Positron annihilation coincidence Doppler broadening spectrometer based on beta < + >-gamma-gamma coincidence

Abstract

The invention discloses a positron annihilation coincidence Doppler broadening spectrometer based on beta +-gamma-gamma coincidence, which belongs to the technical field of nuclear detection and comprises a coincidence signal generation module, an annihilation gamma photon detection module, a signal acquisition module and a control and analysis module, the coincidence signal generation module is used for detecting positron generated by decay of the positron source and generating coincidence signals for case judgment and selection; the annihilation gamma photon detection module is used for detecting gamma photons generated by positron annihilation and generating pulse signals containing energy information; the signal acquisition module is used for acquiring coincidence signals and pulse signals; and the control and analysis module is used for controlling the signal acquisition module and acquiring a positron annihilation coincidence Doppler broadening spectrum from the acquired coincidence signal and the pulse signal. According to the positron annihilation coincidence Doppler broadening spectrometer, the extremely low source component proportion can be achieved, and then the accuracy of a test result is improved.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear detection technology, specifically to the field of positron annihilation spectroscopy, and particularly relates to a method based on β + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer. Background Art

[0002] Positrons are the antiparticles of electrons, and they can be used as sensitive probes for non-destructive studies of the internal microstructure of materials. The annihilation characteristics of positrons in matter are related to the electron density distribution, which is determined by the microstructure. Positron annihilation spectroscopy, which was developed based on the principle of positron annihilation, has been widely used in solid state physics, material science, physical metallurgy, chemistry, national defense science and technology, biomedicine and other fields. Positron annihilation spectroscopy includes a wealth of measurement methods and means, among which positron annihilation conforms to Doppler broadening, which is one of the most basic methods, and can provide information such as the momentum of the electrons involved in the annihilation and the chemical environment at the annihilation site.

[0003] After a positron with a certain kinetic energy enters the material, it will first undergo a rapid thermalization process. The thermalized positron will diffuse in the material and eventually meet an electron and annihilate. The kinetic energy of the thermalized positron is about 0.025 eV at room temperature, while the kinetic energy of the electron in the sample can usually reach several electron volts. Since the total kinetic energy of the annihilated positron-electron pair is not zero, the energy of the γ photon generated when two-photon annihilation occurs will deviate from 0.511 MeV (if the total kinetic energy of the annihilated positron-electron pair is zero, in order to satisfy the conservation of energy and momentum, the energy of the γ photon generated when two-photon annihilation occurs should be 0.511 MeV). Compared with the kinetic energy of the electron, the contribution of the positron to the total kinetic energy can be ignored, so the energy deviation of the annihilated γ photon is mainly caused by the kinetic energy of the electron in the annihilation pair. Therefore, by detecting the energy of the annihilated γ photon, the momentum information of the electron involved in the annihilation can be obtained. Based on this principle, the positron annihilation coincidence Doppler broadening technique was developed.

[0004] Conventional positron annihilation coincidence Doppler broadening spectrometers use a variety of electronic plug-ins, mainly composed of high-purity germanium detectors, preamplifiers, main amplifiers, single-channel analyzers, coincidencers, analog-to-digital converters, dual-channel multi-channel analyzers, computers, etc. For conventional positron annihilation coincidence Doppler broadening spectrometers, it is usually necessary to use two identical samples to clamp the positron source during testing to form a "sample-positron source-sample" sandwich structure. The positron source is composed of radioactive materials wrapped in polyimide Kapton film or metal film. The thicker the film used to wrap the radioactive material, the better the mechanical properties of the positron source, so the risk of damage to the positron source and leakage of radioactive materials during use is lower, and the corresponding service life is longer. However, the thicker the film, the higher the proportion of source components in the positron annihilation coincidence Doppler broadening test results, that is, the higher the proportion of positrons contained in the detected coincidence events that are annihilated in non-samples. For example, when wrapped with two layers of 7.5-micron-thick Kapton film 22 When using a Na positron source, the source component accounts for about 13%; when using two layers of 15-micron-thick Kapton film wrapped 22 When using a Na positron source, the source component accounts for about 23%. Obviously, the lower the source component, the more accurate the test results of the positron annihilation Doppler broadening spectrometer. It is worth noting that for the positron annihilation Doppler broadening technology, there is currently no method to effectively deduct the source component from the test results. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a β-based + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer. By adding a coincidence detector to the conventional positron annihilation coincidence Doppler broadening spectrometer, the positrons generated by the decay of the positron source are detected for event selection. + -γ-γ coincidence method is used to obtain the positron annihilation coincidence Doppler broadening spectrum. On this basis, high-purity germanium detectors are placed in specific positions, which can effectively reduce the proportion of source components in the positron annihilation coincidence Doppler broadening spectrum obtained by the test. In addition, the new spectrometer uses a semi-digital method combining a digital oscilloscope and an electronic plug-in to collect the output signals of the coincidence detector and the high-purity germanium detector.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A beta-based +-γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, the spectrometer includes a coincidence signal generation module, an annihilation γ photon detection module, a signal acquisition module, and a control and analysis module. The coincidence signal generation module and the annihilation γ photon detection module are respectively connected to the signal acquisition module, and the signal acquisition module is connected to the control and analysis module, wherein,

