CN201286192Y - Photoneutron conversion target and photoneutron X ray source - Google Patents

Abstract

The utility model discloses a photoneutron conversion target and a photoneutron-X-ray source using the photoneutron conversion target, which can be used to simultaneously generate photoneutrons and X-rays. The photoneutron-X-ray source includes: an X-ray generator for generating a main beam of X-rays; a photoneutron conversion target, the main beam of X-rays can bombard the photoneutron conversion target to generate photoneutrons, the The neutron conversion target has a body and a channel defined by the body, the channel passing through the body; wherein the first X-ray beam in the main X-ray beam can pass through the channel without reacting with the body, At the same time, the second X-ray beam in the main X-ray beam can enter the body and react with the body to generate photoneutrons.

Description

Photoneutron conversion target and photoneutron-X-ray source

technical field

The utility model relates to a photoneutron conversion target. The photoneutron conversion target is used to generate photoneutrons by using X-rays. The photoneutron conversion target can be used in a contraband detection system in particular.

Background technique

At present, terrorism poses a great threat to the stability of international and domestic societies, and governments of all countries are working hard to solve the problem of counter-terrorism. The detection technology of contraband such as explosives is at the heart of counter-terrorism issues.

One existing contraband detection technology is X-ray imaging detection technology. X-ray imaging detection technology is a security inspection technology that has been widely used. Many devices based on X-ray imaging detection technology can be seen in airports and railway stations. Since X-rays mainly react with electrons outside the nucleus and have no ability to distinguish the characteristics of the nucleus, X-rays can only measure the density (mass thickness) of the detected object, but cannot determine the element type of the detected object. In practice, when contraband is mixed with daily necessities and the density is difficult to distinguish, it is difficult to detect it with X-ray imaging detection technology. Although some new X-ray imaging detection technologies, such as: dual-energy X-ray, CT technology, etc., have improved their recognition capabilities, they still cannot overcome the inherent shortcomings of not being able to identify element types.

Another class of existing dangerous goods detection technology is the neutron type detection technology. For neutron detection technology, neutrons can react with the nucleus of the substance and emit characteristic γ-rays. According to the energy spectrum of γ-rays, the element type of the analyzed substance can be judged. The defect of neutron detection technology lies in its low imaging resolution. At present, the best spatial resolution can only reach 5cm×5cm×5cm, which is far lower than the mm-level resolution of X-ray imaging. Also, individual neutron sources are usually expensive, have a limited lifetime, and do not produce neutrons of high intensity.

Therefore, it is desirable to have a method and/or system that can combine the above-mentioned X-ray imaging detection technology and neutron-type detection technology to obtain high resolution of X-ray imaging detection technology and element identification of neutron-type detection technology these advantages. US Patent No. 5078952 discloses an explosives detection system combining multiple detection means, including an X-ray imaging device and a neutron detection device, so as to achieve a higher detection probability and a lower false alarm rate. Moreover, the U.S. patent also discloses to correlate the data obtained by the X-ray imaging device with the data obtained by the neutron detection device, so as to use high-resolution X-ray images to make up for the defect of low resolution of neutron detection technology . However, in this US patent, an independent X-ray source and a neutron source are used, and the cost thereof is relatively high.

It is worth noting that one way of producing neutrons is to bombard a conversion target with X-rays and generate neutrons from the conversion target, which may be called photoneutrons. This mode of neutron generation offers the possibility to generate both X-rays and neutrons in one source, which is less costly than having two sources for X-rays and neutrons respectively.

A system for detecting and identifying contraband using photoneutrons and X-ray imaging is disclosed in International Application Publication WO 98/55851. The system works in a two-step manner. Specifically, the system first uses a linear accelerator X-ray source to generate an X-ray beam, and uses X-ray imaging to detect the inspected object. If no abnormality is found, the inspected object is allowed to pass through. If a suspected area is found, a beam of light is temporarily turned on. A neutron conversion target (beryllium) is inserted into the X-ray beam to generate photoneutrons, and the suspected area is further detected based on the characteristic gamma rays released by the radiation capture reaction between photoneutrons and material nuclei. The system only uses X-rays for the first step of detection, and has a low probability of detection (PD) due to the limitation of the recognition ability of the X-ray imaging detection technology 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 separately in two steps, that is, only X-rays are generated in one step without photoneutrons, and in another step X-rays are used to generate photoneutrons, but the X-rays are only used to generate photoneutrons and not for detection purposes. Further, the photoneutrons generated by it are only used to detect the suspected area of the object to be inspected, and are not used for overall detection of the object to be inspected.

