Electron annihilation characteristic measuring device

Abstract

To provide a device measuring a positron annihilation characteristic, with a technique for making the device compact and light-weight while ensuring measurement accuracy.SOLUTION: A positron annihilation characteristic measurement device comprises: a positron beam source that is disposed close to or tightly contact with a surface of a measured body; a positron detector that detects a positron not incident upon the measured body, of the positron generated by the positron beam source; a first radiation detector that is disposed at a first position which is set relative to the positron beam source, and detects a first radiation generated when the positron is generated by the positron source; and a second radiation detector that is disposed in a second position which is set relative to the positron beam source, and detects a second radiation generated when the positron generated by the positron beam source is annihilated. The positron detector is shaded by a thin film deposited with aluminum and / or a scintillator deposited with the aluminum evaporated.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] This specification relates to an apparatus for measuring the annihilation characteristics of positrons that are incident on a measurement object and annihilate within the measurement object.

Background Art

[0002] 22 When positrons emitted from a positron source (positron-emitting nuclide) such as Na or 68 Ge are irradiated onto a substance, the irradiated positrons fly through the substance and then combine with electrons in the substance to undergo pair annihilation. If there are defects (vacancies or dislocations) in the substance, the positrons are captured by the defects. Therefore, the time until the positrons annihilate becomes longer compared to the case where there are no defects in the substance. Also, when there are defects in the substance, the distribution of the energy spectrum of the radiation (γ-rays) emitted when the positrons annihilate is different from the distribution of the energy spectrum of the radiation in the case where there are no defects in the substance. Therefore, if the time until the positrons irradiated in the substance annihilate and the energy spectrum distribution of the radiation when the positrons irradiated in the substance annihilate are known, the material characteristics of the substance related to the defects can be estimated. In recent years, research has been conducted on the evaluation of shot peening processing and the observation of the fatigue state of members used in nuclear reactors and members used in infrastructure such as bridges by measuring the annihilation characteristics of positrons.

[0003] For example, Patent Document 1 discloses an apparatus for measuring the annihilation characteristics of positrons. In the apparatus of Patent Document 1, a positron source is sandwiched between two test pieces cut out from a measurement object (measurement substance), and the radiation (γ-rays) when the positrons annihilate is measured.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] To observe the fatigue state of components used in infrastructure such as bridges, there is a need for on-site positron lifetime measurement technology. This requires miniaturizing and lightweighting the equipment while ensuring measurement accuracy.

[0006] In positron annihilation characteristic measurement devices, a larger radiation detector allows for the detection of more radiation emitted during positron creation and annihilation, thus improving the radiation detection efficiency (counting rate). Conversely, a smaller radiation detector reduces its internal volume, suppressing variations in detection time for each incoming radiation source and improving temporal resolution.

[0007] Furthermore, conventional methods involved placing the positron detector inside a dark box to shield it from external light. However, dark boxes require a large amount of space, making it difficult to use them for on-site measurement equipment.

[0008] This specification provides techniques for miniaturizing and reducing the weight of a device for measuring the annihilation characteristics of positrons while ensuring measurement accuracy. [Means for solving the problem]

[0009] A first positron annihilation characteristic measuring device disclosed herein measures the annihilation characteristics of positrons that are incident on an object to be measured and annihilated within the object to be measured. The first positron annihilation characteristic measuring device comprises: a positron source positioned in close proximity to or in close contact with the surface of the object to be measured; a positron detector for detecting positrons generated by the positron source that were not incident on the object to be measured; a first radiation detector positioned at a first position set with respect to the positron source for detecting a first radiation generated when positrons are produced by the positron source; and a second radiation detector positioned at a second position set with respect to the positron source for detecting a second radiation generated when positrons generated by the positron source annihilate. The positron detector is shielded by a thin film coated with aluminum and / or a scintillator coated with aluminum.

[0010] In the first positron annihilation characteristic measurement apparatus described above, the positron detector is shielded from light by aluminum. That is, the positron detector itself possesses light-shielding properties. By covering the positron detector with a thin film coated with aluminum, suitable light-shielding properties can be provided for thin films with high mechanical strength. Therefore, there is no need to provide a dark box or the like to shield the positron detector from light, and the apparatus can be made smaller and lighter.

[0011] A second positron annihilation characteristic measuring device disclosed herein measures the annihilation characteristics of positrons that are incident on an object to be measured and annihilated within the object to be measured. The second positron annihilation characteristic measuring device comprises: a positron source positioned in close proximity to or in close contact with the surface of the object to be measured; a positron detector for detecting positrons generated by the positron source that were not incident on the object to be measured; a first radiation detector positioned at a first position set with respect to the positron source for detecting first radiation generated when positrons are generated by the positron source; and a second radiation detector positioned at a second position set on the opposite side of the positron source from the first position for detecting second radiation generated when positrons generated by the positron source annihilate. The first central axis of the first radiation detector and the second central axis of the second radiation detector intersect, and the positron source is positioned closer to the first and second radiation detectors than the intersection of the first and second central axes.

