JP7216954B2 - Positron Annihilation Characteristic Measuring Apparatus and Positron Annihilation Characteristic Measuring Method - Google Patents

Description

The present specification relates to a technique for measuring annihilation characteristics of positrons that are incident on an object and annihilated in the object.

When a material is irradiated with positrons emitted from a positron beam source (positron emitting nuclide) such as ²² Na or ⁶⁸ Ge, the irradiated positrons fly through the material and then combine with electrons inside the material to annihilate. . If there are defects (vacancies or dislocations) in the material, the positrons are captured by the defects. For this reason, the positrons take longer to annihilate than if there were no defects in the material. In addition, when there are defects in the material, the distribution of the energy spectrum of the radiation (γ-rays) emitted when the positron annihilates is compared to the energy spectrum distribution of the radiation when there are no defects in the material. differ. Therefore, if we know the time it takes for positrons irradiated into the material to annihilate and the energy spectrum distribution of the radiation when the positrons irradiated into the material annihilate, we can estimate the material properties of the material related to defects. be able to. Therefore, by measuring the positron annihilation characteristics, research is being conducted to evaluate shot peening processing and to observe the fatigue state of members used in nuclear reactors.

For example, Patent Document 1 discloses an apparatus for measuring positron annihilation characteristics. In the apparatus of Patent Document 1, a positron beam source is sandwiched between two test pieces cut out from an object to be measured (substance to be measured) to measure radiation (γ-rays) when positrons are annihilated. there is In this method, all the positrons emitted from the positron beam source are irradiated onto the specimen, so the positron lifetime (that is, positron annihilation characteristics) can be calculated with high accuracy.

JP 2017-198552 A

Using the method described in Patent Document 1, the positron lifetime when a test piece is irradiated with positrons can be calculated. However, the positron lifetime alone cannot sufficiently evaluate the internal state of the object. This specification discloses a technique for more accurately evaluating the internal state of an object to be measured based on positron annihilation characteristics.

The positron annihilation characteristics measuring apparatus disclosed in the present specification measures the annihilation characteristics of positrons that enter an object and are annihilated in the object. The positron annihilation characteristics measuring apparatus includes a positron beam source arranged at a position close to or in close contact with the surface of the object to be measured, and a first position set with respect to the positron beam source, and the positrons are emitted from the positron beam source. and a first radiation detector arranged at a second position set with respect to the positron beam source for detecting a first radiation generated when the positron is generated and annihilated when the positron generated by the positron beam source is annihilated. second radiation detection means for detecting the generated second radiation; positron detection means for detecting positrons not incident on the object among the positrons generated by the positron beam source; and first and second radiation detection An annihilation characteristic calculation means for calculating annihilation characteristics of positrons in the body to be measured based on the detection result of the means and the detection result of the positron detection means. The annihilation characteristic calculation means calculates the measurement result of "life time-positron number" from the time difference between the time when the first radiation detection means detects the first radiation and the time when the second radiation detection means detects the second radiation. By fitting the measurement result of "lifetime - number of positrons", the time when the positron lifetime spectrum of the object to be measured is 0 hours is calculated, and the reference time obtained according to the type of object to be measured , the amount of shift from the calculated 0 hour.

In the positron annihilation characteristic measuring apparatus described above, the amount of shift between the 0 hour (reference time) of the positron lifetime spectrum acquired for each type of the object to be measured and the 0 hour of the positron lifetime spectrum acquired for the object to be measured. Calculate This makes it possible to evaluate the internal state of the object, which cannot be determined by the evaluation based only on the positron lifetime.

Also, the positron annihilation property measuring method disclosed in the present specification measures the annihilation property of positrons that are incident on an object and annihilate in the object. A positron annihilation characteristic measuring method includes a positron beam source, a first detector for detecting a first radiation generated when positrons are generated by the positron beam source, an adjusting step of adjusting the position between a second detector that detects the generated second radiation; From the time difference between the time when the first radiation is detected by the first detector and the time when the second radiation is detected by the second detector for one object to be measured, the first time which is 0 hours of the positron lifetime spectrum is determined. A second object to be measured, which is different from the first object to be measured, using a first time calculating step of calculating the time of the positron beam source, the first detector, and the second detector whose positions are adjusted in the adjusting step is calculated from the time difference between the time when the first radiation is detected by the first detector and the time when the second radiation is detected by the second detector. A second time calculation step and a shift amount calculation step of calculating a shift amount between the first time and the second time are provided.

In the above positron annihilation characteristic measuring method, the 0 hour (first time) of the positron lifetime spectrum of the first object to be measured and the 0 hour (second time) of the positron lifetime spectrum of the second object to be measured The difference between the internal state of the first object to be measured and the internal state of the second object to be measured can be evaluated by calculating the amount of shift between. As a result, the internal state of the first measured object and the internal state of the second measured object cannot be evaluated only by comparing the positron lifetimes of the first measured object and the positron lifetimes of the second measured object. difference can be evaluated.

