HK1127640A - Method and system for contraband detection using a photoneutron x-ray - Google Patents

Description

Method and system for detecting contraband by photoneutron-X ray

Technical Field

The invention relates to the technical field of contraband detection, in particular to a method and a system for detecting a photo-neutron-X-ray contraband.

Background

At present, terrorism poses a great threat to the stability of the world and the domestic society, and governments of various countries are all dedicated to solving the anti-terrorism problem. And the detection technology of contraband such as explosives is the core of the anti-terrorism problem.

One existing contraband detection technique is X-ray imaging detection. The X-ray imaging detection technology is a security inspection technology which has been widely used, and many devices based on the X-ray imaging detection technology can be seen in airports and railway stations. Since X-rays mainly react with electrons outside the atomic nuclei and have no ability to distinguish the characteristics of the atomic nuclei, only the density (mass thickness) of the object to be detected can be measured by X-rays, and the element type of the object to be detected cannot be determined. In practice, when contraband is mixed with daily supplies and the density is difficult to distinguish, it is difficult to find by X-ray imaging detection technology. Although some new X-ray imaging detection techniques, such as: dual energy X-ray, CT techniques, etc. have improved recognition capabilities, but have not overcome the inherent disadvantages of being unable to recognize the element species.

Another type of existing hazardous material detection technology is neutron-based detection technology. In the neutron detection technology, neutrons can react with nuclei of a substance to emit characteristic gamma rays, and the element type of the substance to be analyzed can be determined from the energy spectrum of the gamma rays. The drawback of the medium-class detection techniques is their low imaging resolution, which at present is best only up to a spatial resolution of 5cm X5 cm, which is much lower than the mm-level resolution of X-ray imaging. Furthermore, individual neutron sources are generally expensive, have limited time to use, and produce low neutron intensities.

Accordingly, it would be desirable to have a method and/or system that combines the X-ray imaging detection techniques and neutron detection techniques described above to obtain the benefits of high resolution of the X-ray imaging detection techniques and the element identification capabilities of the neutron detection techniques. U.S. patent No.5078952 discloses an explosives detection system that combines multiple detection means, including an X-ray imaging device and a neutron detection device, to achieve a higher probability of detection and a lower false alarm rate. Also, the us patent discloses correlating data obtained by an X-ray imaging device with data obtained by a neutron detection device in order to compensate for the lack of resolution of neutron-based detection techniques with high resolution X-ray images. However, in this us patent, separate X-ray and neutron sources are used, which is costly.

It is noted that one way to generate neutrons is to bombard a conversion target with X-rays and generate neutrons from the conversion target, and the neutrons thus generated may be referred to as photoneutrons. This neutron production approach provides the possibility of producing both X-rays and neutrons in one source, which is a cost savings over using two sources to produce X-rays and neutrons separately.

In international application publication WO98/55851 a system for detecting and identifying contraband using photoneutron and X-ray imaging is disclosed. The system operates in a two-step manner. Specifically, the system firstly generates an X-ray beam by a linear accelerator X-ray source, detects an object to be detected by X-ray imaging, allows the object to be detected to pass through if no abnormality is found, temporarily inserts a photoneutron conversion target (beryllium) into the X-ray beam if a suspected area is found so as to generate photoneutrons, and further detects the suspected area according to characteristic gamma rays emitted by the radiation capture reaction of the photoneutrons and substance nuclei. This system performs the first step of detection with X-rays only, which has a low Probability of Detection (PD) due to the limitations of the recognition capabilities of the X-ray imaging detection technique as described above. Moreover, the system does not generate X-rays and photoneutrons for detection at the same time, but generates X-rays and photoneutrons for detection in two steps, respectively, i.e., only X-rays and no photoneutrons are generated in one step, while photoneutrons are generated with X-rays in the other step, but the X-rays are only used for generating photoneutrons and are not used for detection purposes. Further, the generated photoneutrons are only used for detecting the suspected area of the detected object and are not used for overall detection of the detected object.

A method for material identification using fast neutrons and X-rays is disclosed in the applicant's chinese patent application No. 200510086764.8. In this application, a method and apparatus for simultaneously generating X-rays and photoneutrons is described that splits the X-rays generated by an accelerator into two beams, one of which is used to generate photoneutrons. However, in this application, neutrons are detected by the intensity of light neutrons transmitted through the object to be detected, and are not characteristic γ rays emitted by neutrons reacting with the object to be detected. In this application, in such a detection system, the X-ray beam and the neutron beam are generally required to be laterally spaced apart by a predetermined distance so that the detection of the X-ray beam and the neutron beam does not interfere with each other.

The above applications and patents are all incorporated by reference in their entirety.

Disclosure of Invention

The invention aims to provide a method and a system for detecting contraband by photoneutron-X ray, which overcome the defects of the prior art and combine the high-resolution imaging capability of X-ray imaging detection with the material identification capability of neutron detection to more effectively detect the contraband.

