Radiation detector and method for estimating incident direction of radiation

Abstract

To provide a radiation detector that can detect the direction of entrance of radiation and can be made smaller and lighter at low cost.SOLUTION: A radiation detector 1 detects radiation, and includes a plurality of radiation sensors 10 arranged to form a fractal structure and a radiation shielding material 20 for burying the space between the radiation sensors 10 adjacent to each other.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a radiation detector and a method for estimating the incident direction of radiation.

Background Art

[0002] Patent Document 1 discloses a radiation detector that detects X-rays, which are a type of radiation. This radiation detector includes a scintillation layer that converts X-rays into light, and a plurality of silicon photomultipliers (SiPMs) that are coupled to the scintillation layer and detect the light.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, in indoor areas or outer space contaminated with radioactivity, radiation enters from various directions. For this reason, there is a demand for a radiation detector that can detect the incident direction of radiation. In response to such a demand, conventionally, there are radiation detectors such as a pinhole camera and a Compton camera that can detect the incident direction of radiation within a certain range. In a pinhole camera, a detector body that detects radiation is housed inside a shield formed with a pinhole. Since the shield is made of a material such as lead that shields radiation, the pinhole camera has the drawback of being heavy. On the other hand, a Compton camera has the advantage of being lighter in weight compared to a pinhole camera, but has a complex structure and the number of radiation sensors (channel number) required for radiation detection increases, resulting in a high cost.

[0005] The present invention has been made in view of the above circumstances, and an object thereof is to provide a radiation detector capable of detecting the incident direction of radiation and achieving miniaturization and weight reduction at low cost, and a method for estimating the incident direction of radiation using the same.

Means for Solving the Problems

[0006] The present invention includes the following aspects. 〔1〕 A radiation detector comprising: a plurality of radiation sensors arranged to detect radiation and form a fractal structure; and a radiation shielding material filling between adjacent radiation sensors. Three-dimensionally 〔2〕 The radiation detector according to 〔1〕, wherein the fractal dimension of the fractal structure is 1.5 or more and 2.5 or less. 〔3〕 The radiation detector according to 〔1〕 or 〔2〕, wherein the plurality of radiation sensors each have the appearance of a regular tetrahedron, and the fractal structure is a Sierpinski tetrahedron. 〔4〕 The radiation detector according to any one of 〔1〕 to 〔3〕, wherein the radiation sensor includes a scintillation portion made of a scintillation material that converts the radiation into light, and a light detection portion fixed to the surface of the scintillation portion, transmitting the radiation, and detecting the light. 〔5〕 A method for estimating the incident direction of radiation in a three-dimensional space based on the counting rate of radiation obtained respectively in the plurality of radiation sensors of the radiation detector according to any one of 〔1〕 to 〔4〕. Solve the inverse problem of the equation shown below and

Number

Effects of the Invention

[0007] According to the present invention, it is possible to provide a radiation detector that can detect the incident direction of radiation and can be inexpensively made small and lightweight.

Brief Description of the Drawings

[0008]

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Mode for Carrying Out the Invention

[0009] Hereinafter, an embodiment of the present invention will be described with reference to FIGS. 1 to 7. As shown in FIGS. 1 and 2, the radiation detector 1 according to this embodiment includes a plurality of radiation sensors 10 and a radiation shielding material 20.

[0010] Each radiation sensor 10 detects radiation (for example, γ-rays or β-rays) incident on the radiation sensor 10. The radiation sensor 10 outputs an electrical signal according to the incident radiation. As shown in FIG. 4, the radiation sensor 10 of this embodiment has a scintillation part 11 and a light detection part 12, but is not limited thereto. The radiation sensor 10 may be constituted by, for example, a semiconductor detector. The scintillation part 11 is constituted by a scintillation material that converts radiation into light. The scintillation part 11 forms the appearance of the radiation sensor 10. In this embodiment, since the scintillation part 11 is formed in a regular tetrahedron, the radiation sensor 10 has the appearance of a regular tetrahedron.

