Source-based positron annihilation lifetime measurement system and method

Abstract

The application provides a source device-based positron annihilation lifetime measurement system and method, which comprises a source chamber, a start gamma detector, a stop gamma detector and a positron detector. 22 The Na radioactive source is fixed on the surface of the source chamber, the source chamber and the positron detector are placed on the two sides of the thin film sample and are symmetrically opposite, and the positions are relatively fixed; the start gamma detector and the stop gamma detector are placed on the two sides of the positron detector and are responsible for detecting the 1.28 MeV gamma photons generated by the cascade when the positron is emitted by the radioactive source and the 0.511 MeV gamma photons generated after the positron annihilation. The application can effectively remove the noise positrons by improving the positron range on the non-sample side, reducing the detection efficiency of the far-end annihilation gamma photons, and reducing the size of the electronics; the positron annihilation position on the sample side can be accurately identified, the non-sample positron annihilation components can be excluded, the sample thickness limitation can be broken, and the thin film sample positron annihilation lifetime measurement can be realized.

Description

Technical Field

[0001] This invention belongs to the field of nuclear spectroscopy and nuclear detection technology, specifically relating to a positron annihilation lifetime measurement system and method based on a source device. On the non-sample side, by increasing the positron range, positrons are annihilated at a far end, reducing the detection efficiency of annihilated gamma photons and achieving accurate removal of noisy positrons, effectively reducing the electronic volume. On the sample side, by identifying the positron annihilation position, positron annihilation components in non-samples are excluded, overcoming the limitation of sample thickness measurement and realizing the measurement of positron annihilation lifetime of thin film samples. Background Technology

[0002] Positron annihilation spectroscopy detects the time, energy, and momentum of annihilated gamma photons released after positrons annihilate with electrons in materials, reflecting information such as the electronic density of states and electron momentum at microscopic defects within the material. It exhibits particularly high sensitivity to atomic-scale vacancy defects. Spectroscopic methods for measuring positron annihilation lifetimes, by detecting the time information of annihilated gamma photons, characterize microscopic information such as defect types and concentrations in materials, playing a crucial role in the study of material defect distribution and evolution.

[0003] Currently, conventional positron annihilation lifetime spectrometers based on direct measurement using radioactive sources employ a "sample-radioactive source-sample" measurement structure, which imposes strict requirements on sample thickness to ensure that positrons of all energies are annihilated within the sample. This method has significant limitations when measuring thin film samples with thicknesses on the micrometer scale. High-energy positrons have long ranges and are highly likely to penetrate the thin film and annihilate in other materials, introducing measurement noise and hindering accurate characterization of the microstructure in thin film samples.

[0004] Therefore, how to exclude positron annihilation cases in materials other than thin film samples will be the key to applying conventional methods based on radioactive sources to the measurement of positron annihilation lifetime of thin film samples. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a positron annihilation lifetime measurement system and method based on a source device, comprising a source chamber, an initiation gamma detector, a stop gamma detector, and a positron detector; 22The Na radioactive source is fixed on the surface of the source chamber, which is symmetrically positioned against both sides of the thin film sample, with relatively fixed positions. The initiation gamma detector and the stop gamma detector are placed on either side of the positron detector with the radioactive source at the center, respectively responsible for detecting the 1.28 MeV gamma photons generated during the cascade emission of positrons from the radioactive source and the 0.511 MeV gamma photons generated after positron annihilation. This invention effectively removes noisy positrons by increasing the positron range on the non-sample side and reducing the detection efficiency of far-end annihilated gamma photons, thus reducing the electronic volume. It also accurately identifies the positron annihilation location on the sample side, excluding positron annihilation components from non-sample areas, overcoming the limitations of sample thickness measurement, and enabling the measurement of positron annihilation lifetimes in thin film samples.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A positron annihilation lifetime measurement system based on a source device includes a source chamber, an initiation gamma detector, a stop gamma detector, and a positron detector; 22 The Na radioactive source is fixed on the surface of the source chamber. The source chamber and the positron detector are placed symmetrically facing each other and in close contact with both sides of the thin film sample, with their positions relatively fixed. The starting gamma detector and the stopping gamma detector are placed on both sides of the positron detector with the radioactive source as the center. They are responsible for detecting the 1.28 MeV gamma photons generated by the cascade when the radioactive source emits positrons and the 0.511 MeV gamma photons generated after the positrons annihilate, respectively.