According to an aspect of the present invention, there is provided a method for detecting contraband in photoneutrons and X-rays, which is used for detecting an object to be detected, and comprises:

simultaneously generating a first X-ray beam and photoneutrons;

carrying out X-ray imaging detection on the detected object by using the first X-ray beam;

and simultaneously carrying out the X-ray imaging detection, and carrying out neutron detection on the detected object by utilizing the photoneutron and the characteristic gamma ray emitted by the reaction of the photoneutron and the detected object.

According to another aspect of the present invention, there is provided an optical neutron-X-ray contraband detection system for detecting an object to be detected, the system comprising:

an X-ray generator for generating a main X-ray beam including a first X-ray beam and a second X-ray beam split therefrom, the first X-ray beam being directed into and through the object;

a photoneutron conversion target arranged to receive the second X-ray beam, thereby generating photoneutrons which are directed into the object under examination and react with the object under examination to emit characteristic gamma rays;

an X-ray detection device arranged to receive the first X-ray beam after passing through the object to be inspected so as to perform X-ray imaging detection on the object to be inspected;

a gamma ray detection device arranged to receive the characteristic gamma ray for neutron detection of the object under examination from the characteristic gamma ray;

wherein the system performs the X-ray imaging detection and the neutron detection on the detected object simultaneously.

Compared with other existing schemes, the device of the invention has the following advantages:

1) compared with the single X-ray imaging detection, the method has the advantages that the types of dangerous objects can be distinguished; compared with a neutron detection method based on characteristic gamma rays, the method has the advantages of clear imaging and accurate determination of the position of an object.

2) The solution in international application publication WO98/55851 also uses photoneutron detection and X-ray imaging for detection, but it works in two steps. The invention can simultaneously carry out photoneutron detection and X-ray imaging detection, and the detection accuracy is higher than that of the proposal in international application publication WO 98/55851.

3) In one embodiment of the invention, the body of the photoneutron conversion target is shaped to substantially match the intensity distribution of the main beam of X-rays generated by the X-ray generator, so that intense X-rays can propagate a greater distance within the body of the photoneutron conversion target. Therefore, the yield of photoneutrons is high, the speed of neutron analysis is high, and the acquisition of element distribution information can be completed while X-ray imaging is performed.

4) In one embodiment of the invention, the X-ray image and the neutron image are combined into one image such that the points in the neutron image and the X-ray image corresponding to the same location of the detected object coincide exactly. The operator can obtain the element distribution information and the density information of the detected object by only observing one image.

5) In one embodiment of the present invention, there is also provided an improved gamma ray detector that shields X-rays, neutrons, and extraneous gamma rays so that a detection system can obtain a detection result with high accuracy.

Drawings

FIG. 1 illustrates a schematic structural diagram of an optical neutron-X-ray contraband detection system according to an embodiment of the present invention;

fig. 2 shows an enlarged schematic plan view of the photoneutron conversion target of fig. 1, wherein a channel defined by the photoneutron conversion target is shown;

FIG. 3 shows an end view of the photoneutron conversion target of FIG. 2;

figure 4 shows an improved gamma ray detector.

Detailed Description

Exemplary embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.

Referring to the example shown in fig. 1, an object to be inspected (e.g., the closed container 8) is disposed on a platform 19. It should be noted that the container 8 is shown in cross-section in fig. 1 in order to show the various cargo 10 carried therein, and that these cargo 10 may comprise various materials such as metal 11, wood blocks 12 and explosives 13. The platform 19 is dragged by the dragging device 20 into the detection area of the detection system of the invention. The container 8 is typically made of corrugated steel and aluminum. Other containers such as air containers may also be similarly inspected.

When a position sensor (not shown) detects that the container 8 has moved to a predetermined position, the position sensor may trigger the X-ray generator in the system of the present invention to operate. In one embodiment, the X-ray generator includes an electron accelerator (not shown) and an electron target 2. The electron accelerator, not shown, generates an electron beam 1 which is directed to an electron target 2. The electron target 2 is typically made of a material with a relatively high atomic number, such as tungsten or gold, and the electrons, after being blocked by atoms of tungsten or gold, emit a primary beam 3 of X-rays due to bremsstrahlung radiation. From this main X-ray beam 3, a first X-ray beam and a second X-ray beam are to be split, wherein the first X-ray beam is used for X-ray imaging detection and the second X-ray beam is used for neutron detection, as will be described below. Herein, the X-ray imaging detection means that X-rays transmit a test object and detect density information of the test object by detecting attenuation of the X-rays; the neutron detection is a method in which neutrons react with atoms of a test object to emit characteristic gamma rays, and the characteristic gamma rays are detected to detect element type information of the test object. It should be noted that in the system and method of the present invention, the object to be inspected is inspected by using both the X-ray imaging inspection and the neutron inspection.

