Multi-column compton camera and gamma-ray event data analysis device, method, and program

Abstract

To provide a Compton camera capable of identifying the incident angle of gamma rays with high angular resolution.SOLUTION: A multi-column Compton camera 100 comprises: a gamma-ray detection unit 20 having a plurality of columnar sensor units 21, each of which includes a plurality of pixel sensors 22 in which scattering detection pixels 23 for detecting Compton scattering of incident gamma rays are arranged in a lattice pattern and are stacked; and a control circuit 11 that supplies operating power to each pixel sensor 22 in each columnar sensor unit 21 and reads signals from each pixel sensors 22. The plurality of columnar sensor units 21 is arranged in the same direction with ends in the longitudinal direction aligned and spaced apart, and the end surfaces in the longitudinal direction of the plurality of columnar sensor units 21 are flush and exposed to the outside.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to gamma ray imaging technology, and in particular to a Compton camera suitable for detecting gamma ray events of around 500 KeV using Compton scattering, and a technology for identifying the angle of incidence of the gamma ray being observed by analyzing gamma ray event data recorded by such a Compton camera. [Background technology]

[0002] Gamma rays (hereinafter, mainly referring to gamma rays with an energy of around 500 keV) are a type of radiation that is widely used in medical settings, nuclear power plants, accelerators, space observation, etc., along with other types of radiation (X-rays, proton beams, electron beams, heavy particle beams, muons, etc.). In addition to detecting gamma rays, technology for imaging with gamma rays has also been steadily developed. The importance of gamma ray imaging is recognized in a wide range of applications, including PET, which visualizes treatment sites inside the human body by injecting specific radioisotopes into the body, environmental radiation measurements that measure the distribution of radioisotopes that spread in the Fukushima nuclear accident, exploration of rare metals that contain radioisotopes, and elucidation of high-energy phenomena in space by observing gamma rays coming from space.

[0003] Unlike X-rays and visible light, gamma rays cannot be focused and imaged with mirrors, and are difficult to detect, so imaging them has not been easy. Until now, there have been two main methods. One is to place a pinhole or coded mask made of lead or other material between the detector and the radiation source, and reproduce the gamma-ray image by analyzing the shadow pattern. Since gamma rays are not easily absorbed by substances, heavy elements such as lead are required as the mask material. In this case, the detector only needs to have two-dimensional position sensitivity. The resolution is determined by the fineness of the mask pattern, the distance between the mask and the detector, and the position resolution of the detector to detect gamma rays. The workability of lead is about 1 mm, the distance between the detector and the mask is about 1 m, and the detector often uses a solid scintillator to absorb gamma rays, and its position determination accuracy is about 1 mm, so the resolution is 1 / 1000rad = 0.06 degrees. However, radiation other than the target gamma rays is also detected and becomes noise, so the weak target gamma rays are buried in the noise, making it difficult to use in PET, rare element exploration, and space observation.

[0004] The other is a Compton camera, which performs Compton scattering of gamma rays multiple times in a detector that can record the radiation reaction position in three dimensions, and performs imaging from the reaction pattern. Gamma rays are not absorbed by light elements such as silicon, but are mainly scattered, and reactions (energy and position) are recorded at multiple scattering points. Using these reaction patterns and the equations of Compton mechanics, the direction of arrival of gamma rays can be restricted to a ring shape. If the direction of travel of the electrons generated at the scattering point is known, it can be restricted to a part of the ring (arc). When these rings or arcs are drawn for multiple gamma rays, the intersection of them can be identified as the gamma ray source. Devices using detector materials such as semiconductor sensors, solid scintillators, gases, and liquids have been developed and are used all over the world (see, for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] T. Orita et al., "Double-Photon Emission Imaging With High-Resolution Si / CdTe Compton Cameras," in IEEE Transactions on Nuclear Science, vol. 68, no. 8, pp. 2279-2285, Aug. 2021 Summary of the Invention [Problem to be solved by the invention]

[0006] In Compton mechanics, the resolution is theoretically limited to a few degrees due to the uncertainty of the momentum of electrons in atoms. However, Compton cameras have the advantage of being able to image weak gamma-ray sources because they can distinguish gamma rays from sources other than the target object using the three-dimensional signal pattern in the detector, and are used in PET and space observations. However, a resolution of a few degrees is much lower than the resolution of the human eye, which is 0.05 degrees.

