Moderation and collimation device and thermal neutron radiography system

Abstract

The present invention provides a moderator and collimator for thermal neutron imaging, capable of providing high-flux, well-directional thermal neutrons for neutron imaging equipment such as mobile neutron radiography systems and neutron therapy devices. The moderator and collimator comprises a uniform moderator, a hole array moderator, and a hole array collimator, which are sequentially connected in a first direction; and a neutron reflector surrounding the uniform moderator, the hole array moderator, and the hole array collimator. The uniform moderator comprises at least one neutron moderating material. The hole array moderator comprises a plurality of first holes extending in a two-dimensionally closely spaced arrangement along the first direction, the hole array moderator having hole walls made of neutron moderating material. The hole array collimator comprises a plurality of second holes extending in a two-dimensionally closely spaced arrangement along the first direction, the hole array collimator having hole walls made of neutron absorbing material. The neutron reflector extends in the first direction and is attached to and wrapped around the uniform moderator, the hole array moderator, and the hole array collimator. The neutron reflector 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 to a moderation and collimation device for thermal neutron imaging and a thermal neutron photography system. Background Art

[0002] Neutron radiography is a high-performance nondestructive testing technique that uses the difference in the number of transmitted neutrons after neutrons interact with a sample to generate an image, revealing information about the internal structure and defects of the object. Neutron radiography can be categorized into cold, thermal, and fast neutron radiography, depending on the neutron energy used. Thermal neutron radiography is widely used in aerospace, national defense, security, precision manufacturing, and other industrial fields due to its excellent image quality, low cost, and high sensitivity to specific radionuclides.

[0003] The moderator collimator is a crucial component of thermal neutron radiography systems. It moderates the high-energy neutrons generated by the neutron source into thermal neutrons and constrains the neutron emission angle distribution to meet the imaging requirements of thermal neutron radiography. Currently, thermal neutron radiography systems generally use a uniform moderator combined with a divergent collimator. This approach has low moderation and collimation efficiency. While effective in large neutron source installations such as reactors and spallation neutron sources, it struggles to meet the requirements of miniaturized thermal neutron radiography systems, such as those using small accelerator and isotope neutron sources. Therefore, there is a need for more efficient moderator collimators to improve the performance of mobile thermal neutron radiography systems. Summary of the Invention

[0004] Technical problems to be solved by the present invention

[0005] Current thermal neutron radiography systems typically utilize a uniform moderator combined with a divergent collimator. This technology is widely used in large neutron sources, such as reactors and spallation neutron sources. However, this solution suffers from low moderation and collimation efficiency. When applied to small, mobile neutron sources, it fails to produce high-quality thermal neutron beams, leading to imaging defects such as reduced resolution, prolonged imaging time, and spatial distortion. Therefore, developing more efficient thermal neutron radiography moderators has become a key technical goal.

[0006] Technical means to solve technical problems

[0007] To solve the above technical problems, the present invention provides a moderator and collimator for thermal neutron imaging, comprising a uniform moderator, a hole array moderator, and a hole array collimator sequentially arranged 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 identical shape arranged in a two-dimensional close-packed manner, the first holes extending along the first direction, and the hole walls of the hole array moderator are formed of the neutron moderating material, the hole array collimator comprises a plurality of second holes of identical shape arranged in a two-dimensional close-packed manner, the second holes extending along the first direction, and the hole walls of the hole array collimator are formed of a neutron absorbing material, and 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 and collimator, 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, 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 moderator-collimator device, when observing any position of the hole array moderator and the hole array collimator along the first direction, one or more second holes are contained in one first hole.

[0010] Preferably, in the above-mentioned moderator and collimator, the hole walls of the hole array moderator include 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 a material, and the filling material includes one or more of a single-component gas, a multi-component gas, silica, 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 moderation collimator, the measured size difference of different channels of the second holes with the same shape is less than 5%, the channels of the second holes are filled 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 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 moderation collimator is the above-mentioned moderation collimator device of the present invention.

[0014] Beneficial effects

[0015] (1) The moderator collimator of the present invention adopts a moderator with the characteristic of directional enhancement 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 fluence 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 fluence rate in the imaging direction. At the same time, it can effectively solve the spatial distortion of the image caused by the diffusion 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 collimator, both of which utilize a uniform array of holes with identical hole features. When observing any position of the hole array moderator and hole array collimator along the hole channel, one or more secondary holes are contained within one primary hole. Thus, by using a collimator that is highly matched to the hole array moderator, the neutron extraction efficiency in the imaging direction can be improved, thereby enhancing the neutron fluence rate in the imaging direction.

