CN116609371A - Positron annihilation life measurement system and method based on source device - Google Patents

Positron annihilation life measurement system and method based on source device Download PDF

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Publication number
CN116609371A
CN116609371A CN202310726345.4A CN202310726345A CN116609371A CN 116609371 A CN116609371 A CN 116609371A CN 202310726345 A CN202310726345 A CN 202310726345A CN 116609371 A CN116609371 A CN 116609371A
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positron
gamma
detector
annihilation
sample
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张红强
王宝义
刘福雁
王英杰
曹兴忠
张鹏
况鹏
于润升
魏龙
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Institute of High Energy Physics of CAS
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
    • G01N23/22—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by measuring secondary emission from the material
    • G01N23/225—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by measuring secondary emission from the material using electron or ion
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/36—Measuring spectral distribution of X-rays or of nuclear radiation spectrometry
    • G01T1/361—Measuring spectral distribution of X-rays or of nuclear radiation spectrometry with a combination of detectors of different types, e.g. anti-Compton spectrometers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/36—Measuring spectral distribution of X-rays or of nuclear radiation spectrometry
    • G01T1/362—Measuring spectral distribution of X-rays or of nuclear radiation spectrometry with scintillation detectors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2223/00—Investigating materials by wave or particle radiation
    • G01N2223/10—Different kinds of radiation or particles
    • G01N2223/108—Different kinds of radiation or particles positrons; electron-positron annihilation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E30/00—Energy generation of nuclear origin
    • Y02E30/30—Nuclear fission reactors

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  • General Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
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  • High Energy & Nuclear Physics (AREA)
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  • Chemical & Material Sciences (AREA)
  • Analysing Materials By The Use Of Radiation (AREA)

Abstract

The invention provides a positron annihilation life measurement system and method based on a source device, comprising a source chamber, a starting gamma detector, a stopping 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 closely attached to two sides of the film sample and are oppositely and symmetrically placed, and the positions of the source chamber and the positron detector are relatively fixed; the start gamma detector and the stop gamma detector are arranged at two sides of the positron detector and are respectively responsible for detecting 1.28MeV gamma photons generated by cascade connection when the radioactive source emits positive electrons and 0.511MeV gamma photons generated after positron annihilation. According to the invention, by improving the positron range of the non-sample side, the detection efficiency of the remote annihilation gamma photons is reduced, so that the noise positrons are effectively removed, and the electron body quantity is reduced; accurately identifying positron annihilation position at sample side, eliminating positron annihilation component in non-sample, breaking through thickness limitation of measurement sample, realizing positron annihilation of film sampleAnd (5) measuring the service life.