[0011] The light detection part 12 is fixed to the surface of the scintillation part 11 and detects the light emitted in the scintillation part 11. Specifically, the light detection part 12 is a silicon photomultiplier (SiPM), which converts the detected light into an electrical signal and outputs the electrical signal to the outside. The light detection part 12 is formed so as to transmit radiation. The light detection part 12 is formed, for example, thinly enough for radiation to pass through. In this embodiment, the light detection part 12 is fixed to one plane (triangular plane) of the scintillation part 11 formed as a regular tetrahedron. Also, only one light detection part 12 is provided for the same scintillation part 11.

[0012] In the radiation sensor 10 of the present embodiment configured as described above, radiation can pass through the light detection unit 12 fixed to the surface of the scintillation unit 11. Thereby, the radiation sensor 10 can detect radiation from all directions in the three-dimensional space. Further, since the radiation sensor 10 of the present embodiment has a simple structure in which the light detection unit 12 is fixed to the surface of the scintillation unit 11, it can be easily miniaturized. By miniaturizing the radiation sensor 10, its weight can also be reduced. Furthermore, by miniaturizing the radiation sensor 10, the number of radiations incident on the radiation sensor 10 can be reduced, and the occurrence of pile-up (saturation of the counting rate) can be effectively suppressed.

[0013] Although not shown, the radiation sensor 10 preferably further includes a covering portion that covers the scintillation unit 11 and the light detection unit 12. The covering portion is made of a material that transmits the radiation to be measured and shields light. The covering portion may be formed, for example, by coating the surfaces of the scintillation unit 11 and the light detection unit 12. Also, the covering portion may be, for example, a housing that houses the scintillation unit 11 and the light detection unit 12. The covering portion may be formed of a thin metal such as aluminum foil. By the radiation sensor 10 having the covering portion, it is possible to prevent the light detection unit 12 from detecting light from the outside of the radiation sensor 10 that is paired. That is, it is possible to correctly detect the radiation incident on the paired radiation sensor 10.

[0014] As shown in FIGS. 1 to 3, a plurality of radiation sensors 10 are arranged so as to form a fractal structure. The fractal structure is a structure in which the overall shape formed by arranging a plurality of radiation sensors 10 having the same shape and the same size is the same as the shape of a single radiation sensor 10 (that is, a structure having self-similarity). Specific examples of the fractal structure include, but are not limited to, the Sierpinski tetrahedron and the Menger sponge. The fractal dimension of the fractal structure formed by arranging a plurality of radiation sensors 10 is preferably around 2. Further, the fractal dimension is preferably, for example, 1.5 or more and 2.5 or less, and more preferably closer to 2 than these upper and lower limit values.

[0015] The fractal structure in the present embodiment is the Sierpinski tetrahedron. That is, a plurality of radiation sensors 10 each having a regular tetrahedron shape are arranged such that the overall appearance of the whole (radiation detector 1) is a regular tetrahedron. The fractal dimension of the Sierpinski tetrahedron is 2.

[0016] In the radiation detector 1 shown in FIGS. 1 to 3, four radiation sensors 10 are arranged so as to constitute one sensor unit 5 having a regular tetrahedron appearance as a whole. Further, four sensor units 5 are arranged so as to have a regular tetrahedron appearance as a whole. That is, the radiation detector 1 shown in FIGS. 1 to 3 has 16 radiation sensors 10.

[0017] The radiation shielding material 20 fills the space between adjacent radiation sensors 10 among the plurality of radiation sensors 10 arranged so as to form a fractal structure. In other words, the radiation shielding material 20 fills the void or space inside the structure formed in a self-similar shape by a plurality of radiation sensors 10. Thereby, the region (plane) of the surface of each radiation sensor 10 arranged so as to form a fractal structure and facing an adjacent other radiation sensor 10 is covered with the radiation shielding material 20. It is preferable to use a high-density metal (for example, tungsten, tungsten alloy, lead, etc.) having high radiation shielding performance for the radiation shielding material 20. In the radiation detector 1 of this embodiment, since the fractal structure is a Sierpinski tetrahedron, the voids or spaces inside the structure formed in a self-similar shape by the plurality of radiation sensors 10 are connected together. For this reason, in FIGS. 1 and 2, the radiation shielding material 20 is exposed in a divided state at a plurality of locations on the surface of the radiation detector 1, but actually the radiation shielding material 20 is integrated into one.