[0008] Furthermore, the source chamber and positron detector are used to eliminate interference from annihilated gamma photons outside the sample. The source chamber is an air or vacuum environment, and its geometry and internal space size are arranged according to the specific application scenario. It can be spherical, cylindrical, or other shapes to ensure that positrons that have not entered the sample are annihilated on the source chamber wall at a sufficient distance from the sample. This effectively reduces the detection efficiency of the gamma detector for these annihilated gamma photons, thereby eliminating these positron annihilation cases. The positron detector detects positrons that penetrate the thin film sample, and then the annihilation components of the positrons that penetrate the thin film in materials outside the sample are eliminated by the back-end coincidence / anti-coincidence measurement circuit.

[0009] Furthermore, both the start and stop gamma detectors employ scintillators with high time resolution and fast-response photoelectric converters coupled thereto. The scintillators include BaF2, LaBr3, LYSO, etc., and the photoelectric converters include photomultiplier tubes, silicon photomultiplier tubes, etc.

[0010] Furthermore, the positron detector consists of a plastic scintillator coupled with a fast-response photoelectric converter, responsible for detecting positrons that penetrate the thin-film sample and enter it. When positrons enter the plastic scintillator and deposit energy, they generate fluorescent photons. The photoelectric converter converts the fluorescence into an electrical signal, which is then output to the back-end circuit for further processing. The plastic scintillator used in the positron detector should have sufficient thickness to ensure that positrons deposit enough energy in the plastic scintillator, thereby generating a sufficient number of fluorescent photons. This ensures that the detector signal amplitude is higher than the electronic noise, satisfying the complete detection of incident positrons and thus achieving the complete exclusion of annihilation cases not in the sample. At the same time, the thickness of the scintillator should not be too thick to minimize the detection efficiency of gamma photons and reduce the false exclusion of positron annihilation cases in the sample.

[0011] The present invention also provides a measurement method for a positron annihilation lifetime measurement system based on a source device, comprising:

[0012] Step 1: During measurement, the entire measurement system is placed in a dark room to ensure a completely light-free environment and eliminate the influence of external light on the detector;

[0013] Step 2: After the start gamma detector and the stop gamma detector detect gamma photons, they generate the first gamma signal and the second gamma signal, respectively; after the positron detector detects positrons penetrating the thin film sample, it generates a positron signal; the signal acquisition module is responsible for acquiring the first gamma signal, the second gamma signal, and the positron signal, and transmitting the waveform data to the data processing module for processing;

[0014] Step 3: The data processing module performs energy identification, timing, selection of annihilation cases, and lifetime spectrum statistics on the waveform data.

[0015] Furthermore, in step 3, the energy threshold of the positron signal is set to the electronic noise level of the circuit. As long as the positron detector detects the positron signal, it will output a selection signal, which will be used as the basis for determining whether or not a positron annihilation case is recorded.

[0016] The energy thresholds of the first and second gamma signals are set to a photoelectric peak of 1.28 MeV and 0.511 MeV gamma, respectively, to improve the accuracy of time measurement and eliminate other gamma background interference. After timing, start and stop timing signals are generated. Within a certain time window, the time difference between the start and stop timing signals is calculated as the lifetime of each positron annihilation case. When no selection signal is output, it indicates that the case is a valid positron annihilation case in the sample, and the time difference will be calculated. When a selection signal is output, it indicates that the positrons in the case are not annihilated in the sample, the time difference calculation will be suppressed, and the data processing module will directly proceed to the processing of the next case, thereby excluding positron annihilation cases in non-thin film samples. Finally, the time difference corresponding to each valid positron annihilation case is statistically analyzed, and after sufficient case accumulation, a positron annihilation lifetime spectrum is generated.