Description

Positron annihilation life measurement system and method based on source device
Technical Field
The invention belongs to the technical fields of nuclear spectroscopy and nuclear detection, and particularly relates to a positron annihilation life measurement system and method based on a source device, wherein positron annihilation is promoted at a far end on a non-sample side by improving positron emission range, so that annihilation gamma photon detection efficiency is reduced, accurate removal of noise positrons is realized, and electron body quantity is effectively reduced; and identifying positron annihilation positions on the sample side, removing positron annihilation components in non-samples, breaking through the thickness limitation of the measurement sample, and realizing the measurement of the positron annihilation life of the film sample.
Background
Positron annihilation spectroscopy reflects information such as electron state density, electron momentum and the like at microscopic defects in a material by detecting information such as time, energy, momentum and the like of annihilation gamma photons released after positrons annihilate with electrons in the material, and particularly has unique sensitivity characteristics for vacancy type defects in atomic scale. The positron annihilation life measurement spectroscopy method characterizes microscopic information such as defect types, concentration and the like in a material by detecting time information of annihilation gamma photons, and plays an important role in research on defect distribution and evolution rules of the material.
At present, a conventional positron annihilation lifetime spectrometer based on direct measurement of a radioactive source adopts a sample-radioactive source-sample measuring structure, and has strict requirements on the thickness of a sample, so that annihilation of all energy positrons in the sample is ensured. The method has obvious limitation in the measurement of the film sample with the thickness of micron level, has long high-energy positron range, is very likely to penetrate the film and annihilate in other materials, introduces measurement noise, and is not beneficial to the accurate characterization of the microstructure in the film sample.
Therefore, how to exclude positron annihilation cases in other materials than the film sample is the key to applying the conventional method based on the radioactive source to the measurement of the positron annihilation lifetime of the film sample.
Disclosure of Invention
In order to solve the technical problems, the invention provides a positron annihilation life measuring system and method based on a source device, wherein the system 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 closely attached to two sides of the film sample and are oppositely and symmetrically placed, and the positions of the source chamber and the positron detector are relatively fixed; the starting gamma detector and the stopping gamma detector are arranged on two sides of the positron detector by taking the radioactive source as a center and are respectively responsible for detecting 1.28MeV gamma photons generated by cascade connection when the radioactive source emits positive electrons and 0.511MeV gamma photons generated after positron annihilation. According to the invention, by improving the positron range of the non-sample side, the detection efficiency of the remote annihilation gamma photons is reduced, so that the noise positrons are effectively removed, and the electron body quantity is reduced; and precisely identifying positron annihilation positions on the sample side, eliminating positron annihilation components in non-samples, breaking through the thickness limit of a measurement sample, and realizing the measurement of the positron annihilation life of the film sample.
In order to achieve the above purpose, the invention adopts the following technical scheme:
a positron annihilation life measurement system based on a source device 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 closely attached to two sides of the film sample and are oppositely and symmetrically placed, and the positions of the source chamber and the positron detector are relatively fixed; the starting gamma detector and the stopping gamma detector are arranged on two sides of the positron detector by taking the radioactive source as a center and are respectively responsible for detecting 1.28MeV gamma photons generated by cascade connection when the radioactive source emits positive electrons and 0.511MeV gamma photons generated after positron annihilation.
Further, the source chamber and positron detector are configured to exclude interference of annihilation gamma photons in the non-sample with the measurement; the inside of the source chamber is an air or vacuum environment, the geometric shape and the size of the internal space are specifically arranged according to specific application scenes, and the internal space can be spherical, cylindrical and the like, so that positron which does not enter the sample is annihilated on the wall of the source chamber far enough away from the sample, the detection efficiency of the annihilation gamma photon of the part by the stop gamma detector is effectively reduced, and the positron annihilation case is eliminated; the positron detector detects positrons penetrating through the film sample, and annihilation components of the positrons penetrating through the film in materials except the sample are removed through the rear-end coincidence/anti-coincidence measuring circuit.
Further, the start and stop gamma detectors each employ a scintillator with high temporal resolution and a fast response photoelectric converter coupled thereto, the scintillator comprising BaF 2 、LaBr 3 LYSO, etc., including photomultiplier tubes, silicon photomultiplier tubes, etc.
Further, the positron detector consists of a plastic scintillation sheet coupled with a fast response photoelectric converter and is responsible for detecting positrons penetrating through a film sample and entering the film sample; when positrons enter a plastic scintillation sheet to deposit energy, fluorescent photons are generated, and fluorescence can be converted into an electric signal through a photoelectric converter and then is output to a back-end circuit for further processing; the plastic scintillation sheet adopted by the positron detector has enough thickness to ensure that positrons deposit enough energy in the plastic scintillation sheet, and enough fluorescent photons are correspondingly generated, so that the signal amplitude of the detector is higher than that of electronic noise, and the complete detection of incident positrons is satisfied, thereby realizing the complete elimination of annihilation cases in non-samples; meanwhile, the thickness of the scintillation sheet is not too thick, so that the detection efficiency of gamma photons is as low as possible, and the error elimination of positron annihilation in a sample is reduced.
