Fast neutron imaging detector and detection system

Abstract

The invention discloses a fast neutron imaging detector and a detection system, and belongs to the technical field of ray detection, the fast neutron imaging detector comprises a wave-shifting optical fiber array, scintillators and a photoelectric conversion array, the wave-shifting optical fiber array comprises a plurality of crisscrossed wave-shifting optical fibers, and gaps among the wave-shifting optical fibers are filled with the scintillators; and each photoelectric detector in the photoelectric conversion array is correspondingly coupled with the wave-shifting optical fiber. The detection efficiency of fast neutrons is effectively improved by increasing the thickness of the sensitive area of the detector, meanwhile, the collection efficiency of photons generated in the sensitive area of the detector can be remarkably improved through the crisscrossed wave-shifting optical fiber array, and the position information of a detection event can be accurately obtained in combination with a gravity center positioning method. Compared with a traditional fast neutron imaging detector based on combination of a camera and a conversion screen, the imaging detector has the advantages of fast response, large sensitive area, high detection efficiency and the like.

Description

Technical Field

[0001] The present invention relates to the field of ray detection technology, and in particular to a fast neutron imaging detector and a detection system. Background Art

[0002] Fast neutrons have strong penetrating power and are sensitive to both light and heavy materials. They have unique advantages in nondestructive testing of large-scale, heavily shielded, and light-heavy material staggered samples. However, fast neutron imaging technology is greatly affected by the gamma background and scattered neutrons. Monte Carlo simulation data shows that among the neutron events detected by fast neutron imaging detectors, the proportion of scattered neutrons may exceed 80%. Coupled with the influence of gamma interference, the outline of fast neutron imaging images is blurred, the boundaries of different materials are unclear, and the fine structures inside the samples are difficult to show. In order to effectively suppress the above interference, the use of flight time to select transmitted neutron events can greatly reduce the interference of scattered neutrons, fission neutrons and gamma rays on fast neutron imaging, which is one of the most feasible technical means to improve the quality of fast neutron imaging.

[0003] To this end, many scholars and institutions at home and abroad are exploring methods that can use flight time to suppress background interference such as scattered neutrons, fission neutrons and gamma rays. For example, Japanese scholars poured liquid scintillator into a container equipped with an aluminum honeycomb plate and a photon exit window to make a fast neutron conversion screen, and built a fast neutron imaging detector in combination with a gated image intensifier, a lens group and a CCD camera. In this imaging detector, the fast neutron conversion screen uses a honeycomb plate to divide the liquid scintillator into a very small grid array, so that the photons generated in it can only pass through the end face of the grid, avoiding the thickness limit of the camera depth of field on the fast neutron conversion screen, so that the fast neutron conversion screen has the advantages of high position resolution and high detection efficiency. At the same time, taking advantage of the short decay time of liquid scintillator, a large proportion of photons can be collected within a time window of the order of 10ns. Therefore, combined with the switching function of the gated image intensifier and the pulsed neutron source, the time resolution of the imaging detector can reach the order of 10ns. However, although this technology has achieved good experimental results, its ability to suppress interference such as scattered neutrons is relatively limited. This is because the flight distance of neutrons is limited, and the time resolution of 10ns is not enough to distinguish scattered neutrons whose energy is not much different from that of transmitted neutrons. Other fast neutron imaging systems based on CCD cameras and gated image intensifiers also face the problem of insufficient time resolution. In addition, due to the limitation of the solid angle of the CCD camera to detect photons, the size of the fast neutron conversion screen is limited. The common size of the fast neutron conversion screen is about 20cm, which makes it difficult to image and detect large samples. For this reason, fast neutron imaging detectors with higher time resolution and larger sensitive area still need to be studied.