FIG. 1 is a perspective view schematically showing a positron annihilation characteristic measuring device according to an example. FIG. 1 is a diagram schematically showing the configuration of a positron annihilation characteristic measuring apparatus according to an example; FIG. 2 is an exploded perspective view of the positron detection unit; The figure which shows the structure of a light-shielding container. The figure which shows the state which installed the sample holder in the light-shielding container. FIG. 2 is a block diagram showing the configuration of an arithmetic unit; FIG. 4 is a diagram for explaining the process of removing noise based on the detection result of the positron detector; FIG. 4 is a diagram for explaining the process of removing noise based on the detection result of the positron detector; 4 is a flowchart showing an example of a process for calculating a time shift amount at which the positron lifetime spectrum of an object to be measured becomes 0 hours; FIG. 4 is a schematic diagram for explaining the process of calculating the time when the positron lifetime spectrum of the object to be measured is 0 hours; FIG. 4 is a graph showing the relationship between the time required to introduce defects into an object to be measured by shot peening processing, the positron lifetime, the lifetime distribution, and the shift amount of the time when the positron lifetime spectrum of the object to be measured becomes 0 hours. FIG. 4 is a diagram showing the relationship between the positron lifetime and lifetime distribution reproduced using Monte Carlo simulation, the amount of time shift at which the positron lifetime spectrum of the object to be measured becomes 0 hours, and the speed at which positrons are captured by defects;

The main features of the embodiments described below are listed. It should be noted that the technical elements described below are independent technical elements, and exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims as of the filing. do not have.

(Feature 1) In the positron annihilation property measuring apparatus disclosed in this specification, the object to be measured may be a test object made of metal. The reference time is set in a state in which a reference specimen made of the same kind of metal as the object to be measured and from which internal defects have been removed is placed in a position close to or in close contact with the positron beam source. It may be the time at which the positron lifetime spectrum of the reference specimen is 0 hours, which is calculated based on the detection results detected by the detection means and the second radiation detection means. According to such a configuration, the reference time is calculated using the reference specimen which is made of the same kind of metal as the object to be measured and has no internal defects. By calculating the time at which the positron lifetime spectrum reaches 0 hours using such a reference specimen, the internal state of the object to be measured (that is, the state of internal defects) can be appropriately evaluated.

(Feature 2) The positron annihilation characteristics measuring apparatus disclosed in the present specification may further include a storage section for storing a reference time and an input section for inputting the type of the object to be measured. The storage unit may be capable of storing the reference time for each type of object to be measured. The extinction characteristic calculation means may calculate the shift amount using the reference time stored in the storage section corresponding to the type of the object to be measured input by the input section. According to such a configuration, it is possible to appropriately calculate the shift amount corresponding to the type of the object to be measured. Also, the reference time can be calculated in advance and stored in the storage unit. Therefore, the shift amount can be calculated using the pre-calculated reference time without performing processing for calculating the reference time when measuring the positron lifetime of the object to be measured.

(Feature 3) In the positron annihilation property measuring apparatus disclosed in this specification, the annihilation property calculation means detects the time when the first radiation is detected by the first radiation detection means and the second radiation is detected by the second radiation detection means. A measurement result of the lifetime distribution may be further acquired from the time difference from the time when the measurement was performed. According to such a configuration, in addition to the amount of shift between the reference time and the time at which the positron lifetime spectrum of the object is 0 hours, the measurement result of the lifetime distribution is used to evaluate the internal state of the object. can be done. This makes it possible to evaluate the internal state of the object to be measured more accurately.

A positron annihilation characteristic measuring apparatus 10 according to an embodiment will be described below. A positron annihilation measuring apparatus 10 (hereinafter also simply referred to as apparatus 10) measures gamma rays (1.27 MeV) generated when positrons are generated and gamma rays (511 keV) generated when positrons are annihilated. This device measures the positron lifetime in the body to be measured from the time difference, and evaluates the internal state of the body to be measured from the measurement results. The object to be measured for which the positron lifetime can be measured by the apparatus 10 may be a sample made of a material in which orthopositronium (o-Ps) is not formed. A sample formed of an object can be used as an object to be measured. As shown in FIGS. 1-3, the device 10 has a first γ-ray detector 12, a second γ-ray detector 14, a positron detector 40, a light shielding container 60, and a computing device 50. FIG. In FIG. 2, space A is the inner space of light-shielding container 60 and space B is the inner space of device 10 .