The invention also provides a measuring method of the positron annihilation life measuring system based on the source device, which comprises the following steps:
step 1, during measurement, the whole measurement system is placed in a darkroom to ensure a completely light-proof environment, and the influence of external light on a detector is eliminated;
step 2, after the initial gamma detector and the stop gamma detector detect gamma photons, respectively generating a first gamma signal and a second gamma signal; generating a positron signal after the positron detector detects positrons penetrating through the film sample; the signal acquisition module is responsible for acquiring a first gamma signal, a second gamma signal and a positron signal, and transmitting waveform data to the data processing module for processing;
and 3, the data processing module performs energy discrimination, timing and annihilation case judgment and life spectrum statistics on the waveform data.
Further, in the step 3, the energy threshold of the positron signal is set to be the electronic noise level of the circuit, and as long as the positron detector detects the positron signal, a judgment signal is output, and the judgment signal is used as the judgment basis for recording whether the positron annihilation case exists or not;
the energy thresholds of the first gamma signal and the second gamma signal are respectively set to be a photoelectric peak of 1.28MeV gamma and a photoelectric peak of 0.511MeV gamma so as to improve time measurement accuracy, eliminate other gamma back interference and respectively generate a start timing signal and a stop timing signal after timing; calculating the time difference between the start timing signal and the stop timing signal as the life of each positron annihilation instance within a certain time window; when no judgment signal is output, the case is an effective positron annihilation case in the sample, and the time difference is calculated; when the judging signal is output, the fact that the positrons are not annihilated in the sample is indicated, the time difference calculation is restrained, and meanwhile, the data processing module directly enters the processing of the next instance, so that the positron annihilation instance in the non-film sample is eliminated; finally, the time difference corresponding to each effective positron annihilation instance is counted, and a positron annihilation lifetime spectrum is generated after enough instance accumulation.
The beneficial effects are that:
(1) The invention has simple measurement structure, and can eliminate the positron instance which does not enter the sample at the non-sample side by arranging the source chamber, thereby effectively reducing the electron volume and the signal processing time; and the positron annihilation position can be accurately identified through the positron detector, so that noise annihilation instances of the positrons penetrating through the film sample in other materials are sufficiently eliminated, and accurate measurement of the positron annihilation life in the film sample is realized.
(2) The invention has no limitation on the thickness of the sample, and is suitable for measuring thick samples while meeting the measurement requirement of thin film samples. For thick samples, all energy positrons cannot penetrate the material and annihilate in the sample, the positron detectors do not generate a judgment signal, and gamma photons from the effective annihilation instance of the sample in the detectors are fully reserved.
Drawings
FIG. 1 is a schematic diagram of a positron annihilation lifetime measurement system based on a source device of the present invention;
FIG. 2 is a schematic diagram of a measurement circuit according to the present invention;
FIG. 3 is a schematic diagram of a positron detector of the present invention;
fig. 4 is a plot of the proportion of annihilation gamma photons from a source chamber to all detected annihilation photons.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, the technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
For film sample measurement characterization, the invention can screen annihilation cases of positrons in film samples only, exclude positron annihilation information in non-samples and realize accurate measurement of positron annihilation life in film samples.
As shown in fig. 1, the positron annihilation lifetime measurement system based on the source device of the present invention includes a source chamber, a start gamma detector, a stop gamma detector, and a positron detector. Unlike the conventional "sample-radiation source-sample" configuration, the present invention requires only the sample to be placed on one side of the radiation source, 22 the Na radioactive source is fixed on the surface of the source chamber, and the source chamber and the positron detector are tightly attached to the filmThe two sides of the sample are symmetrically placed in opposite directions, and the positions of the two parts of structures are relatively fixed. The start gamma detector and the stop gamma detector are arranged at two sides of the positron detector with the radioactive source as the center and are respectively responsible for the pair of 22 The Na radiation source emits 1.28MeV gamma photons generated by the cascade of electrons and 0.511MeV gamma photons generated after positron annihilation.