In fig. 1, a photoneutron conversion target 4 is shown in partial cross-section. The X-ray main beam 3 bombards the photoneutron conversion target 4 to obtain photoneutrons 6, and the photoneutrons 6 are used for neutron detection of the container 8. In particular, in this embodiment, the photoneutron conversion target 4 is also used to split a first X-ray beam and a second X-ray beam from the main X-ray beam 3.

The photoneutron conversion target 4 in fig. 1 is shown enlarged in fig. 2 and 3. As shown in fig. 2, the photoneutron conversion target 4 includes a body 401. In one embodiment, the body 401 is an elongated body extending along the propagation direction of the main X-ray beam 3, having a first end 402 and a second end 403. The body 401 has a passage 404 extending through the body 401, the passage 404 extending from a first end 402 to a second end 403. In the embodiment of fig. 2 and 3, the channel 404 is formed as a slit extending sufficiently in plane P (perpendicular to the plane of the paper of fig. 2 and 3) so as to divide the body 401 into two mutually separated portions. Preferably, the channel 404 passes through the center of symmetry of the body 401, dividing it into two symmetrical parts. The channel 404 is defined between the two separate portions. When the X-ray main beam 3 is incident towards the body 401 of the photoneutron conversion target 4, a portion of the X-ray beam 405 passes directly through the photoneutron conversion target 4 via the channel 404 without any reaction with the photoneutron conversion target 4, which portion is defined as the first X-ray beam 405. Another portion of the X-ray beam 406 enters the body 401 and travels in a direction from the first end 402 to the second end 403, and reacts with the nuclei of the photoneutron conversion target 4 during the travel to emit photoneutrons, and this portion of the X-ray beam 406 is defined as a second X-ray beam 406. It can be seen that this channel 404 in fact functions as a beam splitter for splitting the first X-ray beam and the second X-ray beam from the main X-ray beam 3. In other embodiments not shown, the channel 404 may take other forms, for example, the channel may not divide the body 401 into two parts, but may be formed as a through hole (not shown) through the body 401, or as another channel defined by the body 401, as long as it is ensured that a fan-shaped X-ray beam for X-ray imaging can pass through the body 401.

In order to make full use of the X-ray main beam 3 emitted from the electron target 2 to improve the photo-neutron yield of the photo-neutron conversion target 4, the shape of the photo-neutron conversion target 4 may be designed to substantially match the intensity distribution of the X-ray main beam 3, i.e. to enable the strong X-rays to travel a greater distance within the body 401 of the photo-neutron conversion target 4. Referring to fig. 1 and 2, the main beam 3 of X-rays exiting from the electron target 2 generally has an axisymmetric intensity distribution with an intensity distribution symmetry axis along the direction of the electron beam 1, and generally the closer to the intensity distribution symmetry axis, the greater the intensity of the X-rays. Accordingly, the photoneutron conversion target 4 may generally have an axisymmetric shape and define a target symmetry axis 409, and the axisymmetric shape of the photoneutron conversion target substantially matches the axisymmetric distribution of the X-ray main beam 3, ignoring the channel 404 in the photoneutron conversion target 4. In this use, the target symmetry axis 409 coincides with the intensity distribution symmetry axis of the main X-ray beam 3. Preferably, at least a portion of the photoneutron conversion target 4 is preferably a tapered portion that tapers toward the second end 403, so that the photoneutron conversion target 4 has a longer length closer to the target axis of symmetry. In the embodiment shown in fig. 2, the photoneutron conversion target 4 includes a tapered portion 408 adjacent to the second end 403 and a cylindrical portion 407 adjacent to the first end 402, and the cylindrical portion 407 may be integrally formed with the tapered portion 408. The tapered portion 408 may terminate at the second end 403. The tapered portion 408 shown in fig. 2 is frustoconical. The cylindrical portion 407 and the tapered portion 408 have a common longitudinal central axis and coincide with the target axis of symmetry. In other embodiments, the tapered portion 408 may also be non-frustoconical or otherwise tapered (e.g., curvilinearly tapered). In other embodiments, the photoneutron conversion target 4 may also taper from the first end 402 to the second end 403.

Although the channel 404 defined by the photoneutron conversion target 4 is shown in fig. 1-3 as a beam splitter, it will be understood by those skilled in the art that other forms of beam splitters may be employed for splitting the first and second X-ray beams from the main X-ray beam 3. For example, the two-channel split collimator disclosed in the applicant's chinese patent application No.200510086764.8 may be employed. The two-channel split collimator can split the X-ray main beam 3 into two beams spaced apart from each other, and a photoneutron conversion target is disposed on a propagation path of one of the beams to generate photoneutrons.

It should also be noted that the feature that the photoneutron conversion target 4 has a tapered portion is not limited to the application described in the embodiments of the present invention. This feature is also applicable to any other application where photoneutrons are generated by bombarding a photoneutron conversion target with an X-ray beam, such as those described in international application publication WO98/55851 and chinese patent application No.200510086764.8, to improve the production of photoneutrons. In these other applications, the photoneutron conversion target may or may not have the aforementioned channel used as a beam splitter.