[0007] One way to increase the angular resolution of a Compton camera is to place a coded mask in front of the camera. With a Compton camera, multiple reaction positions can be obtained for one gamma ray, but the shadow of the mask reflects the initial scattering position, and the initial scattering position can be determined using Compton mechanics. Thus, the pattern of the shadow of the coded mask can be obtained, resulting in a resolution of less than 0.1 degrees, while also enabling imaging of weak gamma rays. However, this method has the disadvantage that the resolution is limited due to the workability of the lead mask, and the detector is also large. In addition, in space observations, when the camera is installed on a satellite, the lead reacts with the cosmic rays that fly in and becomes a radiation source, which becomes a noise source and makes it difficult to detect dark celestial bodies.

[0008] In view of the above problems, the present invention aims to provide a Compton camera that can identify the angle of incidence of gamma rays with high angular resolution, and a data analysis technique that analyzes gamma ray events recorded by the Compton camera to identify the angle of incidence of the gamma rays of the object being observed with high angular resolution. [Means for solving the problem]

[0009] According to one aspect of the present invention, there is provided a multi-column Compton camera comprising: a gamma ray detection unit having a plurality of columnar sensor units each including a stack of pixel sensors, each of which has scattering detection pixels arranged in a grid pattern to detect Compton scattering of incident gamma rays; and a control circuit which supplies operating power to each of the pixel sensors of each of the columnar sensor units and reads out signals from each of the pixel sensors, wherein the plurality of columnar sensor units are arranged in the same direction with their longitudinal ends aligned and spaced apart, and the longitudinal end faces of the plurality of columnar sensor units are aligned flush and open to the outside.

[0010] According to another aspect of the present invention, there is provided a gamma ray event data analysis device for analyzing gamma ray event data recorded at a predetermined sampling period by the above-mentioned multi-column Compton camera to identify the angle of incidence of a gamma ray to be observed, the device including a scattering detection pixel identification unit for identifying the scattering detection pixel that detected the first Compton scattering for each gamma ray event, a scattering detection number counting unit for counting the number of times the first Compton scattering was detected for each scattering detection pixel, and a scattering detection number counting unit for counting the number of times the first Compton scattering was detected for each layer in the columnar sensor unit. The present invention provides a gamma ray event data analysis device including a scattering detection count aggregation unit that aggregates data into a one-dimensional distribution in each of the X and Y directions, a detection pattern maximum likelihood determination unit that identifies, for each of the X and Y directions of the pixel sensor, a stratified one-dimensional distribution set that is most similar to the stratified one-dimensional distribution set obtained by the aggregation, among the stratified one-dimensional distribution sets that are prepared in advance for each gamma ray incidence angle, and a gamma ray incidence angle determination unit that determines the incidence angle of the gamma ray of the observation target from the gamma ray incidence angles related to the stratified one-dimensional distributions identified for each of the X and Y directions of the pixel sensor, and a corresponding gamma ray event data analysis method and program. Effect of the Invention

[0011] According to the present invention, there is provided a multi-column Compton camera capable of identifying the angle of incidence of gamma rays with high angular resolution. Also, the angle of incidence of gamma rays incident on the gamma ray detection unit of the multi-column Compton camera can be identified with high resolution from gamma ray event data recorded by the multi-column Compton camera. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram of a multi-column Compton camera according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram of a columnar sensor unit according to an example. [Diagram 3] FIG. 2 is a cross-sectional view of a pixel sensor according to an example. [Figure 4] FIG. 13 illustrates signal readout from a scatter detection pixel. [Diagram 5] 1 is a diagram for explaining the tendency of the initial Compton scattering position of a gamma ray incident on a gamma ray detection unit. FIG. [Figure 6] 1 is a schematic diagram of a gamma ray event data analysis device according to an embodiment of the present invention; [Figure 7] 1 is a flowchart showing a procedure for analyzing gamma ray event data. [Figure 8] 13 is a graph showing an example of a one-dimensional distribution of the number of first Compton scattering detections for each layer of a columnar sensor unit. [Figure 9] 1 is a graph showing the angular resolution of a multi-column Compton camera. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings as appropriate. However, more detailed explanations than necessary may be omitted. For example, detailed explanations of already well-known matters or duplicate explanations of substantially identical configurations may be omitted. This is to avoid the following explanation becoming unnecessarily redundant and to facilitate understanding by those skilled in the art. Note that the inventor provides the accompanying drawings and the following explanation so that those skilled in the art can fully understand the present invention, and does not intend to limit the subject matter described in the claims by them. In addition, the dimensions of each member depicted in the drawings, detailed shapes of details, etc. may differ from the actual ones.