[0004] To solve this difficulty, many scholars at home and abroad have conducted research on fast neutron imaging detectors based on four-way photomultiplier tube readout, drawing on the structure of PET imaging detectors. This type of imaging detector uses a specially designed optical guide to couple the scintillator array to four photomultiplier tubes, and uses the pulse amplitude information of the four photomultiplier tubes to reconstruct the incident position information of the neutron. The scintillator array of this detector is relatively thick, so the detection efficiency of fast neutrons is relatively high. At the same time, using the pulse information of the four-way photomultiplier tubes, sub-ns time resolution can be achieved, which can effectively suppress the interference of non-transmitted neutrons on fast neutron imaging. In order to obtain high-quality fast neutron imaging images, multiple detectors can be combined into a linear array or a planar array, and combined with amplified imaging technology, fast neutron imaging images with high contrast and good resolution can be obtained. However, despite the many advantages of this type of detector, its own spatial position resolution is slightly poor. Even if a detector is developed using a 1-inch photomultiplier tube, its position resolution can only reach ~5mm. In order to further improve the position resolution of this type of detector, multi-anode photomultiplier tubes or SiPM arrays can be used to obtain the photon information emitted by the scintillator. Based on this technical route, some scholars have achieved a position resolution of ~3mm for cross-sections ~50×50mm 2 Development of fast neutron imaging detectors. Nevertheless, in order to obtain sub-mm resolution fast neutron imaging images using 3mm position resolution and magnified imaging technology, the beam spot size of the neutron source must be small enough, which will restrict the yield and life of the neutron source to a certain extent. In addition, because magnified imaging requires that the solid angle covered by the detector must be large enough, if imaging detection is to be performed, a large number of detectors must be used to form an array to cover a larger detection solid angle. This will result in a very large and expensive imaging system. In addition, there will inevitably be gaps between detectors, which will also bring some inconvenience to the application of fast neutron imaging technology, especially when obtaining fast neutron projection images.

[0005] In summary, there are two main technical routes for fast neutron imaging detectors at present: one is based on fast neutron conversion screens and CCD cameras; the other is based on scintillator arrays and photomultiplier tubes. Among them, the imaging detectors developed based on fast neutron conversion screens and CCD cameras are limited by the luminescence decay time of the conversion screen and the switching time response of the gated image intensifier. The time resolution of the imaging detector is poor, and it is difficult to use the flight time to suppress interference such as scattered neutrons. The imaging detectors developed based on scintillator arrays and photomultiplier tubes have a very fast time response, and combined with the magnified imaging technology, high-quality fast neutron imaging results can be obtained. However, due to the limitation of the spatial resolution of the detector itself, in order to obtain high-resolution fast neutron imaging images, the beam spot of the neutron source must be limited to a very small scale, which will restrict the yield and life of the neutron source to a certain extent. At the same time, when obtaining the projection image, the gap between the detectors will also bring some adverse effects in practical applications. Summary of the invention

[0006] The purpose of the present invention is to overcome the problems of the prior art and to provide a fast neutron imaging detector and a detection system.

[0007] The object of the present invention is achieved through the following technical solutions: a fast neutron imaging detector, comprising a wave-shifting optical fiber array, a scintillator and a photoelectric conversion array;

[0008] The wave-shifting fiber array includes a plurality of crisscrossed wave-shifting fibers, the gaps between the wave-shifting fibers are filled with scintillators, and each photodetector in the photoelectric conversion array is coupled to the wave-shifting fibers respectively;

[0009] Scintillators are used to receive neutrons and produce photons;

[0010] The wave-shifting fiber array receives the photons generated by the scintillator and transmits the photons to the photoelectric conversion array;

[0011] The photoelectric conversion array is used to convert photons into electrical signal output.

[0012] In one example, the wave-shifting fiber array includes a transverse wave-shifting fiber group and a longitudinal wave-shifting fiber group, the transverse wave-shifting fiber group includes several layers of transverse wave-shifting fibers, and the longitudinal wave-shifting fiber group includes several layers of longitudinal wave-shifting fibers.

[0013] In one example, the cross-sectional size of the wave-shifting optical fiber is less than 1 mm 2 The center spacing of the wave-shifting optical fiber should be 0.5 to 2 mm.

[0014] In one example, the scintillator is a composite scintillator or a neutron sensitive scintillator;

[0015] The composite scintillator is a mixture of hydrogen-rich material powder such as polyethylene or polypropylene and scintillator powder, which is used for fast neutron imaging; the neutron-sensitive scintillator is a mixture of neutron-sensitive material and scintillator powder, wherein the neutron-sensitive material includes a mixture of hydrogen-rich material such as polyethylene or polypropylene and a scintillator powder. 6 Li or 10 B. One or more materials suitable for epithermal neutron imaging.

