Slow-down collimation device and thermal neutron photographing system

Abstract

The invention provides a moderation collimation device for thermal neutron imaging. The moderation collimation device can provide emergent thermal neutrons with high flux and good directivity for neutron imaging equipment such as a mobile neutron photographing system and a neutron treatment device. The slowing-down collimation device comprises a uniform slowing-down body, a hole array slowing-down body and a hole array collimator which are sequentially connected in the first direction. The neutron reflecting layer surrounds the uniform slowing-down body, the hole array slowing-down body and the hole array collimator, the uniform slowing-down body comprises at least one neutron slowing-down material, the hole array slowing-down body comprises a plurality of first holes which are densely arranged in a two-dimensional mode and extend in the first direction, the hole walls of the hole array slowing-down body are made of the neutron slowing-down material, and the hole walls of the hole array slowing-down body are made of the neutron slowing-down material. The hole array collimator comprises a plurality of second holes which are densely laid and arranged in a two-dimensional mode and extend in the first direction, the hole walls of the hole array collimator are made of neutron absorbing materials, the neutron reflecting layer extends in the first direction and is attached to the periphery of the uniform moderator, the hole array moderator and the hole array collimator in a wrapping mode, and the neutron reflecting layer comprises at least one neutron reflecting material.

Description

Technical Field

[0001] The present invention relates to the field of neutron imaging (neutron photography), and in particular to improvements on a moderation collimation device for thermal neutron imaging and a thermal neutron photography system. Background Art

[0002] Neutron photography is a high-performance nondestructive testing technology that uses the difference in the number of transmitted neutrons after the interaction between neutrons and samples to obtain information such as the internal structure and defects of the object being photographed. Neutron photography can be divided into different types such as cold neutron photography, thermal neutron photography and fast neutron photography according to the different neutron energies used. Thermal neutron photography is widely used in industrial fields such as aerospace, national defense security, and precision manufacturing due to its excellent imaging quality, low cost of use, and high detection sensitivity to specific nuclides.

[0003] The moderator collimator is an important component of the thermal neutron radiography system. It slows down the high-energy neutrons produced by the neutron source into thermal neutrons and constrains the neutron emission angle distribution to meet the needs of thermal neutron radiography. At present, the thermal neutron radiography system generally adopts a technical solution of a uniform moderator combined with a divergent collimator. The moderation and collimation efficiency of this solution is low. Although it has a good application effect on large neutron source devices such as reactors and spallation neutron sources, it is difficult to meet the application needs of miniaturized thermal neutron radiography systems such as small accelerator neutron sources and isotope neutron sources. Therefore, it is necessary to develop a more efficient moderator collimator device to improve the performance of mobile thermal neutron radiography systems. Summary of the invention

[0004] Technical Problems to be Solved by the Invention

[0005] The moderator-collimator device in the current thermal neutron radiography system generally adopts the technical solution of uniform moderator combined with divergent collimator, and is widely used in large neutron sources such as reactors and spallation neutron sources. However, the moderation and collimation efficiency of this technical solution is low. When applied to small neutron sources with mobility, it is impossible to obtain high-quality thermal neutron beams, resulting in imaging defects such as reduced resolution, prolonged imaging time, and image spatial distortion. Therefore, the development of a more efficient thermal neutron radiography moderator-collimator device has become an important technical goal of the current thermal neutron radiography technology.

[0006] Technical means to solve technical problems

[0007] In order to solve the above technical problems, the present invention provides a moderation and collimation device for thermal neutron imaging, comprising a uniform moderator, a hole array moderator and a hole array collimator which are sequentially connected in a first direction; and a neutron reflecting layer surrounding the uniform moderator, the hole array moderator and the hole array collimator, wherein the uniform moderator comprises at least one neutron moderating material, the hole array moderator comprises a plurality of first holes of the same shape which are arranged in a two-dimensional close-packed manner, the first holes extend along the first direction, the hole walls of the hole array moderator are neutron moderating material, the hole array collimator comprises a plurality of second holes of the same shape which are arranged in a two-dimensional close-packed manner, the second holes extend along the first direction, the hole walls of the hole array collimator are neutron absorbing material, the neutron reflecting layer extends along the first direction and is attached and wrapped around the uniform moderator, the hole array moderator and the hole array collimator, and the neutron reflecting layer comprises at least one neutron reflecting material.

[0008] Preferably, in the above-mentioned moderator collimator, the shape of the first hole is the same as or different from the shape of the second hole, the shape of the first hole and the shape of the second hole in a cross section perpendicular to the first direction are polygonal or circular, and the grid cell width D0 of the hole array moderator and the grid cell width D1 of the hole array collimator satisfy the following relationship: D0=n*D1, where n=1, 2, 3, 4...

[0009] Preferably, in the above-mentioned moderator and collimator, when observing any position of the hole array moderator and the hole array collimator along the first direction, one or more of the second holes are contained in one of the first holes.