The first γ-ray detector 12 detects γ-rays (1.27 MeV) generated when positrons are produced. The first γ-ray detector 12 may be composed of, for example, a scintillator that emits scintillation light upon incidence of γ-rays, and a photomultiplier tube that converts the scintillation light into an electrical signal. The first γ-ray detector 12 is connected to the computing device 50 . The first γ-ray detector 12 outputs a pulsed electrical signal to the computing device 50 when it detects a γ-ray (1.27 MeV) generated when a positron is generated. A first gamma ray detector 12 is housed within the device 10 . The first γ-ray detector 12 is an example of "first radiation detection means" and "first detector".

The second γ-ray detector 14 detects γ-rays (511 keV) generated when positrons are annihilated. The second γ-ray detector 14 may also be configured similarly to the first γ-ray detector 12 . The second γ-ray detector 14 is connected to the computing device 50 . The second γ-ray detector 14 outputs a pulsed electrical signal to the arithmetic device 50 when it detects a γ-ray (511 keV) generated when the positron annihilates. The first γ-ray detector 12 and the second γ-ray detector 14 are arranged at positions facing the measurement surface of the object S to be measured. A second gamma-ray detector 14 is housed within the device 10 . The second γ-ray detector 14 is an example of "second radiation detection means" and "second detector".

The positron detector 40 comprises a photomultiplier tube 41 and a positron detection unit 42, as shown in FIG. The positron detection unit 42 includes a positron detection scintillator 422 and a light guide 44 . The positron detection unit 42 and the photomultiplier tube 41 are connected by a light guide 44 . The light guide 44 is connected to the light receiving surface of the photomultiplier tube 41 and the scintillator 422 . The positron detector 40 is an example of "positron detection means".

The scintillator 422 emits scintillation light upon incidence of positrons. The light guide 44 transmits scintillation light emitted from the scintillator 422 to the light receiving surface of the photomultiplier tube 41 . The light guide 44 is made of, for example, a highly transmissive material coated with a reflective material. The photomultiplier tube 41 converts the transmitted scintillation light into an electrical signal. An electrical signal from the photomultiplier tube 41 is input to the arithmetic device 50 .

A positron beam source 424 is placed between two thin film light shielding covers 423 . That is, the positron beam source 424 is sandwiched between the thin film light shielding covers 423 . A positron beam source 424 sandwiched between thin film light shielding covers 423 is arranged at a position facing the scintillator 422 . The thin film light shielding cover 423 blocks external light from entering the scintillator 422 . Positrons emitted from the positron beam source 424 are incident on one of the thin film light shielding covers 423 arranged above and below the positron beam source 424 . When positrons enter the thin film light shielding cover 423 arranged on the upper side, the positrons pass through the thin film light shielding cover 423 and enter the object S to be measured. On the other hand, when positrons pass through the lower thin film light shielding cover 423 and enter the scintillator 422, scintillation light is emitted from the scintillator 422, and the inside of the scintillator 422 or the outside of the scintillator 422 (that is, other than the object S to be measured) The positron annihilates. Scintillation light from the scintillator 422 enters the photomultiplier tube 41 through the light guide 44 . As a result, an electrical signal is output from the photomultiplier tube 41 to the arithmetic device 50 . The positron beam source 424 may be arranged so as to be sandwiched between the thin film light shielding cover 423 and the scintillator 422 . That is, the positron beam source 424 may be placed directly on the top surface of the scintillator 422 .

Here, as the positron beam source 424, a positron beam source (positron emitting nuclide) such as ²² Na or ⁶⁸ Ge can be used. Also, the positron beam source 424 is a weak positron beam source such that no other positron is generated after one positron is generated until it annihilates. As a result, it is possible to prevent a situation in which a plurality of positrons exist at the same time and the positron generation time and annihilation time cannot be specified.

The light-shielding container 60 accommodates the positron beam source 424, the object S to be measured, and the positron detector 40 (scintillator 422), as shown in FIG. The light-shielding container 60 has an opening 60a (see FIG. 1), and an opening/closing door 62 is provided in the opening 60a. The opening/closing door 62 can be switched between a closed state in which the opening 60a is closed and an open state in which the opening 60a is opened. By opening the opening/closing door 62, the inside of the light shielding container 60 can be accessed from the outside. In addition, since the opening/closing door 62 is closed, light is prevented from entering the light shielding container 60 from the outside. The switching mechanism between the open state and the closed state of the open/close door 62 is not particularly limited, and may be configured to be manually switched by the user, or automatically switched by the control device driving the actuator. It may be configured as

A sample holder 70 for holding the object S to be measured is installed in the light shielding container 60 . As shown in FIG. 5, the sample holder 70 has a mounting plate 70a formed with a through hole 70b and a fitting portion 70c. An object S to be measured is mounted on the mounting plate 70a. Specifically, the object S to be measured is mounted on the mounting plate 70a so that the object S to be measured is positioned above the through hole 70b. The thickness of the mounting plate 70 a is higher than the height of the positron beam source 424 . The sample holder 70 is made of, for example, SUS (Steel Use Stainless). A thin film light shielding cover may be interposed between the mounting plate 70a and the object S to be measured.