[0016] In one example, the scintillator powder is ZnS scintillator powder.

[0017] In one example, the photoelectric conversion array is a SiPM (silicon photomultiplier tube) array or a multi-anode photomultiplier tube.

[0018] It should be further explained that the technical features corresponding to the above-mentioned imaging detector examples can be combined or replaced with each other to form a new technical solution.

[0019] The present invention also includes a fast neutron imaging detection system, which includes the fast neutron imaging detector formed by any one of the above examples or a combination of multiple examples, and also includes a data processing unit connected to the photoelectric conversion array, which is used to obtain the output signal of the photoelectric conversion array and calculate the neutron incident position.

[0020] In one example, in the wave-shifting fiber array, the transverse wave-shifting fibers are divided into n1 groups, and the longitudinal wave-shifting fibers are divided into n2 groups, where n1 and n2 are the number of rows of the transverse wave-shifting fibers and the longitudinal wave-shifting fibers, respectively; the number of fibers in each group of transverse wave-shifting fibers is the number of layers of the transversely distributed wave-shifting fibers, and the number of fibers in each group of longitudinal wave-shifting fibers is the number of layers of the longitudinally distributed wave-shifting fibers; the longitudinal coordinates of each group of transverse wave-shifting fibers are the same, and the horizontal coordinates of each group of longitudinal wave-shifting fibers are the same.

[0021] Each group of transverse wave-shifting optical fibers has two ends that are bundled and aligned and then coupled to the photoelectric conversion array; each group of longitudinal wave-shifting optical fibers has two ends that are bundled and aligned and then coupled to the photoelectric conversion array.

[0022] In one example, the output end of the photoelectric conversion array is connected to a simplified position readout circuit, including a symmetric charge distribution circuit (SCDC) and a local center of gravity positioning circuit (TCOG) connected in sequence. The number nx of the photoelectric conversion arrays coupled to the transverse wave shifting fiber is greater than n1 / n p The smallest integer of the number of photoelectric conversion arrays coupled to the longitudinal wave-shifting fiber ny is greater than n2 / n p The smallest integer, n p is the number of units of the photoelectric conversion array used.

[0023] In one example, the n1 groups of transverse wave-shifting optical fibers are further grouped, wherein the numbers 1 to nx are the first group, the numbers nx+1 to 2nx are the second group, and the numbers Nnx+1 to (N+1)nx are the Nth group, and all the transverse wave-shifting optical fibers are grouped in this way; the n2 groups of longitudinal wave-shifting optical fibers are further grouped, wherein the numbers 1 to ny are the first group, the numbers ny+1 to 2ny are the second group, and the numbers Nny+1 to (N+1)ny are the Nth group, and all the longitudinal wave-shifting optical fibers are grouped in this way; the photoelectric conversion array of the transverse wave-shifting optical fibers is coupled, and each unit of the first array is coupled in sequence to the optical fiber groups with the numbers 1, nx+1, 2nx+1..Nnx+1, and each unit of the second array is coupled in sequence to the optical fiber groups with the numbers 2, The optical fiber groups are nx+2, 2nx+2...Nnx+2, and each unit of the Nth array is coupled with the optical fiber groups with serial numbers N, nx+N, 2nx+N...Nnx+N in sequence, and the coupling of all the lateral wave-shifting optical fiber groups is completed in this way; the photoelectric conversion array of coupled longitudinal wave-shifting optical fiber, each unit of the first array is coupled with the optical fiber groups with serial numbers 1, ny+1, 2ny+1...Nny+1 in sequence, each unit of the second array is coupled with the optical fiber groups with serial numbers 2, ny+2, 2ny+2...Nny+2 in sequence, and each unit of the Nth array is coupled with the optical fiber groups with serial numbers N, ny+N, 2ny+N...Nny+N in sequence, and the coupling of all the longitudinal wave-shifting optical fiber groups is completed in this way.