[0010] Preferably, in the above-mentioned moderator collimator, the hole wall of the hole array moderator includes a hydrogen-rich, beryllium-rich or carbon-rich material with an atomic ratio (including isotopes) of not less than 50%, and the channel of the first hole is filled with or not filled with material, and the filling material includes one or more of single-component gas, multi-component gas, silicon dioxide, and silicon, and the filling material in the channel of any cross-section perpendicular to the first direction is the same.

[0011] Preferably, in the above-mentioned slowing down collimator, the measured size difference between different channels of the second holes with the same shape is less than 5%, the channels of the second holes are filled with or not filled with material, the filling material includes a single-component gas or a multi-component gas, and the channel length of the hole array collimator is within the range of 5 cm to 30 cm.

[0012] Preferably, the above-mentioned moderation and collimation device further includes a neutron shielding layer, and the neutron shielding layer includes two parts: a non-imaging direction part and an imaging direction part, wherein the non-imaging direction part is arranged in a manner of surrounding the neutron reflection layer on all sides, and the imaging direction part is arranged in a manner of covering the neutron reflection layer from the imaging direction.

[0013] In addition, the present invention provides a thermal neutron radiography system, comprising: a neutron source for generating a neutron beam, a moderation collimator for slowing down and collimating the neutron beam from the neutron source, a sample to be inspected irradiated by the neutron beam from the moderation collimator, a conversion screen for converting the neutron beam after irradiating the sample to be inspected into a visible light image, and an imaging detector for recording the visible light image, characterized in that the moderation collimator is the above-mentioned moderation collimator of the present invention.

[0014] Beneficial Effects

[0015] (1) The moderator collimator of the present invention adopts a moderator with directional enhancement characteristics of neutrons in the imaging direction. Through the uniformly arranged hole array structure, the thermal neutron emission area after moderation and collimation is increased, the probability of neutron emission at small angles is increased, and the neutron injection rate in the imaging direction with the same collimation ratio is effectively improved.

[0016] (2) The moderation collimation device and thermal neutron radiography system of the present invention adopt a collimator with a uniform hole array structure, which improves the probability of neutron emission at small angles, that is, the extraction efficiency of neutrons in the imaging direction, and realizes the enhancement of the neutron injection rate in the imaging direction. At the same time, it can effectively solve the spatial distortion of the image caused by the diffuse collimator under low collimation ratio.

[0017] (3) The moderator collimator and thermal neutron radiography system of the present invention match the structures of the moderator and the collimator, and both the moderator and the collimator use a uniform hole array with hole features of the same shape. When observing any position of the hole array moderator and the hole array collimator along the hole channel direction, one or more second holes are contained in one first hole. In this way, the neutron extraction efficiency in the imaging direction can be improved by the collimator that is highly matched with the hole array moderator, and the neutron fluence rate in the imaging direction can be enhanced.

[0018] Therefore, the moderation and collimation device of the present invention can provide high-flux, well-directional emitted thermal neutrons for neutron imaging equipment such as mobile neutron radiography systems and neutron therapy devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic cross-sectional view showing a slowing-down collimator according to an embodiment of the present invention;

[0020] Figure 2a and Figure 2bis a schematic diagram showing the shapes of a hole array moderator and a hole array collimator according to an embodiment of the present invention, wherein: Figure 2a The hole shape shown is a square. Figure 2b The hole shape shown is an equilateral triangle;

[0021] Figure 3 is a schematic cross-sectional view showing the structure and size relationship of a hole array moderator and a hole array collimator according to an embodiment of the present invention;

[0022] Figure 4a and Figure 4b is a schematic diagram showing examples of hole shapes of a hole array moderator and a hole array collimator, wherein: Figure 4a The example shown here is that both holes have the same regular triangle shape. Figure 4b Shown are examples of hole shapes where one is a regular hexagon and the other is a regular triangle;

[0023] Figure 5 is a schematic diagram showing the structure of a neutron radiography system according to one embodiment of the present invention;

[0024] Figure 6a and Figure 6b is a graph showing the technical effect of a slowing-down collimator according to an embodiment of the present invention, wherein: Figure 6a The results show the comparison of thermal neutron injection of three types of moderation and collimation devices. Figure 6b Shown are the comparison results of neutron emission angles of three types of moderation and collimation devices;

[0025] Figure 7 The figure shows the test results of neutron radiography bright field of the moderation and collimation device according to one embodiment of the present invention. DETAILED DESCRIPTION

[0026] Hereinafter, specific embodiments of the present invention will be described with reference to the accompanying drawings.

[0027] (Slowing-down collimator according to one embodiment of the present invention)

[0028] Figure 1 1 is a schematic cross-sectional view showing a slowing down collimator according to an embodiment of the present invention. Figure 1 A slowing-down collimator according to an embodiment of the present invention will be described.

