Low-source component positron annihilation lifetime spectrometer

Abstract

The invention discloses a low-source component positron annihilation lifetime spectrometer, and relates to the technical field of positron annihilation lifetime spectroscopy. In the spectrometer, a coincidence module is used for detecting accompanying gamma photons generated by positron as initial signals and detecting gamma photons generated by annihilation of the positron in a sample to be detected as termination signals; the anti-coincidence module is used for detecting a positron anti-coincidence signal generated by decay of a radioactive source; the data acquisition module is used for acquiring an initial signal, a termination signal and an anti-coincidence signal and transmitting the signals to the computer data processing module; and the computer data processing module is used for receiving and processing the signal data acquired by the data acquisition module to obtain the positron annihilation lifetime spectrum of the sample to be detected. The anti-coincidence module can well remove annihilation signals of positron in the thin plastic scintillator, so that a final positron annihilation lifetime spectrum only contains positron annihilation lifetime information of the sample to be detected.

Description

Technical Field

[0001] The invention belongs to the technical field of positron annihilation lifetime spectroscopy, and in particular relates to a low-source component positron annihilation lifetime spectrometer. Background Art

[0002] Positron annihilation lifetime technology is a special technical means to use positrons to non-destructively detect the microstructure of materials. It generally obtains the positron annihilation lifetime information of positrons inside the material by detecting the gamma photons annihilated by positrons in the material and the signals of the accompanying gamma photons generated by positrons, and then analyzes the information related to material defects.

[0003] Generally used in positron annihilation lifetime experiments 22 Na is used as a radiation source. When it produces positrons, it also produces a gamma photon as a start signal. The gamma photon produced by positron annihilation is used as the end signal. The pulse signal is detected by the scintillation detector and enters various electronic plug-ins. After energy identification and timing, the time difference between the start signal and the end signal is converted into voltage amplitude information and recorded on the computer to obtain the positron annihilation lifetime spectrum.

[0004] In traditional positron annihilation lifetime experiments, the radiation source 22 Na is encapsulated with a thin film (metal or Kapton film) to form a thin film - 22 The sandwich structure of Na-film. Since positrons will pass through the film before entering the sample to be tested, there is a probability that annihilation will occur in the radiation source and the film (called source component). The composition of the source component includes the part that is annihilated when it first enters the film and the part that is annihilated when it enters the film again after being scattered by the sample. The former is related to the density and thickness of the film, and the latter is related to the density and atomic number of the sample. Therefore, the size of the source component is determined by the material of the film, the density and atomic number of the sample.

[0005] The source component generally needs to be removed by measuring the standard sample. Usually, the source component can be reduced by reducing the thickness of the film, but since the radioactive source used in the positron annihilation experiment needs to have certain mechanical support properties, the film needs to have a certain thickness. When the 7.5-micron Kapton film is used as the packaging material of the radioactive source and the standard sample measured is yttrium-stabilized zirconia (YSZ), the source component generally accounts for 10%.

[0006] At present, the standard samples used by traditional positron annihilation lifetime spectrometers to measure source components are difficult to make consistent with the atomic number and density of the sample to be measured, so the source components are difficult to remove accurately. This will interfere with the positron lifetime results of the sample to be measured, and sometimes even positron annihilation lifetimes that do not belong to the sample to be measured will appear. Although pulsed slow positron beam devices can also measure low source components, their devices are complex and costly. Summary of the invention

[0007] In order to solve the above technical problems, the present invention provides a low source component positron annihilation lifetime spectrometer. The spectrometer can achieve an extremely low source component ratio and reduce the influence of the source component on the positron annihilation lifetime spectrum analysis.

[0008] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] A low-source component positron annihilation lifetime spectrometer, the spectrometer comprises a coincidence module, an anti-coincidence module, a data acquisition module and a computer data processing module; wherein,

[0010] The coincidence module is used to detect the accompanying gamma photons generated by the positron as a starting signal, and is also used to detect the gamma photons generated by the annihilation of the positron in the sample to be tested as a termination signal;

[0011] The anti-coincidence module is used to detect the positron anti-coincidence signal generated by the decay of the radioactive source;

[0012] The data acquisition module is used to collect the start signal, the stop signal, and the anti-coincidence signal, and transmit them to the computer data processing module;

[0013] The computer data processing module is used to receive and process the signal data collected by the data acquisition module. Within a preset time window, when the anti-coincidence module does not detect the positron anti-coincidence signal, and the coincidence module detects the start signal and the end signal respectively, it is recorded as a valid coincidence event. The time difference between the start and end signals in several valid coincidence events is counted by frequency statistics to obtain the positron annihilation lifetime spectrum of the sample to be tested.

[0014] Furthermore, the coincidence module includes two coincidence detectors formed by coupling a first scintillator and a second scintillator with a first photodetector and a second photodetector respectively, wherein one detector is used to detect accompanying gamma photons generated by positrons as a starting signal, and the other detector is used to detect gamma photons generated by annihilation of positrons in a sample to be tested as a termination signal.