[0024] In one example, the calculation expression used by the data processing unit to calculate the neutron incident position is:

[0025]

[0026] Among them, x and y are the horizontal and vertical coordinates of the neutron incident position respectively; x1, x2, x n They represent the positions of the transverse wave-shifting optical fibers with signals; y1, y2, y n They represent the positions of the longitudinal wave-shifting optical fibers with signals; w1, w2, w n They respectively represent the position weight parameters of the transverse or longitudinal wave-shifting optical fiber with signals, and the position weight parameter values ​​are determined according to the signal amplitude.

[0027] It should be further explained that the technical features corresponding to the above-mentioned detection system examples can be combined or replaced with each other to form a new technical solution.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. In one example, wave-shifting optical fibers are used to transmit photon signals, and the photon attenuation length is several meters. Therefore, the sensitive area of ​​the detector can be made relatively large, so that imaging detection can be carried out on large-sized samples; the criss-crossing wave-shifting optical fibers in the wave-shifting optical fiber array can increase the photon collection area, and filling the gaps between the wave-shifting optical fibers with scintillators can improve the conversion efficiency of photons to electrons. The wave-shifting optical fiber array, combined with the scintillators filling the gaps, increases the mass thickness of the elements in which neutrons undergo nuclear reactions, and can effectively lead out the photons generated by the scintillators. Even if the beam spot size of the neutron source is small, the generated photons can be effectively collected and transmitted through the wave-shifting optical fiber array and the scintillators, thereby improving the detection efficiency and imaging quality of the detector, while maintaining high-resolution imaging, reducing the negative impact on the yield and life of the neutron source.

[0030] 2. In one example, the wave-shifting fiber array can reach dozens of layers, and its overall thickness can reach several centimeters. In addition, the powders such as polyethylene or polypropylene in the composite scintillator have a high hydrogen content, so the imaging detector has the characteristic of high detection efficiency.

[0031] 3. In one example, the spacing between the wave-shifting fibers is in the range of 0.5 to 2 mm, and the accuracy of reconstructing the neutron incident position can be several times lower than the spacing between the wave-shifting fibers, achieving sub-mm position resolution.

[0032] 4. In one example, a ZnS scintillator is used, which is insensitive to γ. Its pulse signal can also be subjected to pulse shape discrimination (PSD) to further suppress the γ background signal. Therefore, the detector has a strong γ background suppression capability.

[0033] 5. In one example, the number of wave-shifting optical fibers that are excited to generate signals is determined based on the attenuation distance of photons in the scintillator, and the optical fibers are then regrouped. This enables the coupled photoelectric conversion array to obtain the signal amplitude information caused by each group of optical fibers while using a position simplification network (position readout circuit), thereby achieving higher position resolution.

[0034] 6. In one example, different weights are assigned to wave-shifting optical fibers at different positions by using the amplitude of the optical signal, thereby paying more attention to the wave-shifting optical fibers corresponding to large signal amplitudes, thereby improving the accuracy of position calculation and further improving the position resolution of the detection system. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The specific implementation methods of the present invention are further described in detail below in conjunction with the accompanying drawings. The accompanying drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The same reference numerals are used in these drawings to represent the same or similar parts. The schematic embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

[0036] Figure 1 A schematic diagram of a wave-shifting optical fiber array provided as an example of the present invention, wherein (a) is an overall diagram of the wave-shifting optical fiber array, and (b) is a partial diagram of the wave-shifting optical fiber array;

[0037] Figure 2 A schematic diagram of readout grouping of a wave-shifted optical fiber bundle provided as an example of the present invention;

[0038] Figure 3 A schematic diagram of a position readout circuit design provided for an example of the present invention.

[0039] In the figure: 11-X group of wave-shifting optical fibers; 12-Y group of wave-shifting optical fibers. DETAILED DESCRIPTION

[0040] The technical solution of the present invention is described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] In the description of the present invention, it should be noted that the directions or positional relationships indicated by "center", "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside", etc. are directions or positional relationships based on the drawings, which are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the use of ordinal numbers (e.g., "first and second", "first to fourth", etc.) is to distinguish objects, and is not limited to this order, and cannot be understood as indicating or implying relative importance.

