JP7258353B2 - Positron annihilation property measuring device and measuring method - Google Patents

Description

The present invention relates to a positron annihilation property measuring apparatus and method, and more particularly to a measuring apparatus and method for evaluating the state inside a substance by measuring the positron annihilation property inside the substance.

Conventionally, the state (defects, etc.) within a material has been evaluated and estimated by measuring the time it takes for the positron irradiated in the material to disappear (hereinafter also referred to as the "positron lifetime"). there is

For example, in Patent Document 1, a positron beam source is sandwiched between two pieces of material to be measured, and a scintillator and a photomultiplier tube are arranged outside each of the pieces of material to be measured to detect gamma rays incident on the two scintillators. Disclosed is an apparatus and method for measuring positron lifetimes using

Further, Patent Documents 2 and 3 by the present inventors disclose a positron beam source arranged near an object to be measured, a first γ-ray detector for detecting γ-rays generated when positrons are annihilated, and a γ-ray detector for detecting γ-rays generated when positrons are generated. Disclosed is a method for measuring positron lifetimes by means of a positron annihilation characteristic measuring apparatus including a second γ-ray detector for detecting radiation and a positron detector for detecting positrons that have not been incident on an object to be measured.

JP 2017-198552 A Japanese Patent No. 5843315 JP 2018-40645 A

Positron lifetime measurements as disclosed in Patent Documents 1 to 3 may not always be able to sufficiently, accurately, and stably evaluate the internal state of the object to be measured. Therefore, the present invention focuses on new positron annihilation characteristics in the measurement of positron lifetimes, and provides a positron annihilation characteristics measurement apparatus and apparatus capable of sufficiently, accurately and stably evaluating the internal state of an object to be measured. The purpose is to provide a measurement method.

A positron annihilation property measuring device for measuring the annihilation property of positrons incident on and annihilated in the object to be measured according to one aspect of the present invention is (a) arranged at a position close to or in close contact with the surface of the object to be measured; and (b) first radiation and positron beams disposed at first and second positions set with respect to the positron beam source and generated when positrons are produced by the positron beam source. (c) first radiation detection means and second radiation detection means for detecting second radiation produced when positrons produced by the source are annihilated; (d) positron annihilation characteristics 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; and an extinction characteristic calculation means for calculating.

The extinction characteristic calculation means of one aspect of the present invention includes:
(d1) When the first radiation detection signal from the first radiation detection means is used as the start signal and the second radiation detection signal from the second radiation detection means is used as the stop signal, the difference between the start signal and the stop signal obtain the measurement result of the first "life time one positron number" from the time difference between
(d2) Between the start signal and the stop signal when the detection signal of the first radiation by the second radiation detection means is the start signal and the detection signal of the second radiation by the first radiation detection means is the stop signal Obtain the measurement result of the second "life time one positron number" from the time difference of
(d3) Two positron lifetimes calculated by fitting each of the positron lifetime spectra after obtaining two positron lifetime spectra of the object to be measured from the measurement results of the first and second “lifetime number of positrons” Calculate the 0 hour average time of the spectrum,
(d4) Calculate the amount of shift between the reference time obtained according to the type of the object to be measured and the calculated average time.

According to one aspect of the present invention, the method of measuring the extinction characteristics of positrons that are incident on an object to be measured and are lost in the object to be measured includes:
(i) detecting a positron beam source, first radiation produced when positrons are produced by said positron beam source, and second radiation produced when positrons produced by said positron beam source are annihilated; , an adjusting step of adjusting the position between the first detector and the second detector;
Using the positron beam source, the first detector, and the second detector whose positions are adjusted in the adjustment step, for the first object to be measured,
(ii) calculating the first time, which is the 0 hour of the positron lifetime spectrum, 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; 1 hour calculation step;
(iii) calculating a second time that is 0 hours of the positron lifetime spectrum from the time difference between the time when the first radiation is detected by the second detector and the time when the second radiation is detected by the first detector; 1 hour calculation step;
(iv) calculating an average time of the first time and the second time;
Using the positron beam source, the first detector, and the second detector whose positions are adjusted in the adjustment step, for a second object to be measured that is different from the first object to be measured,
(v) calculating a third time that is 0 hours of the positron lifetime spectrum 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; 3 hours calculation process;
(vi) calculating a fourth time that is 0 hours of the positron lifetime spectrum from the time difference between the time when the first radiation is detected by the second detector and the time when the second radiation is detected by the first detector; a 4-hour calculation step;
(vii) calculating the average time of the third time and the fourth time;
(viii) Positron annihilation characteristics, comprising a shift amount calculation step of calculating a shift amount between the average time of the first time and the second time and the average time of the third time and the fourth time. Measuring method.