The through hole 70b is formed substantially in the center of the mounting plate 70a. The through hole 70b penetrates from the front surface to the rear surface of the mounting plate 70a. Although the shape of the through hole 70b is not particularly limited, it is circular in plan view in this embodiment. In plan view, the through-hole 70b has a size capable of enclosing the positron beam source 424 (the thin film light shielding cover 423). As described above, since the height of the positron beam source 424 is smaller than the thickness of the mounting plate 70a, the positron beam source 424 can be accommodated in the space inside the through hole 70b.

The fitting portion 70c is a reference portion (positioning portion) for setting the sample holder 70 at the set position. For example, as shown in FIG. 5, when the sample holder 70 is placed at a predetermined set position (fitting portion 70c is fitted into the fitting wall 60c in the light shielding container 60), the object S to be measured passes through. It faces the positron beam source 424 through the hole 70b, and a constant space sp (see FIG. 4) is provided between the object S to be measured and the positron beam source 424. FIG. Spacing sp is, for example, 0.5 mm. 5, a part of the wall of the light shielding container 60, the opening/closing door 62, and the like are omitted for the sake of clarity. The shape of the sample holder 70 is not limited to the above, and may be box-shaped, for example. With such a configuration, the positron annihilation characteristics can be measured even when the object to be measured is particulate or liquid.

The arithmetic unit 50 can be configured by a computer or processor having a CPU, ROM, and RAM, and a dedicated circuit such as a digital storage oscilloscope (DSO) or NIM module. The computing device 50 includes a first signal processing section 20 connected to the first γ-ray detector 12 and the second γ-ray detector 14 and a second signal processing section 30 connected to the positron detector 40 . The second signal processing unit 30 processes the electrical signal output from the positron detector 40 (more specifically, the photomultiplier tube 41) and identifies the time when the positron entered the positron detector 40. FIG. The time specified by the second signal processing section 30 is input to the first signal processing section 20 . In addition, the arithmetic unit 50 executes a time shift amount calculation process at which the positron lifetime spectrum of the object S to be measured becomes 0 hours, which will be described later. The first signal processing section 20 and the second signal processing section 30 are examples of the "extinction characteristic calculation means".

As shown in FIG. 6, the first signal processing unit 20 includes a positron generation time determination unit 21, a positron annihilation time determination unit 22, a time difference calculation unit 23, a noise information exclusion unit 24, and a positron lifetime calculation unit 25. It has The positron generation time specifying unit 21 specifies the time when the positron was generated by the positron beam source 424 based on the signal from the first γ-ray detector 12 . The positron annihilation time specifying unit 22 specifies the positron annihilation time based on the signal from the second γ-ray detector 14 . The time difference calculator 23 calculates the time during which the positron lived from the time difference between the time specified by the positron generation time specifier 21 and the time specified by the positron annihilation time specifier 22 . The positron generation time specifying unit 21, the positron annihilation time specifying unit 22, and the time difference calculating unit 23 can be configured in the same manner as the corresponding parts of a conventionally known positron annihilation characteristic measuring apparatus.

The noise information exclusion unit 24 selects the measurement object S Exclude those associated with positrons that have not been injected into . That is, as shown in FIG. 7, when positrons emitted from the positron beam source 424 are incident on the object S (sample) to be measured, the positrons are annihilated in the object S to generate gamma rays (511 keV). . On the other hand, when positrons emitted from the positron beam source 424 are incident on the scintillator 422, they generate scintillation light and are annihilated outside the object S to generate gamma rays (511 keV). Therefore, when the γ-ray (1.27 MeV) when the positron is generated is detected, then the scintillation light is detected, and then the γ-ray (511 keV) when the positron is annihilated is detected, the positron beam Positrons emitted from source 424 can be identified as incident on scintillator 422 . On the other hand, if a gamma ray (1.27 MeV) is detected when a positron is generated and then a gamma ray (511 keV) is detected when a positron is annihilated without detecting scintillation light, the positron beam It can be determined that the positrons emitted from the source 424 have entered the object S (sample) to be measured. For example, as shown in FIG. 8A, when no positron is detected by the positron detector 40 between the positron generation time t1 and the positron annihilation time t2 (that is, when no scintillation light is detected), , the positron generation time t1 and the positron annihilation time t2 are valid data. Then, the time difference (t2-t1) is used to calculate the lifetime of the positron in the object S to be measured. On the other hand, as shown in FIG. 8B, when positrons are detected by the positron detector 40 at time t4 between positron generation time t3 and positron annihilation time t5 (that is, scintillation light is detected) ), the positron generation time t3 and the positron annihilation time t5 are excluded from the data for calculating the lifetime of the positron in the object S as invalid data. The positron generation (time t3), the positron detection (time t4), and the positron annihilation (time t5) occur within an extremely short period of time. Therefore, when the time difference between the positron generation time t3 and the positron detection time t4 is within the predetermined first time difference, the positron generation time t3 and the positron annihilation time t5 detected thereafter can be excluded as invalid data. good. Alternatively, when the time difference between the positron detection time t4 and the positron annihilation time t5 is within a predetermined second time difference, the positron generation time t3 and the positron annihilation time t5 may be excluded as invalid data.

