Compton scattering imaging device based on focusing collimation and defect detection method

Abstract

The invention discloses a Compton scattering imaging device based on focusing collimation and a defect detection method. The Compton scattering imaging device comprises a ray source used for generating X-rays or gamma-rays with certain energy and angle distribution; the front collimator is used for collimating rays output by the ray source to form a beam to perform point-by-point scanning on a measured object; the rear collimator with the focusing structure is formed by splicing a plurality of focusing collimation structures which are distributed and arranged, effective light-passing axes of the focusing collimation structures are converged in the same focusing area, and the rear collimator with the focusing structure is used for receiving scattering signals generated in the effective light-passing axis direction within the range of the focusing area with the set depth of the measured object; the focusing area covers the scanning point; the large-area detector is used for receiving the scattering signal output by the rear collimator and converting the scattering signal into an analog electric signal; and the electronics acquisition system is used for receiving the analog electric signal, converting the analog electric signal into a digital signal, processing the digital signal and transmitting the processed digital signal to the upper computer to generate an internal structure image of the measured object. According to the invention, the spatial resolution capability is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of X-ray imaging and non-destructive testing, and relates to a Compton scatter imaging (CSI) device based on focusing and collimation and a method for detecting near-surface defects in large components. Background Technology

[0002] Large components are widely used in important industrial sectors such as aerospace, energy and power, and high-end equipment manufacturing. These components are often bulky and fixed in place, and disassembly or movement is usually not permitted at the site. Therefore, achieving in-situ non-destructive testing of their internal defects is highly specialized and technically challenging. Conventional non-destructive testing methods include ultrasonic testing, infrared thermography, and microwave testing. Although each has its own characteristics, they are limited by factors such as material properties, structural shape, and probe depth in practical applications, making it difficult to meet the in-situ testing requirements under complex working conditions.

[0003] X-ray imaging is one of the most effective non-destructive testing (NDT) techniques currently available. It offers intuitive imaging results, high spatial resolution, and good reliability. However, traditional transmission X-ray imaging requires the X-ray source and detector to be placed on opposite sides of a large component, and the X-ray must penetrate the entire thickness of the component to form an image. This not only relies on high-energy X-ray sources (such as accelerators) but also limits its application in scenarios where only one side is accessible. Compton scattering (CSI) technology is a NDT technique based on the Compton scattering effect. When high-energy X-rays or gamma rays are incident on the object under test, they undergo inelastic scattering with electrons within the object. The resulting backscattered photons have spatial and energy distributions closely related to the atomic number and local electron density of the material. By collecting and analyzing these backscattered signals, density variations in different regions within the object can be reconstructed. Unlike transmission X-ray imaging, CSI technology allows the X-ray source and detector to be placed on the same side of a large component. The X-ray only needs to reach near the surface of the component, without completely penetrating it, to form an image. This makes it particularly suitable for in-situ NDT of near-surface defects in large components or in scenarios where only one side is accessible.

[0004] After years of development, CSI technology has mainly formed two typical technical routes: flying-spot scanning CSI and direct push-broom CSI. Most imaging devices use flying-spot scanning technology, employing a high-speed rotating collimator to collimate the incident rays into a beam with a constantly changing orientation, scanning the object being inspected point by point. To distinguish voxels at different depths along the beam path, a collimating slit needs to be added at the front of the detector to restrict the rays in different directions. Due to the limitations of the front collimator (which collimates the incident rays into a beam) and the rear collimator (which collimates the scattered rays to different detector pixels), the overall imaging efficiency is very low. Furthermore, the constantly changing beam orientation causes changes in imaging geometry parameters, resulting in significant image distortion. Direct push-broom CSI, on the other hand, collimates the incident rays into a fixed fan beam, scanning the object being inspected line by line, significantly improving the utilization efficiency of the radiation source. By combining it with an array detector with pinhole or parallel-hole collimators, two-dimensional or three-dimensional imaging can also be achieved. However, due to the limitations of pinhole or parallel-hole collimators, the scattered photon collection efficiency is low, making it difficult to meet the requirements of high imaging efficiency and high resolution, which restricts its further application in high-precision non-destructive testing scenarios.