[0018] Therefore, the moderation and collimation device of the present invention can provide high-flux, well-directed 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 moderation collimator according to one embodiment of the present invention;

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

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

[0022] Figure 4a and Figure 4b is a schematic diagram showing examples of the hole shapes of the hole array moderator and the hole array collimator, wherein: Figure 4a The example shown here shows that both holes have the shape of an equilateral triangle. Figure 4b Shown are examples of hole shapes where one is a regular hexagon and the other is an equilateral 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 the moderation collimator according to one embodiment of the present invention, wherein: Figure 6a The results show the comparison of thermal neutron injection amount 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 1 is a diagram showing the test results of neutron radiography bright field of the moderator collimator 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] (Moderator Collimator According to One Embodiment of the Present Invention)

[0028] Figure 1 This is a schematic cross-sectional view of a moderation collimator according to an embodiment of the present invention. Figure 1 A moderation and collimation device according to one embodiment of the present invention will be described.

[0029] like Figure 1 As shown, a moderator and 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 aligned in a first direction ( Figure 1 The neutron reflector 103 is arranged 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 to surround the neutron reflector 103, and the imaging direction part is arranged to extend from the imaging direction ( Figure 1 The hole array moderator 102 and the hole array collimator 104 are provided in a manner that covers the neutron reflector 103. The plurality of holes in the hole array moderator 102 and the hole array collimator 104 extend along the 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 after being moderated and collimated by the moderation and collimation device 1 are led out from the hole array collimator 104 to the right side, that is, 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 uniform moderator is surrounded by a reflective layer to improve thermal neutron utilization.

[0031] The hole array moderator 102 has a two-dimensional repeating structure, consisting of a neutron moderator material and a neutron-transparent material alternating in a two-dimensional arrangement to form a hole array. The hole array moderator 102 is composed of a moderator material of appropriate shape and size to increase moderation efficiency, broaden the emission area of ​​thermal neutrons after small-angle moderation, and enhance the probability of small-angle neutron emission. The pore walls of the moderator are required to have the highest possible moderation capacity and the lowest possible neutron potential. Preferably, the pore wall material of the hole array moderator 102 is a high-neutron moderator material comprising hydrogen-rich, beryllium-rich, or carbon-rich materials with an atomic fraction (including isotopes) of not less than 50%, such as one or more of polyethylene, graphite, water, and beryllium oxide. The filling material within the moderator pores is required to have the lowest possible neutron absorption and the highest possible neutron potential. Preferably, the moderator hole is not filled with any material or is filled with a neutron transparent material including a single-component gas, a multi-component gas, silicon dioxide or silicon, such as one or more of nitrogen, air, vacuum (no filling), silicon dioxide and silicon.

[0032] Neutron reflector 103, composed of a neutron-reflecting material of appropriate thickness, is used to reflect neutrons emitted from non-imaging directions, thereby increasing source neutron utilization and improving neutron moderation efficiency. Preferably, neutron reflector 103 is made of 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 emit thermal neutrons in the 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 is a material that can achieve a measured size difference of less than 5% between different holes of multiple holes of the same shape and has high neutron absorption, it can be used as a preferred hole wall material. Regarding the filling material in the hole of the hole array collimator 104, in order to reduce the reduction in neutron injection rate, it is preferred that the hole 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] Neutron shielding layer 105 is composed of neutron-absorbing material of appropriate size to absorb thermal neutrons in the non-emission imaging direction and gamma rays generated during neutron moderation. Preferably, the neutron shielding layer is made of thermal neutron-absorbing and gamma-absorbing materials, such as one or more of boron-containing polyethylene, lead, and gadolinium-containing 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 one embodiment of the present invention, and is a cross-sectional view perpendicular to a first direction. Figure 2a and Figure 2b The shape shown in can 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 above, the aperture array moderator 102 and aperture 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 shapes of the aperture array moderator and aperture array collimator are not limited to squares and may also be rectangular, hexagonal, circular, elliptical, and other shapes. Preferably, the dimensions of the aperture array moderator and aperture 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. Traditional diverging collimators have very small channel dimensions (much smaller than the cross-sectional dimensions of the moderator) to achieve thermal neutron collimation. This reduces the thermal neutron flux after moderation and collimation, leading to a decrease in the thermal neutron fluence rate. The present invention, however, utilizes an aperture array collimator that is completely different from traditional diverging collimators. This not only increases the exit area of ​​thermal neutrons after small-angle moderation, but also, by matching the aperture array collimator with the moderator, improves the neutron extraction efficiency in the imaging direction, thereby enhancing the neutron injection 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 illustrated diagram may also be an example of the arrangement of the multiple holes and the shape of each hole of the hole array collimator 104. In the following description, for convenience, each hole of the hole array moderator 102 is referred to as a first hole, and each hole of the hole array collimator 104 is referred to as 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 dense manner, and the shapes of the plurality of holes are substantially the same, that is, a uniform hole array is formed. Figure 2a The hole shape shown is 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 shapes shown may be, for example, polygonal shapes such as rectangles and regular hexagons, or circular shapes.