[0042] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0043] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0044] In one example, a fast neutron imaging detector includes a wave-shifting fiber array, a scintillator, and a photoelectric conversion array, all of which need to work in a dark environment. Preferably, the imaging detector is installed in an optical dark box. The wave-shifting fiber array includes a plurality of criss-crossed wave-shifting fibers, which can be a single layer of criss-crossed wave-shifting fibers, which only includes a layer of transverse (X direction) wave-shifting fibers and a layer of longitudinal (Y direction) wave-shifting fibers, and the number of transverse wave-shifting fibers and longitudinal wave-shifting fibers is a plurality. Preferably, as Figure 1 As shown, the wave-shifting fiber array is a multi-layer crisscross wave-shifting fiber, that is, the wave-shifting fiber array includes a transverse wave-shifting fiber group (X group of wave-shifting fiber 11) and a longitudinal wave-shifting fiber group (Y group of wave-shifting fiber 12), the X group of wave-shifting fiber 11 includes several layers of transverse wave-shifting fiber subgroups, each layer of transverse wave-shifting fiber subgroup includes several transverse wave-shifting fibers; the Y group of wave-shifting fiber 12 includes several layers of longitudinal wave-shifting fiber subgroups, each layer of longitudinal wave-shifting fiber subgroup includes several longitudinal wave-shifting fibers. Preferably, the transverse / longitudinal wave-shifting fiber can be square or circular, and its cross-sectional size should but does not have to meet the requirement of less than 1 mm 2 The center spacing of the wave-shifting optical fiber should be 0.5 to 2 mm.

[0045] Preferably, the scintillator is a composite material scintillator or a neutron sensitive scintillator. The composite material scintillator is a mixture of hydrogen-rich material powder such as polyethylene or polypropylene and ZnS scintillator powder. ZnS scintillator is not sensitive to γ, and its pulse signal can be distinguished by pulse shape to further suppress γ background interference, so the detector has a strong γ background suppression capability. The neutron sensitive scintillator is a mixture of neutron sensitive material and ZnS scintillator powder, and the neutron sensitive material includes 6 Li or 10 One or more of B materials, preferably a neutron sensitive scintillator 6 The mixture of LiF powder and ZnS scintillator powder can be made into an epithermal neutron imaging detector with good position resolution, high detection efficiency, fast time response and large area.

[0046] Preferably, the photoelectric conversion array includes a plurality of photodetectors, which are any one of a SiPM (silicon photomultiplier) array or a multi-anode photomultiplier tube. In this example, an 8×8 SiPM array is preferred. The fast neutron imaging detector uses the SiPM array to detect the light signal of each event. According to the pulse signal waveform output by the SiPM array and the horizontal ratio timing technology, it is expected to achieve a time resolution of ~1ns or even better, thus having the advantage of fast time response.

[0047] In this example, the fast neutron detection part of the imaging detector mainly includes criss-crossed wave-shifting optical fibers (also called wave-shifting optical fibers), the gaps of which are filled with a mixture of ZnS scintillator powder and hydrogen-rich material powders such as polyethylene or polypropylene. Each photodetector in the photoelectric conversion array is coupled one-to-one with the transverse / longitudinal wave-shifting optical fibers.

[0048] The working principle of this example imaging detector is:

[0049] After the fast neutrons are incident on the fast neutron detection part, the fast neutrons are elastically scattered with the H nuclei in polyethylene, polypropylene or wave-shifting optical fiber, and part or all of the energy is transferred to the H nuclei to generate energy-carrying recoil protons. Once the recoil protons interact with the ZnS scintillator powder, the ZnS scintillator powder will emit light. A part of the photons emitted by the ZnS scintillator powder can enter the criss-crossing wave-shifting optical fibers. After receiving these photons, the wave-shifting optical fibers will absorb them with a high probability and emit photons with a longer wavelength. The emission direction of the photons is nearly isotropic. Therefore, some of the secondary emitted photons will be transmitted to both ends along the wave-shifting optical fiber. The photodetectors in the photoelectric conversion array collect the photons and convert them into electrical signals and then transmit them to the subsequent data processing unit for detection and analysis, thereby obtaining the position information of the incident neutrons. Specifically, when the fast neutrons interact with the scintillator, the photons generated will be collected by the surrounding wave-shifting optical fibers. Since the transmission distance of photons in ZnS is limited, only the wave-shifting optical fibers that are closer to the interaction point of the incident neutrons can effectively collect these photons. By measuring the amplitude differences of the light signals received by these optical fibers and combining them with the spatial arrangement information of the wave-shifting optical fibers, the position of the incident neutron can be inferred.