In the applicant's Chinese patent application No. 200510086764.8, a method for material identification using fast neutrons and X-rays is disclosed. In this application a method and apparatus for the simultaneous generation of X-rays and photoneutrons is described, which splits the X-rays generated by an accelerator into two beams, one of which is used to generate photoneutrons. However, in this application, for neutrons, the intensity of optical neutrons passing through the object to be inspected is used for detection, rather than the characteristic gamma rays emitted by neutrons reacting with the object to be inspected. Moreover, in this application, in such a detection method, in order to prevent the detection of the X-ray beam and the neutron beam from interfering with each other, it is usually necessary to keep a certain distance between the X-ray beam and the neutron beam laterally.

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

Utility model content

The purpose of the utility model is to provide a photoneutron conversion target and a photoneutron-X-ray source using the photoneutron conversion target, which allows simultaneous generation of photoneutrons and X-rays.

According to one aspect of the present invention, a photoneutron conversion target is provided, which is used to bombard the photoneutron conversion target with an X-ray main beam to generate photoneutrons. The photoneutron conversion target has a body and the body a defined passageway extending through the body;

Wherein, the first X-ray beam in the X-ray main beam can pass through the channel without reacting with the body, and at the same time, the second X-ray beam in the X-ray main beam can enter the body, And react with this body to produce photoneutrons.

According to another aspect of the present invention, there is provided a photoneutron-X-ray source for simultaneously generating photoneutrons and X-rays, comprising:

X-ray generator for generating the main beam of X-rays;

a photoneutron conversion target, the X-ray main beam can bombard the photoneutron conversion target to generate photoneutrons, the photoneutron conversion target has a body and a channel defined by the body, and the channel runs through the body;

Wherein, the first X-ray beam in the X-ray main beam can pass through the channel without reacting with the body, and at the same time, the second X-ray beam in the X-ray main beam can enter the body, And react with this body to produce photoneutrons.

In both above aspects, the channel extends along a target axis of symmetry of the body.

The photoneutron conversion target of the utility model and the photoneutron-X-ray source using the photoneutron conversion target can simultaneously generate photoneutrons and X-rays. Moreover, the photoneutron conversion target and the photoneutron-X-ray source can be applied to any application requiring both photoneutrons and X-rays, not limited to the occasions described in the following embodiments.

Description of drawings

Fig. 1 shows a schematic structural view 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 in Fig. 1, wherein the channel defined by the photoneutron conversion target is shown;

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

Figure 4 shows an improved gamma ray detector.

Detailed ways

Referring to the accompanying drawings, typical specific embodiments of the utility model will be described in detail below. The following examples are used to illustrate the utility model, but not to limit the scope of the utility model.

Referring to the example shown in FIG. 1 , the object to be inspected (for example, a closed container 8 ) is set on a platform 19 . It should be noted that the container 8 is shown in cross-section in FIG. 1 in order to illustrate the various cargoes 10 contained therein, which may include various materials such as metal 11 , wood blocks 12 and explosives 13 . The platform 19 is pulled by the dragging device 20 and enters the detection area of the detection system of the present invention. The container 8 is generally manufactured from corrugated steel and aluminium. Other containers such as air containers can also be tested similarly.

When the position sensor (not shown) detects that the container 8 moves to a predetermined position, the position sensor can trigger the X-ray generator in the system of the present invention to start working. 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 towards an electron target 2 . The electron target 2 is usually made of a material with a high atomic number such as tungsten or gold. After the electrons are blocked by tungsten or gold atoms, they will emit the main X-ray beam 3 due to bremsstrahlung. As will be described below, the first X-ray beam and the second X-ray beam will be separated from the X-ray main beam 3, wherein the first X-ray beam is used for X-ray imaging detection, and the second X-ray beam For neutron detection. In this paper, X-ray imaging detection refers to X-ray transmission through the object to be inspected, and the density information of the object to be inspected is detected by detecting the attenuation of X-rays; neutron detection refers to the reaction of neutrons with the atoms of the object to be inspected to release characteristic gamma rays, and detect the element type information of the inspected object by detecting the characteristic gamma rays. It should be noted that in the system and method of the present invention, the object to be inspected is detected by using X-ray imaging detection and neutron detection at the same time.