[0029] like Figure 1 As shown, a moderator collimator 1 according to an embodiment of the present invention comprises a uniform moderator 101, a hole array moderator 102, a neutron reflector 103, a hole array collimator 104 and a neutron shielding layer 105. The uniform moderator 101, the hole array moderator 102 and the hole array collimator 104 are arranged in a first direction ( Figure 1 The neutron reflector 103 is disposed in a manner of being attached and surrounding the uniform moderator 101, the hole array moderator 102 and the hole array collimator 104 ( Figure 1 In the cross-sectional view, only the neutron reflectors 103 disposed above and below the uniform moderator 101, the hole array moderator 102 and the hole array collimator 104 can be seen. The neutron shielding layer 105 includes two parts: a non-imaging direction part and an imaging direction part. The non-imaging direction part is arranged in a manner of surrounding the neutron reflector 103, and the imaging direction part is arranged in a manner of extending from the imaging direction ( Figure 1 The hole array moderator 102 and the hole array collimator 104 are arranged in a manner of covering the neutron reflector 103. The plurality of holes in the hole array moderator 102 and the hole array collimator 104 extend along a first direction (refer to Figure 3 ). Figure 1 In the moderation and collimation device 1 shown, the neutron source is arranged on the left side in the first direction, opposite to the uniform moderator 101, and the thermal neutrons slowed down and collimated by the moderation and collimation device 1 are led out from the hole array collimator 104 to the right side, i.e., the imaging direction.

[0030] The uniform moderator 101 is composed of a uniform moderator material of appropriate thickness and is used to initially moderate neutrons. Depending on the neutron source, the material of the uniform moderator can be one or more of lead, depleted uranium, tungsten, polyethylene, graphite, heavy water, water, liquid hydrogen, and liquid deuterium. Figure 1 The right side in the middle is connected to the hole array moderator 102 to further moderate and direct the gain neutrons. The uniform moderator is surrounded by a reflective layer to improve the utilization rate of thermal neutrons.

[0031] The hole array moderator 102 has a two-dimensional repeating structure, and is formed into a hole array by alternating arrangement of neutron moderation materials and neutron transparent materials in the two-dimensional direction. The hole array moderator 102 is composed of moderation materials of appropriate shape and size to increase the moderation efficiency, improve the emission area of ​​thermal neutrons after small-angle moderation, and increase the probability of small-angle neutron emission. The hole wall of the moderator is required to have as high a moderation ability as possible and as low a neutron potential as possible. Preferably, the hole wall material of the hole array moderator 102 is a high neutron moderation material including hydrogen-rich, beryllium-rich or carbon-rich materials with an atomic proportion (including isotopes) of not less than 50%, such as one or more of polyethylene, graphite, water, and beryllium oxide. The filling material in the moderator hole is required to have as low a neutron absorption performance as possible and a large neutron potential. Preferably, the moderator hole is not filled with material or is filled with neutron transparent material including single-component gas, multi-component gas, silicon dioxide or silicon, such as one or more of nitrogen, air, vacuum (no filling), silicon dioxide and silicon.

[0032] The neutron reflector 103 is composed of a neutron reflective material of appropriate thickness, and is used to reflect neutrons emitted in non-imaging directions, increase the utilization rate of source neutrons, and improve the neutron moderation efficiency. Preferably, the material of the neutron reflector 103 is a high neutron scattering material, such as one or more of heavy water, graphite, and beryllium.

[0033] The hole array collimator 104 is a multi-channel collimator composed of multiple groups of grid elements, and these channels are composed of multiple small channels. The hole array collimator 104 is composed of neutron absorbing materials of appropriate shape and size, which are used to control the parallelism of the outgoing neutron beam and the thermal neutrons in the outgoing imaging direction. Preferably, the hole wall material of the hole array collimator 104 adopts a material with high mechanical strength, especially not easy to deform, and high neutron absorption, such as one or more of aluminum-based boron carbide and gadolinium steel. However, the hole wall material is not limited to the above materials. As long as it can achieve a measured size difference of less than 5% between different channels of multiple channels with the same shape and high neutron absorption, it can be used as a preferred hole wall material. Regarding the filling material in the channel of the hole array collimator 104, in order to reduce the reduction of the neutron injection rate, it is preferred that the channel is not filled with material, or is filled with a single-component gas or a multi-component gas, such as vacuum (no filling) or air.

[0034] The neutron shielding layer 105 is composed of neutron absorbing materials of appropriate size, and is used to absorb thermal neutrons in the non-emergent imaging direction and gamma rays generated by neutrons during the slowing down process. Preferably, the material of the neutron shielding layer is thermal neutron absorbing material and gamma absorbing material, such as one or more of boron-containing polyethylene, lead, and gadolinium steel.

[0035] Figure 2a and Figure 2b 1 is a schematic diagram showing the shapes of an aperture array moderator and an aperture array collimator according to an embodiment of the present invention, and is a view of a cross section perpendicular to a first direction. Figure 2a and Figure 2b The shape shown in , may be the shape of the hole array moderator 102 , or the shape of the hole array collimator 104 .

