Positron annihilation lifetime measurement system and method based on position positioning

Abstract

The present invention provides a positron annihilation lifetime measurement system and method based on position positioning, comprising a source chamber, a starting gamma detector, a first stopping gamma detector and a second stopping gamma detector; a sample is arranged on one side of a radiation source, and the sample and the radiation source are fixed at the center of the source chamber; the first stopping gamma detector and the second stopping gamma detector are symmetrically placed outside the source chamber with the radiation source as the center, and the starting gamma detector is placed at any position outside the source chamber; the starting gamma detector is responsible for 22 The Na radioactive source detects the accompanying gamma rays produced by the cascade when positrons are emitted. The first and second stop gamma detectors respectively detect the two oppositely emitted annihilation gamma rays produced after the positron annihilation. The present invention accurately locates the positron annihilation location by measuring the time difference between the detection of the annihilation gamma rays, enabling the measurement of the positron annihilation lifetime in thin film samples.

Description

Technical Field

[0001] The present invention belongs to the technical field of nuclear spectroscopy and nuclear detection, and in particular relates to a positron annihilation lifetime measurement system and method based on position positioning. Background Art

[0002] Positron annihilation is a nondestructive and sensitive probe of atomic-level defects in materials, offering unique advantages in studying the distribution and evolution of material defects. Positron annihilation lifetime measurement characterizes the type and concentration of defects in a material by measuring the annihilation lifetime of positrons in the material.

[0003] Conventional positron annihilation lifetime measurements often use two identical and sufficiently thick samples tightly packed together. 22 Na radioactive source, forming a "sample-radioactive source-sample" sandwich measurement structure. 22 When a Na radioactive source decays and releases a positron, it emits a 1.28 MeV initial gamma ray. When the positron annihilates in the sample, it produces two 0.511 MeV annihilation gamma rays in opposite directions. Typically, a gamma detector is used to detect the occurrence of the initial gamma ray as the start time of positron generation, and another gamma detector is used to detect the occurrence time of one of the two annihilation gamma rays as the stop time of the positron. The time difference between the start and stop times is calculated to obtain the positron annihilation lifetime spectrum. Because the positrons generated by the radioactive source have a continuous energy spectrum and a certain range distribution within the material, current measurement structures have strict requirements for the thickness of the measured sample, ensuring that positrons of all energies are annihilated in the sample. However, for thin film samples, positrons may penetrate the film and annihilate in other materials, which will introduce measurement noise into the positron annihilation lifetime spectrum and hinder the accurate characterization of the microstructure of the thin film sample.

[0004] Therefore, how to accurately locate the positron annihilation position, exclude the positron annihilation components in materials other than thin film samples, and screen out positron annihilation cases in samples will be the key to applying conventional methods based on radioactive sources to measure the positron annihilation lifetime of thin film samples. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides a positron annihilation lifetime measurement system and method based on position positioning, comprising a source chamber, a starting gamma detector, a first stopping gamma detector and a second stopping gamma detector; a sample is arranged on one side of the radiation source, and the sample and the radiation source are fixed at the center of the source chamber; the first stopping gamma detector and the second stopping gamma detector are symmetrically placed outside the source chamber with the radiation source as the center, and the starting gamma detector is placed at any position outside the source chamber; the starting gamma detector is responsible for 22The Na radioactive source detects the 1.28 MeV gamma rays generated by the cascade when positrons are emitted. The first and second stop gamma detectors detect the two oppositely emitted 0.511 MeV annihilation gamma rays produced after the positron annihilation, respectively. By measuring the time difference between the detection of the two annihilation gamma rays, the present invention accurately locates the positron annihilation location and screens for positron annihilation instances in the sample, overcoming the limitations of sample thickness and enabling measurement of the positron annihilation lifetime in thin film samples.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A positron annihilation lifetime measurement system based on position positioning includes a source chamber, a starting gamma detector, a first stopping gamma detector and a second stopping gamma detector; a sample is set on one side of the radiation source, and the sample and the radiation source are fixed in the center of the source chamber; the first stopping gamma detector and the second stopping gamma detector are symmetrically placed outside the source chamber with the radiation source as the center, and the starting gamma detector is placed at any position outside the source chamber; the starting gamma detector is responsible for 22 The 1.28MeV gamma rays generated by the cascade when the Na radioactive source emits positrons are detected, and the first stop gamma detector and the second stop gamma detector respectively detect the two oppositely emitted 0.511MeV annihilation gamma rays generated after the positron annihilation.