[0008] The coincidence signal generating module is used to detect positrons generated by decay of a positron source and generate coincidence signals for event selection;

[0009] The annihilation gamma photon detection module is used to detect gamma photons generated by positron annihilation and generate a pulse signal containing energy information;

[0010] The signal acquisition module is used to collect the coincidence signal generated by the coincidence signal generation module and the pulse signal generated by the annihilation gamma photon detection module;

[0011] The control and analysis module is used to control the signal acquisition module and obtain the positron annihilation coincidence Doppler broadening spectrum from the collected coincidence signal and pulse signal.

[0012] Furthermore, the coincidence signal generating module includes a silicon photomultiplier tube, a scintillator, a photoconductor, a positron source, a common-phase amplifier, an inverting amplifier, and a time extender. The positrons generated by the decay of the positron source enter the scintillator and deposit energy, causing the scintillator to generate scintillation photons. When the silicon photomultiplier tube detects the scintillation photons, it generates an electrical pulse signal, which is processed by the common-phase amplifier, the inverting amplifier, and the time extender and then connected to the signal acquisition module.

[0013] Furthermore, by changing the thickness of the scintillator and setting the energy window of the silicon photomultiplier tube detector within a preset range on both sides of the single peak appearing in the energy spectrum, the detected events are mainly events of positron annihilation in the sample.

[0014] Furthermore, the annihilation γ photon detection module includes a first high-purity germanium detector, a second high-purity germanium detector, a first preamplifier, a second preamplifier, a first main amplifier, and a second main amplifier. When the γ photons generated by positron annihilation deposit energy in the sensitive areas of the first and second high-purity germanium detectors, the first and second high-purity germanium detectors generate first and second pulse signals, which are amplified by the first and second preamplifiers and then transmitted to the first and second main amplifiers. The first and second main amplifiers amplify and shape the received signals and transmit them to the signal acquisition module.

[0015] Furthermore, the height is adjusted so that the side surfaces of the cylindrical sensitive areas in the first high purity germanium detector and the second high purity germanium detector are tangent to the upper surface of the scintillator in the coincidence signal generating module.

[0016] Furthermore, the gamma photons detected by the first high-purity germanium detector and the second high-purity germanium detector in a coincidence event mainly come from a pair of gamma photons generated by the annihilation of positrons in the sample.

[0017] Furthermore, the signal acquisition module includes a digital oscilloscope, and the digital oscilloscope includes four channels, wherein one channel is used to acquire the coincidence signal of the coincidence signal generating module, and two channels are used to acquire the output signals of the first main amplifier and the second main amplifier respectively.

[0018] Furthermore, the control and analysis module includes a computer and a C++ program, which uses the C++ program to achieve real-time interaction between the digital oscilloscope and the computer, and controls the working mode of the digital oscilloscope to extract effective signals for positron annihilation coincidence Doppler broadening spectrum analysis, and performs two-dimensional frequency statistics on the extracted effective signals to obtain a positron annihilation coincidence Doppler broadening spectrum with an extremely low source component ratio.

[0019] The beneficial effects of the present invention are:

[0020] (1) The positron annihilation coincidence Doppler broadening spectrometer of the present invention adds a coincidence signal generated by directly detecting positrons, which is used to screen out events that are not annihilated in the sample. In addition, by adjusting the geometric positions of the two high-purity germanium detectors, the events that are not annihilated in the sample can be further eliminated. Under these two event screening strategies, the positron annihilation coincidence Doppler broadening spectrometer of the present invention will be able to achieve an extremely low source component ratio, thereby improving the accuracy of the test results.