[0005] Therefore, there is a lack of efficient and practical in-situ detection methods for near-surface defects of large components (such as interface debonding), and existing CSI technology has the prominent problem of being unable to balance high imaging efficiency and high spatial resolution. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention aims to provide a Compton scattering imaging device and defect detection method based on focusing and collimation. The present invention achieves effective spatial focusing and directional constraint of scattered photons by specifically optimizing the geometry and focusing characteristics of the collimator, thereby simultaneously improving the signal-to-noise ratio of the imaging system and its spatial resolution of minute defects.

[0007] This invention employs an innovative focused collimator structure, composed of multiple continuously spliced ​​conical collimating rings. The parameters of each collimating ring are precisely designed to converge at a single focal point, achieving directional collection of scattered photons within a specific depth range. The geometric parameters of each collimating ring are recursively determined according to a preset relationship, and focal constraints ensure that the light transmission directions of each collimating ring converge at the focal point, forming a spatial focusing effect. This structure significantly improves the utilization rate and signal-to-noise ratio of scattered photons, enabling efficient focusing and imaging of the target depth region.

[0008] This invention optimizes the focusing characteristics and directional constraints of scattered photons, and combines them with spectral feature analysis to achieve single-sided in-situ non-destructive testing of internal defects within a certain depth range near the surface of large structural components. In particular, it enables efficient and high-precision detection of minute interface defects such as debonding and delamination that are difficult to detect by traditional transmission imaging, thus making up for the shortcomings of existing technologies.

[0009] The present invention relates to a CSI device based on focusing and collimation, such as... Figure 1 As shown, it includes at least one X-ray or gamma-ray source, one front collimator, one rear collimator with a focusing structure, one large-area detector, an electronic acquisition system, and host computer software, among which, X-ray or gamma-ray sources can be radioactive sources, X-ray tubes, or small accelerators, etc., used to generate X-rays or gamma rays with a certain energy and angular distribution; The precollimator is usually made of high-density materials such as lead and tungsten alloys. It is used to collimate the X-ray or gamma-ray source into a pencil beam of a certain size, which can scan the sample being tested point by point. The post-collimator with focusing structure is usually made of high-density materials such as lead and tungsten alloys. It is an important component that distinguishes this invention from other CSI devices. It is composed of multiple distributed focusing and collimating structures spliced ​​together. Through fine design, the effective X-ray receiving range of each focusing and collimating structure can be simultaneously focused on a small area (called the focusing area) near a specific depth inside the sample, thereby achieving efficient X-ray focusing. Large-area detectors can be scintillator detectors, flat panel detectors, or fluorescent screens, etc., and can be used in combination with a collimator with a focusing structure to efficiently detect scattered X or γ rays emitted in the focal area of ​​the sample, and convert the X or γ ray signals into analog electrical signals through photoelectric conversion. The electronic acquisition system is used to receive analog signals transmitted from the detector, perform high-speed sampling of the analog signals and convert them from analog to digital signals, and then transmit the data information to the host computer after signal processing such as filtering, baseline correction, peak finding, and signal integration. The host computer software analyzes and reconstructs the data information to generate one-dimensional spectral lines or two-dimensional images, and identifies and judges the internal defects of the sample based on the changes in spectral line or image characteristics.

[0010] This invention also provides a design method for a focusing collimation structure. This method achieves spatial focusing of scattered rays by precisely controlling the geometric vertex coordinates of each conical collimation ring. Its key feature is the use of spatial geometric constraints to achieve directional filtering and spatial focusing of scattered rays. The core idea and steps of this method are as follows.

[0011] 1. Establish the focusing principle: The basic geometric principle for designing the post-collimator is that the effective light transmission axis of each independent collimator unit, when extended backward in space, converges into a predefined small area inside the sample being measured, namely the "focusing area".