[0018] In the radiation detector 1 configured as described above, only the regions on the surfaces of the respective radiation sensors 10 that do not face other radiation sensors 10 are exposed. The regions of each radiation sensor 10 that do not face other radiation sensors 10 are exposed surfaces facing the outside of the radiation detector 1. In the radiation detector 1 shown in FIGS. 1 and 2, three surfaces of the four radiation sensors 10 (10-1, 10-6, 10-11, 10-16) corresponding to the vertices of the regular tetrahedron correspond to the above-described exposed surfaces. Also, two surfaces of the radiation sensors 10 (10-2 to 10-5, 10-7 to 10-10, 10-12 to 10-15) that do not correspond to the vertices of the regular tetrahedron but correspond to the edges of the regular tetrahedron correspond to the above-described exposed surfaces. In this radiation detector 1, radiation incident from the exposed surfaces of the respective radiation sensors 10 can be detected.

[0019] In the radiation detector 1 of this embodiment configured as described above, the incident direction of radiation in three-dimensional space can be detected. Hereinafter, this point will be described.

[0020] For example, as shown in FIG. 5, consider the case where radiation is incident on the radiation detector 1 from two different directions (the first and second incident directions D1, D2). The first incident direction D1 is a direction in which radiation travels from above to the upper end (radiation sensor 10-1) of the radiation detector 1 downward. The second incident direction D2 is a direction toward the side of the radiation detector 1 (the side where the four radiation sensors 10-1, 10-4, 10-13, 10-16 are arranged).

[0021] FIG. 6 is a graph showing the distribution of the counting rates of a plurality of radiation sensors 10 when radiation is incident on the radiation detector 1 from the first incident direction D1. Further, FIG. 7 is a graph showing the distribution of the counting rates of a plurality of radiation sensors 10 when radiation is incident on the radiation detector 1 from the second incident direction D2. The sensor numbers (1, 2, 3..., 16) in each of the graphs of FIGS. 6 and 7 correspond to the branch numbers (10-1, 10-2, 10-3..., 10-16) of the radiation sensors 10 shown in FIGS. 1 to 3 and 5, respectively.

[0022] When radiation is incident on the radiation detector 1 from the first incident direction D1, as shown in FIG. 6, it can be seen that the larger the exposure area of the radiation sensor 10 as viewed from the first incident direction D1, the larger the counting rate in the radiation sensor 10. Specifically, the exposure area of the radiation sensor 10 as viewed from the first incident direction D1 is the largest for the radiation sensor 10-1 located at the upper end of the radiation detector 1 (the exposure surface is three). Therefore, the counting rate in the radiation sensor 10-1 is the largest.

[0023] Next, the exposure areas of the radiation sensors 10-2 to 10-6, 10-9, 10-11, 10-13, 10-16 (see FIGS. 1 and 2) located along the three sides extending from the upper end (apex) of the radiation detector 1 are large (the exposure surface is two). Therefore, the counting rates in these radiation sensors 10-2 to 10-6, 10-9, 10-11, 10-13, 10-16 are the second largest after the radiation sensor 10-1. Then, the exposure areas of the radiation sensors 10-7, 10-8, 10-10, 10-12, 10-14, 10-15 (see FIGS. 1 and 2) located along the three sides located at the lower end of the radiation detector 1 and not related to the apex of the regular tetrahedron are the smallest (the exposure surface is one). Therefore, the counting rates in these radiation sensors 10-7, 10-8, 10-10, 10-12, 10-14, 10-15 are the smallest. As described above, the distribution of the counting rates of the plurality of radiation sensors 10 as shown in FIG. 6 is obtained.

[0024] On the other hand, when radiation enters the radiation detector 1 from the second incident direction D2, as shown in FIG. 7, it can be seen that the larger the exposed area of the radiation sensor 10 as viewed from the second incident direction D2, the larger the counting rate in the radiation sensor 10. Specifically, the exposed area of the radiation sensor 10 as viewed from the second incident direction D2 is the largest (with two exposed surfaces) for the radiation sensors 10-1, 10-4, 10-13, 10-16 (see FIGS. 1 and 2) located along the side of the radiation detector 1 facing the second incident direction D2. For this reason, the counting rates in these radiation sensors 10-1, 10-4, 10-13, 10-16 are the largest.