[0014] <Configuration of the multi-column Compton camera> FIG. 1 is a schematic diagram of a multi-column Compton camera according to an embodiment of the present invention. The multi-column Compton camera 100 according to this embodiment includes a device body 10 and a gamma ray detection unit 20 provided thereon. The device body 10 includes a power supply circuit and an external interface (not shown), as well as a control circuit 11 that supplies operating power to the gamma ray detection unit 20 and reads out signals from the gamma ray detection unit 20. The gamma ray detection unit 20 includes a plurality of substantially rectangular columnar sensor units 21. These columnar sensor units 21 are aligned in the longitudinal direction and arranged in an array in the same direction with gaps between them. That is, one end surface in the longitudinal direction of each of the plurality of columnar sensor units 21 contacts the upper surface of the device body 10 and protrudes upward from the device body 10, and the other end surface is aligned to be flush and open to the outside. The surface consisting of the other end surface of each columnar sensor unit 21 and the gaps between the columnar sensor units 21 is a gamma ray incident surface 20a onto which the gamma rays to be observed are incident.

[0015] 2 is a schematic diagram of an example of a columnar sensor unit 21. The columnar sensor unit 21 is configured by stacking several tens to hundreds of pixel sensors (AstroPix) 22 at equal intervals. Scattering detection pixels 23 are arranged in a lattice pattern on the surface of each pixel sensor 22. Each of these scattering detection pixels 23 converts the Compton scattering of gamma rays incident on the gamma ray detection section 20 into an electrical signal and detects it.

[0016] The pixel sensor 22 is made of a high-voltage silicon CMOS (HV-CMOS). HV-CMOS has a feature that it has a high resistivity and can apply a very high substrate voltage. FIG. 3 is a cross-sectional structure diagram of an example of the pixel sensor 22. In the pixel sensor 22, a deep n-well layer is present on a p-type substrate, which forms a pn junction. Each deep n-well layer corresponds to a scattering detection pixel 23. The p-type has a high hole concentration, and the n-type has a high electron concentration, and carriers move toward the lower concentration near the junction surface. Then, the carriers recombine with each other, and a depletion layer with almost no carriers is formed near the junction surface. At this time, acceptor ions remain in the p-type and donor ions remain in the n-type, which creates a potential difference between the two regions, a built-in potential. Although detailed formulas are omitted, the higher the resistivity of the silicon substrate and the higher the reverse voltage (-HV) applied to the silicon substrate, the wider the depletion layer becomes.

[0017] When radiation such as X-rays or gamma rays passes through this depletion layer, it interacts with the silicon nuclei, generating electron-hole pairs. The electrons are collected by the n-well layer formed on the surface of the substrate above the deep n-well layer, and are amplified as a signal by the charge-sensitive amplifier in the CMOS circuit. The greater the reverse voltage applied to the substrate, the wider the depletion layer becomes, so the greater the reverse voltage, the larger the sensitive region. Therefore, high-voltage silicon CMOS, which can apply a high reverse voltage, can ensure a large sensitive region.

[0018] Figure 4 is a diagram explaining how signals are read out from the scattering detection pixel 23. The signal amplified by the amplifier enters a comparator in the CMOS circuit, and the time that the signal value is higher than the reference voltage is counted by a clock to become a Time over Threshold (ToT) digital value, which is then output as digital data together with the pixel ID and time information. Meanwhile, the analog output waveform of the amplifier goes directly outside the chip and passes through a buffer on the board before being output as an analog signal, so that the waveform can be viewed on an oscilloscope. The analog output is only output from one selected pixel, and only a limited number of pixels can be selected, but the digital signal can be read out from all pixels.