[0008] Furthermore, the source chamber and the stop gamma detector are used to screen out positron annihilation cases in the sample; the interior of the source chamber is a vacuum environment, and the geometric shape, internal space size and thickness of the source chamber wall are specifically arranged according to the specific application scenario, and it is necessary to ensure that the positrons that have not entered the sample are annihilated on the source chamber wall far enough away from the sample, and the time difference between the two annihilated gamma photons is detected. Where ΔL is the optical path difference between the two detected annihilation gamma photons, c is the speed of light, and TR1 and TR2 are the time resolutions of the first- and second-stop gamma detectors, respectively. The first- and second-stop gamma detectors simultaneously detect the two counter-emitted 0.511 MeV annihilation gamma rays produced after the positron annihilation. By measuring the difference in the time difference ΔT between the two gamma rays annihilated in the sample and in the source chamber wall, the annihilation position of the positron is located, thereby eliminating positron annihilation cases incident on the source chamber wall and retaining only positron annihilation cases in the sample.

[0009] Furthermore, the start and first and second stop gamma detectors all use scintillators with high time resolution and fast-response photoelectric converters coupled thereto, the scintillators include BaF2, LaBr3, LYSO, and the photoelectric converters include photomultiplier tubes and silicon photomultiplier tubes.

[0010] The present invention also provides a measurement method of a positron annihilation lifetime measurement system based on position positioning, comprising:

[0011] Step 1: After the start gamma detector detects the gamma ray, it generates a start gamma signal; after the first stop gamma detector and the second stop gamma detector detect the gamma ray, they generate a first stop gamma signal and a second stop gamma signal respectively;

[0012] Step 2: The signal acquisition module is responsible for collecting the start gamma signal, the first stop gamma signal, and the second stop gamma signal, and transmitting the waveform data to the data processing module for processing;

[0013] Step 3: The data processing module performs energy identification, timing, annihilation case selection and lifetime spectrum statistics on the waveform data.

[0014] Furthermore, in step 3, for the start gamma signal and the stop gamma signal, the energy thresholds are set to the photoelectric peak of 1.28 MeV gamma rays and the photoelectric peak of 0.511 MeV gamma rays, respectively, to improve the time measurement accuracy and eliminate the interference of other gamma rays in the background environment, and the start timing signal, the first stop timing signal and the second stop timing signal are generated after timing respectively; within a certain time window, the time difference between the start timing signal and one of the stop timing signals is calculated as the lifetime of each positron annihilation case; only when the start gamma signal and the stop timing signal are detected at the same time, the start timing signal and the stop timing signal are generated. When the first and second stop gamma signals are present simultaneously within a certain time window, a discrimination signal is output to indicate whether a positron annihilation event has been recorded. If a discrimination signal is output, the event is a valid positron annihilation event in the sample, and a time difference calculation is performed. If no discrimination signal is output, the positron in the event did not annihilate in the sample, and the time difference calculation is suppressed. The data processing module then proceeds directly to the next event, thereby screening out positron annihilation events in the sample. Finally, the time difference corresponding to each valid positron annihilation event is statistically analyzed. After sufficient event accumulation, a positron annihilation lifetime spectrum is generated.

[0015] Beneficial effects:

[0016] (1) The present invention has a simple measurement structure and can accurately locate the positron annihilation position, fully exclude positron annihilation cases outside the thin film sample, screen out positron annihilation cases in the thin film, and realize accurate measurement of the positron annihilation lifetime in the thin film sample.

[0017] (2) The present invention has no restrictions on the thickness of the sample and is applicable to the measurement of thick samples while meeting the measurement requirements of thin film samples.