[0012] 2. Examples of Implementation Methods: Based on the above principles, the focusing and collimating structure can be implemented in various physical forms, including but not limited to an array of elongated aperture collimators with multiple axes intersecting in the focusing area, or an array of multiple concentric conical (or frustum-shaped) collimating rings with their sidewalls pointing towards the focusing area. Among these, the conical collimating ring array is considered the preferred solution because it can provide a larger effective solid angle of reception.

[0013] 3. Design Method Definition: The design method for the conical collimation ring array is characterized by quantification and programming. By establishing a spatial coordinate system with the collimator's central axis as the reference, and presetting key system parameters such as the target focal depth, focal region width, initial ring inner diameter, and total collimator height, the geometric vertex coordinates of each conical collimation ring can be determined ring by ring through an iterative algorithm based on geometric projection relationships.

[0014] 4. Clarifying the control capability: By adjusting the key system parameters, the method of the present invention can quantitatively control the depth position of the focusing area inside the sample under test and its spatial widening in the axial direction of the focal point, thereby achieving selective and sensitive detection and depth resolution of defects of different depths.

[0015] 5. Explanation of design trade-offs: The total number of collimation rings is a core design optimization variable. Increasing the number can improve signal collection efficiency and image signal-to-noise ratio, while reducing the number is beneficial for miniaturizing the device. In actual design, a trade-off must be made based on the detection requirements.

[0016] The technical solution of the present invention is: a Compton scattering imaging device based on focusing and collimation, characterized in that it includes a radiation source, a front collimator, a rear collimator with a focusing structure, a large-area detector, an electronic acquisition system, and a host computer; The radiation source is used to generate X-rays or gamma rays with a certain energy and angular distribution; The precollimator is used to collimate the rays output from the ray source to form a single beam for point-by-point scanning of the object under test. The rear collimator with a focusing structure is composed of multiple distributed focusing collimating structures spliced ​​together. The effective light transmission axes of each focusing collimating structure converge in the same focusing area, which is used to receive the scattered signal in the direction of the effective light transmission axis generated within the focusing area of ​​the object under test at a set depth; the focusing area covers the current scanning point. The large-area detector is used to receive the scattered signal output by the collimator and convert it into an analog electrical signal; The electronic acquisition system is used to receive the analog electrical signal, convert it into a digital signal, process it, and then transmit it to the host computer. The host computer is used to generate an image of the internal structure of the object under test based on the data information of each scanning point.

[0017] Preferably, the rear collimator is an array of multiple coaxial conical collimating rings; the method for designing the rear collimator is as follows: 1) Establish a coordinate system with the central axis of the rear collimator as the Z-axis and the lower surface of the rear collimator as the XY plane; set the height of the rear collimator as... The collimation ring at the center is taken as the first collimation ring; the inner diameter of the first collimation ring is set as... The thickness of the bottom wall of the first collimation ring is According to the inner diameter and height Determine the vertex of the inner diameter of the upper surface of the first collimation ring. According to the inner diameter Determine the vertex of the lower surface inner diameter of the first collimation ring. According to the vertex Extending wall thickness outwards Determine the vertex of the lower surface outer diameter of the first collimation ring. ; 2) Set the allowable widening of the focus in the depth direction to... The upper boundary point of the focus expansion is The lower boundary point is Set focus to expand upper boundary point The vertical distance from the lower surface of the rear collimator is ; 3) Construct the outer boundary of the first collimation ring and the inner boundary of the second collimation ring using the geometric projection method; wherein, 31) Draw a straight line connecting the upper boundary points. With vertex And extend the straight line to intersect the upper surface of the collimator, the intersection point 32) Draw the first straight line connecting the lower boundary points to the vertex of the inner diameter of the upper surface of the second collimation ring. With vertex Draw a second straight line to connect the lower boundary points. With vertex The region formed between the first and second straight lines is defined as the effective ray channel range; the middle line connecting this ray channel range intersects the upper surface of the rear collimator at a point. Intersects the lower surface at point ; The vertex of the outer diameter of the upper surface of the first collimating ring. The vertex of the lower surface inner diameter of the second collimating ring; 4) Generate subsequent collimated ring arrays based on iterative algorithms (1) Set the bottom width of each subsequent collimation ring to a fixed value. ; (2) Using the principle of similar triangles, the first Each vertex of the collimation ring ( , , , ) by the first The parameters of the collimation ring are derived as follows: , , , ; (3) Repeat step (2) above to generate collimation rings layer by layer from the inside out until the total outer diameter of the rear collimator or the number of collimation rings reaches the preset value.