The positron lifetime calculator 25 calculates the lifetime of the positrons in the object S to be measured from the survival time of the positrons that have entered the object S after noise has been removed by the noise information exclusion unit 24 . The positron lifetime calculator 25 can be configured in the same manner as the corresponding part of a conventional positron annihilation characteristic measuring device.

Further, as shown in FIG. 2 , a storage unit 52 and an input unit 54 are connected to the computing device 50 . The storage unit 52 stores the time T0 at which the positron lifetime spectrum of the reference specimen described later becomes ₀ hours, in association with the type of the reference specimen. The time _T0 will be detailed later. The input unit 54 is configured so that the measurer can input the types of the object to be measured S and the reference test object.

Next, in the positron annihilation characteristic measuring apparatus 10 of the present embodiment, the process of calculating the time shift amount at which the positron lifetime spectrum of the object S to be measured becomes 0 hours will be described. The time until the positron annihilates (positron lifetime) varies depending on the presence or absence of internal defects in the object S to be measured. Therefore, by measuring the positron lifetime, it is possible to evaluate the presence or absence of internal defects in the object S to be measured. However, there are multiple types of internal defects (for example, dislocations and vacancies), and the mechanical properties of the object S to be measured differ depending on the types of internal defects. Although the existence or non-existence of internal defects can be evaluated only by the positron lifetime, the detailed state inside the object S to be measured, such as what kind of defects are present in the object S and how much, is also evaluated. It is not possible. Therefore, in this embodiment, the amount of shift in the time at which the positron lifetime spectrum of the object S to be measured is 0 hours is calculated, and the calculated amount of shift is used as an index for evaluating the internal state of the object S to be measured. That is, the present inventors calculated the time at which the positron lifetime spectrum is 0 hours for each of a plurality of objects S to be measured having the same positron lifetime. Notice the time difference. As will be described later, the time at which the positron lifetime spectrum of the object S to be measured is 0 hours is determined by the waveform of the positron lifetime spectrum, and the waveform of the positron lifetime spectrum is determined by the type of internal defects in the object S to be measured. Therefore, in order to absolutely evaluate the internal state of a plurality of objects S to be measured, a specimen having no internal defects and made of the same kind of metal as the objects S to be measured is defined as a "reference specimen". The difference (shift amount) between the time at which the positron lifetime spectrum of the sample S is 0 hours and the time at which the positron lifetime spectrum of the sample S is 0 hours was used as an index for evaluating the internal state of the sample S. . Processing for calculating the amount of time shift at which the positron lifetime spectrum of the object S to be measured is 0 hours will be described below with reference to FIGS. 9 to 12. FIG.

As shown in FIG. 9, the computing device 50 first measures the positron lifetime of the reference specimen (S12). A reference specimen is obtained by annealing a specimen made of the same kind of metal as the object S to be measured. By annealing, it is possible to obtain a specimen (that is, a reference specimen) which is made of the same kind of metal as the specimen S and has internal defects removed.

The positron lifetime measurement of the reference specimen is performed by the following procedure. The apparatus 10 is preset with a positron beam source 424 and a positron detector 40 . First, the opening/closing door 62 is opened to open the light-shielding container 60 , and the sample holder 70 on which the reference specimen is placed is placed at a predetermined setting position inside the light-shielding container 60 . The reference specimen is installed at the position where the object to be measured S is installed in FIGS. Specifically, the reference specimen is placed on the upper portion of the through hole 70b of the placement plate 70a of the sample holder 70, and the sample holder 70 is set at the set position. In this state, the positron beam source 424 and the reference specimen face each other through the through hole 70b while being separated from each other. In addition, as a result, the positron beam source 424 is sandwiched between the reference specimen and the positron detector 40 (specifically, the scintillator 422).

Next, the first γ-ray detector 12 and the second γ-ray detector 14 are set at positions facing the reference specimen. At this time, the distance between the positron beam source 424 and the first γ-ray detector 12 and the distance between the positron beam source 424 and the second γ-ray detector 14 are made equal. Any method may be used to adjust the distance between the positron beam source 424 and the gamma ray detectors 12 and 14 . For example, it can be performed by providing a spacer or the like and adjusting its position. Alternatively, since the position of the positron beam source 424 is constant in this embodiment, the positions of the first γ-ray detector 12 and the second γ-ray detector 14 may be adjusted in advance according to the position.