[0039] As demonstrated in the examples described below, the thermal neutron fluence rate of the hole array moderator of the present invention is significantly improved compared to a conventional uniform moderator. This demonstrates that the hole array moderator can increase neutron moderation efficiency and improve source neutron utilization compared to conventional uniform moderators. Furthermore, the hole array collimator can significantly improve the probability of neutron emission at low angles, that is, the efficiency of neutron extraction in the imaging direction, thereby enhancing the neutron fluence rate in the imaging direction.

[0040] Refer to the following Figure 3 The structure and dimensional relationship of an aperture array moderator and an aperture array collimator according to one embodiment of the present invention will be described. Figure 3 is a schematic cross-sectional view showing the structure and size relationship of the hole array moderator and the hole array collimator according to one 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 wall of the first hole of the hole array moderator 102 and the wall of the second hole of the hole array collimator 104, and the gray parts represent the channels between the first hole and the second hole. Reference numeral 301 represents the wall of the first hole of the hole array moderator 102, reference numeral 302 represents the channel of the first hole of the hole array moderator 102, reference numeral 303 represents the wall of the second hole of the hole array collimator 104, and reference numeral 304 represents the 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 In the left and right directions). Figure 3 In FIG, D0 represents the width of one grid element of the aperture array moderator 102, d0 represents the width of one channel of the aperture array moderator 102, and L0 represents the length of the channel of the aperture array moderator 102. D1 represents the width of one grid element of the aperture array collimator 104, d1 represents the width of one channel of the aperture array collimator 104, and L1 represents the length of the channel of the aperture 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 aperture array moderator 102 is an integer multiple of the cell width D1 of the aperture array collimator 104. Furthermore, 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, i.e., the cell width D0 of the aperture array moderator 102 is equal to the cell width D1 of the aperture array collimator 104. At this point, when observing the moderator-collimator device along the first direction, one second hole is contained within one first hole, i.e., the first hole of the collimator and the second hole of the moderator are completely aligned. Figure 3 The example shown is a case where D0=2*D1, that is, the cell width D0 of the aperture array moderator 102 is twice the cell width D1 of the aperture array collimator 104 .

[0046] exist Figure 3 In the case of a square hole, when the moderator collimator is viewed along the first direction, two second holes are contained in one first hole in the vertical direction. Figure 3 In the horizontal direction perpendicular to the left and right directions, two second holes 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 aperture array moderator is not equal to the cell width D1 of the aperture array collimator, the aperture 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 the hole shapes of the hole array moderator and the hole array collimator, wherein: Figure 4a The example shown here shows that both holes have the shape of an equilateral triangle. 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, 102C represents a grid element of the hole array moderator 102, i.e., a first hole, and 104C represents a grid element of the hole array collimator 104, i.e., a second hole. Figure 4a and Figure 4b In the figure, for the convenience of observation, only one of the multiple grid cells of the hole array collimator 104 is given the label 104C, but those skilled in the art will know that Figure 4a and Figure 4b Each of the figures shows a plurality of grid elements of the hole array collimator 104 of the same size and shape, namely the second holes.