[0012] In the second positron annihilation characteristics measurement device described above, the first radiation detector is positioned at a first position, and the second radiation detector is positioned at a second position opposite to the first position relative to the positron source. That is, the first and second radiation detectors are positioned opposite each other across the positron source. Furthermore, the first central axis of the first radiation detector and the second central axis of the second radiation detector intersect. The positron source is positioned closer to the first and second radiation detectors than the intersection of the first and second central axes. Thus, in this positron annihilation characteristics measurement device, since each radiation detector is positioned relatively close to the positron source with the positron source in between, the radiation generated when positrons are created and annihilated is easily incident on each radiation detector. Therefore, even if the size of each radiation detector is reduced compared to conventional devices, the reduction in radiation detection efficiency can be suppressed. [Brief explanation of the drawing]

[0013] [Figure 1] A perspective view showing the positron annihilation characteristic measurement apparatus according to Example 1. [Figure 2] Exploded cross-sectional view of the positron detector and the positron source according to Example 1. [Figure 3] View of the internal configuration of the positron annihilation characteristic measurement device according to Example 1 as seen from the measurement surface side. [Figure 4] View of the internal configuration of the positron annihilation characteristic measurement device according to Example 1 as seen from above. [Figure 5] Block diagram showing the configuration of the arithmetic unit. [Figure 6] Diagram for explaining the process of removing noise based on the detection result of the positron detector. [Figure 7] Diagram for explaining the process of removing noise based on the detection result of the positron detector. [Figure 8] Diagram showing various geometric arrangements of the positron source and the γ-ray detector. [Figure 9] Diagram showing the counting rate of γ-rays in each geometric arrangement. [Figure 10] Graph showing the relationship between the distance between the γ-ray detector and the positron source and the counting rate. [Figure 11] Diagram showing various geometric arrangements of the positron source and the γ-ray detector. [Figure 12] Diagram showing the values of the time resolution in each geometric arrangement. [Figure 13] Graph showing the average positron lifetime calculated for various light-shielding members. [Figure 14] Graph showing the relative intensity of γ-rays calculated for various light-shielding members. [Figure 15] Diagram observing the light-shielding property of the Kapton film vapor-deposited with aluminum. [Figure 16] View of the internal configuration of the positron annihilation characteristic measurement device according to Example 2 as seen from above.

Mode for Carrying Out the Invention

[0014] The main features of the embodiments described below are listed below. Note that the technical elements described below are independent technical elements that exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing.

[0015] In one embodiment of this technology, the aluminum may be provided in a single layer or multiple layers, and the total thickness of the aluminum may be 500 nm or less.

[0016] In this configuration, the thickness of the aluminum (i.e., the light-shielding material) is relatively thin, which reduces the annihilation of positrons within the aluminum during measurement (i.e., the generation of noise components). Furthermore, by layering aluminum, for example by depositing aluminum on both sides of a thin film, the light-shielding properties can be improved, and the total thickness of the aluminum can be reduced.

[0017] In one embodiment of this technology, the second position may be set on the opposite side from the first position with respect to the positron source. The first central axis of the first radiation detector and the second central axis of the second radiation detector may intersect. The positron source may be positioned closer to the first and second radiation detectors than the intersection of the first and second central axes.

[0018] In this configuration, each radiation detector is positioned relatively close to the positron source, with the source in between. Therefore, radiation generated when positrons are created and annihilated easily enters each radiation detector. Consequently, even if the size of each radiation detector is reduced compared to conventional designs, the reduction in radiation detection efficiency can be suppressed.

[0019] In one embodiment of this technology, the first radiation detector may include a first scintillator that emits scintillation light upon incidence of the first radiation, and a first photosensor that detects the scintillation light emitted from the first scintillator. The second radiation detector may include a second scintillator that emits scintillation light upon incidence of the second radiation, and a second photosensor that detects the scintillation light emitted from the second scintillator. The first scintillator may be located in the first position, and the second scintillator may be located in the second position.

[0020] With this configuration, each scintillator that emits scintillation light upon radiation incidence is positioned close to the positron source, allowing for more efficient radiation detection. By detecting the scintillation light emitted by each scintillator with each photosensor, the generation and annihilation times of positrons can be calculated.

[0021] In one embodiment of this technology, the positron detector may be positioned on the opposite side of the positron source from the object being measured. The first position and the second position may be opposite each other via the positron detector.

[0022] With this configuration, the positron detector is placed between the first and second radiation detectors, allowing for a smaller overall size of the device.

[0023] In one embodiment of this technology, the positron detector may have a thin plate shape.

[0024] In this configuration, the positron detector is relatively thin. Therefore, even if the positron detector is offset from each radiation detector, the distance between the positron source and each radiation detector hardly increases. Consequently, each radiation detector can be placed closer together.

[0025] In one embodiment of this technology, a shielding member may be provided to shield the space between the first radiation detector and the second radiation detector.

[0026] This configuration suppresses the occurrence of radiation scattered within one radiation detector entering the other radiation detector.

[0027] In one embodiment of this technology, an annihilation characteristic calculation device may be further provided, which calculates the annihilation characteristics of positrons in the body being measured based on the detection results of the first radiation detector and the second radiation detector and the detection results of the positron detector.