Next, the opening/closing door 62 attached to the light shielding container 60 is closed. When the opening/closing door 62 is closed, the positron lifetime can be measured. The operator inputs the type of the reference specimen to the input unit 54 before starting the positron lifetime measurement. Arithmetic device 50 causes storage unit 52 to store information input to input unit 54 (that is, information about the type of reference specimen). Then, the computing device 50 is activated to start measuring the positron lifetime.

When positrons are generated by the positron beam source 424 , the γ-rays (1.27 MeV) generated at that time are detected by the first γ-ray detector 12 . The first signal processing unit 20 identifies the time when the positron was generated based on the signal from the first γ-ray detector 12 . Positrons generated by the positron beam source 424 are incident on the reference specimen or the scintillator 422 . The positrons incident on the reference specimen combine with electrons after an appropriate time and annihilate to generate gamma rays (511 keV). This γ-ray (511 keV) is detected by the second γ-ray detector 14 . The first signal processing unit 20 identifies the time when the positron annihilated based on the signal from the second γ-ray detector 14, and calculates the survival time of the positron from the time difference.

On the other hand, the positrons incident on the scintillator 422 generate scintillation light, and then annihilate outside the reference specimen to generate gamma rays (511 keV). The scintillation light is transmitted by the light guide 44 to the photomultiplier tube 41 and converted into an electrical signal by the photomultiplier tube 41 . The second signal processing unit 30 identifies the time when the positron entered the scintillator 422 based on the electrical signal from the photomultiplier tube 41 . A gamma ray (511 keV) generated when the positron is annihilated outside the reference specimen is detected by the second gamma ray detector 14 . Therefore, the time is calculated by the first signal processing section 20 even when the light does not enter the reference specimen. However, due to the relationship between the time calculated by the second signal processing unit 30 and the generation time of positrons, data related to positrons that have not entered the reference specimen are excluded. Therefore, the first signal processing unit 20 calculates the lifetime of positrons based only on the data obtained from the positrons incident from the reference specimen.

As is clear from the above description, in the positron annihilation characteristic measuring apparatus 10 of the present embodiment, positrons that have not entered the reference specimen are detected by the positron detector 40, and positrons that have not entered the reference specimen are generated. remove the radiation that causes it as noise. Therefore, even if the positron beam source 424 is not sandwiched between the reference specimens, the positron annihilation characteristics of the reference specimen can be accurately calculated. Moreover, since the scintillator 422 for detecting positrons is not arranged between the positron beam source 424 and the reference specimen, it is possible to prevent the positrons irradiated to the reference specimen from decreasing.

In the above-described embodiment, the γ-rays (1.27 MeV) generated when positrons are generated and the γ-rays (511 keV) generated when positrons are annihilated are detected by different γ-ray detectors. However, it is not limited to such a configuration. For example, both γ-rays (1.27 MeV, 511 keV) may be detected by one γ-ray detector and processed to obtain the positron generation time, annihilation time and the time difference between them.

After the positron lifetime of the reference specimen is measured, the computing device 50 identifies the time when the positron lifetime spectrum of the reference specimen becomes 0 hours (S14). A positron lifetime spectrum is a frequency distribution in which positron lifetimes measured many times are represented by time and frequency (number of times). As described above, the internal defects are removed from the reference specimen, so the internal defects do not change the positron lifetime. Therefore, from the positron lifetime of the reference specimen obtained in step S12, the time T ₀ (so-called reference time) at which the positron lifetime spectrum of the reference specimen is 0 hours is obtained. The arithmetic unit 50 associates the acquired time T0 at which the positron lifetime spectrum of the reference specimen becomes ₀ hours with the type of the reference specimen and stores it in the storage unit 52 (S16).

Conventionally, a positron beam source is sandwiched between two test pieces cut out from an object to be measured, and the radiation (γ-ray) produced when the positrons are annihilated has been measured. In this method, since the position of the positron beam source is shifted with respect to the measurement target and the detector for each measurement, it is difficult to accurately specify the time _T0 at which the positron lifetime spectrum becomes 0 hours. In the positron annihilation characteristic measuring apparatus 10 of the present embodiment, the positional relationship among the positron beam source 424, the first γ-ray detector 12, and the second γ-ray detector 14 changes constantly. fixed without Therefore, the time T0 at which the positron lifetime spectrum of the reference specimen is ₀ hours can be specified with high accuracy.

Next, the computing device 50 measures the positron lifetime of the object S (S18). Note that the processing for measuring the positron lifetime of the object S to be measured is substantially the same as the processing in step S12 described above, so detailed description thereof will be omitted.