According to the present invention, the following effects can be obtained.
(1) By calculating 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, It is possible to evaluate the internal state of the object to be measured, which cannot be determined by the evaluation based only on the positron lifetime.
(2) At that time, the calculation of the 0 hour of the positron lifetime spectrum is calculated as the average time of the 0 hours of the two positron lifetime spectra, and the reference time obtained according to the type of the object to be measured and the calculated average time By calculating the amount of shift between , the drift of the positron lifetime spectrum can be corrected (offset), and the accuracy of evaluation of the internal state of the object to be measured can be improved.

1 is a perspective view schematically showing a positron annihilation characteristic measuring apparatus according to one embodiment of the present invention; FIG. 1 is a diagram schematically showing the configuration of a positron annihilation characteristic measuring apparatus according to an embodiment of the present invention; FIG. 1 is an exploded perspective view of a positron detection unit according to one embodiment of the present invention; FIG. The figure which shows the structure of the light-shielding container of one Embodiment of this invention. The figure which shows the state which installed the sample holder of one Embodiment of this invention in the light-shielding container. 1 is a block diagram showing the configuration of an arithmetic unit according to one embodiment of the present invention; FIG. FIG. 4 is a diagram for explaining the process of removing noise based on the detection result of the positron detector according to one embodiment of the present invention; FIG. 4 is a diagram for explaining the process of removing noise based on the detection result of the positron detector according to one embodiment of the present invention; A flow chart showing an example of a process of calculating a time shift amount at which the positron lifetime spectrum of the object to be measured is 0 hours according to the first embodiment of the present invention. Schematic diagram for explaining the process of calculating the time at which the positron lifetime spectrum of the object to be measured in Example 1 of the present invention becomes 0 hours. A diagram showing the relationship between the time required to introduce defects into an object to be measured by shot peening processing in Example 1 of the present invention, the positron lifetime, the lifetime distribution, and the shift amount of time when the positron lifetime spectrum of the object to be measured becomes 0 hours. . The relationship between the positron lifetime and the lifetime distribution reproduced using the Monte Carlo simulation of Example 1 of the present invention, the shift amount of time when the positron lifetime spectrum of the object to be measured becomes 0 hours, and the rate at which positrons are captured by defects. illustration. The schematic diagram of the dual measurement of Example 2 of this invention. FIG. 10 is a schematic diagram for explaining the principle of offsetting drift in Example 2 of the present invention; FIG. 3 shows the energy spectrum and energy window of ²² Na in Example 2 of the present invention. The figure which shows the example of the positron lifetime spectrum obtained by the dual measurement of Example 2 of this invention. FIG. 10 is a diagram showing the temporal stability of T ₀ obtained by repeated measurements in Example 2 of the present invention; FIG. 8 is a diagram showing the correlation between the results of Δ and □ in FIG. 7;

Embodiments of the present invention will be described with reference to the drawings (particularly FIGS. 1 to 8). FIG. 1 is a perspective view schematically showing a positron annihilation property measuring apparatus according to one embodiment of the present invention. 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 can detect either one or both of γ-rays (1.27 MeV) generated when positrons are generated and γ-rays (511 keV) generated when positrons are annihilated. . 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 . When the first γ-ray detector 12 detects a γ-ray (1.27 MeV) generated when a positron is generated or a γ-ray (511 keV) generated when a positron annihilates, the first γ-ray detector 12 sends a pulsed electric current to the arithmetic unit 50. Output a signal. 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 can detect either one or both of γ-rays (1.27 MeV) generated when positrons are generated and 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 . When the second γ-ray detector 14 detects a γ-ray (1.27 MeV) generated when a positron is generated or a γ-ray (511 keV) generated when a positron annihilates, a pulse-like electric Output a signal. 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 the "second radiation detector" and the "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 has a scintillator 422 and a light guide 44 for positron detection. 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 disappears at 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, for 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. Further, the mechanism for switching between the open state and the closed state of the open/close door 62 that prevents light from entering the light shielding container 60 from the outside by closing the open/close door 62 is not particularly limited. , may be configured to be switched manually by the user, or may be configured to be automatically switched by the control device driving the actuator.