[0018] Preferably, the rear collimator is an array of multiple coaxial frustum-shaped collimating rings.

[0019] Preferably, the focusing and collimating structure is an elongated aperture collimator structure; each elongated aperture collimator structure is arranged such that its axis intersects in the same focusing area.

[0020] Preferably, the radiation source is a radiation source, an X-ray tube, or a small accelerator.

[0021] A post-collimator with a focusing structure is characterized by comprising a plurality of distributed focusing and collimating structures, wherein the effective light transmission axes of each focusing and collimating structure converge in the same focusing area.

[0022] A defect detection method based on the Compton scattering imaging device includes the following steps: 1) The X-rays output from the X-ray source are collimated using a precollimator to form a single beam that scans the object under test point by point, and Compton scattering occurs with the material of the object under test. 2) Utilize a rear collimator with a focusing structure to receive the scattered signal generated within the focal zone at a set depth of the object being measured; 3) Receive the scattered signal output by the collimator using a large-area detector and convert it into an analog electrical signal; 4) The analog electrical signal is received by the electronic acquisition system and converted into a digital signal for processing. The energy and count information of each valid event are extracted and transmitted to the host computer as the data information of the corresponding scan point. 5) The host computer generates an internal structure image of the object under test based on the data information of each scanning point; then, based on the internal structure image, the defects of the object under test are visualized and spatially located.

[0023] The present invention has the following outstanding advantages in terms of structure and performance.

[0024] 1) Comprehensive improvement in detection capabilities: This method is applicable to low-density, low atomic number materials. It can achieve in-situ, unilateral, real-time non-destructive testing without disassembling or moving the object under test, and has the ability to obtain defect depth information.

[0025] 2) Significantly optimized structural layout: Since the detector and the X-ray source can be arranged on the same side of the object being tested, the problem of limited arrangement on both sides in the detection of large components is effectively overcome, which is conducive to promoting the development of detection devices towards lightweight, portable and mobile.

[0026] 3) Enhanced performance through focusing collimator design: By introducing a focusing collimator structure, the collection efficiency of scattered rays is significantly improved. Compared with traditional pinhole or grid collimators, this design can focus scattered photons within a specific depth region onto the detector, thereby significantly improving the count rate, signal-to-noise ratio, and depth resolution.

[0027] 4) Flexible and scalable imaging methods: One-dimensional scanning is used to collect scattering signals, and internal defects in materials can be identified through spectral line analysis; further, through multiple one-dimensional scanning and data reconstruction, two-dimensional or even three-dimensional structural images can be generated, realizing the visualization, location and quantitative identification of defects. Attached Figure Description

[0028] Figure 1 This is a diagram of a Compton scattering imaging device based on focusing and collimation.

[0029] Figure 2 This is a schematic diagram of the quasi-straight ring construction process.

[0030] Figure 3 This is a conceptual diagram of a focusing collimator.

[0031] Figure 4 A schematic diagram of a device for focusing collimated Compton scattering imaging.

[0032] Figure 5 This is a one-dimensional scan of defects of different sizes at a depth of 10mm. Detailed Implementation

[0033] The present invention will now be described in further detail with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0034] The focus of this invention is on a post-collimator with a focusing and collimating structure, which can be implemented in various ways, such as using multiple elongated aperture collimators to align the focusing area, or using multiple conical collimating rings to align the focusing area. The former is easier to understand but has lower detection efficiency. This invention also provides a design method based on multiple conical collimating rings to form a focusing and collimating structure. This method achieves spatial focusing of scattered rays by precisely controlling the geometric vertex coordinates of each conical collimating ring, and can quantitatively control the focal point position and the focal point broadening in the depth direction. The specific design steps are as follows.