[0036] like Figure 2a and Figure 2bAs described, the hole array moderator 102 and the hole array collimator 104 of one embodiment of the present invention have a roughly square shape in a cross section perpendicular to the first direction. However, the shape of the hole array moderator and the hole array collimator is not limited to a square, and may also be a rectangular, hexagonal, circular, elliptical, and other shapes. Preferably, the outer dimensions of the hole array moderator and the outer dimensions of the hole array collimator in a cross section perpendicular to the first direction are roughly the same, thereby maximizing the thermal neutron flux after moderation and collimation. In order to achieve the collimation of thermal neutrons, the channel size of the traditional divergent collimator is made very small (much smaller than the cross-sectional size of the moderator), which reduces the thermal neutron flux after moderation and collimation, thereby resulting in a reduction in the thermal neutron injection rate. The present invention uses a hole array collimator that is completely different from the traditional divergent collimator, which can not only increase the exit area of ​​thermal neutrons after small-angle moderation, but also improve the extraction efficiency of neutrons in the imaging direction by matching with the moderator, thereby achieving an enhancement of the neutron injection amount in the imaging direction.

[0037] Figure 2a and Figure 2b Also shown are examples of the arrangement of the multiple holes in the hole array moderator 102 and the hole array collimator 104 and the shapes of the individual holes, wherein the black lines represent the hole walls. Figure 2a and Figure 2b In the figure, only the number 102 representing the moderator of the hole array is marked. Figure 2a and Figure 2b The displayed graph may also be an example of the arrangement of multiple holes and the shape of each hole of the hole array collimator 104. In the following description, for the convenience of expression, each hole of the hole array moderator 102 is called a first hole, and each hole of the hole array collimator 104 is called a second hole.

[0038] like Figure 2a and Figure 2b As described above, the plurality of holes in the hole array moderator 102 and the hole array collimator 104 are arranged in a two-dimensional densely packed manner, and the shapes of each of the plurality of holes are substantially the same, that is, a uniform hole array is formed. Figure 2a The hole shape shown is a square. Figure 2b The hole shape of the hole array moderator and the hole array collimator is not limited to Figure 2a and Figure 2b The shape shown may be, for example, a polygonal shape such as a rectangle or a regular hexagon, or may be a circle.

[0039] As shown in the embodiments described later, compared with the traditional uniform moderator, the thermal neutron fluence rate of the hole array moderator of the present invention is greatly improved. It can be seen that the hole array moderator can increase the neutron moderation efficiency and improve the utilization rate of source neutrons compared with the traditional uniform moderator. In addition, the hole array collimator can greatly improve the emission probability of small-angle neutrons, that is, the extraction efficiency of neutrons in the imaging direction, thereby enhancing the neutron fluence rate in the imaging direction.

[0040] Refer to the following Figure 3 The structure and size relationship of a hole array moderator and a hole array collimator according to an embodiment of the present invention are described. Figure 3 is a schematic cross-sectional view showing the structure and size relationship of a hole array moderator and a hole array collimator according to an embodiment of the present invention, Figure 1 The partially enlarged view of the hole array moderator 102 and the hole array collimator 104 is shown. Figure 3 The left and right directions Figure 1 same. Figure 3 The white lines in the figure represent the hole wall of the first hole of the hole array moderator 102 and the hole wall of the second hole of the hole array collimator 104, and the gray parts represent the hole channels of the first hole and the second hole. 301 is the hole wall of the first hole of the hole array moderator 102, 302 is the hole channel of the first hole of the hole array moderator 102, 303 is the hole wall of the second hole of the hole array collimator 104, and 304 is the hole channel of the second hole of the hole array collimator 104.

[0041] like Figure 3 As shown, the plurality of holes in the hole array moderator 102 and the hole array collimator 104 are aligned along a first direction ( Figure 3 The left and right directions of the Figure 3 In FIG. 1 , D0 represents the width of a grid element of the hole array moderator 102, d0 represents the width of a channel of the hole array moderator 102, and L0 represents the length of the channel of the hole array moderator 102. D1 represents the width of a grid element of the hole array collimator 104, d1 represents the width of a channel of the hole array collimator 104, and L1 represents the length of the channel of the hole array collimator 104.

[0042] As a preferred embodiment of the present invention, the cell width D0 of the aperture array moderator 102 and the cell width D1 of the aperture array collimator 104 satisfy the following relationship:

[0043] D0=n*D1, where n=1, 2, 3, 4…

[0044] That is, the cell width D0 of the hole array moderator 102 is an integral multiple of the cell width D1 of the hole array collimator 104. And it also satisfies that when the moderator collimator is observed along the first direction, one or more second holes are contained in one first hole.

[0045] Specifically, when n=1, D0=D1, that is, the cell width D0 of the hole array moderator 102 is equal to the cell width D1 of the hole array collimator 104. At this time, when the moderator collimator is observed along the first direction, one second hole is contained in one first hole, that is, the first hole of the collimator and the second hole of the moderator are completely aligned. Figure 3 The illustrated case is D0=2*D1, that is, the cell width D0 of the hole array moderator 102 is twice the cell width D1 of the hole array collimator 104.

[0046] exist Figure 3 In the example, when the aperture shape is a square, when the slowing down collimator is observed along the first direction, in the vertical direction, the two second apertures are contained in one first aperture. Since it is a two-dimensional closely packed array, when the aperture shape is a square, in the horizontal direction ( Figure 3 Two second holes (in a horizontal direction perpendicular to the left and right directions) are contained in one first hole. Therefore, a total of four second holes are contained in one first hole.