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 SUS, for example. 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 the present 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 dissipation 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 including a CPU, ROM, and RAM, and a dedicated circuit such as a digital storage oscilloscope (DS0) or NIM module. Arithmetic device 50 is connected to first signal processor 20, which is connected to first γ-ray detector 12 and second γ-ray detector 14 via channel 1 (CH1) and channel 2 (CH2), and to positron detector 40. A second signal processing unit 30 is provided. 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 or the second γ-ray detector 14 . The positron annihilation time specifying unit 22 specifies the positron annihilation time based on the signal from the first γ-ray detector 12 or 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 extracts the time difference calculated by the time difference calculation unit 23 from the time specified by the second signal processing unit 30 (that is, the time when the positron entered the scintillator 422). Exclude those associated with non-injected positrons. 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 γ-rays (1.27 MeV) are detected when positrons are generated, then scintillation light is detected, and then γ-rays (511 keV) are detected when positrons are extinguished, the positron beam Positrons emitted from source 424 can be identified as incident on scintillator 422 . On the other hand, if the γ-ray (1.27 MeV) produced when the positron was generated was detected, and then the γ-ray (511 keV) produced when the positron was annihilated was detected without detecting scintillation light, the positron beam source It can be specified that the positron emitted from 424 has 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 dissipation time t5 are excluded from the data for calculating the positron lifetime 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 intestinal electron generation time t3 and the positron detection time t4 is within a predetermined first time difference, the positron generation time t3 and the positron disappearance time t5 detected thereafter are excluded as invalid data. good too. 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 To at which the positron lifetime spectrum of the reference specimen described later becomes 0 hours, in association with the type of the reference specimen. Regarding the time To, the input unit 54, which will be described in detail later, 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 inventors of the present invention calculated the time at which the positron lifetime spectrum is 0 hours for each of a plurality of objects S having the same positron lifetime. I focused on the 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. .

9 to 12, the process of calculating the time shift amount at which the positron lifetime spectrum of the object to be measured S of Example 1 is 0 hours will be described. As shown in FIG. 9, the computing device 50 first measures the positron lifetime of the reference specimen (SI2). 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).

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 the first 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.

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 at which the positrons disappeared based on the signal from the second γ-ray detector 14, and calculates the survival time of the positrons from the time difference.

On the other hand, the positrons incident on the scintillator 422 generate scintillation light, and then disappear 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 first embodiment, the positrons that have not entered the reference specimen are detected by the positron detector 40, and the positrons that have not entered the reference specimen are detected. Eliminate the generated radiation 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.

When 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 is 0 hours (SI4). 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 SI2, the time To (so-called reference time) at which the positron lifetime spectrum in the reference specimen is 0 hours is obtained. The arithmetic unit 50 associates the acquired time To 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 (SI6).

In the conventional method as exemplified in Patent Document 1, the positron beam source is sandwiched between two test pieces cut out from the object to be measured, and the radiation (γ rays) when the positrons are annihilated is measured. In this method, since the position of the positron beam source is shifted with respect to the measurement object and the detector for each measurement, it is difficult to accurately specify the time To at which the positron lifetime spectrum becomes 0 hours. In the positron annihilation characteristics measurement device 10 of the first embodiment, the positional relationship among the positron ray source 424, the first γ-ray detector 12, and the second γ-ray detector 14 is constantly changing. fixed without Therefore, the time To 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 (SI8). 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 SI2 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 obtains the measurement result obtained in the measurement of SI8 (that is, the measurement result of the positron lifetime spectrum (“life time (time) one 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 Tso 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 to the measurement results obtained in the SI8 measurement. 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 Tso at which the positron lifetime spectrum of the object S is 0 hours is calculated to be smaller (on the negative side) than the time To.

Finally, the arithmetic unit 50 calculates the time shift amount ΔTo at which the positron lifetime spectrum of the object S to be measured is 0 hours from the time To specified in step SI4 and the time Tso calculated in step S20. (S22).