Returning to fig. 1, the energy of the electron beam 1 is generally selected taking into account the energy of the X-ray beam required and the material of the photoneutron conversion target. According to the type, detection speed and environmental safety of the detected object, X-ray beams with different energies can be selected to penetrate. For safety reasons and for cost savings, as little energy as possible should generally be selected. The electron beam 1 generated by an electron accelerator, not shown, may have an energy in the range of 1MeV to 15 MeV. The ideal material of the photoneutron conversion target 4 should have a small photoneutron reaction threshold and a large photoneutron reaction cross section, but the two are difficult to satisfy simultaneously. For X-rays of 1MeV to 15MeV, the energy is not high enough, and the photoneutron yield is low for materials with large cross-sections but high threshold, and beryllium (A)⁹Be) or heavy water (D)₂O) is a preferred material.⁹The photoneutron reaction threshold of Be is only 1.67MeV, D₂The reaction threshold for D in O was 2.223 MeV. The X-ray main beam 3 entering the photoneutron conversion target 4 and the X-ray main beam⁹Be or²H generates a photoneutron reaction, releasing photoneutrons 6. Since the energy spectrum of the X-ray main beam 3 is continuously distributed, the energy spectrum of the photoneutron 6 is also continuously distributed. In addition, when the electron accelerator is used to generate the electron beam 1 with high energy, the photoneutron conversion target 4 may also be made of a material with a high threshold value but a large cross section, such as each isotope of tungsten (W) and each isotope of uranium (U).

In one embodiment, an electron accelerator, not shown, may generate an electron beam 1 at a particular frequency, such that the electron beam 1 is an electron beam pulse 1 having the particular frequency after the electron beam pulse 1 strikes an electron target 2, an X-ray pulse 3 is generated at the same frequency. The specific frequency may be determined according to the travelling speed of the detected container 8, and may be in the range of 10Hz to 1000Hz, for example. In one embodiment, the particular frequency may be 250 Hz. The pulse width of the electron beam pulse 1 can be in the range of 1-10 μ s.

It is noted that the time it takes for the X-ray main beam 3 to bombard the photoneutron conversion target 4 to generate photoneutrons 6 is very short (typically less than 1 μ s), and therefore, it can be said that, in the present invention, photoneutrons 6 for neutron detection are generated almost "simultaneously" with the first X-ray beam 405 for X-ray imaging detection in the X-ray main beam 3, which allows for simultaneous X-ray imaging detection and neutron detection, which is clearly different from international application publication WO 98/55851.

Photoneutrons 6 are isotropic when generated within the photoneutron conversion target 4, so only a fraction of the photoneutrons can be directed towards the container 8 being inspected. Conversion of target 4 by photoneutrons⁹Be and²the H pairs have a larger scattering cross-section for neutrons, and therefore, the photoneutrons 6 exiting the photoneutron target 4 are generally emitted backwards (i.e., opposite to the direction in which the X-ray main beam 3 strikes the photoneutron conversion target 4). To improve the efficiency of the photoneutrons 6 reaching the container 8 under inspection, a neutron reflector (not shown) may be disposed behind the photoneutron target 4 (adjacent to the first end 402 of the photoneutron target 4). The neutron reflector is used for reflecting the photoneutrons 6 moving away from the container 8 to be detected so as to move towards the container 8 to be detected.

Referring to fig. 1 and 2, an X-ray collimator 5 is disposed on a propagation path of the first X-ray beam 405 before reaching the object 8 to be inspected, so as to collimate the first X-ray beam 405 into a planar fan-shaped beam. The X-ray collimator 5 is preferably disposed adjacent the second end 403 of the body 402 of the photoneutron conversion target 4 and aligned with the channel 404. Thus, after passing through the photoneutron conversion target 4 via the channel 404, the first X-ray beam 405 is collimated by the X-ray collimator 5 to form a planar fan-beam 7. The X-rays outside the fan-beam 7 will be shielded by the X-ray collimator 5, which reduces the influence of the X-rays on neutron detection, in particular on the gamma-ray detector described below.

The X-ray imaging detection of the container 8 with the first X-ray beam 405 and the neutron detection of the container 8 with the photoneutrons 6 generated by the second X-ray beam 406 will be described separately below. It will be appreciated that X-ray imaging detection and neutron detection, respectively, are per se well known to those skilled in the art. However, in the present invention, since the first X-ray beam 405 and the photoneutrons 6 can be generated simultaneously (or nearly so), beam X-ray imaging detection and neutron detection can be performed simultaneously.