[0025] Next, the exposed areas of the radiation sensors 10-2, 10-3, 10-5, 10-6, 10-8, 10-9, 10-11, 10-12, 10-14, 10-15, which are located on two planes continuous with the side of the above-described radiation detector 1 and not related to the side, are large (with one exposed surface). For this reason, the counting rates in these radiation sensors 10-2, 10-3, 10-5, 10-6, 10-8, 10-9, 10-11, 10-12, 10-14, 10-15 are the second largest after those in the radiation sensors 10-1, 10-4, 10-13, 10-16. Since the remaining radiation sensors 10-7, 10-10 are in positions where they cannot be seen from the second incident direction D2, the counting rates in these radiation sensors 10-7, 10-10 are the smallest. Thus, the distribution of the counting rates of the plurality of radiation sensors 10 as shown in FIG. 7 is obtained.

[0026] As shown in FIGS. 6 and 7, it can be seen that when the incident direction of the radiation with respect to the radiation detector 1 is different, the distribution of the counting rates of the plurality of radiation sensors 10 changes. Therefore, in the radiation detector 1 of the present embodiment, the incident direction of the radiation in the three-dimensional space can be detected based on the distribution of the counting rates of the plurality of measured radiation sensors 10.

[0027] Next, a method for estimating the incident azimuth of radiation using the radiation detector 1 of the present embodiment will be described. In the method for estimating the incident direction of radiation, the inverse problem analysis is performed based on the counting rates of radiation obtained by the plurality of radiation sensors 10 of the radiation detector 1, thereby estimating the incident direction of radiation in the three-dimensional space. Specifically, for example, the incident direction is estimated by solving the inverse problem of the following equation.

[0028]

Equation

[0029] In the above equation, Ci (i = 1, 2, ···, 16) is the counting rate measured by each radiation sensor 10 (10-1 to 10-16). i corresponds to the sensor number (branch number) of the radiation sensor 10. θ and ζ are angles indicating the incident direction of radiation on the radiation detector 1, and are angles in directions orthogonal to each other with the radiation detector 1 as the center in the polar coordinate system. φ is the flux (particle flux of radiation) with the incident angles θ and ζ as parameters. Ri (i = 1, 2, ···, 16) is the response function of each radiation sensor 10, and is obtained in advance by Monte Carlo calculation. The above equation indicates that the counting rate Ci of each radiation sensor 10 is the integral value of the flux φ for each incident angle θ, ζ of the radiation.

[0030] In the method for estimating the incident direction of radiation, based on the counting rate Ci of the radiation measured by the plurality of radiation sensors 10, by solving the inverse problem of the above equation, the incident angles θ, ζ and the flux φ of the radiation on the radiation detector 1 are obtained. Thereby, the incident direction of the radiation on the radiation detector 1 in the three-dimensional space can be estimated. Note that when solving the inverse problem, for example, unfolding calculation or deep learning (deep learning) may be used. In this case, in addition to estimating the incident directions of a plurality of radiations, the position of the radiation source can be estimated.

[0031] As described above, in the radiation detector 1 of the present embodiment, the plurality of radiation sensors 10 are arranged so as to form a fractal structure. Thereby, even if the number of radiation sensors 10 is suppressed to be small, the incident direction of radiation can be detected. And by suppressing the number of radiation sensors 10, it becomes possible to provide a small and inexpensive radiation detector 1. Also, since the plurality of radiation sensors 10 are arranged so as to form a fractal structure, it becomes possible to suppress the amount of the radiation shielding material 20 that fills between the radiation sensors 10 to be smaller than that of a conventional pinhole camera. Thereby, the weight of the radiation detector 1 can be reduced.

[0032] Also, in the radiation detector 1 of the present embodiment, the fractal dimension of the fractal structure in which the plurality of radiation sensors 10 are arranged is within the range of 1.5 or more and 2.5 or less. Since the fractal dimension is 1.5 or more, it becomes possible to efficiently detect radiation from all directions in three-dimensional space. Also, since the fractal dimension is 2.5 or less, it becomes possible to simplify the structure of the radiation detector 1 due to the fractal structure. For example, radiation from all directions can be detected with a small number of channels (the number of radiation sensors 10). Since the shapes of the plurality of radiation sensors 10 are the same, the response characteristics of the radiation sensors 10 to radiation are also the same, so there is also an advantage that the manufacturing of the radiation detector 1, signal data processing such as radiation counting rate, and inverse problem analysis are easily simplified.