In FIG. 1 , the photoneutron conversion target 4 is shown in a partial cross-sectional view. The X-ray main beam 3 bombards the photoneutron conversion target 4 to obtain photoneutrons 6 , and uses the photoneutrons 6 to perform neutron detection on the container 8 . In particular, in this embodiment, the photoneutron conversion target 4 is also used to split the first X-ray beam and the second X-ray beam from the main X-ray beam 3 .

The photoneutron conversion target 4 in FIG. 1 is shown enlarged in FIGS. 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 , and has a first end 402 and a second end 403 . The body 401 has a channel 404 extending through the body 401 , and the channel 404 extends from the first end 402 to the second end 403 . In the embodiment of Figures 2 and 3, the channel 404 is formed as a slit extending sufficiently in a plane P (perpendicular to the paper of Figures 2 and 3) so that the body 401 is divided into two separate parts . Preferably, the channel 404 passes through the center of symmetry of the body 401, dividing it into two symmetric parts. The channel 404 is defined between the two separate parts. When the X-ray main beam 3 is incident on the body 401 of the neutron conversion target 4, a part of the X-ray beam 405 directly passes through the neutron conversion target 4 through the channel 404 without contact with the neutron conversion target 4. Any reaction takes place, this part of the X-ray beam is defined as the first X-ray beam 405 . Another part of the X-ray beam 406 enters the body 401, and propagates toward the direction from the first end 402 to the second end 403, and reacts with the nucleus of the photoneutron conversion target 4 during the propagation, thereby Photoneutrons are emitted, and this portion of the X-ray beam 406 is defined as the second X-ray beam 406 . It can be seen that the channel 404 actually acts 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 not-shown embodiments, the channel 404 may also adopt other forms. For example, the channel may not divide the body 401 into two parts, but be formed as a through hole (not shown) that runs through the body 401. ), or formed as other channels defined by the body 401, as long as the fan-shaped X-ray beam used 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 photoneutron yield of the neutron conversion target 4, the shape of the neutron conversion target 4 can be designed to match the intensity of the X-ray main beam 3 The distributions are substantially matched, ie, so that the X-rays with greater intensity can travel a greater distance within the body 401 of the neutron photo-neutron conversion target 4 . Referring to Fig. 1 and Fig. 2, the X-ray main beam 3 that comes out from the electron target 2 generally has an intensity distribution that is axisymmetric, and its intensity distribution symmetry axis is along the direction of the electron beam 1, and, generally, the closer to the intensity distribution symmetry axis , the greater the intensity of X-rays. Accordingly, in the case of ignoring the channel 404 in the neutron conversion target 4, the neutron conversion target 4 may generally have an axisymmetric shape and define a target symmetry axis 409, and the neutron conversion target The axisymmetric shape of the X-ray main beam 3 substantially matches the axisymmetric distribution. In this use, the target axis of symmetry 409 coincides with the axis of symmetry of the intensity distribution of the main X-ray beam 3 . Preferably, at least a portion of the neutron photoconversion target 4 is preferably a tapered portion that tapers toward the second end 403, so that the neutron photoconversion target 4 has a longer length closer to the axis of symmetry of the target. length. In the embodiment shown in FIG. 2, the neutron 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, the cylindrical portion 407 can be connected to the The tapered portion 408 is integrally formed. The tapered portion 408 may terminate at the second end 403 . The tapered portion 408 shown in FIG. 2 is frusto-conical. The cylindrical portion 407 and the tapered portion 408 have a common longitudinal central axis, coincident with the target axis of symmetry. In other embodiments, the tapered portion 408 may also be non-truncated and tapered, or tapered in other ways (eg, tapered in a curved manner). 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 optical neutron conversion target 4 is shown as a beam splitter in FIGS. A first X-ray beam and a second X-ray beam are branched off from the main X-ray beam 3 . For example, the dual-channel shunt collimator disclosed in the applicant's Chinese patent application No. 200510086764.8 can be used. The double-channel shunt collimator can divide the X-ray main beam 3 into two beams spaced apart from each other, and set a photoneutron conversion target on the propagation path of one of the beams to generate photoneutrons.