[0021] (2) The present invention uses a semi-digital data acquisition system, which can particularly obtain the complete output signal of the high-purity germanium detector, allowing the flexible use of various algorithms to perform de-piling processing on the signal, which is also conducive to improving the accuracy of the test.

[0022] (3) The new spectrometer of the present invention allows two samples to be placed on the same side of the positron source, which simplifies the sample replacement process and helps reduce the length of time the tester is exposed to radiation during the experiment. Therefore, it is tester-friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The present invention is a β-based + -Schematic diagram of the structure of the positron annihilation coincidence Doppler broadening spectrometer for γ-γ coincidence;

[0024] Figure 2 is the simulated distribution of scintillator photon numbers detected by SiPM;

[0025] Figure 3This is a diagram showing the change in the proportion of events that are annihilated in the sample as judged by the coincidence signal generation module under different scintillator thicknesses. DETAILED DESCRIPTION

[0026] The present application is further described below in conjunction with the accompanying drawings and embodiments.

[0027] The specific technical solution of the present invention is as follows: a method based on β + The positron annihilation coincidence Doppler broadening spectrometer of -γ-γ coincidence includes a coincidence signal generation module, an annihilation γ photon detection module, a signal acquisition module, and a control and analysis module. The coincidence signal generation module and the annihilation γ photon detection module are respectively connected to the signal acquisition module, and the signal acquisition module is connected to the control and analysis module. The spectrometer device provided by the present invention is as follows Figure 1 As shown in the figure, the composition and functions of the above modules are as follows:

[0028] Coincidence signal generation module: This module is used to detect positrons generated by the decay of a positron source and generate coincidence signals for event selection. It includes a silicon photomultiplier (SiPM) and its signal readout circuit board, a scintillator, a light guide, a positron source, a common-phase amplifier, an inverting amplifier, a time extender, and a constant low-voltage power supply. After the positrons generated by decay enter the scintillator, they will deposit energy in it, causing the scintillator to generate scintillation photons. The electrical pulse signal generated when the SiPM detects the scintillation photons is processed by the common-phase amplifier, the inverting amplifier, and the time extender and then connected to the signal acquisition module. The main function of the time extender is to increase the time width of the processed electrical pulse signal from a few nanoseconds to the microsecond level. The light guide in this module can be scraped from a two-component optical glue with a thickness of micrometers; the scintillator can be hot-pressed from a thicker scintillator with a thickness of tens of micrometers. The appropriate thickness of the light guide and scintillator can be determined by the Geant4 simulation results, among which the thickness of the scintillator is crucial to reducing the source component and depends on the positron source (commonly used 22 Na, 68 The selection of Ge source. 22 Taking the Na positron source as an example, when the thickness of the light guide is 7.1 microns and the thickness of the scintillator is 83 microns, the simulated SiPM detection scintillation photon number distribution spectrum (approximate integrated energy spectrum) is as follows: Figure 2 As shown in Figure 2, it can be seen that when the energy window of the SiPM detector is set at Figure 2 When the image is between the two dashed lines, the detected events are mainly from the events annihilated in the sample. Figure 3 The simulation shows the change in the proportion of annihilated events in the sample selected by the coincidence signal generation module by setting the appropriate energy window of the SiPM detector (taking into account both the count rate and the proportion of source components) at different scintillator thicknesses. Figure 3The fitted line obtained from the data points in the figure can be inferred that when the thickness of the scintillator is about 20 microns, about 95% of the detected events are cases of positron annihilation in the sample. 68 When using a Ge positron source, a thicker scintillator can be used to achieve the same source component ratio, which is conducive to reducing the difficulty of preparing thin scintillators. By using a scintillator with a thickness of tens of microns (depending on the type of positron source) and setting the energy window of the SiPM detector within a preset range on both sides of the single peak appearing in the energy spectrum, the source component ratio in the test results can be effectively reduced. This is the first strategy of the present invention to reduce the source component of the positron annihilation spectrum that meets the Doppler broadening.