[0035] 1. Set the system reference parameters and the inner boundary of the first collimator ring, and establish a coordinate system with the collimator center axis as the Z-axis and the lower surface of the collimator as the XY plane.

[0036] (1) Set the collimator height to The inner diameter of the first collimation ring is (Reserve space for front collimator installation), the bottom wall thickness of the collimation ring is... It can be set based on X-ray shielding requirements.

[0037] (2) Determine the coordinates of the first three vertices of the first collimation ring: Top right vertex : Coordinates are based on the inner diameter and height Sure.

[0038] Bottom right corner Coordinates are derived from the inner diameter. Determined (located in the Z=0 plane).

[0039] Bottom left vertex Coordinates are derived from the vertex Extending wall thickness outwards Sure.

[0040] 2. Set the focusing target parameters. To balance defect location accuracy and depth resolution, the focus area needs to be defined. (1) Set the allowable widening of the focus in the depth direction as follows: Define the upper boundary point of the focus broadening as... The lower boundary point is ,in . From focus F to the upper boundary point distance, From focus F to the lower boundary point The distance.

[0041] (2) Set the vertical distance between the upper boundary point of the focus expansion and the lower surface of the collimator as: .

[0042] 3. Construct the outer boundary of the first collimation ring and the inner boundary of the second collimation ring. By using the geometric projection method, the geometric path of the air gap is determined by widening the line connecting the boundary point and the known vertex through the focal point, and then the coordinates of each vertex are determined.

[0043] (1) Determine the upper right vertex of the second collimation ring. Draw a straight line connecting the focus and the upper boundary point. With the lower left vertex of the first collimation ring Extend this line to intersect the upper surface of the collimator (Z=H plane), and the intersection point is the... This point actually defines the boundary of the maximum angular span that the scattered rays can pass through.

[0044] (2) Determine the upper left vertex of the first collimation ring. With the lower right vertex of the second collimation ring : a) Draw a straight line connecting the focus and the lower boundary point. With point .

[0045] b) Draw a straight line connecting the focus and the lower boundary point. With point .

[0046] c) The area formed between the two straight lines mentioned above is the effective ray channel range. Based on the collimator wall thickness design principle, the outer slope of the first collimator ring and the inner slope of the second collimator ring are determined within the above range.

[0047] d) To ensure maximum light throughput and focusing effect, select the middle line of this range, intersecting the upper surface of the collimator at [insert line here]. Intersecting with the lower surface .

[0048] 4. Generate subsequent collimated ring arrays based on iterative algorithms. After determining the key nodes of the first two rings, the structural parameters of the subsequent collimated rings are automatically generated using a recursive iterative algorithm.

[0049] (1) Set the bottom width of each subsequent collimation ring to a fixed value. (or a function value based on the radius variation).

[0050] (2) Application of iterative formula: Using the principle of geometrically similar triangles, the first... The coordinates of each vertex of the collimation ring ( , , , (This can also be considered as the vertical distance from the Z-axis) can be derived from the first... The parameters of the collimation ring are derived as follows: (3) Iterative construction: Repeat the above calculation steps to generate collimation rings layer by layer from the inside out until the total outer diameter of the collimator or the number of collimation rings reaches the preset system indicators (such as effective detection area or physical size limit).

[0051] 5. System optimization trade-offs The number of collimation rings during the design process The number of collimating rings is a key optimization variable. Increasing the number of collimating rings can increase the effective receiving area of ​​the detector, significantly improve the count rate of scattered photons and the image signal-to-noise ratio, and enhance the focusing effect; reducing the number of collimating rings is beneficial to reducing the system size and achieving a miniaturized and compact design of the probe. In this embodiment, the optimal number of collimating ring layers is selected according to actual detection requirements to achieve a balance between imaging quality and system size.

[0052] This invention proposes a CSI defect detection method based on focused collimation, and the specific implementation steps are as follows.

