-made rf interference in a nuclear quadrupole resonance detection system

Info

Publication number: WO2006088544A1
Application number: PCT/US2005/045385
Authority: WO
Inventors: Daniel B. Laubacher
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2004-12-13
Filing date: 2005-12-12
Publication date: 2006-08-24

Abstract

This invention relates to the reduction of the man-made radio frequency interference coupling into a sensor in a nuclear quadrupole resonance detection system that has a tunnel through which the object to be scanned passes. The coupling is reduced by placing the sensor so that the normal to the plane of the sensor is perpendicular to the axis of the tunnel.

Description

TITLE

REDUCTION OF MAN-MADE RF INTERFERENCE IN A NUCLEAR QUADRUPOLE RESONANCE DETECTION SYSTEM

This application claims the benefit of U.S. Provisional Application No. 60/635,583, filed December 13, 2004, which is incorporated in its entirety as a part hereof for all purposes .

Technical Field

This invention relates to the reduction of the man-made radio frequency interference that can limit sensor performance in a nuclear quadrupole resonance detection system.

Background The use of nuclear quadrupole resonance (NQR) as a means of detecting explosives and other contraband has been recognized for some time; see, for example, T. Hirshfield et al, J. Molec. Struct. 58, 63 (1980); A.N. Garroway et al, Proc. SPJS 2092, 318 (1993); and A.N. Garroway et al, IEEE Trans, on Geoscience and Remote Sensing 39, 1108 (2001). NQR provides some distinct advantages over other detection methods. NQR requires no external magnet such as required by nuclear magnetic resonance, and it is sensitive to the compounds of interest, i.e. there is a specificity of the NQR frequencies .

One technique for measuring NQR in a sample is to have the sample transported through a tunnel-like apparatus in which the sample passes within a solenoid coil that surrounds the sample. The coil provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample and results in their producing their characteristic resonance signals. The signals are detected by a sensor - either by the solenoid, which performs as a transmit and receive coil; or by a separate receive coil, in which case the solenoid performs as only a transmit coil . In either case, the normal to the sensor is parallel to, and typically coincident with, the axis of the solenoid, i.e. the axis of the tunnel. This is the typical apparatus configuration that might be used for scanning mail, packages or luggage. Man-made RF interference can enter the tunnel and couple into the sensor thereby limiting the sensor performance.

A detection system can have one or more coils that serve as both excitation and receive coils, or it can have separate coils wherein each type of coil performs only one of the functions - i.e. one coil only excites nuclear quadrupole nuclei, and one coil only detects nuclear quadrupole resonance when it is present . An excitation, i.e. transmit, coil of an NQR detection system provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample and results in their producing their characteristic resonance signals that the receive coil, i.e. sensor, detects.

It can be especially advantageous to use a sensor made of a high temperature superconductor (HTS) rather than copper since the HTS self-resonant coil has a quality factor Q of the order of 10³-10^S. The NQR signals have low intensity and short duration. In view of the low intensity NQR signal, it is important to have a signal-to-noise ratio (S/N) as large as possible. The signal-to-noise ratio is proportional to the square root of Q so that the use of a HTS self- resonant coil as a sensor results in an increase in S/N by a factor of 10-100 over that of a copper coil. Therefore, the use of a high temperature superconductor coil with a large Q as the sensor provides a distinct advantage over the use of an ordinary conductor coil .

An object of the present invention is to reduce the man-made RF interference coupling into a NQR sensor in a NQR detection system.

Summary This invention provides a nuclear quadrupole resonance detection system comprising a tunnel through which the object to be scanned passes and at least one sensor, wherein each sensor is located with the normal to the plane of the sensor perpendicular to the axis of the tunnel to thereby reduce the man-made RF interference coupling into the sensor.

Preferably, there are at least two sensors and they are not all placed on the same side of the tunnel . Preferably, when there are two or more sensors the sensors are arranged in pairs with one sensor of a pair placed on one side of the tunnel and the other sensor of a pair placed on the opposite side of the tunnel . Preferably, the sensors are high temperature superconductor self-resonant coils. Most preferably, the sensors are high temperature superconductor self- resonant planar coils. Preferably, the only function of the sensor coils is to detect NQR signals, and a separate transmit coils are used to excite the nuclear quadrupole nuclei.