Due to 22 The positrons generated by the Na radioactive source are emitted towards the full space isotropy, and a part of positrons enter the film sample and are annihilated in the film sample; a part of positrons do not enter the film sample and are randomly emitted into the space opposite to the sample and annihilated; some of the positrons are also able to penetrate the sample and annihilate in other materials with higher positron energies. Annihilation gamma photons are generated after annihilation of positrons in a sample and other non-sample materials, and a stop gamma detector for detecting positron annihilation information cannot distinguish whether detected annihilation gamma photons come from the sample, so that influence of annihilation events of positrons in the non-sample on measurement needs to be eliminated. In addition, during measurement, the whole measuring device needs to be placed in a darkroom to ensure a completely light-proof environment, and the influence of external light on the detector is eliminated.
In the present invention, positron annihilation events in non-samples are excluded by providing a source chamber and a positron detector. The inside of the source chamber is an air or vacuum environment, the geometric shape and the size of the internal space are specifically arranged according to specific application scenes, and the positron which does not enter the sample can be annihilated on the wall of the source chamber far enough away from the sample, so that the detection solid angle of the annihilation photon of 0.511MeV of the part can be increased, the capturing probability of the annihilation photon of the part by the gamma detector is effectively reduced, and the influence of the part of positrons is eliminated. The positron detector can detect positrons penetrating through the sample, and annihilation of the positrons penetrating through the film in materials except the sample is eliminated through the rear-end coincidence/anti-coincidence measuring circuit. In order to sufficiently exclude positron annihilation cases in non-samples, effective annihilation cases in samples are screened out, and a measuring circuit designed by the invention is shown in fig. 2.
After the start gamma detector and the stop gamma detector detect gamma photons, a first gamma signal and a second gamma signal are respectively generated. The positron detector detects positrons penetrating the film sample and generates positron signals. 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.
The data processing module is responsible for carrying out energy discrimination, timing and annihilation case judgment and life spectrum statistics on the waveform data.
For positron signals, since positron energy generated by the radioactive source is continuous, in order to ensure that positrons with different energies are detected, the energy threshold should be as small as possible, and generally set to be the electronic noise level of the circuit, so as to prevent excessive noise from introducing interference to the implementation of circuit functions. If the positron detector detects a positron signal, a judgment signal is output, and the judgment signal is used as a judgment basis for judging whether a positron annihilation case is recorded or not.
For the first gamma signal and the second gamma signal, the energy threshold is respectively set as a photoelectric peak of 1.28MeV gamma and a photoelectric peak of 0.511MeV gamma so as to improve the time measurement precision, eliminate other gamma back interference and respectively generate a start timing signal and a stop timing signal after timing; the time difference between the start timing signal and the stop timing signal is calculated as the lifetime of each positron annihilation event over a time window (on the order of hundred nanoseconds). When no judgment signal is output, the case is an effective positron annihilation case in the sample, and the time difference is calculated; when the judging signal is output, the fact that the positron is not annihilated in the sample is indicated, the time difference calculation is restrained, and meanwhile, the data processing module directly enters the processing of the next instance, so that the positron annihilation instance in the non-sample is eliminated. Finally, the time difference corresponding to each effective positron annihilation instance is counted, and a positron annihilation lifetime spectrum is generated after enough instance accumulation.
To ensure accuracy of positron annihilation lifetime measurements, both start and stop gamma detectors employ scintillator detectors with high temporal resolution, typically made of scintillators with better temporal performance (e.g., baF 2 、LaBr 3 LYSO, etc.) and a fast response photoelectric converter (e.g., photomultiplier, silicon photomultiplier, etc.) coupled thereto.
In the present invention, as shown in fig. 3, the positron detector is composed of a plastic scintillator coupled with a fast response photoelectric converter, and is responsible for detecting positrons penetrating through a film sample and entering the film sample. When the positrons enter the plastic scintillation sheet to deposit energy, fluorescence photons are generated, and fluorescence can be converted into an electric signal through a photoelectric converter and output to a back-end circuit for further processing. The plastic scintillation sheet adopted by the positron detector has enough thickness to ensure that positrons deposit enough energy in the plastic scintillation sheet, and enough fluorescent photons are correspondingly generated, so that the signal amplitude of the detector is higher than that of electronic noise, and the complete detection of incident positrons is satisfied, thereby realizing the complete elimination of annihilation cases in non-samples; meanwhile, the thickness of the scintillation sheet is not too thick, so that the detection efficiency of gamma photons is as low as possible, and the error elimination of positron annihilation in a sample is reduced.
The spherical source chamber of fig. 1 is an embodiment of the present invention, and the radius of the spherical shell of the source chamber depends on the detection efficiency of the start and stop gamma detectors. For example, if the detector employs BaF with dimensions Φ30mm×20mm 2 In the case of crystals, simulation results using Geant4 monte carlo software as shown in fig. 4 indicate that: when the radius of the source chamber in vacuum is larger than 25cm, the proportion of gamma photons from the spherical wall in all detected annihilation gamma photons is less than 5%, so that annihilation positrons on the source chamber can be ensured not to influence measurement. For other types and sizes of gamma detectors, the structural dimensions of the source chamber may also be determined by analog or experimental means.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.