First, X-ray imaging detection is described. Referring to fig. 1, an X-ray fan beam 7 (i.e., a collimated first X-ray beam 405) is directed toward a container 8 under inspection, and the fan beam 7 is attenuated by cargo 10 loaded in the container 8. These attenuated X-rays are measured by an X-ray detection device, which may be an X-ray detector array 15 comprising a plurality of X-ray detectors. The attenuation factor of the X-rays reflects the absorption capacity of the X-rays by the substance on the line from the electron target 2 to the corresponding X-ray detector in the X-ray detector array 15, and its size is related to the density and composition of the substance loaded in the container 8. Two-dimensional X-ray imaging of the container 8 can be achieved using the X-ray detector array 15. The detectors in the X-ray detector array 15 may be gas ionization chambers, cadmium tungstate crystals, CsI crystals, or other types of detectors. As mentioned before, the electron beam 1 strikes the electron target 2 at a certain frequency, so that X-ray pulses of the same frequency are generated. For each X-ray pulse, the array of detectors 15 will obtain a one-dimensional image of a certain section of the container. As the container 8 is pulled forward by the pulling device 20, the plurality of one-dimensional images from the plurality of measurements constitute a two-dimensional transmission image about the container.

Neutron detection, which is performed simultaneously with X-ray imaging detection, will now be described. After photoneutrons 6 are generated via the photoneutron conversion target 4, the container 8 to be inspected will be bathed in a photoneutron field. After the photoneutrons 6 enter the container 8 under inspection, they lose energy by scattering (both inelastic and elastic). It is not necessary to collimate the photoneutrons 6 before they enter the container 8 under examination, since it will diffuse into a fairly wide area during scattering. The photoneutron 6 is a fast neutron when it is generated, and then becomes a slow neutron within a few μ s. The energy of the photoneutrons 6 then enters the energy region of the thermal neutrons. The time interval for a photoneutron 6 from a fast neutron to a thermal neutron is typically about 1 ms. Thermal neutrons can disappear finally, and two methods for disappearing are adopted: absorbed by the substance, or escape. The existence time of the thermal neutrons in the space is 1 ms-30 ms. The neutron can also generate capture reaction in the fast neutron and slow neutron energy regions, but the section is small, and when the neutron energy is reduced, the capture section is in inverse proportion to the moving speed of the neutron, so that the section is rapidly increased. Since the electron accelerator operates in a continuous pulse mode, thermal neutron fields between different pulses are superimposed. For example, when the electron accelerator is operated at a frequency of about 250Hz and a pulse width of 5 μ s, the resulting neutron field in space will be a fast neutron pulse with a frequency of 250Hz and a pulse width of 5 μ s, superimposed on an approximately constant thermal neutron field.

The thermal neutrons may emit characteristic gamma rays after the matter has undergone a radiation-capture reaction, e.g.¹The reaction of H with neutrons may give off 2.223MeV characteristic gamma rays,¹⁴n reacts with neutrons to give off characteristic gamma rays of 10.835MeV,¹⁷the Cl reacts with neutrons to give off gamma rays characteristic of 6.12 MeV. The element types in the detected object can be judged by measuring the characteristic gamma rays. Different materials in the container 8 can give off different characteristic gamma rays under neutron irradiation. According to the difference of the gamma energy spectrum, the type of the substance can be analyzed. For example, if a large number of signals of the N and H elements are found in the container, then there may be explosives and "fertilizer bombs"; if significant gamma radiation of Cl is found, it is possible to find drugs such as heroin and cocaine (which are often stolen in the form of chlorides). In addition, nuclear materials (such as uranium and plutonium) can also be examined by measuring fission neutrons produced by photoneutron capture.

The measurement of the gamma ray energy spectrum is performed by a gamma ray detection device, which may be one or more gamma ray detector arrays 14, each gamma ray detector array 14 including a plurality of gamma ray detectors and being arranged to receive the characteristic gamma rays. Also, as shown in fig. 1, when a plurality of gamma ray detector arrays 14 are included, they may be disposed at both sides of the traveling path of the container 8. Also, the gamma ray detector array 14 may be arranged at a distance from the X ray detector array 15, i.e. offset from the X ray fan beam 7 (first X-ray beam), to minimize the influence of the first X-ray beam on the gamma ray detector. By analyzing the gamma energy spectrum signal of each gamma ray detector array, the two-dimensional distribution information of the concerned element kind is obtained.

The gamma-ray detector can be selected from a wide variety of types, such as: NaI (Tl), BGO, HPGe, LaBr₃And the like.

Two types of detectors are used in the present invention: the detector comprises an X-ray detector and a gamma-ray detector, wherein the two detectors work in an environment where X-rays, neutrons and gamma-rays coexist. The two rays may interfere with each other, especially X-rays which are strong relative to neutrons and gamma rays, and thus it may interfere with the gamma spectrum of gamma ray detection. Therefore, it is very necessary for the gamma detector to shield X-rays and neutron rays.