It should also be noted that the characteristic that the photoneutron conversion target 4 has a tapered portion is not limited to the occasions described in the embodiment of the present invention. This feature is also applicable to any other occasions where X-ray beams are used to bombard photoneutron conversion targets to generate photoneutrons, for example, it can be applied to the occasions described in International Application Publication WO 98/55851 and Chinese Patent Application No. 200510086764.8, to Increase the production of photoneutrons. In these other applications, the neutron photoconversion target may or may not have the aforementioned channel used as a beam splitter.

Returning to FIG. 1 , the selection of the energy of the electron beam 1 generally needs to consider the required energy of the X-ray beam and the material of the photoneutron conversion target. Depending on the type of object to be detected, the detection speed and the safety of the environment, X-ray beams with different energies can be selected for penetration. For safety reasons and to save costs, the energy should generally be chosen to be as small as possible. The energy of the electron beam 1 generated by an electron accelerator not shown may be in the range of 1 MeV to 15 MeV. The ideal material for the photoneutron conversion target 4 should have a smaller photoneutron reaction threshold and a larger photoneutron reaction cross section, but it is difficult to satisfy both. For X-rays from 1MeV to 15MeV, because the energy is not high enough, the photoneutron yield is low for materials with large cross-section but high threshold, while beryllium ( ⁹ Be) or heavy water ( D ₂ O) It is an ideal material. The photoneutron reaction threshold of ⁹ Be is only 1.67MeV, and the reaction threshold of D in D ₂ O is 2.223MeV. The X-ray main beam 3 entering the neutron conversion target 4 undergoes photoneutron reaction with ⁹ Be or ² H therein, releasing photoneutrons 6 . Since the energy spectrum of the X-ray main beam 3 is continuously distributed, the energy spectrum of the photoneutrons 6 is also continuously distributed. In addition, when the electron accelerator used can generate the electron beam 1 with higher energy, the photoneutron conversion target 4 can also use materials with higher threshold but larger cross-section, such as various isotopes of tungsten (W) and uranium (U) of each isotope.

In one embodiment, an electron accelerator not shown can generate an electron beam 1 with a specific frequency, so that the electron beam 1 is an electron beam pulse 1 with the specific frequency. After the electron beam pulse 1 bombards the electron target 2, a X-ray pulses of the same frequency3. The specific frequency can be determined according to the traveling speed of the detected container 8, for example, it can be in the range of 10 Hz-1000 Hz. In one embodiment, the specific frequency may be 250 Hz. The pulse width of the electron beam pulse 1 may range from 1 to 10 μs.

It should be noted that the time taken for the X-ray main beam 3 to bombard the photoneutron conversion target 4 to generate photoneutrons 6 is very short (usually less than 1 μs). The optical neutron 6 and the first X-ray beam 405 used for X-ray imaging detection in the X-ray main beam 3 are generated almost "simultaneously", which allows simultaneous X-ray imaging detection and neutron detection, which is clearly distinguished Published in International Application WO 98/55851.

The photoneutrons 6 are isotropic when generated in the photoneutron conversion target 4 , so only a part of the photoneutrons can be emitted to the detected container 8 . Since ⁹ Be and ² H in the photoneutron conversion target 4 have larger scattering cross-sections for neutrons, the photoneutrons 6 emitted from the photoneutron target 4 will generally go backwards (that is, opposite to the X-ray main beam 3 incident to the direction of photoneutron conversion target 4) emission. In order to improve the efficiency of the photoneutrons 6 reaching the container 8 to be detected, a neutron reflector (not shown) may be arranged behind the photoneutron target 4 (adjacent to the first end 402 of the photoneutron target 4 ). The neutron reflector is used to reflect the optical neutrons 6 moving away from the inspected container 8 so that they move toward the inspected container 8 .