[0053] 1. Generate a pencil beam of X-rays to scan the sample being tested. An industrial microfocus X-ray tube was selected, and the resulting conical continuous spectrum X-rays were filtered and then collimated into a pencil beam with a cross-sectional diameter of approximately 1-2 mm by a tungsten alloy cylindrical aperture collimator. The object under test was scanned point-by-point, and Compton scattering occurred with the material at a specific depth within the sample. In this embodiment, the sample was a multilayer composite material containing artificially pre-fabricated defects. A series of square air gap defects with increasing thicknesses (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5, 2.0 mm), each with a cross-sectional area of ​​0.5 × 0.5 mm² and a center-to-center spacing of 2 cm were embedded at a depth of 10 mm.

[0054] 2. Using a focusing collimator to collect gamma rays from a specific micro-focal region This embodiment employs a preferred "multi-layer concentric conical collimating ring densely packed" scheme. Through the aforementioned design method, the sidewall inclination angle of each conical ring is precisely calculated and processed, ensuring that the spatial extensions of the air gap channels between all conical rings converge within a pre-defined "focusing zone" inside the sample. In this example, the focal point is set to 50mm, with a depth-direction widening of 5mm. This means the focusing collimator can collect scattered signals within a micro-focal zone at depths of 47.5-52.5mm and shield stray signals from other directions, effectively improving detection sensitivity and spatial resolution.

[0055] 3. Large-area detectors detect scattered gamma rays. Large-area detectors use scintillator detectors (such as CsI and GAGG detectors) that can effectively detect gamma rays. These detectors are installed close to the back of the focusing collimator and are used to convert scattered gamma rays from the micro-focal region that pass through the air gap of the collimator into visible light photons, which are then converted into analog electrical signals by internal photoelectric conversion devices (such as SiPM).

[0056] 4. Electronic data acquisition system The analog signal output by the detector is received by a dedicated digital processing board. The board performs high-speed analog-to-digital conversion, digital filtering, baseline correction, pulse peak finding and integration, extracts the energy and count information of each valid event, and packages the digital signal for transmission to the host computer.

[0057] 5. The host computer reconstructs and visualizes the data in real time. The digital processing board transmits the number of photons and energy spectrum information received within a fixed time interval to the host computer in real time. After receiving the data, the host computer performs real-time rendering and imaging reconstruction, identifying potential defect regions inside the sample by analyzing spectral line characteristics. The host computer system has functions such as data storage, spectral line fitting, image reconstruction, and display, and can save the original acquired data and reconstruction results simultaneously after a single scan. In subsequent scans, it can further generate two-dimensional or three-dimensional structural images of the sample's interior, achieving visualization and spatial localization of defects.

[0058] The final one-dimensional scan result of the defect in this embodiment is as follows: Figure 5 As shown.

[0059] Although specific embodiments of the invention have been disclosed for illustrative purposes to aid in understanding and implementing the invention, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the content disclosed in the preferred embodiments, and the scope of protection claimed by the invention is defined by the claims.

Claims

1. A Compton scattering imaging device based on focusing and collimation, characterized in that, Includes a radiation source, a front collimator, a rear collimator with a focusing structure, a large-area detector, an electronic acquisition system, and a host computer; The radiation source is used to generate X-rays or gamma rays with a certain energy and angular distribution; The precollimator is used to collimate the rays output from the ray source to form a single beam for point-by-point scanning of the object under test. The rear collimator with a focusing structure is composed of multiple distributed focusing collimating structures spliced ​​together. The effective light transmission axes of each focusing collimating structure converge in the same focusing area, which is used to receive the scattered signal in the direction of the effective light transmission axis generated within the focusing area of ​​the object under test at a set depth; the focusing area covers the current scanning point. The large-area detector is used to receive the scattered signal output by the collimator and convert it into an analog electrical signal; The electronic acquisition system is used to receive the analog electrical signal, convert it into a digital signal, process it, and then transmit it to the host computer. The host computer is used to generate an image of the internal structure of the object under test based on the data information of each scanning point.