When the positron lifetime of the object to be measured S is measured, the computing device 50 calculates the measurement result obtained in the measurement of S18 (that is, the measurement result of the positron lifetime spectrum (“lifetime (time)−positron number (frequency)” )), the time at which the positron lifetime spectrum of the object S to be measured is 0 hours is calculated (S20). The time T _S0 at which the positron lifetime spectrum of the object S to be measured becomes 0 hours can be calculated using various known analysis programs. For example, the positron lifetime spectrum is not separated into a bulk component and a defect component, but both are averaged and weighted fitting is performed on the measurement result obtained in the measurement of S18. For weighted fitting, for example, a known PATFIT program, CERES, or the like can be used. As shown in FIG. 10, in order to equalize the total number of weighted fittings, the time TS0 at which the positron lifetime spectrum of the object S to be measured is ₀ hours is calculated to be smaller (on the negative side) than the time _T0 .

Finally, the arithmetic unit 50 calculates the shift amount ΔT ₀ of the time at which the positron lifetime spectrum of the object S to be measured is 0 hours from the time T ₀ specified in step S 14 and the time T _{S 0} calculated in step S 20 . Calculate (S22).

FIG. 11 shows the relationship between the positron lifetime, the lifetime distribution, and the shift amount _ΔT0 measured or calculated using the positron annihilation characteristic measuring apparatus 10 of the present embodiment for the object S to which defects have been introduced by shot peening. there is The horizontal axis indicates the processing time (projection time) of the shot peening processing for the object S to be measured. The positron lifetime shown in FIG. 11 is the average value of positron lifetimes measured many times in the object S, and can be used as a parameter reflecting the amount and size of defects in the material. In this case, the lifetime distribution is the result of analysis by a model function assuming that the positron lifetime has a spread of logarithmic normal distribution, and can be used as a parameter reflecting the number of lifetime components. ΔT ₀ can also be used as a parameter reflecting the number of lifetime components in the material. Further, when the projection time is 0, it means that the shot peening process is not performed. Therefore, the positron lifetime and lifetime distribution when the projection time is 0 show the positron lifetime and lifetime distribution of the reference specimen. Further, the shift amount ΔT ₀ when the projection time is 0 coincides with the time T ₀ at which the positron lifetime spectrum of the reference specimen is 0 hours.

As shown in FIG. 11, the positron lifetime increased as the projection time increased from 0 seconds, and changed little when the projection time exceeded about 100 seconds. In addition, the lifetime distribution increased between 0 and 36 seconds of projection time and decreased between 36 and 240 seconds of projection time, but did not reach 0 picoseconds even when projection time reached 240 seconds. . If the defects existing in the object S are either dislocations or vacancies, the lifetime distribution theoretically becomes 0 when the projection time is long (for example, 240 seconds). This is because the positrons are fully trapped in the defects, so there is only one type of annihilation site. For this reason, defects of both dislocations and vacancies are present inside the object S to be measured when the projection time is long.

On the other hand, the absolute value of the shift amount ΔT ₀ increases (that is, changes downward in the graph of FIG. 11) during the projection time from 0 to 72 seconds, and decreases during the projection time from 70 seconds to 240 seconds. (ie, shifted upwards in the graph of FIG. 11). Even in the range where the positron lifetime and lifetime distribution hardly change even if the projection time is lengthened (projection time 120 to 240 seconds), the shift amount ΔT ₀ changed greatly. Therefore, it can be said that the internal defects of the object S to be measured can be evaluated in more detail by using the shift amount ΔT ₀ together with the positron lifetime and the lifetime distribution.

FIG. 12 shows the relationship between the positron lifetime, the lifetime distribution, and the shift amount ΔT ₀ reproduced using the Monte Carlo simulation. The horizontal axis indicates the speed at which positrons are trapped by defects (trapping speed). FIG. 12 shows the positron lifetime, the lifetime distribution, and the shift amount ΔT ₀ when the defects existing in the object S are only dislocations, and the positron Life, life distribution, and shift amount ΔT ₀ are shown.

As shown in FIG. 12, when the defects existing in the object S to be measured were only dislocations, both the lifetime distribution and the shift amount ΔT ₀ were about 0 picoseconds at a trapping speed of 10 ¹³ s ⁻¹ . On the other hand, when both dislocation and vacancy defects are present in the object S, the lifetime distribution is 26.7 picoseconds at a trapping speed of 10 ¹³ s ^-1 and the shift amount ΔT ₀ is -2.1 picoseconds. seconds, and none of them went to about 0 picoseconds. Here, the projection time of 240 seconds shown in FIG. 11 is considered to be greater than or equal to the trapping speed of 10 ¹¹ s ⁻¹ in the graph of FIG. 12 . This is because ΔT ₀ has a wider dynamic range than the positron lifetime and lifetime distribution, and is sensitive even when the trapping speed is 10 ¹¹ s ⁻¹ or more, so the difference in sensitivity can be used for determination. Therefore, since the lifetime distribution is not 0 picoseconds at the projection time of 240 seconds in FIG. Thus, it was shown that the state of internal defects in the object S to be measured can be evaluated with higher accuracy by using the positron lifetime, lifetime distribution, and shift amount ΔT ₀ .