[0047] However, even if the cell width D0 of the hole array moderator is not equal to the cell width D1 of the hole array collimator, the hole shape is not limited to a square, and may be a rectangle, an equilateral triangle, a regular hexagon, or the like.

[0048] Figure 4a and Figure 4b is a schematic diagram showing examples of hole shapes of a hole array moderator and a hole array collimator, wherein: Figure 4a The example shown here is that both holes have the same regular triangle shape. Figure 4b Shown are examples of hole shapes where one is a regular hexagon and the other is a regular triangle. Figure 4a and Figure 4b In FIG. 1 , 102C represents a grid element of the hole array moderator 102, that is, a first hole, and 104C represents a grid element of the hole array collimator 104, that is, a second hole. Figure 4a and Figure 4b In the figure, for the convenience of observation, only one of the multiple grid elements of the hole array collimator 104 is given the label 104C, but those skilled in the art can know that Figure 4a and Figure 4b In each of the figures, a plurality of grid elements, i.e., second holes, of the hole array collimator 104 having the same size and shape are shown.

[0049] Specifically, if Figure 4aAs shown, the hole array moderator and the hole array collimator are both equilateral triangles, and there is a corresponding relationship in position and size between one equilateral triangle grid element 102C of the hole array moderator and four equilateral triangle grid elements 104C of the hole array collimator. Figure 4a In the example shown, in terms of positional relationship, four second holes are contained in one first hole. In terms of size relationship, the cell width D0 of the hole array moderator and the cell width D1 of the hole array collimator satisfy the relationship D0=2*D1, where the cell width is the side length of the regular triangle.

[0050] exist Figure 4b In the example shown, the hole shape of the hole array moderator is a regular hexagon, and the hole shape of the hole array collimator is a regular triangle, that is, the hole shapes of the two are different. Figure 4b As shown, there is a corresponding relationship in position and size between one regular hexagonal grid element 102C of the aperture array moderator and six regular triangle grid elements 104C of the aperture array collimator. Figure 4b In the example shown, in terms of positional relationship, 6 second holes are contained in 1 first hole. In terms of size relationship, the cell width D0 of the hole array moderator and the cell width D1 of the hole array collimator satisfy the relationship D0=2*D1. Wherein, when the hole shape is a regular triangle, the cell width is the side length of the triangle, and when the hole shape is a regular hexagon, the cell width is the diagonal length of the regular hexagon.

[0051] By adopting the above structure, the hole array collimator and the hole array moderator can be fully matched in space, the emission area of ​​thermal neutrons after small-angle moderation can be increased, thereby improving the neutron extraction efficiency in the imaging direction and enhancing the neutron injection amount in the imaging direction.

[0052] In the above embodiment, it is preferred that the hole wall of the hole array collimator is thinner, that is, the value of D1-d1 is smaller. The main influence of the collimator wall thickness on the imaging effect is that it affects the thermal neutron flux rate reaching the imaging position by affecting the effective neutron emission area. Therefore, the smaller the hole wall thickness, the better, and as close to 0 as possible, but due to the limitation of material strength, the lower limit of the wall thickness is determined by the necessary mechanical strength, etc. As described above, the hole wall material of the hole array collimator 104 adopts a high-neutron absorption material with high mechanical strength, such as one or more of aluminum-based boron carbide and gadolinium steel. Due to the high mechanical strength of the hole wall material, the hole wall thickness of the collimator can be reduced as much as possible while maintaining the required mechanical strength.

[0053] In the above embodiment, it is preferred that the hole length L1 of the hole array collimator 104 is 5 cm to 30 cm. The definition of the collimation ratio of the hole array collimator of the present invention is different from that of the conventional divergent collimator. For the conventional divergent collimator, the collimation ratio is determined by the collimator aperture entrance diameter D and the distance L from the collimator aperture to the detector plane, while the collimation ratio of the hole array collimator is determined by the hole length L1 and the hole width d1 of the hole array collimator, and has nothing to do with the distance to the imaging plane. That is, the collimation ratio of the hole array collimator is defined as L1 / d1. For example, when L1=9 cm and d1=0.45 cm, the collimation ratio is 20. In order to ensure the structural strength of the collimator, the thickness of the hole wall is roughly fixed. When the collimation is relatively large, if the hole length L1 is less than 5 cm, the size of each grid element D1 is very small. At this time, the hole wall accounts for a large proportion, which will reduce the thermal neutron fluence rate of imaging. When the channel length L1 exceeds 30 cm, the excessively long distance will lead to a decrease in the thermal neutron flux due to the certain absorption of thermal neutrons by air.

[0054] In the above embodiment, it is preferred that the cell width D0 of the hole array moderator 102 is 2 to 3 times the cell width D1 of the hole array collimator 104, that is, n=2 or 3. When the n value is small, because the cell width D1 is large, in order to meet the collimation ratio, the length of the collimator will be longer, increasing the absorption of thermal neutrons by air, thereby reducing the thermal neutron fluence rate. When the n value is too large, because the cell width D1 of the collimator is small, the hole wall of the collimator will account for too large a proportion, reducing the effective neutron emission area, and also reducing the thermal neutron fluence rate.