[0049] Specifically, if Figure 4aAs shown, the apertures of the aperture array moderator and the aperture 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 aperture array moderator and four equilateral triangle grid elements 104C of the aperture array collimator. Figure 4a In the example shown, four second apertures are contained within one first aperture. Regarding size, the cell width D0 of the aperture array moderator and the cell width D1 of the aperture array collimator satisfy the relationship D0 = 2 * D1, where the cell width is the side length of an equilateral 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 triangular grid elements 104C of the aperture array collimator. Figure 4b In the example shown, six secondary apertures are contained within one primary aperture. Regarding size, the cell width D0 of the aperture array moderator and the cell width D1 of the aperture array collimator satisfy the relationship D0 = 2 * D1. For an equilateral triangle, the cell width is the length of the triangle's side; for a regular hexagon, the cell width is the length of the hexagon's diagonal.

[0051] The above structure can fully match the hole array collimator and the hole array moderator in space, increase the emission area of ​​thermal neutrons after small-angle moderation, 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 mentioned 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 aperture length L1 of the aperture array collimator 104 is 5 cm to 30 cm. The definition of the collimation ratio of the aperture 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 aperture array collimator is determined by the aperture length L1 and the aperture width d1 of the aperture array collimator, and has nothing to do with the distance to the imaging plane. That is, the collimation ratio of the aperture 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 aperture wall is roughly fixed. When the collimation ratio is large, if the aperture length L1 is less than 5 cm, the size of each grid element D1 is very small. At this time, the aperture wall accounts for a large proportion, which will reduce the thermal neutron fluence rate of the imaging. When the channel length L1 exceeds 30 cm, the air has a certain absorption of thermal neutrons, and the excessive distance will lead to a decrease in the thermal neutron flux.

[0054] In the above embodiment, it is preferred that the cell width D0 of the aperture array moderator 102 is 2 to 3 times the cell width D1 of the aperture array collimator 104, i.e., n = 2 or 3. When the value of n is small, because the cell width D1 is large, the collimator length will be longer to meet the collimation ratio, increasing the absorption of thermal neutrons by air, thereby reducing the thermal neutron fluence rate. When the value of n is too large, because the cell width D1 of the collimator is small, the collimator hole wall will occupy a large proportion, reducing the effective neutron emission area and similarly 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 collimator, 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 referred to as 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 This is a 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 FIG. Figure 5 As shown, a thermal neutron radiography system illustrating one embodiment of the present invention includes a neutron source 501, a moderator collimator 502, a sample under examination 503, a dark box 504, a conversion screen 505, a reflector 606, and an imaging detector 507. Neutron source 501 provides the neutron beam and, in a small thermal neutron radiography system, can be an accelerator DT neutron generator. 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 sample under examination 503, the neutron beam is converted into a visible light image by conversion screen 505. This visible light image is reflected by reflector 506 and projected onto imaging detector 507, where it is converted into an image signal. Although not illustrated here, the neutron radiography system according to one embodiment of the present invention may also include an image processing device for processing the image signal output by imaging detector 507, a memory for storing image data, and a display screen for displaying the image signal.

[0059] exist Figure 5 In the thermal neutron radiography system shown, the conversion screen 505, the reflector 506 and the imaging detector 507 are placed in the 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 and the like 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. The reflector 506 can also 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 according to one embodiment of the present invention is characterized in that the moderator collimator 502 adopts Figures 1 to 3 The moderator collimator 1 of the present invention is shown. 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 arranged to surround the uniform moderator 101 and the hole array moderator 102.

[0061] As demonstrated in the embodiments described below, the thermal neutron radiography system employing the above-described structure significantly improves the thermal neutron fluence rate of the aperture array moderator of the present invention compared to a conventional uniform moderator. This indicates that the aperture array moderator can increase neutron moderation efficiency and improve source neutron utilization compared to conventional uniform moderators. Furthermore, the aperture array collimator can significantly improve the probability of neutron emission at low angles, i.e., the neutron extraction efficiency in the imaging direction, thereby enhancing the neutron fluence rate in the imaging direction. Furthermore, the aperture array collimator and aperture array moderator can be fully spatially matched, increasing the emission area of ​​thermal neutrons after low-angle moderation, thereby improving the neutron extraction efficiency in the imaging direction and enhancing the neutron injection rate 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 with reference to 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 arranged in a row (in the left and right directions). Hole array moderator 102 utilizes a uniform rectangular square hole array structure, with hole walls made of polyethylene. Specific dimensions are L0 = 20 cm, D0 = 1 cm, and d0 = 0.9 cm. A neutron reflector layer is 20 cm thick graphite, surrounding uniform moderator 101, hole array moderator 102, and hole array collimator 104. Hole array collimator 104 utilizes a uniform rectangular square hole array structure, with hole walls made of aluminum-based boron carbide with a 20% boron carbide content. Specific dimensions are L1 = 9 cm, D1 = 0.5 cm, and d1 = 0.45 cm. The hole array collimator is constructed 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 grid cell width D0 of the hole array moderator 102 is twice the grid cell width D1 of the hole array collimator 104. Figure 3 As shown, when observing the moderation collimator along the first direction, in the vertical direction, the two second holes are contained in one first hole. In the horizontal direction ( Figure 3 In the horizontal direction perpendicular to the left and right directions, two second holes are contained in one first hole. Therefore, a total of four second holes are contained in one first hole.