Figure 4 shows an improved gamma ray detector in which a NaI crystal 22 and a photomultiplier tube 23 form the body of the detector. The NaI crystal 22 has a front face 30 for receiving gamma rays, a rear face 31 opposite the front face 30, and a circumferential surface 32. When gamma rays are incident on the NaI crystal 22, photoelectric effect, compton scattering, or electron pair effect occurs. The gamma photon gives up energy to a secondary electron, which ionizes in the crystal and the electron-hole pairs produced by the ionization will fluoresce. The fluorescence photons punch photoelectrons out on the photocathode of the photomultiplier tube 23. The photoelectrons are then multiplied by a photomultiplier tube to form a voltage signal through a front discharge path. To provide shielding for the NaI crystal 22 from X-rays and neutrons. The gamma ray detector shown in fig. 4 also includes a neutron shielding material 28, the neutron shielding material 28 surrounding at least a circumferential surface 32 of the NaI crystal 22 and exposing a front face 30 of the NaI crystal 22. The neutron shielding material 28 also preferably surrounds the rear end face 31 of the NaI crystal 22. The neutron shielding material 28 is generally composed of a H-rich material, such as paraffin, polyethylene, waterAre suitable materials. Polyethylene is generally chosen in view of structural and fire protection requirements. The H atoms in the neutron shielding material 28 have a large scattering cross section for neutrons, and are capable of reflecting the neutrons and rapidly reducing and absorbing the energy of the neutrons. However, the neutron shielding material 28 emits 2.223MeV characteristic H γ rays after radiation capture of neutrons, which would interfere with the γ signal to be measured by the detector. Thus, inside the neutron shielding material 28, the gamma ray detector further comprises an X/gamma ray shield 26, the X/gamma ray shield 26 surrounding at least the circumferential surface of the detector crystal and exposing the front face 30 of the NaI crystal 22. Preferably, the X/gamma ray shield 26 also surrounds the rear end face 31 of the NaI crystal 22. The X/gamma ray shield 26 is capable of not only absorbing gamma rays emitted by the neutron shielding material 28 when reacting with neutrons, but also shielding a substantial portion of the X rays and their scattered radiation from the electron target 2, so that the gamma ray detector can be in a normal operating environment. The material of the X/γ -ray shield 26 is a heavy metal having an atomic number of 74 or more, such as lead Pb or tungsten W. A neutron absorber 27 is also provided in front of the gamma detector crystal 22, facing the front face 30 of the NaI crystal 22. Unlike the requirement of the neutron shielding material 28, the neutron absorber 27 is required to be capable of not only absorbing neutrons but also emitting 2.223MeV gamma rays of H. The neutron absorber 27 may be formed of paraffin or polyethylene with boron having a high thermal neutron absorbing capacity¹⁰B material (e.g., boron-containing polyethylene) that leaves H no longer with the opportunity to emit gamma photons. In order for the gamma-ray detector to measure only the area of the detected object in front of it, and not to be interested in signals from other directions (such as X-ray scatter, gamma count background of N in air), the gamma-ray detector further comprises a collimator 29. The collimator 29 is arranged in front of the NaI crystal 22 and the neutron absorber 27 and is used for shielding an X-ray scattering background in the surrounding space and a gamma background generated by neutrons in the surrounding substances. The collimator 29 comprises a through-hole aligned with the front face 30 of the NaI crystal, the through-hole defining an extension direction for allowing only X/gamma-rays reaching the front face substantially along the extension direction and via the through-hole to enter the NaI crystal, thereby performing gamma-rays detectionAnd (6) collimation. The diameter of the through hole can be the same as that of the NaI crystal 22, the length can be determined according to the required collimation effect, and the length range of 5-30 cm is generally selected. The collimator 29 may typically be made of a heavy metal with an atomic number greater than or equal to 74, such as lead Pb or tungsten W, or of steel.

In addition, although not shown in the drawings, a time gating circuit may be provided for the gamma-ray detector to control the measurement time of the gamma-ray detector so that the measurement time of the gamma-ray detector avoids the beam-out time of the X-ray beam generated by the X-ray generator in the system of the present invention, which may further suppress the interference of the X-ray to the gamma-ray detector.

Based on the signals from the X-ray detector array 15 and the gamma-ray detector array 14, the container 8 to be inspected can be X-ray imaged and neutron imaged, respectively, to obtain X-ray images and neutron images. Returning to FIG. 1, in the system of the present invention, X-ray imaging signal processing circuitry 17 receives signals from the X-ray detector array 15 and processes them to obtain an X-ray image. The gamma-ray signal processing circuit 18 receives the voltage signal from the gamma-ray detector array 14 and analyzes the gamma energy spectrum, thereby obtaining a two-dimensional neutron image containing two-dimensional element distribution information of the object to be detected. The two-dimensional neutron image is combined with the obtained two-dimensional X-ray image to realize the identification and discovery of contraband in the container.

In the process of detecting the detected object, the X-ray image and the neutron image cannot be obtained simultaneously in the process of moving the detected object due to different placing positions of the X-ray detector array and the gamma-ray detector array, and the neutron images obtained by the gamma-ray detector arrays are different due to different positions. In order to combine the X-ray image with the neutron image to better realize the contraband inspection, the following method is adopted:

for different gamma-ray detector arrays, because the distance relation of the gamma-ray detector arrays is determined, the position relation between neutron images of the gamma-ray detector arrays is also determined, and for successively obtained neutron images, the positions of the neutron images are respectively adjusted, so that the gamma-ray detector arrays at different positions can jointly form a neutron image reflecting element distribution.