[0018] (3) The present invention has no restrictions on the type and quantity of samples. In principle, as long as the distance between the samples is greater than the position resolution capability of the system, the system can filter out the annihilation information of samples at different positions. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the positron annihilation lifetime measurement system based on position positioning of the present invention;

[0020] Figure 2 Schematic diagram of the measuring circuit of the present invention;

[0021] Figure 3 Schematic diagram of signal selection time window;

[0022] Figure 4 The minimum radius of the source chamber required to stop the gamma detector when using crystals of different diameters;

[0023] Figure 5 Schematic diagram of a measurement structure for systematically screening positron annihilation cases in different types of thin film samples. DETAILED DESCRIPTION

[0024] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0025] The position-based positron annihilation lifetime measurement system of the present invention can simultaneously detect two 0.511MeV gamma rays generated by positron annihilation. By measuring the time difference between the detection of these two annihilation gamma rays, the positron annihilation position can be accurately located, and positron annihilation cases in the sample can be screened out, breaking through the thickness limit of the measured sample and realizing the measurement of the positron annihilation lifetime in thin film samples.

[0026] like Figure 1 As shown, the position-based positron annihilation lifetime measurement system of the present invention includes a source chamber, a starting gamma detector, a first stopping gamma detector and a second stopping gamma detector. Different from the traditional "sample-radiation source-sample" measurement structure, the present invention only needs to set the sample on one side of the radiation source, and the sample and the radiation source are fixed in the center of the source chamber. The first stopping gamma detector and the second stopping gamma detector are symmetrically placed outside the source chamber with the radiation source as the center, and the starting gamma detector is placed at any position outside the source chamber. The starting gamma detector is responsible for 22The 1.28MeV gamma rays generated by the cascade when the Na radioactive source emits positrons are detected, and the first stop gamma detector and the second stop gamma detector respectively detect the two oppositely emitted 0.511MeV annihilation gamma rays generated after the positron annihilation.

[0027] because 22 Positrons generated by a Na radioactive source are emitted isotropically throughout space. Some positrons enter the sample and annihilate, while others do not enter or penetrate the sample, ultimately annihilating on the source chamber walls. Positron annihilation in both the sample and source chamber walls produces annihilation gamma photons, and the effect of positron annihilation events in the source chamber walls on the measurement must be eliminated.

[0028] In the present invention, two stop gamma detectors are set up to simultaneously detect the two reversely emitted 0.511 MeV annihilation gamma rays generated after positron annihilation, so as to exclude positron annihilation cases in non-samples and screen out positron annihilation cases in samples. Figure 1 As shown, the two annihilation gamma rays generated by the positrons incident on the source chamber wall outside the shaded solid angle will not be detected simultaneously by the two stop gamma detectors, so this part of the positron annihilation case can be ruled out.

[0029] The two annihilation gamma rays produced by the positrons that enter the sample and are incident on the source chamber wall within the solid angle of the shadow part can be detected simultaneously by the two stop gamma detectors. The time difference between the two annihilation gamma rays detected by the two stop detectors is ΔL is the optical path difference between the two annihilated gamma photons, and c is the speed of light. The time difference between the two gamma rays produced by annihilation in the sample and detected by the two stop detectors can be regarded as having an average value of 0 and a standard deviation of The single Gaussian function R(δ) can be expressed as follows:

[0030]

[0031] Where δ is the deviation of the time difference caused by the time resolution of the two stop gamma detectors, TR1 and TR2 are the time resolutions of the first and second stop gamma detectors respectively. The time difference between the two annihilation gamma rays generated by the annihilation of the source chamber wall and detected by the two stop detectors can be regarded as the average value The standard deviation is Therefore, the annihilation position of the positron can be determined by measuring the difference in the time difference between the two annihilation gamma rays being detected, thereby excluding the positron annihilation cases that are incident on the source chamber wall within the shadowed solid angle, and ultimately only retaining the positron annihilation cases in the sample.

[0032] In order to fully exclude positron annihilation cases in non-samples and screen out effective annihilation cases in samples, the measurement circuit designed by the present invention is as follows: Figure 2 shown.