[0033] Also, in the radiation detector 1 of the present embodiment, the plurality of radiation sensors 10 each have the appearance of a regular tetrahedron. On top of that, the fractal structure in which the plurality of radiation sensors 10 are arranged is the Sierpinski tetrahedron. Thereby, since the fractal dimension of the fractal structure becomes 2, radiation from all directions can be detected with the minimum number of channels (the number of radiation sensors 10).

[0034] In addition, in the radiation detector 1 of the present embodiment, the light detection unit 12 of each radiation sensor 10 is configured to transmit radiation. As a result, even if the light detection unit 12 is disposed on the surface of the scintillation unit 11 corresponding to the exposed surface of the radiation sensor 10, radiation can be detected.

[0035] In addition, in the radiation detector 1 of the present embodiment, it is also possible to discriminate and measure a plurality of types of radiation, specifically γ-rays and β-rays, by using the energy difference corresponding to the type of radiation, and also to discriminate and measure the direct rays emitted directly from the radiation source and incident on the radiation detector 1 and the scattered rays scattered around and incident on the radiation detector 1. That is, the incident directions of various types of radiation can be individually detected. Hereinafter, this point will be described with reference to FIGS. 8 and 9.

[0036] FIG. 8 is a graph showing the energy spectra of γ-rays and β-rays. In the graph of FIG. 8, the energy spectra of γ-rays emitted by γ-ray nuclides (Cs137, Ba137m) and the energy spectra of β-rays emitted by β-ray nuclides (Sr90, Y90) are shown. The vertical axis of the graph in FIG. 8 corresponds to the amount of radiation incident on the radiation detector 1 (radiation sensor 10) (the number of photons emitted by the scintillation unit 11 in response to the incident radiation). In the graph of FIG. 8, the β-ray energy of the measured β-ray nuclide (Y90) includes the β-ray energy of the β-ray nuclide (Y90) formed by the decay of the β-ray nuclide (Sr90).

[0037] As shown in FIG. 8, the characteristics of the energy spectrum of radiation are significantly different between gamma rays and beta rays. For example, energy of 0.8 MeV or more is not measured for gamma rays (Ba137m produced by the decay of Cs137), but energy of 0.8 MeV or more is also measured for beta rays (Y90). Note that energy of 0.8 MeV or more is not measured for beta rays (Sr90). For this reason, by utilizing the difference in characteristics between gamma rays (Ba137m produced by the decay of Cs137) and beta rays (Y90), for example, gamma rays can be measured by setting the energy range measured by the radiation detector 1 (radiation sensor 10) to 0.8 MeV or less. In addition, beta rays can be measured by setting the energy range measured by the radiation detector 1 (radiation sensor 10) to 0.8 MeV or more. In other words, gamma rays and beta rays can be measured separately, and similarly, direct rays and scattered rays can be discriminated by utilizing the energy difference. By discriminating between direct rays and scattered rays, the incident direction can be estimated with higher accuracy.

[0038] In addition, taking into consideration the difference in the characteristics of the energy spectra of gamma rays and beta rays, gamma rays and beta rays can be discriminated and measured, for example, by using the counting rate measured by the radiation detector 1 (radiation sensor 10). Specifically, the counting rate C total and the count rate C at energies above 0.8 MeV high Ratio to (C high / C total ) is used. As shown in FIG. 9, the count rate C total and the ratio of counting rates (C high / C total ) to determine the relationship between the counting rate ratio (C high / C total In the range where the ratio of count rates (C high / C total In the range where the counting rate ratio (C high / C totalBy using (0), it becomes possible to discriminate and measure γ-rays and β-rays.

[0039] Note that the method of discriminating and measuring γ-rays and β-rays is not limited to the above. For example, as shown in FIGS. 12 and 13, by utilizing the difference in the ranges of γ-rays and β-rays, it may be possible to discriminate and measure γ-rays and β-rays. This will be described below. As shown in FIG. 12, the β-rays incident on the radiation sensor 10 stop on or near the surface of the radiation sensor 10. On the other hand, as shown in FIG. 13, the γ-rays incident on the radiation sensor 10 reach the inside of the radiation sensor 10 or penetrate the radiation sensor 10. That is, the range of β-rays is shorter than that of γ-rays.