[0066] The neutron shielding layer consists of two layers: a non-imaging section and an imaging section. The non-imaging section is composed of 1 cm thick boron-containing polyethylene and 0.5 cm thick lead, while the imaging section 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 the moderation collimator according to one embodiment of the present invention, wherein: Figure 6a The results show the comparison of thermal neutron injection amount 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: the moderator and collimator 1 (hole array moderator + hole array collimator) of one embodiment of the present invention, the conventional moderator + divergent collimator and the conventional moderator + hole array collimator. Figure 6a As shown in the curve, when the collimation ratio L / D = 20, the normalized thermal neutron flux rates of the three moderator 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 flux 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 flux rate can reach 6.75×10 3 n·cm -2 ·s -1 , fully meeting the needs of neutron radiography.

[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: the moderator and collimator 1 (hole array moderator + hole array collimator) of an embodiment of the present invention, the conventional moderator + hole array collimator and the conventional moderator and collimator (uniform moderator + divergent collimator). Figure 6b As shown in the graph, the low-angle neutron emission probability 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. This demonstrates that the hole array collimator can significantly improve the low-angle neutron emission probability, that is, the neutron extraction efficiency in the imaging direction, thereby enhancing the neutron fluence rate in the imaging direction. Furthermore, compared to the conventional moderator + hole array collimator, the moderator collimator of the present invention significantly improves the thermal neutron fluence rate within the same low-angle range. This demonstrates that the hole array moderator can increase the neutron moderation efficiency and improve the utilization rate of source neutrons compared to traditional uniform moderators. Thus, 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 low-angle neutron emission probability, that is, the neutron extraction efficiency in the imaging direction, thereby enhancing the neutron fluence rate in the imaging direction.

[0071] Figure 7 1 is a diagram showing the test results of neutron radiography bright field of the moderator collimator 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 areas appear darker than the central area. This is due to the unevenness of the 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 hole array collimator wall 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-down collimator 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 and collimation device 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 the characteristic of directional enhancement 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 fluence 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 fluence rate in the imaging direction. At the same time, it can effectively solve the spatial distortion of the image caused by the diffusion 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 collimator, both of which utilize a uniform array of holes with identical hole features. When observing any position of the hole array moderator and hole array collimator along the hole channel, one or more secondary holes are contained within one primary hole. Thus, by using a collimator that is highly matched to the hole array moderator, the neutron extraction efficiency in the imaging direction can be improved, thereby enhancing the neutron fluence rate in the imaging direction.

Claims

1. A moderator collimator for thermal neutron imaging, characterized in that: comprising a uniform moderator, a hole array moderator, and a hole array collimator sequentially connected in a first direction; and a neutron reflector surrounding the uniform moderator, the hole array moderator and the hole array collimator, The uniform moderator comprises at least one neutron moderating material, The hole array moderator includes a plurality of first holes of the same shape arranged in a two-dimensional close-packed manner, wherein the first holes extend along the first direction. The hole walls of the hole array moderator are made of neutron moderation material. The hole array collimator includes a plurality of second holes of the same shape arranged in a two-dimensional close-packed manner, and the second holes extend along the first direction. The hole walls of the hole array collimator are made of 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. The neutron reflecting layer includes at least one neutron reflecting material.

2. The moderation and collimation device according to claim 1, wherein: 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 moderation and collimation device according to claim 2, wherein: When observing any position of the hole array moderator and the hole array collimator along the first direction, one or more second holes are contained in one first hole.

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

5. The moderation and collimation device according to any one of claims 1 to 3, characterized in that: The measured size difference between 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 within the range of 5 cm to 30 cm.

6. The moderation and collimation 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 to surround the neutron reflecting layer on all sides, and the imaging direction part is arranged to cover 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 to 6.

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