[0028] In this configuration, the annihilation characteristics of positrons can be calculated.

[0029] In one embodiment of this technology, the positron annihilation characteristic measurement device may be portable.

[0030] This technology allows for miniaturization and weight reduction while ensuring measurement accuracy, making it useful for portable positron annihilation characteristic measurement devices.

[0031] (Example 1) The following describes the positron annihilation characteristic measuring device 10 according to Example 1. The positron annihilation characteristic measuring device 10 (hereinafter also simply referred to as device 10) is a device that detects gamma rays (1.27 MeV) generated when a positron is produced and gamma rays (511 keV) generated when a positron annihilates, measures the positron lifetime in the object being measured from the time difference, and evaluates the internal state of the object being measured from the measurement result. In particular, the device 10 of this embodiment is a portable device that measures the positron lifetime on-site by pressing the measuring surface 11a against the object being measured (substance being measured) without cutting out the object being measured. As shown in Figures 1 to 5, the device 10 has a main body 11, a positron detector 12, a first gamma-ray detector 14, a second gamma-ray detector 16, a positron source 44, and a computing device 50.

[0032] Although not shown in the diagram, the main body 11 houses a positron detector 12, a first gamma-ray detector 14, a second gamma-ray detector 16, and a positron source 44. A measurement window 11b is formed on the measurement surface 11a of the main body 11. The measurement of the positron lifetime in the body under test by the device 10 is performed with the measurement window 11b of the device 10 in close proximity to the body under test.

[0033] As shown in Figure 2, the positron detector 12 comprises a photomultiplier tube 41 and a scintillator 42 for positron detection. The light-receiving surface 41a of the photomultiplier tube 41 is connected to the scintillator 42. The positron detector 12 has a cylindrical shape. The positron detector 12 is covered with a Kapton® film 60. More specifically, the Kapton film 60 covers the outer surface of the scintillator 42. An aluminum film 62 is provided on the inner surface of the Kapton film 60. The aluminum film 62 is provided by depositing aluminum onto the Kapton film 60. The Kapton film 60 and the aluminum film 62 are provided to prevent external light from entering the scintillator 42. The thickness of the Kapton film 60 is not particularly limited, but can be, for example, 5 μm. The thickness of the aluminum film 62 is also not particularly limited, but can be, for example, 500 nm or less.

[0034] In this embodiment, the scintillator 42 is a plastic scintillator. The scintillator 42 is not limited to the above and may be made of other known materials. The scintillator 42 emits scintillation light when positrons are incident on it. The emitted scintillation light enters the light-receiving surface 41a of the photomultiplier tube 41. Since the scintillator 42 is covered with an aluminum film 62, even if the scintillation light is emitted in a direction different from the light-receiving surface 41a, it is reflected by the aluminum film 62 and guided to the light-receiving surface 41a. The photomultiplier tube 41 converts the scintillation light into an electrical signal. The electrical signal from the photomultiplier tube 41 is input to the computing unit 50.

[0035] As shown in Figure 2, the positron source 44 is positioned between two Kapton films 46. That is, the positron source 44 is sandwiched between the Kapton films 46. The positron source 44, sandwiched between the Kapton films 46, is positioned facing the scintillator 42. The positron detector 12 is positioned so that the positron source 44 faces the measurement window 11b of the main body 11 shown in Figure 1. That is, the positron detector 12 is positioned on the opposite side of the positron source 44 from the object to be measured. The Kapton film 60 and the aluminum film 62 block external light from entering the scintillator 42 through the measurement window 11b. Positrons emitted from the positron source 44 are incident on either of the Kapton films 46 positioned above or below the positron source 44. When positrons are incident on the upper Kapton film 46, they pass through the Kapton film 46 and the measurement window 11b and are incident on the object to be measured. On the other hand, when a positron passes through the lower Kapton film 46 and enters the scintillator 42, scintillation light is emitted from the scintillator 42, and the positron is annihilated inside or outside the scintillator 42 (i.e., outside the object being measured). The scintillation light from the scintillator 42 enters the photomultiplier tube 41. As a result, an electrical signal is output from the photomultiplier tube 41 to the computing unit 50. The positron source 44 may be positioned between the Kapton film 60 and the scintillator 42. In other words, the positron source 44 may be placed directly on the upper surface of the scintillator 42.

[0036] Here, the positron source 44 is, 22 Positron sources (positron-emitting nuclides) such as sodium can be used. Furthermore, positron source 44 is a weak positron source such that no other positrons are generated between the time one positron is generated and the time it annihilates. This prevents situations where multiple positrons exist simultaneously, making it impossible to determine the generation and annihilation times of positrons.

[0037] The first gamma-ray detector 14 detects gamma rays (for example) that are generated when positrons are produced. 22In the case of Na, it detects 1.27 MeV. As shown in Figures 3 and 4, the first gamma-ray detector 14 has a scintillator 14a that emits scintillation light upon the incidence of gamma rays, and a photomultiplier tube 14b that converts the scintillation light into an electrical signal. The scintillator 14a has a roughly truncated square pyramidal shape. The photomultiplier tube 14b has a light-receiving surface (not shown) connected to the scintillator 14a. The first gamma-ray detector 14 is connected to the computing unit 50. When the first gamma-ray detector 14 detects gamma rays generated when positrons are produced, it outputs a pulsed electrical signal to the computing unit 50. The first gamma-ray detector 14 is an example of a "first radiation detector".