The spatial position relation of the X-ray image and the X-ray image is also determined, and the neutron image and/or the X-ray image can be translated and combined into one image, so that points corresponding to the same position of the detected object in the neutron image and the X-ray image are completely coincided. Thus, for the merged image, each point includes the element distribution information and the density information of the detected object. In the system of the present invention, an image merging device (not shown) may be used to perform the above-mentioned positional adjustment of the X-ray image and the neutron image so as to merge the X-ray image and the neutron image into one image. Therefore, an operator can obtain the element distribution information and the density information of the detected object by only observing one image so as to carry out relatively accurate positioning on suspicious contraband in the detected object.

Although exemplary embodiments of the present invention have been described, it is to be understood that the present invention is not limited to these embodiments and that various changes and modifications of the present invention may be effected therein by one skilled in the art without departing from the spirit and scope of the present invention as defined in the appended claims.

Claims (47)

1. A method for detecting photo-neutron-X-ray contraband, which is used for detecting an object to be detected, the method comprising:

simultaneously generating a first X-ray beam and photoneutrons;

carrying out X-ray imaging detection on the detected object by using the first X-ray beam;

and simultaneously carrying out the X-ray imaging detection, and carrying out neutron detection on the detected object by utilizing the photoneutron and the characteristic gamma ray emitted by the photoneutron and the detected object.

2. The method of claim 1, wherein the photoneutrons are produced by a bombardment of a photoneutron conversion target by a second X-ray beam.

3. The method of claim 2 wherein the first and second X-ray beams are split from the same main X-ray beam.

4. The method of claim 3, wherein the first X-ray beam is split from the main X-ray beam with the photoneutron conversion target.

5. The method of claim 4, wherein the photoneutron conversion target has a channel through which the first beam of rays passes.

6. The method of any one of claims 1-5,

the X-ray imaging detection generates an X-ray image of the detected object, and the neutron imaging generates a neutron image of the detected object;

and combining the X-ray image and the neutron image, so that points corresponding to the same position of the detected object in the neutron image and the X-ray image are completely coincided.

7. The method of claim 6, wherein the X-ray image reflects density information of the object under test and the neutron image reflects element distribution information of the object under test.

8. An optical neutron-X-ray contraband detection system for detecting an object under inspection, the system comprising:

an X-ray generator for generating a main X-ray beam including a first X-ray beam and a second X-ray beam split therefrom, the first X-ray beam being directed into and through the object;

a photoneutron conversion target arranged to receive the second X-ray beam, thereby generating photoneutrons which are directed into the object under examination and react with the object under examination to emit characteristic gamma rays;

an X-ray detection device arranged to receive the first X-ray beam after passing through the object to be inspected so as to perform X-ray imaging detection on the object to be inspected;

a gamma ray detection device arranged to receive the characteristic gamma ray for neutron detection of the object under examination from the characteristic gamma ray;

wherein the system performs the X-ray imaging detection and the neutron detection on the detected object simultaneously.

9. The system of claim 8, wherein the primary beam of X-rays generated by the X-ray generator is a pulse of X-rays having a particular frequency.

10. The system according to claim 9, wherein the specific frequency is in the range of 50Hz to 250Hz and/or the pulse width of the X-ray pulses is about 5 μ s.

11. The system of claim 8, wherein the X-ray generator comprises:

an electron accelerator for generating an electron beam; and

an electron target to which the electron beam is directed to generate the primary beam of X-rays.

12. The system of claim 11, wherein the electron beam is an electron beam pulse having a particular frequency, the electron accelerator generating the electron beam pulse at the particular frequency such that the primary X-ray beam is an X-ray pulse having the particular frequency.

13. The system of claim 11, wherein the electron accelerator generates the electron beam at an energy between 1MeV and 15 MeV.

14. The system of claim 13, wherein the electron beam generated by the electron accelerator has an energy of at least 1.67 MeV.

15. The system of claim 8, wherein the material of the photoneutron conversion target is beryllium or heavy water.

16. The system of claim 8, wherein the photoneutron conversion target has an elongated body having a first end and a second end, the second X-ray beam entering the interior of the body and propagating in a direction from the first end to the second end.

17. The system of claim 16, wherein the body of the photoneutron conversion target is shaped to substantially match an intensity distribution of the primary beam of X-rays produced by the X-ray generator such that intense X-rays can propagate a greater distance within the body of the photoneutron conversion target.

18. The system of claim 17, wherein the intensity distribution of the primary X-ray beam is an axisymmetric distribution defining an intensity distribution symmetry axis;

the body of the photoneutron conversion target is shaped to be axisymmetrical about a target symmetry axis;

in use, the target axis of symmetry coincides with the intensity distribution axis of symmetry.