[0050] The present invention also includes a fast neutron imaging detection system, including the above-mentioned fast neutron imaging detector, and also includes a data processing unit connected to the photoelectric conversion array, which is used to measure the output signal of the photoelectric conversion array and calculate the neutron incident position.

[0051] In one example, the transverse wave-shifting optical fibers in the wave-shifting optical fiber array are divided into n1 groups, and the longitudinal wave-shifting optical fibers are divided into n2 groups, where n1 and n2 are the number of rows of the transverse wave-shifting optical fibers and the longitudinal wave-shifting optical fibers, respectively. Furthermore, the number nx of the photoelectric conversion arrays coupled to the transverse wave-shifting optical fibers is greater than n1 / n p The smallest integer of the number of photoelectric conversion arrays coupled to the longitudinal wave-shifting fiber ny is greater than n2 / n p The smallest integer, n p is the number of units in the photoelectric conversion array used. Furthermore, although the attenuation distance of photons in ZnS is only ~0.5 mm, most events will at most trigger the SiPM arrays corresponding to the three adjacent wave-shifting fiber bundles to generate signals. p or n2 / n pToo small, preferably the minimum value of nx and ny is set to 3 (that is, when n1 / n p or n2 / n p When nx or ny is less than 3, nx or ny is set to 3). Furthermore, it is preferred that the transverse (or longitudinal) wave-shifting optical fibers are numbered in a cycle for every nx (or ny) groups of wave-shifting optical fibers, and the two ends of each numbered wave-shifting optical fiber are bundled and aligned before being coupled to the SiPM array.

[0052] Specifically, in order to obtain the position information of the incident neutron, the crisscrossed wave-shifting fibers are divided into two groups, namely, X groups of wave-shifting fibers and Y groups of wave-shifting fibers. In the X group (or Y group), starting from the first column, every nx (or ny) columns form a cycle, recorded as X1, X2, ..., X nx , X nx+1 , X nx+2 , …, X 2nx , X 2nx+1 ,…(or Y1, Y2,…, Y ny , Y ny+1 , Y ny+2 , …, Y 2ny , Y 2ny+1 ,…),like Figure 2 As shown. nx+1 , X 2nx+1 , …, X 63nx+1 The optical fibers at both ends are bundled together and the end faces are aligned. Then, these 64 wave-shifted optical fiber bundles are coupled to the 8×8 SiPM array, with each optical fiber bundle corresponding to one SiPM. nx+2 , X 2nx+2 , …, X 63nx+2 The fiber bundle is coupled to the second 8×8 SiPM array, and so on. If the above numbers are less than 64, they can be left blank.

[0053] In one example, in order to reduce the signal channels of the data acquisition system of the SiPM array, the SiPM array can use a position readout network (position readout circuit) to simplify the 64-way signal readout to 4-way signal readout, and the position readout network is connected to the photoelectric conversion array, including Figure 3The simplified network of the symmetrical charge distribution circuit SCDC combined with the local center of gravity positioning circuit TCOG shown in the figure, and the SiPM array matched with the simplified position network, can greatly reduce the signal channels of the data acquisition system of the imaging detector. Optionally, the position readout circuit can also be a simplified resistor position readout network or a simplified capacitor position readout network. In this way, the intensity of the light signal detected by the corresponding SiPM array and the serial number of the fiber bundle can be obtained using 4 signals. Because each fiber bundle corresponds to only one group of wave-shifting fibers (X group of wave-shifting fibers or Y group of wave-shifting fibers), and the X (or Y) coordinates of the group of wave-shifting fibers are exact. Therefore, once the SiPMs coupled to the X and Y groups of wave-shifting fibers detect the photon signal within the set ns-level time window, the serial numbers of the X and Y groups of wave-shifting fibers corresponding to the signal can be obtained, and then the X and Y coordinates of the signal can be obtained. Obviously, for an event, it is possible to cause a signal on the SiPM array corresponding to the wave-shifting fiber bundles of multiple serial numbers in the X (or Y) group. Once such a situation occurs, the position of the wave-shifting fiber bundle with the largest signal amplitude can be selected as the event for location. If the wave-shifting fiber spacing is set to less than 1 mm, sub-mm position resolution can be achieved. Preferably, a data analysis algorithm can also be used to achieve higher positioning accuracy. In this case, the calculation expression for the neutron incident position calculated by the data processing unit is:

[0054]

[0055] Among them, x and y are the horizontal and vertical coordinates of the neutron incident position respectively; x1, x2, x n are the positions of the transverse wave-shifting fibers with signals in the X groups of wave-shifting fibers; y1, y2, y n are the positions of the longitudinal wave-shifting optical fibers with signals in the Y group of wave-shifting optical fibers; w1, w2, w n Respectively represent the position weight parameters of the transverse or longitudinal wave-shifting fibers with signals in the X group of wave-shifting fibers or the Y group of wave-shifting fibers. The position weight parameter value is determined according to the signal amplitude. The larger the amplitude, the greater the weight. If the amplitude is 0, the weight is 0. Combined with the above-mentioned data analysis algorithm with higher position resolution, the positioning accuracy of the event can be several times less than the wave-shifting fiber spacing.

[0056] The wave-shifting optical fiber used in the present invention can reach dozens of layers, and its overall thickness can reach several centimeters, and the mixture of scintillator and polyethylene or polypropylene powder contains a large amount of hydrogen, so the fast neutron detection efficiency is high. At the same time, because the attenuation distance of photons in the wave-shifting optical fiber can reach several meters, the sensitive area of ​​the detector can be very large. In addition, by obtaining the pulse information of each event and combining the horizontal ratio timing method, the time resolution of the detector can be better than 1ns. In addition, because the spacing between the wave-shifting optical fibers is small and the event positioning accuracy can be several times less than the spacing between the wave-shifting optical fibers, the imaging detector has extremely high position resolution. Therefore, the fast neutron imaging detector proposed by the present invention has the characteristics of high detection efficiency, large sensitive area, fast time response, and good position separation, and can obtain high-quality fast neutron imaging images.

[0057] The above specific implementation methods are detailed descriptions of the present invention. It cannot be determined that the specific implementation methods of the present invention are limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions and substitutions can be made without departing from the concept of the present invention, which should be regarded as belonging to the protection scope of the present invention.

Claims

1. A fast neutron imaging detector, characterized in that: It includes a wave-shifting optical fiber array, a scintillator, and a photoelectric conversion array; The wave-shifting fiber array includes a plurality of crisscrossed wave-shifting fibers, the gaps between the wave-shifting fibers are filled with scintillators, and each photodetector in the photoelectric conversion array is coupled to the wave-shifting fibers respectively; Scintillators are used to detect neutrons and produce photons at the corresponding locations; The wave-shifting fiber array receives the photons generated by the scintillator and transmits the photons to the photoelectric conversion array; The photoelectric conversion array is used to convert photons into electrical signal output.

2. The fast neutron imaging detector according to claim 1, characterized in that: The wave-shifting fiber array includes a transverse wave-shifting fiber group and a longitudinal wave-shifting fiber group. The transverse wave-shifting fiber group includes several layers of transverse wave-shifting fibers, and the longitudinal wave-shifting fiber group includes several layers of longitudinal wave-shifting fibers.

3. The fast neutron imaging detector according to claim 1, characterized in that: The cross-sectional size of the wave-shifting optical fiber is less than 1 mm 2 , the center spacing of the wave-shifting optical fiber is 0.5 to 2 mm.

4. The fast neutron imaging detector according to claim 1, characterized in that: The scintillator is a composite material scintillator or a neutron sensitive scintillator; The composite scintillator is a mixture of hydrogen-rich material powder and scintillator powder, and the hydrogen-rich material is polyethylene or polypropylene; The neutron sensitive scintillator is a mixture of neutron sensitive material and scintillator powder. The neutron sensitive material includes 6 Li or 10 One or more of B materials.

5. The fast neutron imaging detector according to claim 4, characterized in that: The scintillator powder is ZnS scintillator powder; the photoelectric conversion array is a SiPM array or a multi-anode photomultiplier tube.