In this embodiment, prior to the calculation of the time _TS0 at which the positron lifetime spectrum of the object S to be measured is 0 hours in steps S18 and S20, the positron lifetime spectrum of the reference specimen S in steps S12 and S14 is 0 hours. Although the process for identifying the time _T0 at which is achieved is performed, the configuration is not limited to this. It is sufficient if the computing device 50 can acquire the time _T0 at which the positron lifetime spectrum of the reference specimen is 0. For example, the time T0 at which the positron lifetime spectrum of the reference specimen is ₀ can be obtained together with the type of the reference specimen. , may be stored in the storage unit 52 in advance.

In this embodiment, as an index for evaluating the internal state of the object S, the time T0 at which the positron lifetime spectrum of the reference specimen is ₀ hours and the positron lifetime spectrum of the object S are 0 hours. Although the difference (shift amount) ΔT ₀ from the time T _S0 is used, the configuration is not limited to this. For example, for two objects to be measured, the difference (shift amount) between the two may be calculated from the time at which the positron lifetime spectrum is 0 hours, and the difference (shift amount) between the two may be relatively evaluated. .

Although specific examples of the technology disclosed in this specification have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. In addition, the technical elements described in this specification or in the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing.

10: Positron annihilation characteristic measuring device 12: First γ-ray detector 14: Second γ-ray detector 40: Positron detector 50: Arithmetic device 52: Storage unit 54: Input unit 60: Light shielding container 424: Positron beam source S: Subject measuring body

Claims (5)

A positron annihilation characteristic measuring device for measuring the annihilation characteristics of positrons that enter a body and are annihilated in the body,
a positron beam source disposed at a position close to or in close contact with the surface of the object to be measured;
a first radiation detection means arranged at a first position set with respect to the positron beam source and detecting a first radiation generated when positrons are generated by the positron beam source;
a second radiation detection means arranged at a second position set with respect to the positron beam source for detecting a second radiation generated when the positrons generated by the positron beam source are annihilated;
Positron detection means for detecting positrons not incident on the object among the positrons generated by the positron beam source;
annihilation characteristics calculation means for calculating annihilation characteristics of positrons in the body to be measured based on the detection results of the first and second radiation detection means and the detection results of the positron detection means,
The extinction characteristic calculation means includes:
acquiring a measurement result of "lifetime-positron count" from the time difference between the time when the first radiation detection means detects the first radiation and the time when the second radiation detection means detects the second radiation;
By fitting the measurement result of the "lifetime - number of positrons", the time 0 of the positron lifetime of the object to be measured is calculated,
A positron annihilation characteristic measuring apparatus for calculating a shift amount between a reference time acquired according to the type of the object to be measured and the calculated time 0 . The reference time is set in a state in which a reference specimen, which is made of the same kind of material as the kind of the object to be measured and from which internal defects have been removed, is arranged in a position close to or in close contact with the positron beam source, The time 0 of the positron lifetime estimated by fitting the positron lifetime spectrum of the reference specimen, which is calculated based on the detection results detected by the first radiation detection means and the second radiation detection means. Item 2. The positron annihilation characteristic measurement device according to item 1. further comprising a storage unit for storing the reference time and an input unit for inputting the type of the object to be measured;
The storage unit is capable of storing the reference time for each type of object to be measured,
3. The method according to claim 1, wherein said extinction characteristic calculation means calculates said shift amount using said reference time stored in said storage section corresponding to the type of said object to be measured input by said input section. The positron annihilation property measurement device described. The annihilation characteristic calculation means further acquires the measurement result of the lifetime distribution from the time difference between the time when the first radiation detection means detects the first radiation and the time when the second radiation detection means detects the second radiation. The positron annihilation characteristics measuring device according to any one of claims 1 to 3. A method for measuring annihilation characteristics of positrons that enter a body and are annihilated in the body,
A positron beam source, a first detector for detecting first radiation generated when positrons are generated by the positron beam source, and a second detector for detecting positrons generated when the positrons generated by the positron beam source are annihilated. an adjustment step of adjusting the position between the second detector that detects radiation;
Using the positron beam source, the first detector, and the second detector whose positions have been adjusted in the adjusting step, the first radiation is detected by the first detector with respect to the first object to be measured. A first time calculation step of calculating a first time that is the time 0 of the positron lifetime estimated by fitting the positron lifetime spectrum from the time difference between the time when the second radiation is detected by the second detector and the time when the second radiation is detected. and,
using the positron beam source, the first detector, and the second detector whose positions are adjusted in the adjusting step, the first detection for a second object to be measured which is different from the first object to be measured; A second time that is the time 0 of the positron lifetime estimated by fitting the positron lifetime spectrum from the time difference between the time when the first radiation is detected by the detector and the time when the second radiation is detected by the second detector. a second time calculation step of calculating
and a shift amount calculating step of calculating a shift amount between the first time and the second time.

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