[0038] The second gamma-ray detector 16 detects gamma rays (511 keV) generated when a positron annihilates. The second gamma-ray detector 16 has a scintillator 16a that emits scintillation light upon the incidence of gamma rays, and a photomultiplier tube 16b that converts the scintillation light into an electrical signal. The scintillator 16a has the same shape as the scintillator 14a (i.e., a roughly truncated square pyramidal shape). The photomultiplier tube 16b has its light-receiving surface (not shown) connected to the scintillator 16a. The second gamma-ray detector 16 is connected to the computing unit 50. When the second gamma-ray detector 16 detects gamma rays generated when a positron annihilates, it outputs a pulsed electrical signal to the computing unit 50. The second gamma-ray detector 16 is an example of a "second radiation detector".

[0039] As shown in Figures 3 and 4, the first gamma-ray detector 14 and the second gamma-ray detector 16 are positioned so as to sandwich the positron source 44 and the positron detector 12. More specifically, the positron source 44 and the positron detector 12 are positioned between scintillator 14a and scintillator 16a. As shown in Figure 4, the sides of the first gamma-ray detector 14 and the second gamma-ray detector 16 are positioned approximately parallel to the measurement surface 11a of the main body 11. As shown in Figure 3, when viewed from the measurement surface 11a side, the central axis X1 of the first gamma-ray detector 14 and the central axis X2 of the second gamma-ray detector 16 are positioned so as to intersect. The central axis X3 of the positron detector 12 is positioned approximately perpendicular to the measurement surface 11a of the main body 11. As shown in Figure 3, the positron source 44 is positioned closer to the first gamma-ray detector 14 and the second gamma-ray detector 16 than to the intersection point P1 of the central axes X1 and X2.

[0040] The positron detector 12 is provided with a shielding member 52. The shielding member 52 is made of a plate-shaped lead. The shielding member 52 is positioned to shield the space between scintillators 14a and 16a. This prevents gamma rays scattered inside one scintillator (e.g., scintillator 14a) from entering the other scintillator (e.g., scintillator 16a).

[0041] The arithmetic unit 50 can be composed of a computer or processor equipped with a CPU, ROM, and RAM, and dedicated circuits such as a digital storage oscilloscope (DSO) or NIM module. As shown in Figure 5, the arithmetic unit 50 includes a first signal processing unit 20 connected to the first gamma-ray detector 14 and the second gamma-ray detector 16, and a second signal processing unit 30 connected to the positron detector 12. The second signal processing unit 30 processes the electrical signal output from the positron detector 12 (specifically the photomultiplier tube 41) and identifies the time when a positron was incident on the positron detector 12. The time identified by the second signal processing unit 30 is input to the first signal processing unit 20. The first signal processing unit 20 and the second signal processing unit 30 are examples of an "annihilation characteristic calculation device".

[0042] The first signal processing unit 20 includes a positron generation time identification unit 21, a positron annihilation time identification unit 22, a time difference calculation unit 23, a noise information exclusion unit 24, and a positron lifetime calculation unit 25. The positron generation time identification unit 21 identifies the time when a positron was generated in the positron source 44 based on the signal from the first gamma-ray detector 14. The positron annihilation time identification unit 22 identifies the time when the positron annihilated based on the signal from the second gamma-ray detector 16. The time difference calculation unit 23 calculates the lifetime of the positron from the time difference between the time identified by the positron generation time identification unit 21 and the time identified by the positron annihilation time identification unit 22. The positron generation time identification unit 21, the positron annihilation time identification unit 22, and the time difference calculation unit 23 can be configured in the same way as the corresponding parts of a conventionally known positron annihilation characteristic measuring device.

[0043] The noise information exclusion unit 24 excludes the time difference calculated by the time difference calculation unit 23 from the time identified by the second signal processing unit 30 (i.e., the time when positrons were incident on the scintillator 42), specifically those related to positrons that were not incident on the object being measured. That is, as shown in Figure 6, when positrons emitted from the positron source 44 are incident on the object being measured (sample), the positrons annihilate within the object being measured, generating gamma rays (511 keV). On the other hand, when positrons emitted from the positron source 44 are incident on the scintillator 42, scintillation light is generated, and the positrons annihilate outside the object being measured, generating gamma rays (511 keV). Therefore, if gamma rays (1.27 MeV) are detected when the positrons are generated, then scintillation light is detected, and then gamma rays (511 keV) are detected when the positrons annihilate, it can be determined that positrons emitted from the positron source 44 were incident on the scintillator 42. On the other hand, if a gamma ray (1.27 MeV) is detected when a positron is generated, and then, without detecting scintillation light, a gamma ray (511 keV) is detected when the positron annihilated, it can be determined that a positron emitted from the positron source 44 was incident on the object being measured (sample). For example, as shown in Figure 7(a), if no positron is detected by the positron detector 12 between the positron generation time t1 and the positron annihilation time t2 (i.e., no scintillation light is detected), then the positron generation time t1 and the positron annihilation time t2 are considered valid data. The time difference (t2-t1) is then used to calculate the lifetime of the positron in the object being measured. On the other hand, as shown in Figure 7(b), when a positron is detected by the positron detector 12 at time t4, between the positron generation time t3 and the positron annihilation time t5 (i.e., when scintillation light is detected), the positron generation time t3 and the positron annihilation time t5 are treated as invalid data and excluded from the data used to calculate the lifetime of the positron in the object being measured. Note that positron generation (time t3), positron detection (time t4), and positron annihilation (time t5) occur within a very short period of time. Therefore, when the time difference between the positron generation time t3 and the positron detection time t4 falls within a predetermined first time difference, the positron generation time t3 and the subsequent detected positron annihilation time t5 may be excluded as invalid data.Alternatively, if the time difference between the positron detection time t4 and the positron annihilation time t5 falls within a predetermined second time difference, the positron generation time t3 and the positron annihilation time t5 may be excluded as invalid data.