[0055] In the above embodiment, it is preferred that the imaging position is between L1+2(n-1)(D1-d1) and L1+2n(D1-d1) from the collimator outlet, wherein L1 is the hole length of the hole array collimation, D1 is the grid width of the hole array collimator, and d1 is the hole width of the hole array collimator. Thus, a uniform neutron fluence rate distribution characteristic can be obtained in the imaging plane. Here, the imaging plane that can obtain a uniform neutron fluence rate distribution characteristic is called the focal plane. It is more preferred that the imaging position is between L1 and L1+2(D1-d1) from the collimator outlet, that is, the imaging position when n=1. Thus, the clearest thermal neutron imaging and the highest imaging thermal neutron fluence rate can be obtained in the imaging plane.

[0056] (Neutron Radiography System According to One Embodiment of the Present Invention)

[0057] Figure 5 Schematic diagram showing the structure of a thermal neutron radiography system according to one embodiment of the present invention. Figure 5 The structure of a thermal neutron radiography system according to one embodiment of the present invention will be described.

[0058] Figure 5 The structure of a small thermal neutron radiography system is shown in Figure 1. Figure 5 As shown, a thermal neutron radiography system according to an embodiment of the present invention includes a neutron source 501, a moderator collimator 502, a sample to be inspected 503, a dark box 504, a conversion screen 505, a reflector 606, and an imaging detector 507. Among them, the neutron source 501 is a part that provides a neutron beam, and an accelerator DT neutron generator, etc., can be used in a small thermal neutron radiography system. The moderator collimator 502 includes a moderator and a collimator, the moderator is used to convert high-energy neutrons into thermal neutrons, and the collimator is used to collimate and focus the neutron beam. After irradiating the sample to be inspected 503, the neutron beam is converted into a visible light image on the conversion screen 505, and the visible light image is reflected by the reflector 506 and projected onto the imaging detector 507, which converts it into an image signal. The illustration is omitted here, and the neutron radiography system according to an embodiment of the present invention may also include an image processing device for processing the image signal output by the imaging detector 507, a memory for storing image data, and a display screen for displaying image signals, etc.

[0059] exist Figure 5 In the thermal neutron radiography system shown, a conversion screen 505, a reflector 506 and an imaging detector 507 are placed in a dark box 504. The dark box 504, the conversion screen 505, the reflector 506 and the imaging detector 507 can all use existing equipment. For example, the conversion screen 505 can include neutron conversion materials and fluorescent materials. After the thermal neutrons interact with the neutron conversion materials, α, β, γ rays, etc. are emitted. These secondary rays or charged particles make the fluorescent materials emit light, thereby converting the neutron beam image into a visible light image. As a conversion screen suitable for thermal neutron radiography, for example, a transient thermal neutron conversion screen made of a uniform mixture of lithium, boron, cadmium, gadolinium and fluorescent materials can be used. Since the dark box 504, the reflector 506 and the imaging detector 507 can all use existing equipment, their detailed description is omitted here. For example, the imaging detector 507 can use an Andor iXon ultra-888 deep-cooled CCD camera. In addition, although Figure 5 The reflector 506 is shown in FIG. 5 , but it is not necessary and the reflector 506 may be omitted so that the imaging detector 507 directly records the visible light image from the conversion screen 505 .

[0060] The thermal neutron radiography system of one embodiment of the present invention is characterized in that the moderator collimator 502 adopts Figures 1 to 3 The slowing down collimator 1 of the present invention is shown. The slowing down collimator includes Figure 1 The uniform moderator 101 and the hole array moderator 102 shown as collimators include Figure 1The hole array collimator 104 is shown. In addition, the moderator collimator 502 further includes a neutron reflector 103 which is arranged to surround the uniform moderator 101 and the hole array moderator 102.

[0061] The thermal neutron radiography system using the above structure, as shown in the embodiments described later, has a greatly improved thermal neutron fluence rate of the hole array moderator of the present invention compared to the traditional uniform moderator. It can be seen that the hole array moderator can increase the neutron moderation efficiency and improve the utilization rate of source neutrons compared to the traditional uniform moderator. In addition, the hole array collimator can greatly improve the emission probability of small-angle neutrons, that is, the extraction efficiency of neutrons in the imaging direction, thereby enhancing the neutron fluence rate in the imaging direction. In addition, the hole array collimator and the hole array moderator can be fully matched in space, which can increase the emission area of ​​thermal neutrons after small-angle moderation, thereby improving the extraction efficiency of neutrons in the imaging direction, thereby enhancing the neutron injection amount in the imaging direction.

[0062] (Example)

[0063] The specific embodiments and technical effects of the slowing-down collimator 1 of the present invention are described below in conjunction with the accompanying drawings.