Referring to FIG. 1 and FIG. 2 , the X-ray collimator 5 is arranged on the propagation path of the first X-ray beam 405 before reaching the object 8 to collimate the first X-ray beam 405 into a plane fan beam. The X-ray collimator 5 is preferably disposed adjacent to the second end 403 of the body 402 of the neutron conversion target 4 and aligned with the channel 404 . In this way, after the first X-ray beam 405 passes through the neutron conversion target 4 through the channel 404 , it is collimated by the X-ray collimator 5 to form a plane fan beam 7 . X-rays outside the fan beam 7 will be shielded by the X-ray collimator 5, which can reduce the influence of X-rays on neutron detection (especially the γ-ray detector described below).

The X-ray imaging detection of the container 8 by the first X-ray beam 405 and the neutron detection of the container 8 by the optical neutrons 6 generated by the second X-ray beam 406 will be respectively described below. It should be appreciated that X-ray imaging detection and neutron detection are each well known per se to those of ordinary skill in the art. However, in the present invention, since the first X-ray beam 405 and the optical neutron 6 can be generated simultaneously (or almost simultaneously), the X-ray imaging detection of the X-ray beam and the neutron detection can be performed simultaneously.

First, X-ray imaging detection will be described. Referring to FIG. 1 , the X-ray fan beam 7 (ie, the collimated first X-ray beam 405 ) is directed towards the inspected container 8 , and the fan beam 7 is attenuated by the goods 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 multiple of X-ray has reflected the material on the connection of corresponding X-ray detector from electronic target 2 to X-ray detector array 15 to X-ray absorption capacity, and its size and container 8 are loaded material density and Composition related. Two-dimensional X-ray imaging of the container 8 can be realized by using the X-ray detector array 15 . The detectors in the X-ray detector array 15 can be gas ionization chambers, cadmium tungstate crystals, CsI crystals, or other types of detectors. As mentioned above, the electron beam 1 bombards the electron target 2 with a certain frequency, thereby generating X-ray pulses of the same frequency. For each X-ray pulse, the array of detectors 15 will obtain a one-dimensional image of a section of the container. As the dragging device 20 pulls the container 8 forward, multiple one-dimensional images obtained from multiple measurements constitute a two-dimensional transmission image about the container.

Neutron detection performed simultaneously with X-ray imaging detection will now be described. After the photoneutrons 6 are generated by the photoneutron conversion target 4, the inspected container 8 will be bathed in the photoneutron field. After the photoneutron 6 enters the detected container 8, it loses energy through scattering (inelastic and elastic scattering). There is no need to collimate the photoneutrons 6 before they enter the inspected container 8, because they diffuse into a relatively wide area during the scattering process. Photoneutrons 6 are fast neutrons when they are produced, and then become slow neutrons within a few μs. After that, the energy of photoneutrons 6 enters the energy region of thermal neutrons. The time interval of photoneutron 6 from fast neutron to thermal neutron is generally about 1 ms. Thermal neutrons eventually disappear, in two ways: by being absorbed by matter, or by escaping. The existence time of thermal neutrons in space is 1ms ~ 30ms. Neutrons can also undergo capture reactions in the fast neutron and slow neutron energy regions, but the cross section is very small. When the neutron energy decreases, because the capture cross section is inversely proportional to the neutron speed, the cross section rises rapidly . Since the electron accelerator works in a continuous pulse mode, the thermal neutron field between different pulses will be superimposed. For example, when the electron accelerator works with a frequency of about 250 Hz and a pulse width of 5 μs, the final neutron field formed in space will be a fast neutron pulse with a frequency of 250 Hz and a pulse width of 5 μs, superimposed on an approximately constant on the thermal neutron field.

Thermal neutrons can emit characteristic γ-rays after the radiation capture reaction of matter, for example, ¹ H reacts with neutrons to emit 2.223MeV characteristic γ-rays, ¹⁴ N reacts with neutrons to emit 10.835MeV characteristic γ-rays , ¹⁷ Cl reacts with neutrons to release characteristic gamma rays of 6.12MeV. The type of elements in the detected object can be judged by measuring these characteristic gamma rays. Different materials in the container 8 can emit 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 N and H elements are found in the container, then there may be explosives and "fertilizer bombs"; if a large amount of Cl gamma rays are found, it is possible to find drugs such as heroin and cocaine (they usually smuggled in the form of chloride). In addition, nuclear materials such as uranium and plutonium can also be inspected by measuring fission neutrons produced by optical neutron capture.