[0044] The positron lifetime calculation unit 25 calculates the lifetime of a positron in the object under test from the lifespan of a positron incident on the object under test, after noise has been removed by the noise information removal unit 24. The positron lifetime calculation unit 25 can be configured in the same way as the corresponding part of a conventional positron annihilation characteristic measurement device.

[0045] Next, the procedure for measuring the positron lifetime of an object to be measured using the positron annihilation characteristic measuring device 10 described above will be explained. The device 10 is pre-set with a positron source 44. First, a guide rail 80 is set to guide the device 10 toward the object to be measured (for example, a bridge). Next, a slider (not shown) attached to the bottom surface of the main body 11 is fitted to the guide rail 80, and the measuring surface 11a of the device 10 is brought close to the object to be measured while sliding along the guide rail 80. The main body 11, which slides along the guide rail 80, is positioned close to the object to be measured by a stopper 82 provided at the end of the guide rail 80. At the positioned position, the main body 11 is fixed to the guide rail 80 using, for example, a screw 84. This allows the positron source 44 to be positioned facing the object to be measured through the measurement window 11b. After fixing the main body 11, the calculation unit 50 is activated to start measuring the positron lifetime.

[0046] When a positron is generated in the positron source 44, the resulting gamma ray (1.27 MeV) is detected by the first gamma-ray detector 14. The first signal processing unit 20 determines the time of positron generation based on the signal from the first gamma-ray detector 14. The positron generated in the positron source 44 is incident on the object to be measured or the scintillator 42. After a suitable amount of time, the positron incident on the object to be measured combines with an electron and annihilates, generating a gamma ray (511 keV). This gamma ray (511 keV) is detected by the second gamma-ray detector 16. The first signal processing unit 20 determines the time of annihilation of the positron based on the signal from the second gamma-ray detector 16 and calculates the positron's lifespan from the time difference.

[0047] Meanwhile, positrons incident on the scintillator 42 generate scintillation light, which then annihilates outside the object being measured, generating gamma rays (511 keV). The scintillation light travels through the scintillator 42 and is guided to the light-receiving surface 41a of the photomultiplier tube 41, where it is converted into an electrical signal. The second signal processing unit 30 determines the time when the positron was incident on the scintillator 42 based on the electrical signal from the photomultiplier tube 41. In addition, gamma rays (511 keV) generated when the positron annihilates outside the object being measured are detected by the second gamma-ray detector 16. Therefore, even if the positron was not incident on the object being measured, the time is calculated by the first signal processing unit 20. However, based on the relationship between the time calculated by the second signal processing unit 30 and the time of positron generation, data regarding positrons that were not incident on the object being measured is excluded. Therefore, the first signal processing unit 20 calculates the lifetime of the positron based only on the data obtained from positrons incident on the object being measured.

[0048] As is clear from the above description, in the positron annihilation characteristic measuring device 10 of this embodiment, positrons that were not incident on the object to be measured are detected by the positron detector 40, and the radiation generated by positrons that were not incident on the object to be measured is removed as noise. For this reason, the positron annihilation characteristics of the object to be measured can be calculated accurately without having to sandwich the positron source 44 between the object to be measured. In addition, since a scintillator 42 for positron detection is not placed between the positron source 44 and the object to be measured, it is possible to prevent a decrease in the number of positrons irradiated onto the object to be measured.

[0049] Next, we will describe the simulations and experiments conducted to evaluate the gamma-ray detection efficiency (counting rate) and the time resolution when detecting gamma rays, depending on the geometrical arrangement of the positron source 44 and each gamma-ray detector 14, 16. Counting rate and time resolution are important parameters when evaluating the characteristics of a positron annihilation characteristic measurement device. In these simulations and experiments, the relative positions of the positron source 44 and each gamma-ray detector 14, 16 were changed, and the counting rate and time resolution for each geometrical arrangement were calculated.