[0033] The start gamma detector generates a start gamma signal upon detecting gamma rays. The first and second stop gamma detectors generate a first and second stop gamma signal, respectively, upon detecting gamma rays. The signal acquisition module collects the start gamma signal, the first and second stop gamma signals and transmits the waveform data to the data processing module for processing.

[0034] The data processing module is responsible for energy identification, timing, annihilation case selection and lifetime spectrum statistics of waveform data.

[0035] For the start gamma signal and stop gamma signal, the energy thresholds are set to the photoelectric peak of 1.28MeV gamma rays and the photoelectric peak of 0.511MeV gamma rays, respectively, to improve the time measurement accuracy and eliminate other gamma background interference. After timing, the start timing signal, the first stop timing signal, and the second stop timing signal are generated. Within the first time window (on the order of hundreds of nanoseconds), the time difference between the start timing signal and one of the stop timing signals is calculated as the lifetime of each positron annihilation event. Figure 3 As shown, only when the start gamma signal, the first stop gamma signal, and the second stop gamma signal are detected simultaneously within the first time window, and the first stop gamma signal and the second stop gamma signal exist simultaneously within the second time window, will the judgment signal for whether the positron annihilation case is recorded or not be output; the second time window should contain the vast majority of the first stop gamma signal and the second stop gamma signal, and the time window can be set to one-tenth of R(δ) (FWTM, Full width at one-tenth maximum), When a discrimination signal is output, it indicates that the event is a valid positron annihilation event in the sample, and a time difference calculation is performed. However, when no discrimination signal is output, it indicates that the positron in the event did not annihilate in the sample, and the time difference calculation is suppressed. The data processing module then directly proceeds to the next event, thereby eliminating positron annihilation events not in the sample. Finally, the time difference corresponding to each valid positron annihilation event is statistically analyzed. After sufficient events have been accumulated, a positron annihilation lifetime spectrum is generated.

[0036] To ensure the accuracy of positron annihilation lifetime measurement, both the start and stop gamma detectors use scintillator detectors with high time resolution, which are generally composed of a scintillator with good time performance (such as BaF2, LaBr3, LYSO, etc.) and a fast-response photoelectric converter coupled with it (such as a photomultiplier tube, silicon photomultiplier tube, etc.).

[0037] The interior of the source chamber is a vacuum environment. In principle, it can be of any geometric shape. The size of the internal space and the thickness of the source chamber wall are arranged according to the specific application scenario. It is necessary to ensure that the positrons that have not entered the sample are annihilated on the source chamber wall far enough away from the sample, and the time difference between the two annihilated gamma photons is required to be detected. Figure 1 The spherical source chamber is a specific embodiment of the present invention. The radius of the source chamber shell depends on the geometric dimensions and time resolution of the stopped gamma detector. If the time resolution of the two stopped gamma detectors is 100 ps, ​​the optical path difference between the two annihilated gamma photons annihilated on the source chamber wall should be greater than 8 cm. Assuming that the diameter of the stopped gamma detector crystal is D and the radius of the source chamber is R, according to the geometric relationship, the minimum optical path difference for the annihilated gamma photons detected on the source chamber is like Figure 4 The figure shows the minimum radius of the source chamber required when the gamma detector uses crystals of different diameters. If the detector uses a BaF2 crystal with a size of Φ30mm×20mm, the radius of the source chamber should be >43mm.

[0038] The present invention has no restrictions on the thickness of the sample. It can be applied to the measurement of thick samples while meeting the measurement requirements of thin film samples. At the same time, the present invention has no restrictions on the type and quantity of samples. In principle, as long as the distance between the samples is greater than the position resolution capability of the system, the system can filter out the annihilation information of samples at different positions. Figure 5 This is a possible “sample-source” measurement structure. The specific measurement structure is arranged according to the specific application scenario.