[0015] Furthermore, the anti-coincidence module includes an anti-coincidence detector formed by coupling a thin plastic scintillator and a third photoelectric detection device, and the anti-coincidence detector is used to detect positron anti-coincidence signals generated by decay of a radiation source.

[0016] Furthermore, the thin plastic scintillator comprises a blind hole, and a radiation source is dripped into the blind hole.

[0017] Furthermore, the three input channels of the digital acquisition module are connected to the outputs of the anti-coincidence module and the coincidence module through coaxial cables, and three-channel pulse data are collected and transmitted to the computer data processing module for storage and processing.

[0018] Furthermore, the computer data processing module deconstructs the positron annihilation lifetime spectrum to obtain the positron annihilation lifetime component of the sample to be tested.

[0019] Furthermore, the first scintillator and the second scintillator are any one of BaF2, LaBr3, and LYSO scintillator crystals, the first, second, and third photoelectric detection devices are any one of photomultiplier tubes and silicon photomultiplier tubes, and the radiation source is 22 Na, 68 Any one of Ge.

[0020] Furthermore, within a preset time window, an anti-coincidence threshold is set for the anti-coincidence detector. When the positron anti-coincidence signal received by the anti-coincidence detector is lower than the anti-coincidence threshold and the two coincidence detectors receive start and stop signals, it is recorded as a valid coincidence event. The time difference between the start and stop signals of the valid coincidence event is determined by a constant ratio timing method, and the time differences of several valid coincidence events are statistically processed to obtain a positron annihilation lifetime spectrum.

[0021] The beneficial effects of the present invention are:

[0022] The anti-coincidence module in the present invention can effectively remove the signal of positron annihilation in the thin plastic scintillator, so that the final positron annihilation lifetime spectrum only contains the positron annihilation lifetime information of the sample to be tested; the use of the anti-coincidence module can eliminate the need to measure standard samples to determine the source composition, thereby improving the accuracy of the positron annihilation lifetime spectrum; compared with a pulsed slow positron beam device, the device of the present invention has a simple structure, low cost, and is easy to maintain and expand. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the structure of a low source component positron annihilation lifetime spectrometer of the present invention;

[0024] Figure 2 This is an enlarged schematic diagram of the anti-coincidence module;

[0025] Figure 3Schematic diagram of the positron annihilation lifetime spectrum. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0027] like Figure 1 As shown, the present invention provides a low source component positron annihilation lifetime spectrometer, which is used to reduce the influence of source components and accurately measure the positron lifetime of samples. The low source component positron annihilation lifetime spectrometer comprises a coincidence module, an anti-coincidence module, a data acquisition module and a computer data processing module;

[0028] The coincidence module comprises two coincidence detectors formed by coupling the first scintillator and the second scintillator with the first photodetector and the second photodetector, respectively. One of the detectors detects the accompanying gamma photons generated by the positron as a start signal, and the other detector detects the gamma photons generated by the annihilation of the positron in the material as a stop signal, so the two coincidence detectors can also be called a start detector and a stop detector.

[0029] The anti-coincidence module includes an anti-coincidence detector formed by coupling a thin plastic scintillator and a third photoelectric detection device. A blind hole of a certain size is dug on one side of the thin plastic scintillator, and a radiation source is dripped in. The anti-coincidence detector is used to detect the positron anti-coincidence signal generated by the decay of the radiation source.

[0030] The data acquisition module includes a data acquisition card, which acquires a total of three signals from the anti-coincidence detector and the two coincidence detectors, and transmits them to the computer data processing module. Within a certain time window, when the anti-coincidence detector does not detect the positron signal (that is, it indicates that the positron is annihilated in the sample to be tested), and the two coincidence detectors detect the start and end gamma photon signals respectively, it is considered a valid coincidence event. When the valid coincidence event occurs, the two pulse signals of the two coincidence detectors will be recorded by the computer data processing module according to a predetermined data format.

[0031] The computer data processing module is used to receive and process the signal data collected by the data acquisition module, and the computer data processing module performs constant ratio timing on the two pulse signals to obtain the time difference between the start and end gamma photon signals. The positron annihilation lifetime spectrum of the sample to be tested can be obtained by performing frequency statistics on the time differences of a large number of the effective coincidence events. The positron annihilation lifetime spectrum is analyzed by the computer terminal to obtain the positron annihilation lifetime component of the sample to be tested.

[0032] like Figure 2As shown, the radiation source is placed in the blind hole of the thin plastic scintillator, and the thin plastic scintillator is coupled to the third photodetector using silicone oil to form an anti-coincidence module. When the positron generated by the decay of the radiation source enters the thin plastic scintillator, a pulse signal is generated. The pulse signal of the thin plastic scintillator can be used to determine whether the positron has entered the sample. If there is no pulse signal on the thin plastic scintillator, it is considered that the positron has entered the sample. The standard sample size for positron annihilation lifetime measurement is generally 10×10×1 mm. The size of the blind hole can be set to about 8×8×0.5 mm, which can support the standard sample on the one hand and maximize the solid angle of the positron directly emitted to the sample on the other hand.