[0050] First, as shown in Figure 8, the gamma-ray count rate was calculated by Monte Carlo simulation for three different geometric configurations (configurations A, B, and C). Configuration A is a configuration commonly used in positron lifetime measurement research. Configuration B is a configuration commonly used in benchtop positron lifetime measurement devices. Configuration C is the configuration used in the device 10 of this embodiment. In configuration A, the central axes of each gamma-ray detector 14 and 16 are pointed toward the positron source 44. In configuration B, the central axes of each gamma-ray detector 14 and 16 are pointed toward the central axis of the positron detector 12 (the axis passing through the positron source 44). In each configuration, the same positron source 44, positron detector 12, and each gamma-ray detector 14 and 16 are used. In this simulation, the position of the positron source 44 is... 22 The procedure involved placing two Fe samples, each containing a Na positron source, in between. The positron source was enclosed in two 7.5 μm thick Kapton films.

[0051] In this simulation, 1.27 MeV gamma rays (generated when a positron is created) and 511 keV gamma rays (generated when a positron is annihilated) were alternately generated 10 million times at the position of the positron source 44. The detection efficiency (count rate) of the pair of gamma-ray detectors 14 and 16 was calculated from the frequency of events in which the 1.27 MeV gamma rays were absorbed by one scintillator 14a and the 511 keV gamma rays were absorbed by the other scintillator 16a. The results are shown in Figure 9. Note that the count rates shown in Figure 9 are relative values ​​to configuration A.

[0052] As is clear from Figure 9, there was not a significant difference in the counting rates of arrangements A and B. In contrast, arrangement C was found to be able to detect gamma rays with 1.89 times the efficiency compared to arrangement A. In other words, the apparatus 10 of this embodiment can detect radiation efficiently with an extremely high gamma ray counting rate compared to conventional geometric arrangements.

[0053] Furthermore, in configuration C, simulations were also performed to examine the difference in counting rate due to changes in the distance between each gamma-ray detector 14, 16 and the positron source 44. The distance between each gamma-ray detector 14, 16 and the positron source was calculated as the distance between the centroid position of scintillators 14a, 16a and the positron source 44. As shown in Figure 10, the simulation results showed that the shorter the distance between the centroid position of scintillators 14a, 16a and the positron source 44, the higher the counting rate.

[0054] Next, as shown in Figure 11, the time resolution was experimentally calculated for three different geometric configurations (configurations D, E, and F). In configurations D, E, and F, the relative position of the positron source 44 to the two gamma-ray detectors 14 and 16, which are arranged parallel to each other, is different. The positron source 44 and the gamma-ray detectors 14 and 16 (scintillators 14a and 16a) are farthest apart in configuration D and closest in configuration F. In this experiment, the position of the positron source 44 was determined by placing annealed stainless steel as the sample and a sample sealed in Kapton film. 22The experiment was conducted using a Na positron source. The obtained positron lifetime spectrum was analyzed using one component (excluding the positron source component), and the time resolution was calculated as the full width at half maximum (FWHM) of the resolution curve. The results are shown in Figure 12.

[0055] As shown in Figure 12, the time resolution was highest in configuration D, and as the positron source 44 was moved in a direction that sandwiched it between the gamma-ray detectors 14 and 16, the time resolution decreased. This is thought to be because each scintillator 14a and 16a extends long in the axial direction of each gamma-ray detector 14 and 16. In other words, as the position of the positron source 44 shifts from configuration D to configuration F, the scintillation light is scattered greatly inside each scintillator 14a and 16a, so it takes a long time for the scintillation light to reach the light-receiving surface of the photomultiplier tubes 14b and 16b.

[0056] Here, the difference in time resolution between the three configurations D, E, and F is less than 25 ps in all cases. For example, if the time resolution decreases from 200 ps to 250 ps, ​​when measuring an object with a positron lifetime of 150 ps, ​​the estimation error of the positron lifetime relative to the time resolution becomes approximately 1.14 times. On the other hand, the estimation error relative to the cumulative number of gamma-ray detections n is the reciprocal of the square root of n. That is, when changing from configuration A to configuration C in Figure 8, the estimation error of the positron lifetime is reduced to approximately 0.73 times due to the improvement in the counting rate (1.89 times). In other words, by adopting configuration C in Figure 8, the effect of the decrease in time resolution caused by positioning the positron source 44 between the gamma-ray detectors 14 and 16 can be suppressed compared to the conventional configurations A and B.

[0057] Next, we will describe an experiment conducted to evaluate the light-shielding properties of the positron detector 12 due to differences in the light-shielding materials provided to the positron detector 12. As shown in Figure 13, in this experiment, the reliability of the measurement was evaluated by calculating the average positron lifetime of annealed stainless steel when the positron detector 12 was shielded with four types of light-shielding materials: Kapton film only, titanium foil and Kapton film, 300 nm aluminum-deposited Kapton film, and 500 nm aluminum-deposited Kapton film. Titanium foil is a light-shielding material that has been commonly used in the past. Note that "Tabletop type" shown in (a) in the graphs of Figure 13 and Figure 14 (described later) indicates the average positron lifetime calculated using a tabletop positron annihilation characteristic measurement device (hereinafter referred to as the tabletop device). In the tabletop device, the positron lifetime is measured by placing a test piece cut from the object to be measured in a dark box. In other words, in a benchtop device, the positron detector is placed inside a dark box, completely blocking external light from entering the detector. Therefore, the closer the calculated average positron lifetime is to the value obtained with a benchtop device, the higher the reliability of the measurement can be considered.