FIG. 11 shows the relationship between the positron lifetime, the lifetime distribution, and the shift amount ΔTo measured or calculated using the positron dissipation characteristics measuring apparatus 10 of the first embodiment for the object S to which defects have been introduced by shot peening. ing. 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. ΔTo 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 ΔTo when the projection time is 0 coincides with the time To when 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 that 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 ΔTo increased during the projection time from 0 to 72 seconds (that is, changed downward in the graph of FIG. 11), and decreased 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 ΔTo changed greatly. Therefore, by using the shift amount ΔTo together with the positron lifetime and lifetime distribution, it can be said that the internal defects of the object S to be measured can be evaluated in more detail.

FIG. 12 shows the relationship between the positron lifetime, the lifetime distribution, and the shift amount ΔTo 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 ΔTo when the defects existing in the object S are only dislocations, and the positron lifetime when the object S has both defects of dislocations and vacancies. , life distribution and shift amount ΔTo.

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 ΔTo were about 0 picoseconds at a trapping speed of 10 ¹³ /S. 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, and the shift amount ΔTo is -2.1 picoseconds. None of them reached 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 in the graph of FIG. 12 . This is because ΔTo has a wider dynamic range than the positron lifetime and lifetime distribution, and is sensitive even when the trapping speed is 10 ¹¹ /S or more, so the difference in sensitivity can be used for determination. Therefore, it can be said that both dislocation and vacancy defects are present in the object S because 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 high accuracy by using the positron lifetime, the lifetime distribution, and the shift amount ΔTo.

Note that in the first embodiment, prior to the calculation of the time Tso at which the positron lifetime spectrum of the object S to be measured is 0 hours in steps SI8 and S20, the positron lifetime spectrum of the reference specimen is 0 hours in steps SI2 and SI4. Although the process of identifying the time To at which .theta. It is sufficient if the computing device 50 can acquire the time To at which the positron lifetime spectrum of the reference specimen is 0. For example, the time To at which the positron lifetime spectrum of the reference specimen is 0 is preliminarily set together with the type of the reference specimen. It may be stored in the storage unit 52 .

In addition, in the first embodiment, the time To at which the positron lifetime spectrum of the reference test sample is 0 hours and the positron lifetime spectrum of the sample S at 0 hours are used as indexes for evaluating the internal state of the object S to be measured. Although the difference (shift amount) ΔTo from the time Tso 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. .

Next, as Example 2, a method for improving the accuracy of calculation of the shift amount in Example 1 and more accurately and stably evaluating the state of the internal defect of the object to be measured will be described. In Example 2, as a basic flow (process), the measuring method using the positron annihilation characteristic measuring apparatus 10 of one embodiment of the present invention described with reference to FIGS. 1 to 8 and FIGS. 12 is utilized. At that time, when scintillation radiation detectors are used as the first γ-ray detector 12 and the second γ-ray detector 14 of the apparatus 10, the positron lifetime due to the temporal stability of the photomultiplier tubes and high-voltage power supply constituting them Spectral drift occurs. Specifically, drift of the positron lifetime spectrum occurs due to, for example, fluctuations in the output of the high-voltage power supply and instability of the measurement system.

As a result, there is a possibility that the measurement accuracy of the shift amount ΔTo between the time To and Tso at which the positron lifetime spectrum becomes 0 hours, and thus the accuracy of the defect evaluation may be lowered. In particular, since the shift amount ΔTo requires a measurement accuracy of 1 ps or less, in order to enhance the effect of ΔTo evaluation, it is necessary to minimize the drift of the positron lifetime spectrum due to the temporal stability of the photomultiplier tube, high-voltage power supply, etc. There is a need to. Therefore, in the second embodiment, as described below, a method of correcting (offsetting) the drift of the positron lifetime spectrum using dual measurement is proposed.

In Example 1 described above, decay γ-rays (1.27 MeV) generated when positrons are generated by the positron beam source ( ²² Na) 424 are detected by the first γ-ray detector 12 . An annihilation gamma ray (511 keV) generated when the positrons incident on the reference specimen and the object S are annihilated by combining with electrons is detected by the second gamma ray detector 14 . That is, the first γ-ray detector 12 does not detect annihilation γ-rays (511 keV), and the second γ-ray detector 14 does not detect decay γ-rays (1.27 MeV).