[0039] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A positron annihilation lifetime measurement system based on position positioning, characterized by: It includes a source chamber, a starting gamma detector, a first stopping gamma detector and a second stopping gamma detector; a sample is set on one side of the radiation source, and the sample is 22 The Na radioactive source is fixed in the center of the source chamber; the first stop gamma detector and the second stop gamma detector are fixed in the center of the source chamber; 22 The Na radioactive source is placed symmetrically outside the source chamber, and the starting gamma detector is placed at any position outside the source chamber. The starting gamma detector is responsible for 22 The 1.28 MeV gamma rays generated by the cascade when a Na radioactive source emits positrons are detected. The first-stop gamma detector and the second-stop gamma detector respectively detect the two oppositely emitted 0.511 MeV annihilation gamma rays produced after the positron annihilation. By measuring the time difference between the two annihilation gamma rays, the positron annihilation position is accurately located, positron annihilation cases in the sample are screened out, and the positron annihilation lifetime in the thin film sample can be measured. The source chamber and the stop gamma detector are used to screen out positron annihilation cases in the sample; the interior of the source chamber is a vacuum environment, and the geometric shape, internal space size and thickness of the source chamber wall are specifically arranged according to the specific application scenario to ensure that the positrons that have not entered the sample are annihilated on the source chamber wall far enough away from the sample to meet the time difference between the two annihilation gamma rays being detected. ,in, is the optical path difference between the two annihilation gamma photons detected, c is the speed of light, and The time resolution of the first stop gamma detector and the second stop gamma detector are respectively; the first stop gamma detector and the second stop gamma detector simultaneously detect the two oppositely emitted 0.511 MeV annihilation gamma rays generated after the positron annihilation, and measure the time difference between the two annihilation gamma rays annihilated in the sample and the source chamber wall. The annihilation position of the positron is located by the difference of the two, thus excluding the positron annihilation cases incident on the source chamber wall and only retaining the positron annihilation cases in the sample; The start gamma detector, the first stop gamma detector and the second stop gamma detector all use scintillators with high time resolution and fast-response photoelectric converters coupled thereto. The scintillators include BaF2, LaBr3 and LYSO, and the photoelectric converters include photomultiplier tubes and silicon photomultiplier tubes.

2. The measurement method of a positron annihilation lifetime measurement system based on position positioning according to claim 1, characterized in that: The steps include: Step 1: After the start gamma detector detects the gamma ray, it generates a start gamma signal; after the first stop gamma detector and the second stop gamma detector detect the gamma ray, they generate a first stop gamma signal and a second stop gamma signal respectively; Step 2: The signal acquisition module is responsible for collecting the start gamma signal, the first stop gamma signal, and the second stop gamma signal, and transmitting the waveform data to the data processing module for processing; Step 3: The data processing module performs energy identification, timing, annihilation case selection and lifetime spectrum statistics on the waveform data.

3. The measuring method according to claim 2, characterized in that In step 3, the energy thresholds for the start gamma signal and the stop gamma signal are set to the photoelectric peak of 1.28 MeV gamma rays and the photoelectric peak of 0.511 MeV gamma rays, respectively. The photoelectric peak of MeV gamma rays is detected to improve the time measurement accuracy and eliminate the interference of other gamma rays in the background environment. After timing, a start timing signal, a first stop timing signal, and a second stop timing signal are generated respectively. Within a certain time window, the time difference between the start timing signal and one of the stop timing signals is calculated as the lifetime of each positron annihilation case. Only when the start gamma signal, the first stop gamma signal, and the second stop gamma signal are detected simultaneously, and the first stop gamma signal and the second stop gamma signal exist simultaneously within a certain time window, a discrimination signal is output to indicate whether the positron annihilation case is recorded or not. When a discrimination signal is output, it indicates that the case is a valid positron annihilation case in the sample, and a time difference calculation is performed. When no discrimination signal is output, it indicates that the positron in the case is not annihilated in the sample, the time difference calculation is suppressed, and the data processing module directly enters the processing of the next case, thereby screening out positron annihilation cases in the sample. Finally, the time difference corresponding to each valid positron annihilation case is statistically analyzed, and after accumulating enough cases, a positron annihilation lifetime spectrum is generated.

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