2. The Compton scattering imaging device according to claim 1, characterized in that, The rear collimator is an array of multiple coaxial conical collimating rings; the method for designing the rear collimator is as follows: 1) Establish a coordinate system with the central axis of the rear collimator as the Z-axis and the lower surface of the rear collimator as the XY plane; set the height of the rear collimator as... The collimation ring at the center is the first collimation ring; Set the inner diameter of the first collimation ring as The thickness of the bottom wall of the first collimation ring is According to the inner diameter and height Determine the vertex of the inner diameter of the upper surface of the first collimation ring. According to the inner diameter Determine the vertex of the lower surface inner diameter of the first collimation ring. According to the vertex Extending wall thickness outwards Determine the vertex of the lower surface outer diameter of the first collimation ring. ; 2) Set the allowable widening of the focus in the depth direction to... The upper boundary point of the focus expansion is The lower boundary point is ; Set focus to expand upper boundary point The vertical distance from the lower surface of the rear collimator is ; 3) Construct the outer boundary of the first collimation ring and the inner boundary of the second collimation ring using the geometric projection method; wherein, 31) Draw a straight line connecting the upper boundary points. With vertex And extend the straight line to intersect the upper surface of the collimator, the intersection point 32) Draw the first straight line connecting the lower boundary points to the vertex of the inner diameter of the upper surface of the second collimation ring. With vertex Draw a second straight line to connect the lower boundary points. With vertex The region formed between the first and second straight lines is defined as the effective ray channel range; the middle line connecting this ray channel range intersects the upper surface of the rear collimator at a point. Intersects the lower surface at point ; The vertex of the outer diameter of the upper surface of the first collimating ring. The vertex of the lower surface inner diameter of the second collimating ring; 4) Generate subsequent collimated ring arrays based on iterative algorithms (1) Set the bottom width of each subsequent collimation ring to a fixed value. ; (2) Using the principle of similar triangles, the first Each vertex of the collimation ring ( , , , ) by the first The parameters of the collimation ring are derived as follows: , , , ; (3) Repeat step (2) above to generate collimation rings layer by layer from the inside out until the total outer diameter of the rear collimator or the number of collimation rings reaches the preset value.

3. The Compton scattering imaging device according to claim 1, characterized in that, The rear collimator is an array of multiple coaxial frustum-shaped collimating rings.

4. The Compton scattering imaging device according to claim 1, characterized in that, The focusing and collimating structure is a slender aperture collimator structure; each slender aperture collimator structure is arranged such that its axis intersects in the same focusing area.

5. The Compton scattering imaging apparatus according to claim 1, 2, or 3, characterized in that, The radiation source is a radiation source, an X-ray tube, or a small accelerator.

6. A rear collimator with a focusing structure, characterized in that, It includes multiple focusing and collimating structures arranged in a distributed manner, and the effective light transmission axes of each of the focusing and collimating structures converge in the same focusing area.

7. The rear collimator according to claim 6, characterized in that, The rear collimator is an array of multiple coaxial conical or frustum-shaped collimating rings.

8. The rear collimator according to claim 6, characterized in that, The focusing and collimating structure is a slender aperture collimator structure; each slender aperture collimator structure is arranged such that its axis intersects in the same focusing area.

9. A defect detection method based on the Compton scattering imaging device of claim 1, comprising the following steps: 1) The X-rays output from the X-ray source are collimated using a precollimator to form a single beam that scans the object under test point by point, and Compton scattering occurs with the material of the object under test. 2) Utilize a rear collimator with a focusing structure to receive the scattered signal generated within the focal zone at a set depth of the object being measured; 3) Receive the scattered signal output by the collimator using a large-area detector and convert it into an analog electrical signal; 4) The analog electrical signal is received by the electronic acquisition system and converted into a digital signal for processing. The energy and count information of each valid event are extracted and transmitted to the host computer as the data information of the corresponding scan point. 5) The host computer generates an internal structure image of the object under test based on the data information of each scanning point; then, based on the internal structure image, the defects of the object under test are visualized and spatially located.

10. The method according to claim 9, characterized in that, The rear collimator is an array of multiple coaxial conical or frustum-shaped collimating rings.

Images