In contrast, in the dual measurement of the second embodiment, the first γ-ray detector 12 detects decay γ-rays (1.27 MeV), the second γ-ray detector 14 detects annihilation γ-rays (511 keV), Furthermore, in parallel, the first γ-ray detector 12 detects annihilation γ-rays (511 keV), the second γ-ray detector 14 detects decay γ-rays (1.27 MeV), and simultaneously obtains two positron lifetime spectra. , the time To at which the positron lifetime spectrum becomes 0 hours is obtained. The method of obtaining the time To at which the positron lifetime value spectrum and the positron lifetime spectrum are 0 hours is the same as in the case of the first embodiment.

FIG. 13 is a diagram showing an outline of dual measurement according to the second embodiment. In Example 1, the lifetime value (Lifetime_1) is measured as shown in FIG. 13(a). In the dual measurement of Example 2, the lifetime value (Lifetime_2) is also measured in parallel as shown in FIG. 13(b). This allows two positron lifetime spectra to be obtained at the same time, effectively doubling the counts. Then, by averaging ((T _O1 +T _O2 )/2) the time To at which the two positron lifetime spectra simultaneously obtained by dual measurement are 0 hours, the drift of the positron lifetime spectra can be canceled.

14A and 14B are schematic diagrams for explaining the principle of drift cancellation in the second embodiment. In Spectrum_1 of FIG. 14(a), the time axis of the positron lifetime spectrum advances to the right, but in Spectrum_2 of (b), the time axis of the positron lifetime spectrum reverses and advances to the left. is also reversed. As a result, drift is canceled by averaging over time To.

15 is a diagram showing the energy spectrum and energy window of ²² Na in Example 2. FIG. In the first embodiment, the energy window range is set as shown in FIG. 15(1) as indicated by the dashed line, and the start signal (photo- Record the stop signal (photo-peak of 0.511 MeV) on Channel_2. In the dual measurement of the second embodiment, a wide energy window range is set as shown in FIG. 15(2) so that both the start signal and the stop signal are included, and The gamma ray detection signal from 14 is recorded. The recorded signals are distinguished into start and stop signals on the program executed in the DSO. In this way, two positron lifetime spectra can be simultaneously obtained by dual measurement using the signals from Detector_1 and Detector_2 as a stop signal and a start signal, respectively.

FIG. 16 shows an example of a positron lifetime spectrum obtained by the dual measurement of Example 2. In FIG. A wide window was set for channel_1 and channel_2 of the DSO (FIG. 15(2)), and after recording the detected signal waveforms in the window, the signals were discriminated into start and stop signals on the program executed in the DSO. Spectrum_1 (FIG. 16(a)) is a positron lifetime spectrum obtained with DSO channel_1 as the start and channel_2 as the stop as shown in FIG. 13(a). Spectrum_2 (FIG. 16(b)) is a positron lifetime spectrum obtained with DSO channel_2 as the start and channel_1 as the stop as shown in FIG. 13(b). The positron lifetime obtained by analyzing the positron lifetime spectrum of Spectrum_1 was about 105.9 ps, and the positron lifetime obtained by analyzing the positron lifetime spectrum of Spectrum_2 was about 105.0 ps.

FIG. 17 shows the temporal stability of time _T0 obtained by repeated measurements using the dual measurement of Example 2. As shown in FIG. In FIG. 17, Δ is the time To of Spectrum_1, □ is the time To of Spectrum_2, and ● is the average of Δ and □ at the same time. Δ and □ correspond to the measurement in Example 1, and ● corresponds to the dual measurement in Example 2. The standard deviation of Δ (To of Spectrum — 1) was 2.20 ps, the standard deviation of □ (To of Spectrum — 2) was 2.30 ps, and the standard deviation of ● (average of Δ and □) was 0.35 ps. The standard deviation of the measurement ● (average of △ and □) by the dual measurement of this Example 2 is about one digit smaller than the standard deviation of the measurement of Example 1, and the change over time of time To is greatly reduced. indicates that

FIG. 18 is a diagram showing correlations for the results of Δ and □ in FIG. 17 . In FIG. 18, a strong negative correlation was observed between Δ and □. When linear approximation was performed, the correlation coefficient R was −0.9527. In FIG. 17, the fact that the change over time of time To is greatly reduced by averaging is considered to be due to this strong correlation. It is considered that the drift of the positron lifetime spectrum was canceled by averaging the time To as in the second embodiment. As a result, according to the second embodiment, it is possible to maximize the accuracy of defect evaluation using the shift amount ΔTo of the time To at which the positron lifetime spectrum becomes 0 hours.