[0019] 2, the multiple pixel sensors 22 in the columnar sensor unit 21 are connected in series to the control circuit 11 in the main device 10. In other words, each pixel sensor 22 is not provided with peripheral circuits such as a power supply circuit or a signal readout circuit, and almost the entire surface of the pixel sensor 22 serves as a sensing surface for detecting Compton scattering.

[0020] The control circuit 11 can supply operating power to each pixel sensor 22 and can read out digital signals from any pixel sensor 22 of any sensor unit 21. Specifically, the control circuit 11 reads out digital signals from each scattering detection pixel 23 of all pixel sensors 22 of all columnar sensor units 21 at a predetermined sampling period (for example, 1 ms) and records the signals in a storage device within the device main body 10, an external storage device via an external interface, or cloud storage. In this way, data on gamma ray events captured by the multi-column Compton camera 100 is recorded.

[0021] Due to the structural characteristics of the multi-column Compton camera 100 as described above, the gamma-ray events detected have characteristics. Figures 5(a) and 5(b) are diagrams for explaining the tendency of the first Compton scattering position of the gamma rays incident on the gamma-ray detection unit 20. Both diagrams are side views of one columnar sensor unit 21, and show how a plurality of pixel sensors 22 are stacked. (a) shows a case where the angle of the incident gamma rays with respect to the normal direction of the gamma-ray incident surface 20a of the gamma-ray detection unit 20 is small, and (b) shows a case where the angle of the incident gamma rays with respect to the normal direction of the gamma-ray incident surface 20a of the gamma-ray detection unit 20 is large. The arrows in the diagrams represent the trajectory of the gamma rays incident on the gamma-ray detection unit 20, and the arrowheads represent the position where the first Compton scattering occurred. Note that in Figure 5, the trajectory of the gamma rays after the first Compton scattering is not depicted.

[0022] The gamma ray to be observed is incident on the end face of the columnar sensor unit 21 at the gamma ray incident surface 20a, that is, on either the sensing surface of the pixel sensor 22 on the top layer of the columnar sensor unit 21 or the gaps of the columnar unit sensors 21. Some gamma ray events incident on the sensing surface of the pixel sensor 22 on the top layer pass through it and reach the pixel sensor 22 on the lower layer side where they are first Compton scattered, but the number of events that pass through several pixel sensors 22 on the upper layer side and reach the pixel sensor 22 on the lower layer side is relatively small. Therefore, there are relatively more events that are first Compton scattered in the pixel sensor 22 on the upper layer side than in the lower layer side. On the other hand, for gamma ray events that are incident on the gaps of the columnar sensor unit 21, the gamma ray reaches the pixel sensor 22 on the lower layer side without any hindrance, so that the first Compton scattering occurs evenly in both the pixel sensors 22 on the upper layer side and the pixel sensor 22 on the lower layer side. Moreover, the scattering positions are concentrated at the ends of the pixel sensors 22 due to the relative positional relationship in which the gamma rays are incident on the side surface of the columnar sensor unit 21 from an obliquely upward direction.

[0023] The lower part of Fig. 5(a) and (b) shows a schematic diagram of the count distribution of the first Compton scattering position of the gamma rays incident on the gap of the columnar sensor unit 21. As described above, the first Compton scattering position of the gamma rays incident on the gap of the columnar sensor unit 21 is concentrated on the edge part of the pixel sensor 22. Moreover, the shape of the distribution changes depending on the incidence angle of the gamma rays. That is, as shown in Fig. 5(a), when the angle of the incident gamma rays with respect to the normal direction of the gamma ray incidence surface 20a is small, the first Compton scattering position is concentrated in a part very close to the edge of the pixel sensor 22, but as the angle of the incident gamma rays with respect to the normal direction of the gamma ray incidence surface 20a becomes larger, the distribution of the first Compton scattering position moves from the edge of the pixel sensor 22 to the center. In this way, there is a correlation between the incidence angle of the gamma rays to the gamma ray detection unit 20 and the count distribution of the first Compton scattering position, and based on this correlation, the incidence angle of the gamma rays can be obtained from the count distribution of the first Compton scattering position. For example, if the columnar sensor unit 21 is 50 cm tall and the pixel sensor 22 has a position detection accuracy of about 0.5 mm, then in principle it is possible to identify the gamma ray incidence angle with an angular resolution of about 1 / 1000 rad = 0.06 degrees.