[0064] In this embodiment, the uniform moderator 101 is a two-layer structure. Specifically, in the first direction ( Figure 1 7 cm thick lead and 4 cm thick polyethylene are sequentially connected in the left and right directions (in the right and left directions). The hole array moderator 102 adopts a uniform right-angle square hole array structure, and the hole wall material is polyethylene, with specific dimensions of L0=20 cm, D0=1 cm, and d0=0.9 cm. The neutron reflection layer is 20 cm thick graphite, surrounding the uniform moderator 101, the hole array moderator 102 and the hole array collimator 104. The hole array collimator 104 adopts a uniform right-angle square hole array structure, and the hole wall material is aluminum-based boron carbide with a boron carbide content of 20%, with specific dimensions of L1=9 cm, D1=0.5 cm, and d1=0.45 cm. The hole array collimator is formed by cross-stacking multiple aluminum-based boron carbide plates.

[0065] In this implementation, the Figure 3 The structure shown is D0=2*D1, that is, the gate cell width D0 of the aperture array moderator 102 is twice the gate cell width D1 of the aperture array collimator 104. And, Figure 3 As shown, when the slowing down collimator is observed along the first direction, in the vertical direction, the two second holes are contained in one first hole. Figure 3 Two second holes (in a horizontal direction perpendicular to the left and right directions) are contained in one first hole. Therefore, a total of four second holes are contained in one first hole.

[0066] The neutron shielding layer includes two parts, a non-imaging direction part and an imaging direction part, both of which are two-layer structures. The neutron shielding layer in the non-imaging direction is composed of 1 cm thick boron-containing polyethylene and 0.5 cm thick lead, and the neutron shielding layer in the imaging direction is composed of 3 cm thick boron-containing polyethylene and 1 cm thick lead.

[0067] Refer to the following Figure 6a , Figure 6b and Figure 7 The technical effects of the present invention are described.

[0068] Figure 6a and Figure 6b is a graph showing the technical effect of a slowing-down collimator according to an embodiment of the present invention, wherein: Figure 6a The results show the comparison of thermal neutron injection of three types of moderation and collimation devices. Figure 6b Shown are the comparison results of neutron emission angles of three types of moderation and collimation devices.

[0069] Figure 6a The comparison results of thermal neutron injection amount of three types of moderators and collimators are shown, wherein the three types of moderators and collimators are: moderator and collimator 1 (hole array moderator + hole array collimator) of one embodiment of the present invention, conventional moderator + divergent collimator and conventional moderator + hole array collimator. Figure 6a As shown in the curve graph, when the collimation ratio L / D = 20, the normalized thermal neutron injection rates of the three types of slowing down collimators are 2.25×10 -7 n cm -2 ·s -1 , 1.15×10 -7 n cm -2 ·s -1 and 1.34×10 -7 n cm -2 ·s -1 , of which thermal neutrons accounted for 29.3%, 5.6% and 13.5% respectively. Compared with the traditional divergent collimator, the thermal neutron injection rate of the hole array moderator collimator increased by 100% and the thermal neutron proportion increased by 400%. When the accelerator neutron source strength is 3×10 10 n·s -1 When the thermal neutron injection rate can reach 6.75×10 3 n cm -2 ·s -1 , fully meeting the needs of neutron photography.

[0070] Figure 6bThe comparison results of neutron emission angles of three types of moderators and collimators are shown, wherein the three types of moderators and collimators are: moderator and collimator 1 (hole array moderator + hole array collimator) of an embodiment of the present invention, conventional moderator + hole array collimator and conventional moderator and collimator (uniform moderator + divergent collimator). Figure 6b It can be seen from the curve graph shown that the emission probability of small-angle neutrons of the moderator collimator of the present invention and the conventional moderator + hole array collimator is much higher than that of the conventional moderator collimator, which reflects that the hole array collimator can greatly improve the emission probability of small-angle neutrons, that is, the extraction efficiency of neutrons in the imaging direction, and achieves the enhancement of the neutron fluence rate in the imaging direction. In addition, compared with the conventional moderator + hole array collimator, the thermal neutron fluence rate in the same small-degree range of the moderator collimator of the present invention is greatly improved, which reflects that the hole array moderator can increase the neutron moderation efficiency and improve the utilization rate of source neutrons compared with the traditional uniform moderator. In this way, the structure of the present invention can not only improve the neutron moderation efficiency to increase the thermal neutron fluence rate, but also improve the emission probability of small-angle neutrons, that is, the extraction efficiency of neutrons in the imaging direction, and achieve the enhancement of the neutron fluence rate in the imaging direction.

[0071] Figure 7 The figure shows the test results of neutron radiography bright field of the moderation and collimation device according to one embodiment of the present invention. Figure 7 The neutron field distribution image obtained by the hole array collimator with a collimation ratio of 20 in the absence of a sample is shown. From the image, it can be seen that the brightness of the central area is higher, and the surrounding area appears darker than the central area. This is due to the uneven neutron beam emitted by the moderator. In addition, a faint grid-like shadow can be seen in the bright field image, which is due to the fact that the imaging plane does not completely coincide with the focal plane. Specifically, the grid-like shadow here is caused by the grid shape of the hole array collimator, and as mentioned above, the imaging plane that can obtain a uniform neutron fluence rate distribution characteristic is called the focal plane. When the image plane does not completely coincide with the focal plane, the shielding of the thermal neutron beam by the wall of the hole array collimator will cause uneven neutron fluence rate, and this unevenness is reflected in the grid-like shadow. However, in Figure 7 In the figure, the grid lines in the grid-shaped shadows are all straight lines, which shows that the slowing collimation device of the present invention does not cause spatial distortion of the image.