19. The system of any of claims 16-18, wherein at least a portion of the body is a tapered portion that tapers toward the second end.

20. The system of claim 19, wherein the tapered portion terminates at the second end.

21. The system of claim 20, wherein the tapered portion is conical or frustoconical.

22. The system of claim 19, wherein the body further comprises a cylindrical body portion, the tapered portion being adjacent the second end, the cylindrical body portion being adjacent the first end.

23. The system of claim 8, further comprising a beam splitter for splitting the first X-ray beam and the second X-ray beam from the primary X-ray beam.

24. The system of claim 23, wherein the beam splitter is a two-channel split collimator.

25. The system of any of claims 16-18, further comprising a beam splitter for splitting the first and second X-ray beams from the primary X-ray beam,

wherein the beam splitter is formed by a channel through the body of the photoneutron conversion target, the channel extending from a first end to a second end of the body,

wherein the X-ray main beam is guided to the X-ray neutron conversion target, a part of the X-ray beam passing through the X-ray neutron conversion target via the channel is defined as the first X-ray beam, and another part of the X-ray beam entering into the body of the X-ray neutron conversion target is defined as the second X-ray beam.

26. The system of claim 25, wherein the channel extends sufficiently in a plane that the body is divided into two separate portions by the channel.

27. The system of claim 25, wherein the channel extends along an axis of symmetry of the body.

28. The system of claim 8, further comprising an X-ray collimator disposed on a propagation path of the first X-ray beam before reaching an object under examination to collimate the first X-ray beam into a planar fan beam.

29. The system of claim 25, further comprising an X-ray collimator disposed adjacent the second end of the body and aligned with the passage so as to collimate the first X-ray beam passing through the passage into a planar fan beam.

30. The system of claim 8, further comprising a neutron reflector for reflecting optical neutrons moving away from the inspected object to move towards the inspected object.

31. The system of claim 8, wherein the X-ray detection device is an X-ray detector array comprising a plurality of X-ray detectors.

32. The system of claim 8, wherein the gamma ray detection device comprises one or more gamma ray detector arrays, each gamma ray detector array comprising a plurality of gamma ray detectors.

33. The system of claim 32, wherein the gamma ray detection device comprises a plurality of gamma ray detector arrays disposed on opposite sides of the inspected object.

34. The system of claim 8, wherein the gamma ray detection device is arranged offset from the first X-ray beam to minimize an effect of the first X-ray beam on the gamma ray detection device.

35. The system of claim 32, wherein the gamma ray detector comprises:

a detector crystal for converting gamma rays incident into the detector crystal into fluorescence photons, the detector crystal having: a front end face for receiving gamma rays, a rear end face opposite the front end face, and a circumferential surface;

a photomultiplier tube disposed adjacent the rear end face of the detector crystal for receiving fluorescence photons from the body of photoelectric conversion material, converting them into photoelectrons and multiplying the photoelectrons;

an X/gamma ray shield surrounding at least a circumferential surface of the detector crystal and exposing a front end face of the detector crystal;

a neutron shield located outside the X/gamma ray shield and surrounding at least a circumferential surface of the detector crystal and exposing a front end face of the detector crystal;

a neutron-absorber disposed adjacent the front face of the detector crystal and preventing neutrons from entering the detector crystal from the front face and not producing characteristic gamma rays of 2.223MeV of hydrogen;

a collimator comprising a through hole aligned with the front face of the detector crystal, the through hole defining an extension direction for allowing only X/gamma rays reaching the front face via the through hole substantially along the extension direction to enter the detector crystal.

36. The system of claim 35, wherein the X/gamma ray shield further surrounds a rear face of the detector crystal.

37. The system of claim 35, wherein the neutron shield further surrounds a rear end face of the detector crystal.

38. The system of claim 35, wherein the detector crystal is NaI.

39. The system of claim 35, wherein the X/gamma ray shield material is a heavy metal having an atomic number greater than or equal to 74.

40. The system of claim 35, wherein the neutron shield is comprised of an H-rich material.

41. The system of claim 40, wherein the neutron shield is comprised of paraffin, polyethylene, or water.

42. The system of claim 35, wherein the neutron absorber is constructed of an H-rich material along with boron.

43. The system of claim 42, wherein the neutron absorber is comprised of boron-containing polyethylene.

44. The system of claim 35, wherein the material of the collimator is a heavy metal having an atomic number greater than or equal to 74 or steel.

45. The system of claim 35, wherein the gamma ray detector further comprises a time gating circuit for controlling a measurement time of the gamma ray detector such that the measurement time of the gamma ray detector avoids an exit time of the main beam of X-rays generated by the X-ray generator.

46. The system of claim 8, further comprising signal processing means for receiving and processing detection signals from said X-ray detection means and said gamma-ray detection means and forming an X-ray image and a neutron image, respectively.

47. The system of claim 46, further comprising image merging means for merging said X-ray image and said neutron image into an image such that points in the neutron image and the X-ray image corresponding to the same location of the detected object are completely coincident.