[0017] Beneficial effects:

[0018] (1) The measurement structure of the present invention is simple. By setting the source chamber, non-sample side, i.e., positron events that have not entered the sample, can be excluded, which effectively reduces the electronic volume and signal processing time. Furthermore, the positron detector can accurately identify the positron annihilation position, fully exclude noise annihilation events of positrons that penetrate the thin film sample in other materials, and realize the accurate measurement of positron annihilation lifetime in the thin film sample.

[0019] (2) This invention does not limit the thickness of the sample and is applicable to the measurement of thick samples while meeting the requirements for thin film sample measurement. For thick samples, all positrons of all energies cannot penetrate the material and are completely annihilated in the sample. The positron detector does not generate a selection signal, and the gamma photons from the effective annihilation cases of the sample in the detector will be fully preserved. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the positron annihilation lifetime measurement system based on the source device of the present invention;

[0021] Figure 2 This is a schematic diagram of the measurement circuit of the present invention;

[0022] Figure 3 This is a schematic diagram of the positron detector structure of the present invention;

[0023] Figure 4 This is a graph showing the proportion of gamma photons from the source chamber to all detected annihilated photons. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be 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 illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0025] For the measurement and characterization of thin film samples, this invention can identify and screen cases where positrons are annihilated only in the thin film sample, exclude positron annihilation information not in the sample, and achieve accurate measurement of positron annihilation lifetime in thin film samples.

[0026] like Figure 1 As shown, the positron annihilation lifetime measurement system based on a source device of the present invention includes a source chamber, an initiation gamma detector, a stop gamma detector, and a positron detector. Unlike the traditional "sample-radioactive source-sample" structure, the present invention only requires the sample to be placed on one side of the radioactive source. 22 The Na radioactive source is fixed to the surface of the source chamber. The source chamber and the positron detector are placed symmetrically opposite each other, closely attached to both sides of the thin-film sample, with the positions of the two structures relatively fixed. The initiation gamma detector and the stop gamma detector are placed on either side of the positron detector with the radioactive source as the center, respectively responsible for... 22 The detectors perform a 1.28 MeV gamma photon generated by the cascaded emission of positrons from the Na radioactive source and a 0.511 MeV gamma photon generated after the annihilation of the positron.

[0027] because 22 Positrons generated by the Na radioactive source are emitted isotropically throughout space. Some positrons enter the thin-film sample and annihilate within it; some positrons do not enter the sample but are randomly emitted into the space opposite the sample and annihilate there as well; and some positrons with higher energy can penetrate the sample and annihilate in other materials. The annihilation of positrons in both the sample and other non-sample materials produces annihilation gamma photons. However, the stop gamma detector used to detect positron annihilation cannot distinguish whether the detected annihilation gamma photons originate from the sample. Therefore, it is necessary to eliminate the influence of positron annihilation cases in non-sample materials on the measurement. Furthermore, during the measurement, the entire measurement apparatus must be placed in a dark room to ensure a completely light-protected environment and eliminate the influence of external light on the detector.

[0028] In this invention, positron annihilation instances outside the sample are eliminated by setting up a source chamber and a positron detector. The source chamber is an air or vacuum environment, and its geometry and internal space size are specifically arranged according to the application scenario. This ensures that positrons that have not entered the sample are annihilated on the source chamber wall at a sufficient distance from the sample. This increases the detection solid angle of the 0.511 MeV annihilated photons, effectively reducing their probability of being captured by the stop gamma detector, thereby eliminating the influence of these positrons. The positron detector can detect positrons that penetrate the sample, and then the annihilation instances of positrons penetrating the thin film in materials outside the sample are eliminated by the back-end coincidence / anti-coincidence measurement circuit. To fully eliminate positron annihilation instances outside the sample and screen out valid annihilation instances in the sample, the measurement circuit designed in this invention, such as... Figure 2 As shown.