[0024] <Analysis of gamma-ray event data>

[0025] 6 is a schematic diagram of a gamma ray event data analysis device according to an embodiment of the present invention. The gamma ray event data analysis device (hereinafter, sometimes simply referred to as a data analysis device) 30 according to this embodiment is a device that analyzes gamma ray event data (hereinafter, sometimes simply referred to as data) 200 and identifies the incidence angle of a gamma ray of an observation target that is incident on the gamma ray detection unit 20 of the multi-column Compton camera 100 with high angular resolution. The gamma ray event data 200 is a collection of data in which gamma ray events detected by the multi-column Compton camera 100 are recorded at a predetermined sampling period, and is stored in the storage device 40. Each piece of data in each sampling period records an event in which one gamma ray is incident on the gamma ray detection unit 20 and is repeatedly scattered by one or more pixel sensors 22 within the sampling period.

[0026] The data analysis device 30 includes a scattering detection pixel specifying unit 31, a scattering detection counting unit 32, a scattering detection count collecting unit 33, a detection pattern maximum likelihood determining unit 34, and a gamma ray incident angle specifying unit 35. The scattering detection pixel specifying unit 31 specifies the scattering detection pixel 23 that detected the first Compton scattering for each gamma ray event. The scattering detection counting unit 32 counts the number of times the first Compton scattering was detected for each scattering detection pixel 23. The scattering detection count collecting unit 33 collects the counted number of times the first Compton scattering was detected for each layer in the columnar sensor unit 21 into a one-dimensional distribution in each of the X and Y directions of the pixel sensor 22. The detection pattern maximum likelihood determining unit 34 specifies, for each of the X and Y directions of the pixel sensor 22, the one that is most similar to the one-dimensional distribution set by layer obtained by the collection among the one-dimensional distribution sets by layer that are prepared in advance for each gamma ray incident angle. The gamma ray incident angle identifying unit 35 identifies the incident angle of the gamma ray of the observation target incident on the gamma ray detection unit 20 from the gamma ray incident angle related to the layer-by-layer one-dimensional distribution identified for each of the X and Y directions of the pixel sensor 22.

[0027] The above-mentioned components of the data analysis device 30 can be realized as hardware such as ASIC or FPGA, or as software that executes a computer program stored in a storage device (not shown) or a computer program stored and provided in a computer-readable non-transitory recording medium on a processor (not shown) such as a CPU or GPU. It can also be realized by appropriately combining hardware and software. All or part of the components of the gamma ray event data analysis device 30 may be located on a cloud. The analysis results by the gamma ray event data analysis device 30 may be passed to another processing system for further processing, or the results may be displayed to the user through a user interface (not shown). The gamma ray event data analysis device 30 and the storage device 40 may be configured as an integrated computer device.

[0028] Next, the analysis of the gamma ray event data 200 by the data analysis device 30 will be described. FIG. 7 is a flow chart showing the procedure for analyzing the gamma ray event data 200. The data analysis device 30 reads out the gamma ray event data 200 one by one from the storage device 40 (S1). As described above, an event in which one gamma ray is repeatedly scattered by one or more pixel sensors 22 is recorded in each data of each sampling period. The read out data is passed to the scattering detection pixel identifying unit 31, which identifies the scattering detection pixel 23 that detected the first Compton scattering for each gamma ray event (S2). When only one scattering detection pixel 23 that detected scattering is recorded in the event, the scattering detection pixel identifying unit 31 can identify that pixel as the scattering detection pixel 23 that detected the first Compton scattering. On the other hand, if multiple scattering detection pixels 23 that detected scattering are recorded in the event, the scattering detection pixel identification unit 31 can perform Compton reconstruction to identify the scattering detection pixel 23 that detected the first Compton scattering.

[0029] When the scattering detection pixel 23 that detected the first Compton scattering is identified, the scattering detection counting unit 32 counts once the number of detections by that scattering detection pixel 23 (S3). The above steps S1 to S3 are repeated until the end of the gamma ray event data 200 (NO in S4).

[0030] When the count of the number of first Compton scattering events for each scattering detection pixel 23 is completed (YES in S4), the scattering detection count aggregation unit 33 aggregates the counted number of first Compton scattering detection events for each layer in the columnar sensor unit 21 into a one-dimensional distribution in each of the X and Y directions of the pixel sensor 22 (S5).