[0072] As mentioned above, although the hole array collimator still has some defects in the bright field image, it can obtain a relatively uniform neutron irradiation plane near the imaging plane, and effectively solve the image spatial distortion caused by the traditional diffusion collimator under low collimation ratio, which can meet the requirements of neutron radiography.

[0073] From the above description, it can be seen that the moderation collimator and thermal neutron radiography system of the present invention have the following technical effects:

[0074] (1) The moderator collimator of the present invention adopts a moderator with directional enhancement characteristics of neutrons in the imaging direction. Through the uniformly arranged hole array structure, the emission area of ​​thermal neutrons after small-angle moderation is increased, the probability of small-angle neutron emission is increased, and the neutron injection rate in the imaging direction is effectively improved.

[0075] (2) The moderation collimation device and thermal neutron radiography system of the present invention adopt a collimator with a uniform hole array structure, which improves the probability of neutron emission at small angles, that is, the extraction efficiency of neutrons in the imaging direction, and realizes the enhancement of the neutron injection rate in the imaging direction. At the same time, it can effectively solve the spatial distortion of the image caused by the diffuse collimator under low collimation ratio.

[0076] (3) The moderator collimator and thermal neutron radiography system of the present invention match the structures of the moderator and the collimator, and both the moderator and the collimator use a uniform hole array with hole features of the same shape. When observing any position of the hole array moderator and the hole array collimator along the hole channel direction, one or more second holes are contained in one first hole. In this way, the neutron extraction efficiency in the imaging direction can be improved by the collimator that is highly matched with the hole array moderator, and the neutron fluence rate in the imaging direction can be enhanced.

Claims

1. A moderation and collimation device for thermal neutron imaging, characterized in that: It includes a uniform moderator, a hole array moderator and a hole array collimator which are sequentially connected in a first direction; and a neutron reflection layer surrounding the uniform moderator, the hole array moderator and the hole array collimator, The uniform moderator comprises at least one neutron moderator material. The hole array moderator comprises a plurality of first holes of the same shape arranged in a two-dimensional manner, wherein the first holes extend along the first direction. The hole walls of the hole array moderator are neutron moderator materials. The hole array collimator comprises a plurality of second holes of the same shape arranged in a two-dimensional manner, wherein the second holes extend along the first direction. The hole walls of the hole array collimator are made of neutron absorbing material. The neutron reflection layer extends along the first direction and is attached and wrapped around the uniform moderator, the hole array moderator and the hole array collimator. The neutron reflection layer includes at least one neutron reflection material.

2. The slowing down and collimating device according to claim 1, characterized in that: The shape of the first hole is the same as or different from the shape of the second hole, and the shape of the first hole and the shape of the second hole in a cross section perpendicular to the first direction are polygonal or circular, The cell width D0 of the aperture array moderator and the cell width D1 of the aperture array collimator satisfy the following relationship: D0=n*D1, where n=1, 2, 3, 4… 3. The slowing down collimator according to claim 2, characterized in that: When any position of the hole array moderator and the hole array collimator is observed along the first direction, one or more of the second holes are contained in one of the first holes.

4. The slowing down collimator according to any one of claims 1 to 3, characterized in that: The hole walls of the hole array moderator include hydrogen-rich, beryllium-rich or carbon-rich materials with an atomic ratio (including isotopes) of not less than 50%. The pores of the first holes are filled or not filled with materials, and the filling materials include one or more of single-component gas, multi-component gas, silicon dioxide, and silicon. The filling materials in the pores of any cross section perpendicular to the first direction are the same.

5. The slowing down and collimating device according to any one of claims 1 to 3, characterized in that: The measured size difference of different channels of the second hole with the same shape is less than 5%, and the channels of the second hole are filled with or not filled with a material, and the filling material includes a single-component gas or a multi-component gas. The hole length of the hole array collimator is in the range of 5 cm to 30 cm.

6. The slowing down and collimating device according to any one of claims 1 to 3, characterized in that: It also includes a neutron shielding layer, which includes a non-imaging direction part and an imaging direction part. The non-imaging direction part is arranged in a manner of surrounding the neutron reflecting layer on all sides, and the imaging direction part is arranged in a manner of covering the neutron reflecting layer from the imaging direction.

7. A thermal neutron radiography system comprising: A neutron source for generating a neutron beam, a moderation collimator for moderating and collimating the neutron beam from the neutron source, a sample to be inspected irradiated by the neutron beam from the moderation collimator, a conversion screen for converting the neutron beam after irradiating the sample to be inspected into a visible light image, and an imaging detector for recording the visible light image, characterized in that: The slowing-down collimator is the slowing-down collimator device according to any one of claims 1-6.

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