[0058] First, as shown in Figure 13(b), when the positron detector 12 was shielded from light using only the Kapton film, values ​​approximately equal to those of the desktop device shown in (a) were obtained. In other words, it was found that when the positron detector 12 is shielded from light using only the Kapton film, measurements can be performed with the same reliability as the desktop device. On the other hand, as shown in (c), when titanium foil was used in addition to the Kapton film as a light-shielding material, the average positron lifetime differed significantly from that of the desktop device. In other words, it was found that using conventionally used titanium foil as a light-shielding material reduces the reliability of the measurement. In contrast, as shown in (d) and (e), when an aluminum-deposited Kapton film was used, values ​​approximately equal to those of the desktop device shown in (a) were obtained, regardless of the thickness of the aluminum film. As described above, as in this embodiment, by covering the positron detector 12 with a Kapton film 60 on which an aluminum film 62 has been deposited, it is possible to shield from light with higher reliability than when using titanium foil, and to ensure the same level of reliability as the desktop device.

[0059] Next, we will describe an experiment conducted to evaluate the incidence of positrons onto the object under test due to differences in the light-shielding members provided for the positron detector 12. Some of the positrons emitted from the positron source 44 are annihilated inside the light-shielding members. Since the gamma rays generated when positrons annihilate inside the light-shielding members become noise components, it is desirable for many positrons to be incident on the object under test during measurement. In this experiment, we measured a sample (quartz glass) when the positron detector 12 was shielded with four types of light-shielding members similar to those in Figure 13, and evaluated the incidence of positrons onto the sample by calculating the relative intensity of the component originating from the sample among the detected gamma rays.

[0060] First, as shown in Figure 14(b), when the positron detector 12 was shielded from light using only Kapton film, the relative intensity of gamma rays from the sample components decreased by approximately 1% compared to the desktop device shown in (a). This indicates that a small amount of positrons were annihilated inside the Kapton film. Next, as shown in Figure 14(c), when titanium foil was used in addition to Kapton film as a light-shielding material, the relative intensity of gamma rays from the sample components decreased significantly compared to the desktop device. This means that using conventionally used titanium foil as a light-shielding material resulted in a larger rate of positron annihilation inside the light-shielding material (i.e., titanium foil). In contrast, as shown in (d) and (e), when an aluminum-deposited Kapton film was used, the relative intensity did not decrease as much (approximately 2% decrease) compared to the desktop device shown in (a), regardless of the thickness of the aluminum film. However, as mentioned above, positrons are annihilated inside the Kapton film as well, so the actual contribution of aluminum to the decrease in relative intensity is small. As described above, by covering the positron detector 12 with a Kapton film 60 on which an aluminum film 62 has been deposited, as in this embodiment, it is possible to efficiently inject positrons into the object to be measured compared to when titanium foil is used, and it is also possible to secure a sufficient positron injection ratio compared to a tabletop device.

[0061] As explained above, in the apparatus 10 of this embodiment, as shown in arrangement C in Figure 8, each gamma-ray detector 14 and 16 is positioned relatively close to the positron source 44, flanking it. Therefore, gamma rays generated when positrons are created and annihilated easily enter each gamma-ray detector 14 and 16. Consequently, the apparatus 10 has a high gamma-ray counting rate. For this reason, even if the size of the apparatus 10 (more specifically, the size of each gamma-ray detector 14 and 16) is reduced to improve the time resolution, the counting rate of each gamma-ray detector 14 and 16 can be ensured. Furthermore, in this embodiment, even if the positron source 44 is positioned so as to be flanked by each gamma-ray detector 14 and 16, as shown in arrangement F in Figure 11, the reduction in time resolution caused by this arrangement can be compensated for by the high counting rate, thereby reducing measurement errors. In other words, the apparatus 10 of this embodiment can achieve both a high counting rate and high time resolution.

[0062] In this embodiment, the positron detector 12 is covered with an aluminum-deposited Kapton film 60. Since the aluminum film 62 is relatively thin, the number of positrons annihilated within the aluminum film 62 is reduced, thereby suppressing noise during measurement. Furthermore, because the very thin aluminum film 62 is deposited onto the Kapton film 60, which has high strength, the aluminum film 62 and Kapton film 60 are less likely to tear during the fabrication of the positron detector 12, making it easy to manufacture the positron detector 12.