6. A fast neutron imaging detection system, characterized in that: The fast neutron imaging detector comprises the fast neutron imaging detector according to any one of claims 1 to 5, and further comprises a data processing unit connected to the photoelectric conversion array, for acquiring the output signal of the photoelectric conversion array and calculating the neutron incident position.

7. The fast neutron imaging detector according to claim 6, characterized in that: The transverse wave-shifting optical fibers in the wave-shifting optical fiber array are divided into n1 groups, and the longitudinal wave-shifting optical fibers are divided into n2 groups, wherein n1 and n2 are the number of rows of the transverse wave-shifting optical fibers and the longitudinal wave-shifting optical fibers, respectively; the number of optical fibers in each group of the transverse wave-shifting optical fibers is the number of layers of the transversely distributed wave-shifting optical fibers, and the number of optical fibers in each group of the longitudinal wave-shifting optical fibers is the number of layers of the longitudinally distributed wave-shifting optical fibers; the longitudinal coordinates of each group of the transverse wave-shifting optical fibers are the same, and the abscissas of each group of the longitudinal wave-shifting optical fibers are the same; Each group of transverse wave-shifting optical fibers has two ends that are bundled and aligned and then coupled to the photoelectric conversion array; each group of longitudinal wave-shifting optical fibers has two ends that are bundled and aligned and then coupled to the photoelectric conversion array.

8. The fast neutron imaging detector according to claim 7, characterized in that: The output end of the photoelectric conversion array is connected to a position readout simplification circuit, including a symmetrical charge distribution circuit and a local center of gravity positioning circuit connected in sequence; The number nx of photoelectric conversion arrays coupled to the transverse wave shifting fiber is greater than n1 / n p The smallest integer of the number of photoelectric conversion arrays coupled to the longitudinal wave-shifting fiber ny is greater than n2 / n p The smallest integer, n p is the number of units of the photoelectric conversion array used.

9. The fast neutron imaging detector according to claim 7, characterized in that: The n1 groups of transverse wave-shifting optical fibers are further grouped, with serial numbers 1 to nx forming the first group, serial numbers nx+1 to 2nx forming the second group, and serial numbers Nnx+1 to (N+1)nx forming the Nth group, and all the transverse wave-shifting optical fibers are grouped in this way; the n2 groups of longitudinal wave-shifting optical fibers are further grouped, with serial numbers 1 to ny forming the first group, serial numbers ny+1 to 2ny forming the second group, and serial numbers Nny+1 to (N+1)ny forming the Nth group, and all the longitudinal wave-shifting optical fibers are grouped in this way; the photoelectric conversion array of the transverse wave-shifting optical fibers is coupled, and each unit of the first array is coupled to the optical fiber groups with serial numbers 1, nx+1, 2nx+1...Nnx+1 in sequence, and each unit of the second array is coupled to the optical fiber groups with serial numbers 2, nx+2 , 2nx+2...Nnx+2, each unit of the Nth array is coupled with the optical fiber groups numbered N, nx+N, 2nx+N...Nnx+N in sequence, and the coupling of all lateral wave-shifting optical fiber groups is completed in this way; the photoelectric conversion array of coupled longitudinal wave-shifting optical fiber, each unit of the first array is coupled with the optical fiber groups numbered 1, ny+1, 2ny+1...Nny+1 in sequence, each unit of the second array is coupled with the optical fiber groups numbered 2, ny+2, 2ny+2...Nny+2 in sequence, each unit of the Nth array is coupled with the optical fiber groups numbered N, ny+N, 2ny+N..Nny+N in sequence, and the coupling of all longitudinal wave-shifting optical fiber groups is completed in this way.

10. The fast neutron imaging detector according to claim 7, characterized in that: The calculation expression for calculating the neutron incident position by the data processing unit is: Among them, x and y are the horizontal and vertical coordinates of the neutron incident position respectively; x1, x2, x n They represent the positions of the transverse wave-shifting optical fibers with signals; y1, y2, y n They represent the positions of the longitudinal wave-shifting optical fibers with signals; w1, w2, w n They respectively represent the position weight parameters of the transverse or longitudinal wave-shifting optical fiber with signals, and the position weight parameter values ​​are determined according to the signal amplitude.

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