[0029] After the start gamma detector and stop gamma detector detect gamma photons, they generate a first gamma signal and a second gamma signal, respectively. The positron detector generates a positron signal after detecting positrons penetrating the thin-film sample. The signal acquisition module is responsible for acquiring the first gamma signal, the second gamma signal, and the positron signal, and transmitting the waveform data to the data processing module for processing.

[0030] The data processing module is responsible for energy identification, timing, selection of annihilation cases, and lifetime spectrum statistics of waveform data.

[0031] For positron signals, since the positrons produced by the radiation source have continuous energy, the energy threshold should be as small as possible to ensure that positrons of different energies are detected. It is generally set to the electronic noise level of the circuit to prevent excessive noise from interfering with the circuit's functionality. Once the positron detector detects a positron signal, it will output a selection signal, which will be used as the criterion for determining whether or not a positron annihilation case should be recorded.

[0032] For the first and second gamma signals, energy thresholds were set to a photoelectric peak of 1.28 MeV and a photoelectric peak of 0.511 MeV, respectively, to improve time measurement accuracy and eliminate background interference from other gamma signals. After timing, start and stop timing signals were generated. Within a certain time window (on the order of hundreds of nanoseconds), the time difference between the start and stop timing signals was calculated as the lifetime of each positron annihilation event. When no selection signal was output, it indicated that the event was a valid positron annihilation event in the sample, and time difference calculation was performed. When a selection signal was output, it indicated that the positrons in the event were not annihilated in the sample, and time difference calculation was suppressed. Simultaneously, the data processing module directly proceeded to the next event, thus excluding positron annihilation events not in the sample. Finally, the time differences corresponding to each valid positron annihilation event were statistically analyzed. After sufficient event accumulation, a positron annihilation lifetime spectrum was generated.

[0033] To ensure the accuracy of positron annihilation lifetime measurement, both the start and stop gamma detectors employ scintillator detectors with high time resolution. These detectors typically consist of a scintillator with good time performance (such as BaF2, LaBr3, LYSO, etc.) and a fast-response photoelectric converter coupled to it (such as a photomultiplier tube, silicon photomultiplier tube, etc.).

[0034] In this invention, such as Figure 3 As shown, the positron detector consists of a plastic scintillator coupled with a fast-response photoconverter, responsible for detecting positrons that penetrate the thin-film sample and enter it. When positrons enter the plastic scintillator and deposit energy, they generate fluorescent photons. The photoconverter converts the fluorescence into an electrical signal, which is then output to the back-end circuit for further processing. The plastic scintillator used in the positron detector should be thick enough to ensure that positrons deposit sufficient energy within it, generating a sufficient number of fluorescent photons. This ensures that the detector signal amplitude is higher than the electronic noise, achieving complete detection of incident positrons and thus completely eliminating annihilation cases not present in the sample. Simultaneously, the scintillator thickness should not be too large to minimize the detection efficiency of gamma photons, reducing the false rejection of positron annihilation cases in the sample.

[0035] Figure 1 The spherical source chamber is a specific embodiment of the present invention, and the radius of the spherical shell of the source chamber depends on the detection efficiency of the starting and stopping gamma detectors. For example, if the detector uses a BaF2 crystal with dimensions of Φ30mm × 20mm, such as Figure 4 Simulation results using Geant4 Monte Carlo software show that when the radius of the vacuum source chamber is greater than 25 cm, less than 5% of all detected annihilated gamma photons originate from the spherical wall, ensuring that annihilated positrons in the source chamber do not affect the measurement. For other types and sizes of gamma detectors, the structural dimensions of the source chamber can also be determined through simulation or experimental methods.

[0036] Those skilled in the art will readily understand 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 within the scope of protection of the present invention.