The measurement of gamma ray energy spectrum is accomplished by gamma ray detection device, and this gamma ray detection device can be one or more gamma ray detector arrays 14, and each gamma ray detector array 14 comprises a plurality of gamma ray detectors , and arranged to receive the characteristic gamma rays. Also, as shown in FIG. 1 , when multiple gamma ray detector arrays 14 are included, they may be arranged on both sides of the travel path of the container 8 . Also, the gamma-ray detector array 14 can be arranged at a certain distance away from the X-ray detector array 15, that is, a certain distance away from the X-ray fan beam 7 (first X-ray beam), so that the first X-ray beam is Gamma ray detector effects are minimized. For each gamma-ray detector array, by analyzing its gamma energy spectrum signal, the two-dimensional distribution information of the concerned element species is obtained.

There are many types of γ-ray detectors to choose from, such as: NaI(T1), BGO, HPGe, LaBr ₃ and so on.

Two types of detectors are used in the present invention: X-ray detectors and gamma-ray detectors, these two detectors work in an environment where X-rays, neutrons and gamma-rays coexist. The two kinds of rays may interfere with each other, especially X-rays are stronger than neutrons and gamma rays, so it may interfere with the gamma energy spectrum detected by gamma rays. Therefore, for gamma detectors, it is very necessary to shield X-rays and neutron rays.

FIG. 4 shows an improved γ-ray detector, wherein NaI crystal 22 and photomultiplier tube 23 constitute the main body of the detector. The NaI crystal 22 has a front end face 30 for receiving γ-rays, a rear end face 31 opposite to the front end face 30 , and a peripheral surface 32 . When γ-rays are incident on the NaI crystal 22, photoelectric effect, Compton scattering or electron pair effect will occur. The gamma photons deliver energy to the secondary electrons, which are ionized in the crystal, and the electron-hole pairs generated by the ionization will produce fluorescence. Fluorescent photons emit photoelectrons on the photocathode of the photomultiplier tube 23 . The photoelectrons are then multiplied by the photomultiplier tube, and a voltage signal is formed through the preamplifier circuit. In order to provide the NaI crystal 22 with shielding from X-rays and neutrons. The gamma ray detector shown in FIG. 4 further includes a neutron shielding material 28 , the neutron shielding material 28 at least surrounds the peripheral surface 32 of the NaI crystal 22 and exposes the front face 30 of the NaI crystal 22 . Preferably, the neutron shielding material 28 also surrounds the rear end surface 31 of the NaI crystal 22 . The neutron shielding material 28 is generally composed of H-rich substances, such as paraffin, polyethylene, and water are suitable materials. Considering the structure and fire protection requirements, polyethylene is generally selected. The H atoms in the neutron shielding material 28 have a large scattering cross-section for neutrons, can reflect neutrons, and quickly reduce and absorb the energy of neutrons. However, the neutron shielding material 28 will emit 2.223 MeV characteristic Hγ rays after radiation capture with neutrons, and the characteristic Hγ rays will interfere with the gamma signal to be measured by the detector. Therefore, on the inner side of the neutron shielding material 28, the gamma-ray detector also includes an X/gamma-ray shielding body 26, which at least surrounds the peripheral surface of the detector crystal and exposes the NaI Front face 30 of crystal 22 . Preferably, the X/γ-ray shielding body 26 also surrounds the rear end surface 31 of the NaI crystal 22 . The X/γ-ray shielding body 26 can not only absorb the γ-rays emitted by the neutron shielding material 28 when it reacts with neutrons, but also shield most of the X-rays and their scattered rays from the electron target 2, so that the γ-ray detector Able to be in a normal working environment. The material of the X/γ-ray shielding body 26 is a heavy metal with an atomic number greater than or equal to 74, such as lead Pb or tungsten W. In front of the gamma detector crystal 22 , facing the front end 30 of the NaI crystal 22 , a neutron absorber 27 is further provided. Different from the requirements of the neutron shielding material 28, the neutron absorber 27 must not only be able to absorb neutrons, but also be unable to emit 2.223 MeV gamma rays of H. The neutron absorber 27 can be made of paraffin or polyethylene and boron ¹⁰ B material with high thermal neutron absorption capacity (such as boron-containing polyethylene), which makes H no longer have the opportunity to emit gamma photons. In order to make the gamma-ray detector only measure the detected object area in front of it, and not be interested in signals from other directions (such as X-ray scattering, gamma counting background of N in the air), the gamma-ray detector also includes a collimator Device 29. The collimator 29 is arranged in front of the NaI crystal 22 and the neutron absorber 27 to shield the X-ray scattering background in the surrounding space and the γ background generated by neutrons in the surrounding materials. The collimator 29 comprises a through hole aligned with the front face 30 of the NaI crystal, the through hole defining a direction of extension for allowing only X substantially along the direction of extension and via the through hole to the front face. /γ-rays enter the NaI crystal, thereby collimating the γ-rays to be detected. The diameter of the through hole can be the same as the diameter of the NaI crystal 22, and the length can be determined according to the required collimation effect, and generally a length range of 5-30 cm is selected. The collimator 29 can usually be made of heavy metal with an atomic number greater than or equal to 74 (such as lead Pb or tungsten W) or steel.