Improving the performance of a nuclear quadrupole resonance detection system is especially important when the sample may be or contain explosives, drugs (controlled substances) and other contraband, and the purpose of the nuclear quadrupole resonance detection system is to determine the presence of such a substance by detecting the nuclear quadrupole resonance thereof. Detailed Description

This invention addresses the problem of the coupling of man-made RF interference into a sensor of a nuclear quadrupole detection system. This invention provides a solution for a NQR detection system that contains a tunnel through which an object to be scanned is passed.

As used herein, "tunnel" means an opening in a device that performs the activities of scanning a sample for the detection of the presence of nuclear quadrupole resonance. The sample is passed into, down the length of the longitudinal axis of, and out of the opening that forms the tunnel. The object is scanned for the detection of nuclear quadrupole resonance while it is in the tunnel. The cross-section of the tunnel, looking down its length, can have various shapes. Typically, the tunnel will have a rectangular cross- section, but the cross-section may be in other shapes such as circular, substantially circular, elliptical or polygonal. Typically, there will be a conveyor belt or other means to transport the object to be scanned through the tunnel, i.e. from one end of the tunnel to the other.

Man-made RF interference can be expected in the environments in which objects are being screened by NQR in such manner for contraband. Made-man RF interference can enter the tunnel, and, when the normal to the plane of a sensor is parallel to the axis of the tunnel, the RF interference can couple to the sensor. This can impair the performance of the sensor. This is especially important when high temperature superconductor coils are used as sensors . These sensors have significantly higher sensitivity than those made of conventional conductor materials. The higher sensitivity of the HTS sensors enables them to detect smaller amounts of contraband. Although the HTS sensors are able to detect these low intensity signals, made-man RF interference could seriously affect that ability.

One way to reduce the effect of the made-man RF interference is to lengthen the tunnel so that the RF field penetrates a smaller portion of the tunnel . A sensor, with the normal to the plane of the sensor parallel to the axis of the tunnel, can then be positioned toward the center of the tunnel, i.e. farther from the ends of the tunnel, where the field is not significantly large to limit the sensor performance .

However, in most environments in which objects are being scanned for contraband, the length of the tunnel is an important consideration. This invention enables the use of a tunnel of convenient length. This invention reduces the man-made RF interference coupling into a sensor by rotating the sensor so that the normal to the plane of the sensor is perpendicular to the axis, i.e. the centerline, of the tunnel. The sensor is placed along the side of the tunnel wall.

The reference to a side of a tunnel is used here in the sense of distinguishing the placement of one sensor (or excitation coil) in a certain location about the wall or perimeter of the tunnel that is different from the location in which another sensor (or excitation coil) is placed. A tunnel that has a rectangular-shaped, or even polygonal-shaped, cross- section will have flat surfaces that clearly qualify as "sides" of the tunnel in this sense. In a tunnel with a rectangular cross-section, for example, "sides" may refer to the top and bottom walls of the tunnel as well as to the two vertical walls. Even a tunnel with a circular, substantially-circular or elliptical cross- section, however, despite not having flat surfaces in the shape of its cross-section, may be thought of as having "sides" in the sense that the location in which one sensor (or excitation coil) is placed may be distinguished from the location in which another sensor (or excitation coil) is placed by reference to other features or attributes that enable distinguishing the location of one "side" from that of another.

Preferably, the sensors are used only for the purpose of detecting NQR signals, and separate excitation coils are used only for the purpose of exciting nuclear quadrupole nuclei. When separate coils are used for excitation and detection, an excitation coil preferably has the same orientation as a sensor, i.e. along a side of the tunnel with the normal to the plane of the coil perpendicular to the axis of the tunnel .

More than one sensor may be needed to provide the desired sensitivity to NQR signals. When two or more sensors are used (e.g. multiple pairs thereof), it is preferable to have them placed on different sides of the tunnel. In one preferred embodiment, pairs of sensors are arranged directly opposite one another on opposite sides of the tunnel with one sensor of a pair placed on one side of the tunnel and the other sensor of a pair placed on the opposite side of the tunnel facing the first sensor. Preferably, the normals to the planes of the two sensors of a pair are collinear, and the planes of the sensors are parallel.

Separate excitation coils are preferably arranged in a similar configuration, i.e. in one or more pairs directly opposite one another on opposite sides of the tunnel with one excitation coil of a pair placed on one side of the tunnel and the other excitation coil of a pair placed on the opposite side of the tunnel facing the first coil. Preferably, the normals to the planes of the two excitation coils of a pair are collinear, and the planes of the excitation coils are parallel.