[0031] FIG. 8 is a graph showing an example of one-dimensional distribution of the number of first Compton scattering detections for each layer of the columnar sensor unit 21. These graphs were obtained by simulation using MEGALib, a tool for Monte Carlo simulation of the interaction between gamma rays and a detector. Specifically, the columnar sensor units 21, each having 1 cm×1 cm pixel sensors 23 each having 20 vertical×20 horizontal scattering detection pixels 23 of 0.5 mm×0.5 mm size, were stacked in 100 layers at a height of 60 cm, and arranged in a 10×10 array at 1 cm intervals to form the gamma ray detection unit 20. A simulation was performed in which 600 keV gamma rays were incident on the gamma ray incident surface 20a of the pixel sensors at an angle of 0.1 degrees with respect to the normal. The horizontal axis of each graph represents the position 6 mm or more in the X or Y direction on the sensing surface of the pixel sensor 22, and the vertical axis represents the total number of first Compton scatterings. Moreover, the numbers enclosed in squares in each graph represent layer numbers, with the topmost layer of the columnar sensor unit 21 being "100" and the bottommost layer being "1."

[0032] As can be seen from these graphs, a lot of initial Compton scattering is detected near the edge (10 mm position) of the sensing surface of the pixel sensor 22, and the lower the layer, the more prominent this characteristic becomes. This is because there are few gamma rays that pass through several pixel sensors 22 on the upper layer side to reach the pixel sensor 22 on the lower layer side, so there is less scattering detected in the center of the sensing surface of the pixel sensor 22 on the lower layer side than in the upper layer side, but even at the edge of the pixel sensor 22 on the lower layer side, gamma rays pass through the gaps between the columnar sensor units 21 and reach the sensor directly without any obstacles, so there is relatively more scattering detected at the edge. Furthermore, the above characteristic becomes more prominent when scattering detected by multiple columnar sensor units 21 is aggregated.

[0033] Returning to FIG. 7, when the one-dimensional distribution aggregation of the counts of the scattering detection counts is completed, the detection pattern maximum likelihood determination unit 34 specifies the one that is most similar to the one-dimensional distribution set by layer obtained by aggregation among the one-dimensional distribution sets by gamma ray incidence angle (referred to as the "reference detection pattern") for each XY direction of the pixel sensor 22. The reference detection pattern is obtained by acquiring the one-dimensional distribution of the first Compton scattering detection count of each layer of the columnar sensor unit 21 while changing the gamma ray incidence angle to the gamma ray detection unit 20 of the multi-column Compton camera 100 in advance by actual measurement or simulation in which gamma rays are actually incident at each incidence angle, and patterning and summarizing the one-dimensional distribution. For example, the one-dimensional distribution of the Compton scattering detection count of each layer shown in each graph of FIG. 8 is patterned to generate the reference detection pattern when the gamma ray incidence angle is 0.1 degrees. The reference detection pattern is stored in the storage device in the data analysis device 30, the storage device 40 that saves the gamma ray event data 200, a cloud storage, or the like.

[0034] The maximum likelihood determination between the reference detection pattern and the layer-by-layer one-dimensional distribution set obtained by aggregation can be performed, for example, by fitting a linear combination of the layer-by-layer one-dimensional distributions. Alternatively, machine learning may be performed in advance using the one-dimensional distribution of the first Compton scattering detection count for each layer obtained by simulation as input data and the gamma ray incidence angle at that time as teacher data, and the one-dimensional distribution of the first Compton scattering detection count for each layer obtained by aggregation is input to the trained model to estimate the gamma ray incidence angle.

[0035] When the closest reference detection pattern is identified for each of the X and Y directions of the pixel sensor 22, the gamma ray incident angle identifying unit 35 identifies the incident angle of the gamma ray of the observation target for each of the X and Y directions of the pixel sensor 22 from the gamma ray incident angle related to the identified layer-by-layer one-dimensional distribution (S7). This identifies the angle at which the gamma ray is incident on the gamma ray incident surface 20a.