[0033] Reference again Figure 1 The block sample to be tested is placed above the thin plastic scintillator, and the third photoelectric detection device is used to detect the fluorescence signal caused by the positrons generated by the decay of the radiation source on the thin plastic scintillator. The first scintillator and the first photoelectric detection device are coupled through silicone oil to form a starting detector, which detects the 1.275 MeV gamma photons accompanying the positrons. The second scintillator and the second photoelectric detection device are coupled through silicone oil to form a termination detector, which detects the 0.511 MeV gamma photons generated by the annihilation of the positrons in the sample.

[0034] The first and second scintillators may be scintillating crystals such as BaF2, LaBr3, LYSO, etc., and the first, second and third photoelectric detection devices may be photomultiplier tubes, silicon photomultiplier tubes, etc. The radiation source may be 22 Na, 68 Ge et al.

[0035] The three input channels of the digital acquisition module are connected to the anti-coincidence detector and the outputs of the two coincidence detectors through coaxial cables. The three pulse data are collected and transmitted to the computer data processing module for storage and processing. Within a certain coincidence time window (such as 100 ns), a certain anti-coincidence threshold is set for the anti-coincidence detector. When the pulse signal received by the anti-coincidence detector is lower than the anti-coincidence threshold and the two coincidence detectors receive the pulse signal, it is a set of valid coincidence events. The pulse signal of the coincidence event is used to determine the time difference between the start and end of the gamma photon signal by the constant ratio timing method. The time difference of the coincidence event is statistically processed to obtain the positron annihilation lifetime spectrum, such as Figure 3 As shown. The positron annihilation lifetime spectrum can be analyzed using software such as LTv9, PATFIT-88, MELT, and CONTIN.

[0036] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. 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 low source component positron annihilation lifetime spectrometer, characterized in that: The spectrometer includes a coincidence module, an anti-coincidence module, a data acquisition module and a computer data processing module; wherein, The coincidence module is used to detect the accompanying gamma photons generated by the positron as a starting signal, and is also used to detect the gamma photons generated by the annihilation of the positron in the sample to be tested as a termination signal; The anti-coincidence module is used to detect the positron anti-coincidence signal generated by the decay of the radioactive source; The data acquisition module is used to collect the start signal, the stop signal, and the anti-coincidence signal, and transmit them to the computer data processing module; The computer data processing module is used to receive and process the signal data collected by the data acquisition module. Within a preset time window, when the anti-coincidence module does not detect the positron anti-coincidence signal, and the coincidence module detects the start signal and the end signal respectively, it is recorded as a valid coincidence event. The time difference between the start and end signals in several valid coincidence events is counted by frequency statistics to obtain the positron annihilation lifetime spectrum of the sample to be tested.

2. A low source component positron annihilation lifetime spectrometer according to claim 1, characterized in that: The coincidence module includes two coincidence detectors formed by coupling a first scintillator and a second scintillator with a first photodetector and a second photodetector, respectively, wherein one detector is used to detect accompanying gamma photons generated by positrons as a starting signal, and the other detector is used to detect gamma photons generated by annihilation of positrons in a sample to be tested as a termination signal.

3. The low source component positron annihilation lifetime spectrometer according to claim 3, characterized in that: The anti-coincidence module comprises an anti-coincidence detector formed by coupling a thin plastic scintillator and a third photoelectric detection device, and the anti-coincidence detector is used to detect a positron anti-coincidence signal generated by decay of a radiation source.

4. A low source component positron annihilation lifetime spectrometer according to claim 3, characterized in that: The thin plastic scintillator comprises a blind hole into which a radiation source is dropped.

5. The low source component positron annihilation lifetime spectrometer according to claim 4, characterized in that: The three input channels of the digital acquisition module are connected with the outputs of the anti-coincidence module and the coincidence module through coaxial cables, and three-channel pulse data are collected and transmitted to the computer data processing module for storage and processing.

6. A low source component positron annihilation lifetime spectrometer according to claim 5, characterized in that: The computer data processing module interprets the positron annihilation lifetime spectrum to obtain the positron annihilation lifetime component of the sample to be tested.

7. A low source component positron annihilation lifetime spectrometer according to claim 6, characterized in that: The first scintillator and the second scintillator are any one of BaF2, LaBr3, and LYSO scintillator crystals; the first, second, and third photoelectric detection devices are any one of photomultiplier tubes and silicon photomultiplier tubes; the radiation source is 22 Na, 68 Any one of Ge.

8. The low source component positron annihilation lifetime spectrometer according to claim 7, characterized in that: Within a preset time window, an anti-coincidence threshold is set for the anti-coincidence detector. When the positron anti-coincidence signal received by the anti-coincidence detector is lower than the anti-coincidence threshold and the two coincidence detectors receive start and stop signals, it is recorded as a valid coincidence event. The time difference between the start and stop signals of the valid coincidence event is determined by the constant ratio timing method. The time differences of several valid coincidence events are statistically processed to obtain the positron annihilation lifetime spectrum.

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