Preferably, the sensors and the excitation coils occupy the same sides of the tunnel so that the planes of the pairs of sensors are parallel to the planes of the excitation coils, i.e. the planes of all coils may thus be parallel . In one such embodiment the NQR detection system is comprised of two pairs of excitation coils and at least two pairs of sensors.

The first pair of excitation coils is arranged directly opposite one another on opposite sides of the tunnel . The second pair of excitation coils is also arranged directly opposite one another on opposite sides of the tunnel. The planes of one or both of the first pair of excitation coils may thus not be parallel with the planes of one or both of the second pair of excitation coils. When those planes are not parallel, they form an angle with each other, and may, for example, be perpendicular to each other.

Each pair of sensors is arranged directly opposite one another on opposite sides of the tunnel with one sensor of a pair placed on one side of the tunnel and the other sensor of a pair placed on the opposite side of the tunnel . The sensors are placed on the same sides being occupied by the excitation coils. At least one pair of sensors is arranged on the sides being occupied by the first pair of excitation coils. At least one pair of sensors is arranged on the sides being occupied by the second pair of excitation coils. The planes of one or both of the first pair of sensors may thus not be parallel with the planes of one or both of the second pair of sensors. When those planes are not parallel, they form an angle with each other, and may, for example, be perpendicular to each other. In a preferred embodiment, the result is two orthogonal or perpendicular pairs of excitation coils and at least two pairs of sensors that are orthogonal or perpendicular. Typically when more than two pairs of sensors are used they would be divided about equally between the two orthogonal positions.

The excitation coils used in this invention can be made of copper, silver, aluminum or a high temperature superconductor. A copper, silver or aluminum coil is preferably in the form of a shielded-loop resonator (SLR) coil. SLR' s have been developed to eliminate the detuning effect of the electrical interaction between the coil and the surrounding material . Preferably, one or more SLR copper excitation coils are used as the excitation coil (s) to apply the RF signal to the sample. Preferably, one or more pairs of excitation coils are used and each pair is arranged directly opposite one another on opposite sides of the tunnel .

The sensors are preferably high temperature superconductor self-resonant coils. The high temperature superconductor self-resonant coil is preferably in the form of a self-resonant planar coil, i.e. a surface coil, with a coil configuration of HTS on one or both sides of a substrate. High temperature superconductors are those that superconduct above 77K. The high temperature superconductors used to form the

HTS self-resonant coil are preferably selected from the group consisting of YBa₀ 2Cu3OV , Tl₀2Ba₀2CaCu2O₀8,

TlBa₀Ca₀Cu O₀ , (TlPb) Sr CaCu O and (TlPb) Sr Ca CuO₀ .

Most preferably, the high temperature superconductor is YBa₀ ACuό0I or TlABa₀ACaCuAOo .

Claims

CLAIMSWhat is claimed is:

1. A system to detect nuclear quadrupole resonance in an object to be scanned, comprising (a) a tunnel, having an axis, through which the object is passed, and (b) one or more planar sensors; wherein the normal to the plane of each sensor is perpendicular to the axis of the tunnel, and each sensor is a high temperature superconductor self-resonant planar coil.

2. The detection system of Claim 1 further comprising one or more excitation coils; wherein each sensor only detects nuclear quadrupole resonance signals, and each excitation coil only excites nuclear quadrupole nuclei .

3. The detection system of Claim 2 wherein each excitation coil is a shielded-loop resonator coil.

4. The detection system of Claim 2 wherein each sensor is a YBa₃Cu₃O₇ or Tl₂Ba₂CaCu₃O₈ high temperature superconductor self-resonant planar coil, and each excitation coil is a copper shielded-loop resonator coil .

5. The detection system of Claim 2 wherein each excitation coil is planar, and the normal to the plane of an excitation coil is perpendicular to the axis of the tunnel .

6. The detection system of Claim 1 which comprises at least two sensors; wherein a first sensor is located on one side of the tunnel, and a second sensor is situated facing and directly across from the first sensor on the opposite side of the tunnel .

7. The detection system of Claim 6 wherein the normal to the plane of the first sensor is collinear with the normal to the plane of the second sensor, and the planes of the first and second sensors are parallel .

8. The detection system of Claim 2 which comprises at least two excitation coils; wherein a first excitation coil is located on one side of the tunnel, and a second excitation coil is situated facing and directly across from the first excitation coil on the opposite side of the tunnel .