In addition, although not shown in the figure, a time gating circuit can also be provided for the gamma ray detector, for controlling the measurement time of the gamma ray detector, so that the measurement time of the gamma ray detector avoids the The beam emission time of the X-ray beam generated by the X-ray generator can further suppress the interference of X-rays to the γ-ray detector.

According to the signals from the X-ray detector array 15 and the γ-ray detector array 14, X-ray imaging and neutron imaging can be performed on the inspected container 8, so as to obtain X-ray images and neutron images. Returning to FIG. 1 , in the system of the present invention, the X-ray imaging signal processing circuit 17 receives signals from the X-ray detector array 15 and processes them to obtain X-ray images. The gamma-ray signal processing circuit 18 receives the voltage signal from the gamma-ray detector array 14 and analyzes the gamma energy spectrum to obtain a two-dimensional neutron image containing two-dimensional element distribution information of the object under inspection. 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.

Considering that when the detected object is detected, due to the different placement positions of the X-ray detector array and the gamma-ray detector array, the X-ray image and the neutron image cannot be obtained simultaneously during the progress of the detected object, Moreover, due to the different positions of the gamma-ray detector arrays, the obtained neutron images are also different. In order to combine X-ray images with neutron images to better realize contraband inspection, the following methods are adopted:

For different γ-ray detector arrays, since their distance relationship is definite, the positional relationship between their neutron images is also definite. For the neutron images obtained successively, their positions can be adjusted respectively, so that they are in Gamma-ray detector arrays at different positions together form a neutron image reflecting the distribution of elements.

For X-ray images and neutron images, their spatial position relationship is also determined, neutron images and/or X-ray images can be translated and merged into an image, so that the neutron images and X-ray images correspond to the detected object Points at the same location are completely coincident. In this way, for the merged image, each point includes element distribution information and density information of the detected object. In the system of the present invention, an image combining device (not shown) can be used to realize the above-mentioned position adjustment of the X-ray image and the neutron image, so that the X-ray image and the neutron image can be combined into one image . In this way, the operator only needs to observe one image to obtain the element distribution information and density information of the detected object, so as to relatively accurately locate the suspected contraband in the detected object.

Although typical embodiments of the present utility model have been described, it should be understood that the present utility model is not limited to these embodiments. For those skilled in the art, various changes and improvements of the present utility model can be realized, but these are all within the scope of this utility model. within the spirit and scope of the novel claims.