Claims (6)

1. A positron annihilation lifetime measurement system based on a source device, characterized in that: comprising a source chamber and an initial gammaA 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 closely attached to two sides of the film sample and are oppositely and symmetrically placed, and the positions of the source chamber and the positron detector are relatively fixed; the starting gamma detector and the stopping gamma detector are arranged on two sides of the positron detector by taking the radioactive source as a center and are respectively responsible for detecting 1.28MeV gamma photons generated by cascading when the radioactive source emits positive electrons and 0.511MeV gamma photons generated after positron annihilation; during measurement, noise positrons are effectively removed by improving the positron range of a non-sample side and reducing the detection efficiency of remote annihilation gamma photons; and precisely identifying positron annihilation positions on the sample side, eliminating positron annihilation components in non-samples, and realizing the measurement of the positron annihilation life of the film sample.
2. The source-based positron annihilation lifetime measurement system as claimed in claim 1, wherein: the source chamber and the positron detector are used for eliminating interference of annihilation gamma photons in a non-sample on measurement; the inside of the source chamber is an air or vacuum environment, the geometric shape and the size of the internal space are arranged according to specific application scenes, and the internal space is spherical or cylindrical, so that positrons which do not enter a sample are annihilated on the wall of the source chamber far enough away from the sample, the detection efficiency of a stop gamma detector on annihilation gamma photons of the part is effectively reduced, and the positron annihilation case of the part is eliminated; the positron detector detects positrons penetrating through the film sample, and annihilation components of the positrons penetrating through the film in materials except the sample are removed through the rear-end coincidence/anti-coincidence measuring circuit.
3. The source-based positron annihilation lifetime measurement system as claimed in claim 1, wherein: the start and stop gamma detectors each employ a photoelectric converter having a high time resolution scintillator and a fast response coupled thereto, the scintillator comprising BaF 2 、LaBr 3 Or LYSO, the photoelectric converter comprises a photomultiplier tube or a silicon photomultiplier tube.
4. The source-based positron annihilation lifetime measurement system as claimed in claim 1, wherein: the positron detector consists of a plastic scintillation sheet coupled fast response photoelectric converter and is responsible for detecting positrons penetrating through a film sample and entering the film sample; when positrons enter a plastic scintillation sheet to deposit energy, fluorescent photons are generated, and fluorescent light is converted into an electric signal through a photoelectric converter and is output to a back-end circuit for further processing; the thickness of the plastic scintillation sheet adopted by the positron detector ensures that positrons deposit enough energy in the plastic scintillation sheet, and enough fluorescent photons are correspondingly generated, so that the signal amplitude of the detector is higher than that of electronic noise, and the complete detection of incident positrons is met, thereby realizing the complete elimination of annihilation cases in non-samples; meanwhile, the thickness of the plastic scintillation sheet enables the detection efficiency of gamma photons to be as small as possible, and error elimination of positron annihilation in a sample is reduced.
5. The method of one of claims 1 to 4, comprising the steps of:
step 1, during measurement, the whole measurement system is placed in a darkroom to ensure a completely light-proof environment, and the influence of external light on a detector is eliminated;
step 2, after the initial gamma detector and the stop gamma detector detect gamma photons, respectively generating a first gamma signal and a second gamma signal; generating a positron signal after the positron detector detects positrons penetrating through the film sample; the signal acquisition module is responsible for acquiring a first gamma signal, a second gamma signal and a positron signal, and transmitting waveform data to the data processing module for processing;
and 3, the data processing module performs energy discrimination, timing and annihilation case judgment and life spectrum statistics on the waveform data.
6. The measurement method according to claim 5, wherein in the step 3, an energy threshold of the positron signal is set to an electronic noise level of the circuit, so long as the positron detector detects the positron signal, a judgment signal is outputted, and the judgment signal is used as a judgment basis for recording whether the positron annihilation event is recorded or not;
the energy thresholds of the first gamma signal and the second gamma signal are respectively set to be a photoelectric peak of 1.28MeV gamma and a photoelectric peak of 0.511MeV gamma so as to improve time measurement accuracy, eliminate other gamma back interference and respectively generate a start timing signal and a stop timing signal after timing; calculating the time difference between the start timing signal and the stop timing signal as the life of each positron annihilation instance within a certain time window; when no judgment signal is output, the case is an effective positron annihilation case in the sample, and the time difference is calculated; when the judging signal is output, the fact that the positrons are not annihilated in the sample is indicated, the time difference calculation is restrained, and meanwhile, the data processing module directly enters the processing of the next instance, so that the positron annihilation instance in the non-film sample is eliminated; finally, the time difference corresponding to each effective positron annihilation instance is counted, and a positron annihilation lifetime spectrum is generated after enough instance accumulation.
CN202310726345.4A 2023-06-19 2023-06-19 Positron annihilation life measurement system and method based on source device Pending CN116609371A (en)

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