[0036] Effect FIG. 9 is a graph showing the angular resolution of the multi-column Compton camera 100. The graph shows event data obtained when gamma rays are incident on the gamma ray detection unit 20 of the multi-column Compton camera 100 configured in the above-mentioned simulation from two gamma ray sources separated from the center of the field of view (normal line of the gamma ray incident surface 20a) by 0.05 degrees in opposite directions, and the data is analyzed to determine the gamma ray incident angle. The vertical axis represents the coefficient related to fitting with the reference detection pattern, and the horizontal axis represents the gamma ray incident angle determined from the gamma ray event data. As can be seen from the graph, two gamma ray incident angles, -0.05 degrees and 0.05 degrees, are correctly determined by data analysis. In other words, it shows that incident angles of 0.1 degrees or less can be distinguished. Thus, according to the multi-column Compton camera 100 of this embodiment, the incident angle of gamma rays incident on the gamma ray detection unit 20 can be determined with a high angular resolution of 0.1 degrees or less.

[0037] <<Variations>> The columnar sensor units 21 do not need to be regularly arranged in a lattice pattern, but may be arranged in a one-dimensional array. Also, the gaps between the columnar sensor units 21 do not need to be all the same.

[0038] In the gamma ray detection section 20, a lead side wall may be provided so as to surround the arranged columnar sensor units 21. This reduces the incidence of gamma rays from the side of the columnar sensor unit 21, thereby reducing noise events and enabling the angle of incidence to be determined with higher accuracy, i.e., even for weaker gamma ray sources.

[0039] The reference detection pattern does not need to be composed of a one-dimensional distribution of the initial Compton scattering detection counts for all layers of the columnar sensor unit 21. For example, the one-dimensional distribution related to the pixel sensors 22 in the top layer may be excluded, or the reference detection pattern may be composed of only the one-dimensional distribution related to the pixel sensors 22 in a representative layer. In this case, it is advantageous to include many one-dimensional distributions related to the pixel sensors 22 in the lower layers in the reference pattern. Note that care must be taken because the angle determination accuracy decreases if the data constituting the reference detection pattern is thinned out too much.

[0040] The size of the scattering detection pixel 23 may be smaller than 0.5 mm, but making the pixels too fine increases the number of pixels arranged in the pixel sensor 22, which in turn increases power consumption and causes heat generation problems. The columnar sensor unit 21 requires stacking a large number of pixel sensors 21, and for this reason it is desirable to suppress heat generation in the pixel sensors 22. In the above example, the size of the scattering detection pixel 23 is 0.5 mm x 0.5 mm, which is larger than that of a typical image sensor, but even with this size, the gamma ray incidence angle can be identified with a sufficiently high angular resolution.

[0041] As described above, the embodiment has been described as an example of the technology of the present invention. For this purpose, the attached drawings and detailed description have been provided. Therefore, among the components described in the attached drawings and detailed description, not only components essential for solving the problem but also components that are not essential for solving the problem in order to illustrate the above technology may be included. Therefore, just because those non-essential components are described in the attached drawings or detailed description, it should not be immediately recognized that those non-essential components are essential. In addition, since the above-mentioned embodiment is intended to illustrate the technology of the present invention, various modifications, substitutions, additions, omissions, etc. may be made within the scope of the claims or their equivalents. [Industrial Applicability]

[0042] INDUSTRIAL APPLICABILITY The multi-column Compton camera according to the present invention is capable of identifying the angle of incidence of gamma rays with high angular resolution, and is therefore useful for medical PET and the observation of gamma rays coming from space. [Explanation of symbols]

[0043] 100 Multi-column Compton Camera 11 Control circuit 20 Gamma ray detector 21 Pillar sensor unit 22 pixel sensor 23 Scattering Detection Pixels 30 Gamma ray event data analysis device 31 Scattering detection pixel identification unit 32 Scattering detection count unit 33 Scattering detection count collection unit 34 Maximum likelihood detection pattern determination unit 35 Gamma ray incident angle determination unit 200 Gamma Ray Event Data

Claims

1. a gamma ray detection unit having a plurality of columnar sensor units each including a plurality of stacked pixel sensors each having a lattice-like arrangement of scattering detection pixels for detecting Compton scattering of incident gamma rays; a control circuit that supplies operating power to each of the pixel sensors of each of the columnar sensor units and reads out signals from each of the pixel sensors; The plurality of columnar sensor units are arranged in the same direction with their longitudinal ends aligned and spaced apart from one another, and the longitudinal end faces of the plurality of columnar sensor units are aligned flush and open to the outside. A multi-column Compton camera characterized by:

2. The control circuit is provided outside the gamma ray detection unit, The multi-column Compton camera according to claim 1 , wherein the plurality of pixel sensors in each of the columnar sensor units are connected in series to the control circuit.