[0040] In order to utilize such a difference in the ranges of γ-rays and β-rays, two types of radiation detectors 1 with different sizes of the radiation sensor 10 are used. As shown in FIGS. 12(a) and 12(b), the radiation dose of β-rays that stop on or near the surface of the radiation sensor 10 is proportional to the surface area of the radiation sensor 10 between the two types of radiation detectors 1. On the other hand, as shown in FIGS. 13(a) and 13(b), the radiation dose of γ-rays that reach the inside of the radiation sensor 10 or penetrate the radiation sensor 10 is proportional to the volume of the radiation sensor 10 between the two types of radiation detectors 1. Therefore, by obtaining the proportional relationship of the radiation doses detected by the two types of radiation detectors 1, it is possible to discriminate and measure γ-rays and β-rays. Note that in FIGS. 12 and 13, the larger the number of lines indicating β-rays and γ-rays, the larger the radiation dose of the β-rays and γ-rays corresponding to the radiation sensor 10.

[0041] As described above, the embodiments according to the present invention have been described, but the present invention is not limited to the above embodiments and can be appropriately modified without departing from the spirit thereof.

[0042] In the above-described embodiment, 16 regular tetrahedral radiation sensors 10 are used to form a Sierpinski tetrahedron. However, as shown in FIG. 10 for example, a Sierpinski tetrahedron may be formed using a larger number of radiation sensors 10. That is, the number of stages of the fractal structure in the radiation detector may be increased. By increasing the number of stages of the fractal structure (the number of radiation sensors 10), even low-dose radiation can be detected by the radiation detector 1. Also, even if the number of stages of the fractal structure is increased, since the geometric arrangement is the same, data processing such as the radiation counting rate and inverse problem analysis are less likely to become complicated.

[0043] Also, in the above-described embodiment, the radiation detector includes only one structure in which a plurality of radiation sensors are arranged to form a fractal structure. However, for example, a plurality of such structures may be provided. When the radiation detector includes a plurality of such structures, as shown in FIG. 11 for example, the orientation or angle of a plurality (two in the illustrated example) of structures 2 formed on the tetrahedron may be changed relative to each other. In this case, it is possible to improve the resolution of the incident angle of radiation with respect to the radiation detector.

Explanation of Reference Numerals

[0044] 1 Radiation detector 10 Radiation sensor 11 Scintillation section 12 Photodetection section 20 Radiation shielding material

Claims

1. A radiation detector comprising: a plurality of radiation sensors that detect radiation and are three-dimensionally arranged to form a fractal structure; and a radiation shielding material that fills the space between adjacent radiation sensors.

2. The radiation detector according to claim 1, wherein the fractal dimension of the fractal structure is 1.5 or more and 2.5 or less.

3. Each of the plurality of radiation sensors has the appearance of a regular tetrahedron, and the fractal structure is a Sierpinski tetrahedron. The radiation detector according to claim 1 or claim 2.

4. The radiation sensor includes a scintillation portion made of a scintillation material that converts the radiation into light, and a light detection portion that is fixed to the surface of the scintillation portion, transmits the radiation, and detects the light. The radiation detector according to any one of claims 1 to 3.

5. By solving the inverse problem of the equation shown below based on the counting rate of the radiation obtained respectively in the plurality of radiation sensors of the radiation detector according to any one of claims 1 to 4, the incident direction of the radiation in three-dimensional space is estimated. 【Number 1】 In the above equation, Ci (i = 1, 2,...) is the counting rate measured in each radiation sensor, and i corresponds to the sensor number of the radiation sensor. θ and ζ are angles indicating the incident direction of the radiation to the radiation detector, and are angles in directions orthogonal to each other centered on the radiation detector in a polar coordinate system. φ is a flux (particle flux of radiation) with the incident angles θ and ζ as parameters. Ri (i = 1, 2,...) is the response function of each radiation sensor, which is obtained in advance by Monte Carlo calculation. A method for estimating the incident direction of radiation.

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