Embodiments of the present invention have been described with reference to the drawings. However, the invention is not limited to these embodiments. Furthermore, the present invention can be implemented with various improvements, modifications, and variations based on the knowledge of those skilled in the art without departing from the scope of the invention.

The apparatus and method for measuring positron dissipation characteristics of the present invention can be used and applied in research and diagnosis of metal fatigue.

10: Positron extinction characteristic measuring device 12: First γ-ray detector 14: Second γ-ray detector 20: First signal processing unit 30: Second signal processing unit 40: Positron detector 50: Arithmetic unit 52: Storage unit 54: Input unit 60: light shielding container 424: positron beam source S: object to be measured

Claims (5)

A positron annihilation characteristic measuring device for measuring the annihilation characteristics of positrons that are incident on and annihilated in the body to be measured,
a positron beam source disposed at a position close to or in close contact with the surface of the object to be measured;
First radiation and positrons generated by the positron beam source, which are arranged at first and second positions set with respect to the positron beam source and are generated when positrons are generated by the positron beam source a first radiation detection means and a second radiation detection means for detecting a second radiation emitted when the is 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:
When the detection signal of the first radiation in the first radiation detection means is set as the start signal and the detection signal of the second radiation in the second radiation detection means is set as the stop signal, the time between the start signal and the stop signal is Acquire the measurement result of the first "life time one positron number" from the time difference,
When the detection signal of the first radiation in the second radiation detection means is set as a start signal and the detection signal of the second radiation in the first radiation detection means is set as a stop signal, the time between the start signal and the stop signal is Acquire the measurement result of the second "lifetime number of positrons" from the time difference,
After obtaining two positron lifetime spectra of the object to be measured from the measurement results of the first and second "number of lifetimes per positron", the two positrons calculated by fitting each of the positron lifetime spectra Calculate the average time of 0 hours of the lifetime spectrum,
A positron dissipation 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 average time. 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 positron according to claim 1, which is a 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 first radiation detection means and the second radiation detection means. An extinction characteristic measuring device. 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 apparatus according to claim 1, wherein said extinction characteristic calculating 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 dissipation characteristic measurement device described. Claims 1 to 3, wherein said extinction characteristic calculating means repeats the acquisition of the measurement result of said first "number of positrons per lifetime" to the calculation of said average time, and further acquires the time-series distribution of said average time. The positron annihilation characteristic measurement device according to any one of . A method for measuring the extinction characteristics of positrons that are incident on an object and disappear in the object, comprising:
detecting a positron beam source, first radiation produced when positrons are produced in the positron beam source, and second radiation produced when the positrons produced in the positron beam source are annihilated; including an adjusting step of adjusting the position between the detector and the second detector;
Using the positron beam source, the first detector, and the second detector whose positions are adjusted in the adjusting step, for the first object to be measured,
A first time that is 0 hours of the positron lifetime spectrum 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 time calculation step;
A second time that is 0 hours of the positron lifetime spectrum is calculated from the time difference between the time when the first radiation is detected by the second detector and the time when the second radiation is detected by the first detector. a time calculation step;
calculating the average time of the first time and the second time;
Using the positron beam source, the first detector, and the second detector whose positions are adjusted in the adjustment step, for a second object to be measured that is different from the first object to be measured,
A third time that is 0 hours of the positron lifetime spectrum 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 time calculation step;
A fourth time that is 0 hours of the positron lifetime spectrum is calculated from the time difference between the time when the first radiation is detected by the second detector and the time when the second radiation is detected by the first detector. a time calculation step;
calculating the average time of the third time and the fourth time;
a shift amount calculating step of calculating a shift amount between the average time of the first time and the second time, and the average time of the third time and the fourth time. Characteristic measurement method.

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