Claims (23)

1. A neutron conversion target for generating photoneutrons by bombarding the neutron conversion target with an X-ray main beam, characterized in that the neutron conversion target has a body and a channel defined by the body, the passage runs through the body; Wherein, the first X-ray beam in the X-ray main beam can pass through the channel without reacting with the body, and at the same time, the second X-ray beam in the X-ray main beam can enter the body, And react with this body to produce photoneutrons. 2. The photoneutron conversion target according to claim 1, wherein the body is an elongated body extending along the propagation direction of the X-ray main beam, and the body has a first end and a second end ;and The extension direction of the channel is the same as the propagation direction of the X-ray main beam. 3. The neutron photoconversion target according to claim 2, characterized in that the shape of the body of the photoneutron photoconversion target is designed to substantially match the intensity distribution of the X-ray main beam. 4. The optical neutron conversion target according to claim 3, wherein the intensity distribution of the main beam of X-rays is an axisymmetric distribution, which defines an intensity distribution symmetry axis; The body of the neutron conversion target is configured to have an axisymmetric shape about a target symmetry axis, the axisymmetric shape of the neutron conversion target substantially matching the axisymmetric distribution of the X-ray main beam. 5. The neutron conversion target according to claim 3 or 4, wherein at least a portion of the body is a tapered portion which tapers towards the second end. 6. The neutron photoconversion target of claim 5, wherein the tapered portion terminates at the second end. 7. The photoneutron conversion target according to claim 6, wherein the tapered portion is conical or frusto-conical. 8. The neutron conversion target of claim 5, wherein the body further comprises a cylindrical portion, the tapered portion is adjacent to the second end, and the cylindrical portion is adjacent to the first end. one end. 9. The neutron light conversion target of claim 4, wherein the channel extends along a target axis of symmetry of the body. 10. A photoneutron-X-ray source for simultaneously producing photoneutrons and X-rays, characterized in that the photoneutron-X-ray source comprises: X-ray generator for generating the main beam of X-rays; a photoneutron conversion target, the X-ray main beam can bombard the photoneutron conversion target to generate photoneutrons, the photoneutron conversion target has a body and a channel defined by the body, and the channel runs through the body; Wherein, the first X-ray beam in the X-ray main beam can pass through the channel without reacting with the body, and at the same time, the second X-ray beam in the X-ray main beam can enter the body, And react with this body to produce photoneutrons. 11. The optical neutron-X-ray source according to claim 10, wherein the body is an elongated body extending along the direction of propagation of the X-ray main beam, and the body has a first end and a second end. ends; and The extension direction of the channel is the same as the propagation direction of the X-ray main beam. 12. The photoneutron-X-ray source according to claim 11, wherein the shape of the body of the photoneutron conversion target is designed to substantially match the intensity distribution of the main X-ray beam. 13. The optical neutron-X-ray source according to claim 12, characterized in that, the intensity distribution of the X-ray main beam is an axisymmetric distribution, which defines an intensity distribution symmetry axis; The body of the neutron conversion target is configured to have an axisymmetric shape about a target symmetry axis, the axisymmetric shape of the neutron conversion target substantially matching the axisymmetric distribution of the X-ray main beam. 14. Optical neutron-X-ray source according to claim 12 or 13, characterized in that at least a part of the body is a tapered portion which tapers towards the second end. 15. The optical neutron-X-ray source of claim 14, wherein the tapered portion terminates at the second end. 16. The optical neutron-X-ray source of claim 15, wherein the tapered portion is conical or frusto-conical. 17. The photoneutron-X-ray source of claim 14, wherein said body further comprises a cylindrical portion, said tapered portion adjacent said second end, said cylindrical portion adjacent said the first end. 18. The optical neutron-X-ray source of claim 13, wherein the channel extends along a target axis of symmetry of the body. 19. The optical neutron-X-ray source according to claim 10, characterized in that, the X-ray main beam generated by the X-ray generator is an X-ray pulse with a specific frequency. 20. The optical neutron-X-ray source according to claim 10, wherein the X-ray generator comprises: electron accelerators for producing electron beams; and an electron target to which the electron beam is irradiated to generate the main X-ray beam. 21. The optical neutron-X-ray source according to claim 20, wherein the electron beam is an electron beam pulse with a specific frequency. 22. The optical neutron-X-ray source according to claim 10, further comprising an X-ray collimator, the X-ray collimator is arranged to be aligned with the channel for the The first X-ray beam of the channel is collimated into a planar fan beam. 23. The optical neutron-X-ray source according to claim 10, further comprising a neutron reflector for reflecting optical neutrons moving in a direction opposite to the propagation direction of the X-ray main beam .

Images