[0063] In the above-described embodiment, the aluminum film 62 was provided on only one side of the Kapton film 60. However, for example, the aluminum film 62 may be provided on both sides of the Kapton film 60 by depositing aluminum on both sides of the Kapton film 60. Figure 15 shows the light-shielding properties observed when aluminum is deposited on the Kapton film in different ways. Figure 15(a) shows the case where aluminum is deposited on one side of the Kapton film to a thickness of 300 nm, Figure 15(b) shows the case where aluminum is deposited on one side of the Kapton film to a thickness of 500 nm, and Figure 15(c) shows the case where aluminum is deposited on both sides of the Kapton film to a thickness of 100 nm. As shown in Figures 15(a) and (b), it can be seen that the light-shielding properties are improved by increasing the thickness of the deposited aluminum. Furthermore, in the embodiment shown in Figure 15(c), the total thickness of the aluminum (200 nm) is thinner than in Figures 15(a) and (b), but the light-shielding properties are high. This is because the overlap of pinholes that may occur in each aluminum film is suppressed by providing multiple layers of aluminum. In this way, the light-shielding properties can be further improved by providing multiple layers of aluminum. In addition to depositing aluminum onto the Kapton film, aluminum may also be deposited on the outer surface of the scintillator 42.

[0064] (Example 2) In the positron annihilation characteristic measurement apparatus of Example 2, the configuration of the positron detector 112 differs from that of Example 1. In Example 1, the positron detector 12 was composed of a photomultiplier tube 41 and a scintillator 42. In Example 2, as shown in Figure 16, the positron detector 112 has an MPPC (Multi-Pixel Photon Counter) 141 and a scintillator 142. The MPPC 141 is an optical semiconductor element that measures photons and has a thin plate shape. In Example 2, the scintillator 142 has a shape corresponding to the MPPC 141. In Example 2, the positron detector 112 is positioned in front of each gamma-ray detector 14, 16 (i.e., on the measurement surface 11a side).

[0065] In Example 2, since the positron detector 112 has a thin plate shape, even if the positron detector 112 and each of the gamma-ray detectors 14 and 16 are offset in the y-axis direction, the distance between each of the gamma-ray detectors 14 and 16 and the positron source 44 does not increase significantly. Therefore, as is clear from comparing Figure 16 with Figure 2, the first gamma-ray detector 14 and the second gamma-ray detector 16 can be placed closer together. This allows for a further improvement in the gamma-ray counting rate. Furthermore, since no member is placed between the first gamma-ray detector 14 and the second gamma-ray detector 16, as shown in Figure 16, the space between them can be suitably blocked by placing a shielding member 152 between the scintillator 14a and the scintillator 16a.

[0066] Although the embodiments described above were explained using a portable positron annihilation characteristic measurement device as an example, the technology disclosed herein may also be applied to a tabletop positron annihilation characteristic measurement device.

[0067] Although specific examples of the technology disclosed herein have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples described above. Furthermore, the technical elements described herein or in the drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. [Explanation of Symbols]

[0068] 10: Positron annihilation characteristics measuring device 12: Positron detector 14: First gamma-ray detector 16: Second gamma-ray detector 44: Positron source 50: Arithmetic device

Claims

1. A positron annihilation characteristic measuring device for measuring the annihilation characteristics of positrons that are incident on an object to be measured and annihilated within that object, A positron source positioned in close proximity to or in close contact with the surface of the object to be measured, A positron detector for detecting positrons generated by the positron source that were not incident on the object to be measured, A first radiation detector is positioned at a first location set relative to the positron source and detects the first radiation generated when positrons are produced in the positron source. A second radiation detector is positioned at a second position opposite to the first position relative to the positron source, and detects a second radiation generated when positrons produced by the positron source annihilate. It is equipped with, The first central axis of the first radiation detector and the second central axis of the second radiation detector intersect. The positron source is positioned closer to the first and second radiation detectors than the intersection of the first and second central axes. Positron annihilation characteristics measuring device.

2. The positron annihilation characteristic measuring apparatus according to claim 1, wherein the positron detector is shielded from light by a thin film on which aluminum has been deposited and / or a scintillator on which aluminum has been deposited.

3. The aluminum is provided in a single or multiple layer, The positron annihilation characteristic measuring apparatus according to claim 2, wherein the total thickness of the aluminum is 500 nm or less.

4. The first radiation detector comprises a first scintillator that emits scintillation light upon incidence of the first radiation, and a first photosensor that detects the scintillation light emitted from the first scintillator. The second radiation detector comprises a second scintillator that emits scintillation light upon incidence of the second radiation, and a second photosensor that detects the scintillation light emitted from the second scintillator. The positron annihilation characteristic measuring apparatus according to any one of claims 1 to 3, wherein the first scintillator is located in the first position and the second scintillator is located in the second position.

5. The positron detector is positioned on the opposite side of the positron source from the object being measured. The positron annihilation characteristic measuring apparatus according to any one of claims 1 to 4, wherein the first position and the second position are opposite each other via the positron detector.

6. The positron detector has a thin plate shape, according to any one of claims 1 to 5, for the positron annihilation characteristic measuring device.

7. The positron annihilation characteristic measuring apparatus according to any one of claims 1 to 6, wherein a shielding member is provided to shield the space between the first radiation detector and the second radiation detector.

8. The positron annihilation characteristic measuring apparatus according to any one of claims 1 to 7, further comprising an annihilation characteristic calculation device that calculates the annihilation characteristics of positrons in the body to be measured based on the detection results of the first radiation detector and the second radiation detector and the detection results of the positron detector.

9. The positron annihilation characteristic measuring device according to any one of claims 1 to 8, wherein the positron annihilation characteristic measuring device is portable.

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