3. The multi-column Compton camera of claim 2 , wherein the pixel sensor is a high voltage silicon CMOS pixel sensor.

4. The multi-column Compton camera of claim 1 , wherein the pixel sensor is a high voltage silicon CMOS pixel sensor.

5. 5. A gamma ray event data analysis device for identifying an incident angle of a gamma ray of an observation target by analyzing gamma ray event data recorded at a predetermined sampling period by a multi-column Compton camera according to claim 1, a scatter detection pixel identifying unit for identifying the scatter detection pixel that detected the first Compton scatter for each gamma ray event; a scattering detection number counting unit that counts the number of times the first Compton scattering is detected for each scattering detection pixel; a scattering detection count aggregation unit that aggregates the counted number of detections of the first Compton scattering for each layer in the columnar sensor unit into a one-dimensional distribution in each of the X and Y directions of the pixel sensor; a detection pattern maximum likelihood determination unit that identifies a detection pattern that is most similar to the stratified one-dimensional distribution set obtained by the aggregation among stratified one-dimensional distribution sets that are prepared in advance for each gamma ray incidence angle for each XY direction of the pixel sensor; a gamma ray incidence angle determination unit that determines an incidence angle of a gamma ray of the observation target from an incidence angle of a gamma ray related to a layer-by-layer one-dimensional distribution determined for each of the X and Y directions of the pixel sensor; A gamma ray event data analysis device comprising:

6. A method for analyzing gamma ray event data, which is for identifying an incident angle of a gamma ray of an observation target by analyzing gamma ray event data recorded at a predetermined sampling period by the multi-column Compton camera according to any one of claims 1 to 4, comprising the steps of: a scatter detection pixel identifying unit identifying the scatter detection pixel that detected the first Compton scatter for each gamma ray event; a scattering detection number counting unit counting the number of times the first Compton scattering is detected for each scattering detection pixel; a scattering detection count aggregation unit aggregating the counted number of detections of the first Compton scattering into a one-dimensional distribution in each of the X and Y directions of the pixel sensor for each layer in the columnar sensor unit; A detection pattern maximum likelihood determination unit specifies a stratified one-dimensional distribution set that is most similar to the stratified one-dimensional distribution set obtained by the aggregation, among stratified one-dimensional distribution sets that are prepared in advance for each gamma ray incidence angle, for each X and Y direction of the pixel sensor; A step of determining an incidence angle of a gamma ray of the observation target from an incidence angle of a gamma ray related to a layer-by-layer one-dimensional distribution determined for each of the X and Y directions of the pixel sensor by a gamma ray incidence angle determination unit. A method for analyzing gamma ray event data.

7. 5. A gamma ray event data analysis program for causing a computer to analyze gamma ray event data recorded at a predetermined sampling period by the multi-column Compton camera according to claim 1 to 4 and to specify an incident angle of a gamma ray to be observed, a scatter detection pixel identifying means for identifying the scatter detection pixel that detected the first Compton scatter for each gamma ray event; a scattering detection number counting means for counting the number of times the first Compton scattering was detected for each of the scattering detection pixels; a scattering detection count aggregation means for aggregating the counted number of detections of the first Compton scattering for each layer in the columnar sensor unit into a one-dimensional distribution in each of the X and Y directions of the pixel sensor; a detection pattern maximum likelihood determination means for determining, for each of the X and Y directions of the pixel sensor, a stratified one-dimensional distribution set that is most similar to the stratified one-dimensional distribution set obtained by the aggregation, among stratified one-dimensional distribution sets that are prepared in advance for each angle of incidence of gamma rays; A computer is caused to function as a gamma ray incidence angle specifying means for specifying an incidence angle of a gamma ray of the observation target from an incidence angle of a gamma ray related to a layer-by-layer one-dimensional distribution specified for each of the X and Y directions of the pixel sensor. A gamma ray event data analysis program comprising:

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