Claims

1. A positron annihilation lifetime measurement system based on a source device, characterized in that: Includes source chamber, starting gamma detector, stopping gamma detector, and positron detector; 22 The Na radioactive source is fixed on the surface of the source chamber, which is symmetrically positioned against both sides of the thin film sample, with relatively fixed positions. The initiation gamma detector and the stop gamma detector are placed on both sides of the positron detector with the radioactive source as the center, respectively responsible for detecting the 1.28 MeV gamma photons generated by the cascade when the radioactive source emits positrons and the 0.511 MeV gamma photons generated after positron annihilation. During measurement, the detection efficiency of far-end annihilated gamma photons is reduced by increasing the positron range on the non-sample side, thereby effectively removing noisy positrons. The positron annihilation position is accurately identified on the sample side to exclude positron annihilation components from the non-sample side, thus realizing the measurement of the positron annihilation lifetime of the thin film sample. The source chamber and positron detector are used to eliminate interference from annihilated gamma photons outside the sample. The source chamber is an air or vacuum environment, and its geometry and internal space size are arranged according to the specific application scenario. It can be spherical or cylindrical to ensure that positrons that have not entered the sample are annihilated on the source chamber wall at a sufficient distance from the sample, effectively reducing the detection efficiency of the gamma detector for annihilated gamma photons and thus eliminating these positron annihilation cases. The positron detector detects positrons that penetrate the thin film sample, and then the annihilation components of the positrons that penetrate the thin film in materials outside the sample are eliminated by the back-end coincidence / anti-coincidence measurement circuit. Both the start and stop gamma detectors employ a scintillator with high time resolution and a fast-response photoelectric converter coupled thereto. The scintillator includes BaF2, LaBr3, or LYSO, and the photoelectric converter includes a photomultiplier tube or a silicon photomultiplier tube. The positron detector consists of a plastic scintillator coupled with a fast-response photoelectric converter, and is responsible for detecting positrons that penetrate the thin-film sample and enter it; When positrons enter the plastic scintillator and deposit energy, they generate fluorescent photons. These photons are then converted into electrical signals by a photoelectric converter and output to the back-end circuit for further processing. The thickness of the plastic scintillator used in the positron detector ensures that positrons deposit sufficient energy within the scintillator, resulting in a sufficient number of fluorescent photons. This ensures that the detector signal amplitude is higher than the electronic noise, allowing for the complete detection of incident positrons and thus the complete exclusion of annihilation cases not present in the sample. Simultaneously, the thickness of the plastic scintillator minimizes the detection efficiency of gamma photons, reducing the false exclusion of positron annihilation cases in the sample.

2. The measurement method of the positron annihilation lifetime measurement system based on a source device according to claim 1, characterized in that, Includes the following steps: Step 1: During measurement, the entire measurement system is placed in a dark room to ensure a completely light-free environment and eliminate the influence of external light on the detector; Step 2: After the start gamma detector and the stop gamma detector detect gamma photons, they generate the first gamma signal and the second gamma signal, respectively; after the positron detector detects positrons penetrating the thin film sample, it generates a positron signal; the signal acquisition module is responsible for acquiring the first gamma signal, the second gamma signal, and the positron signal, and transmitting the waveform data to the data processing module for processing; Step 3: The data processing module performs energy identification, timing, selection of annihilation cases, and lifetime spectrum statistics on the waveform data.

3. The measurement method according to claim 2, characterized in that, In step 3, the energy threshold of the positron signal is set to the electronic noise level of the circuit. As long as the positron detector detects the positron signal, it will output a selection signal, which will be used as the basis for determining whether or not a positron annihilation case is recorded. The energy thresholds of the first and second gamma signals are set to a photoelectric peak of 1.28 MeV and 0.511 MeV gamma, respectively, to improve the accuracy of time measurement and eliminate other gamma background interference. After timing, start and stop timing signals are generated. Within a certain time window, the time difference between the start and stop timing signals is calculated as the lifetime of each positron annihilation case. When no selection signal is output, it indicates that the case is a valid positron annihilation case in the sample, and the time difference will be calculated. When a selection signal is output, it indicates that the positrons in the case are not annihilated in the sample, the time difference calculation will be suppressed, and the data processing module will directly proceed to the processing of the next case, thereby excluding positron annihilation cases in non-thin film samples. Finally, the time difference corresponding to each valid positron annihilation case is statistically analyzed, and after sufficient case accumulation, a positron annihilation lifetime spectrum is generated.

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