9. The detection system of Claim 8 wherein the normal to the plane of the first excitation coil is collinear with the normal to the plane of the second excitation coil, and the planes of the first and second excitation coils are parallel.

10. The detection system of Claim 7 which comprises at least two excitation coils; wherein a first excitation coil is located on one side of the tunnel, and a second excitation coil is situated facing and directly across from the first excitation coil on the opposite side of the tunnel .

11. The detection system of Claim 10 wherein the normal to the plane of the first excitation coil is collinear with the normal to the plane of the second excitation coil, and the planes of the first and second excitation coils are parallel.

12. The detection system of Claim 11 wherein the first sensor and the first excitation coil are located on the same side of the tunnel, and the second sensor and the second excitation coil are located on the same side of the tunnel.

13. The detection system of Claim 11 wherein the planes of the first and second sensors and the planes of the first and second excitation coils are all parallel .

14. The detection system of Claim 11 further comprising third and fourth planar sensors; wherein the normal to the plane of each of the third and fourth sensors is perpendicular to the axis of the tunnel, and each sensor is a high temperature superconductor self- resonant planar coil .

15. The detection system of Claim 14 wherein the third sensor is located on one side of the tunnel, and the fourth sensor is situated facing and directly

^■ across from the third sensor on the opposite side of the tunnel .

16. The detection system of Claim 15 wherein the normal to the plane of the third sensor is collinear with the normal to the plane of the fourth sensor, and the planes of the third and fourth sensors are parallel .

17. The detection system of Claim 11 further comprising third and fourth planar excitation coils; wherein the normal to the plane of each of the third and fourth excitation coils is perpendicular to the axis of the tunnel .

18. The detection system of Claim 18 wherein the third excitation coil is located on one side of the tunnel, and the fourth excitation coil is situated facing and directly across from the third excitation coil on the opposite side of the tunnel .

19. The detection system of Claim 18 wherein the normal to the plane of the third excitation coil is collinear with the normal to the plane of the fourth excitation coil, and the planes of the third and fourth excitation coils are parallel .

20. The detection system of Claim 16 further comprising third and fourth planar excitation coils; wherein the normal to the plane of each of the third and fourth excitation coils is perpendicular to the axis of the tunnel .

21. The detection system of Claim 20 wherein the third excitation coil is located on one side of the tunnel, and the fourth excitation coil is situated facing and directly across from the third excitation coil on the opposite side of the tunnel .

22. The detection system of Claim 21 wherein the normal to the plane of the third excitation coil is collinear with the normal to the plane of the fourth excitation coil, and the planes of the third and fourth excitation coils are parallel.

23. The detection system of Claim 22 wherein the planes of the sensors and the planes of the excitation coils are all parallel to each other.

24. The detection system of Claim 22 wherein the plane of the first sensor is not parallel to the plane of the third sensor or the plane of the fourth sensor, and the plane of the second sensor is not parallel to the plane of the third sensor or the plane of the fourth sensor.

25. The detection system of Claim 22 wherein the plane of the first excitation coil is not parallel to the plane of the third excitation coil or the plane of the fourth excitation coil, and the plane of the second excitation coil is not parallel to the plane of the third excitation coil or the plane of the fourth excitation coil .

26. The detection system of Claim 24 wherein the plane of the first excitation coil is not parallel to the plane of the third excitation coil or the plane of the fourth excitation coil, and the plane of the second excitation coil is not parallel to the plane of the third excitation coil or the plane of the fourth excitation coil .

27. The detection system of Claim 22 wherein the plane of the first sensor is perpendicular to the plane of the third sensor and/or the plane of the fourth sensor, and the plane of the second sensor is perpendicular to the plane of the third sensor and/or the plane of the fourth sensor.

28. The detection system of Claim 22 wherein the plane of the first excitation coil is perpendicular to the plane of the third excitation coil and/or the plane of the fourth excitation coil, and the plane of the second excitation coil is perpendicular to the plane of the third excitation coil and/or the plane of the fourth excitation coil.

29. The detection system of Claim 27 wherein the plane of the first excitation coil is perpendicular to the plane of the third excitation coil and/or the plane of the fourth excitation coil, and the plane of the second excitation coil is perpendicular to the plane of the third excitation coil and/or the plane of the fourth excitation coil .

30. The detection system of Claim 2 wherein an excitation coil applies excitation to a sample to be screened for the detection of the presence of explosives, drugs or other contraband.