[0029] Annihilation γ photon detection module: This module is used to detect γ photons generated by positron annihilation, including a first high-purity germanium detector, a second high-purity germanium detector, a first preamplifier (built-in, not shown in the figure), a second preamplifier (built-in, not shown in the figure), a first main amplifier, a second main amplifier, a first high-voltage constant-steady power supply, and a second high-voltage constant-steady power supply. The first high-voltage constant-steady power supply and the second high-voltage constant-steady power supply are used to power the first and second high-purity germanium detectors. When the γ photons generated by positron annihilation deposit energy in the sensitive area (i.e., high-purity germanium) of the first and second high-purity germanium detectors, the first and second high-purity germanium detectors will generate first and second pulse signals, which are respectively amplified by the first and second preamplifiers and transmitted to the first and second main amplifiers. The first and second main amplifiers further amplify and shape the received signals, and then transmit them to the signal acquisition module. Since the high-purity germanium detector has a high energy resolution and the amplitude of the pulse signal generated is proportional to the energy deposited by the γ photons, accurate annihilation γ photon energy information can be obtained by recording the signals generated by it. When building the module, the height is adjusted so that the side of the cylindrical sensitive area in the first high purity germanium detector and the second high purity germanium detector is tangent to the upper surface of the scintillator in the coincidence signal generating module ( Figure 1 The dotted line connecting the first and second high purity germanium detectors in the above is the corresponding tangent line). Since most positron annihilation cases will produce a pair of gamma photons that are approximately back-to-back, placing the first high purity germanium detector and the second high purity germanium detector in the above manner will make the gamma photons they detect in a coincidence event mainly come from a pair of gamma photons produced by positron annihilation in the sample, which is beneficial to reduce the proportion of source components in the test results. This is the second strategy of the present invention to reduce the source component of the positron annihilation coincidence Doppler broadening spectrum.

[0030] Signal acquisition module: This module is used to collect signals from the coincidence signal generation module and the annihilation γ photon detection module, and includes a digital oscilloscope. Considering that the signal width of the coincidence signal generation module is in the microsecond level, and the signal width of the amplified high-purity germanium detector is in the tens of microseconds, the digital oscilloscope used in this module should have the characteristics of high vertical resolution accuracy and low sampling rate. A digital oscilloscope usually has four channels, A, B, C, and D. Channel B is used to collect the output signal of the coincidence signal generation module, and channels A and C are used to collect the output signals of the first main amplifier and the second main amplifier respectively. Channel D is not used. The digital oscilloscope adopts the "or trigger" mode. When the signal of channel A or channel C meets the trigger condition, it will transmit the analog signals of channels A, B, and C within a certain time window to the control and analysis module after digital processing.

[0031] Control and analysis module: This module is used to control the signal acquisition module and obtain the positron annihilation coincidence Doppler broadening spectrum from the collected signals, including a computer and a C++ program. The C++ program is used to realize the real-time interaction between the digital oscilloscope and the computer, control the digital oscilloscope to work in a specific mode and complete data transmission, and extract the effective signal for positron annihilation coincidence Doppler broadening spectrum analysis from the large number of collected signals. The main steps of signal extraction are as follows: Taking the trigger of channel A as an example, first determine whether the signal of channel C meets the set threshold condition. If not, discard the current case, otherwise determine whether there is obvious accumulation of the signals of channel A and channel C. If there is, discard the current case, otherwise determine whether the signal of channel B in the current case meets the set threshold condition. If not, discard the current case, otherwise record the baseline-corrected amplitude values ​​of the signals in channel A and channel C in the current case, that is, a set of positron annihilation coincidence Doppler broadening spectrum data. Perform two-dimensional frequency statistics on a large number of the above data groups to obtain a positron annihilation coincidence Doppler broadening spectrum with a very low source component ratio.

[0032] Example

[0033] according to Figure 1 The schematic diagram of the new spectrometer involved in the present invention is used to construct the SiPM of Onsemi's MicroFJ-60035. According to the circuit diagram provided in the product manual, the corresponding signal readout circuit board can be made. 22 NaCl liquid (or use 68 A Ge positron source is dropped onto the optical window glass of the SiPM detector. The positron source intensity is usually about 30 microcuries. 22After the NaCl liquid is completely dry, place the micrometer-sized light guide and the scintillator of tens of micrometers (the specific thickness of the two can be determined according to the simulation results) on the positron source spot in sequence. Among them, the extremely thin light guide can be scraped from SL600 two-component optical glue, and the thinner scintillator (plastic scintillator is preferred, such as EJ-228) can be prepared by hot pressing. The light guide just prepared with SL600 has a certain viscosity. One side of it can be tightly adhered to the optical window glass of SiPM to avoid leakage of the positron source, and the other side can be adhered to the scintillator. The in-phase amplifier in the figure can use Mini-circuits' ZFL-1000LN+ low-noise amplifier, the inverting amplifier can use OPA855, and the time extender can use Ortec 460 delay line amplifier. The two high-purity germanium detectors and the corresponding main amplifiers can use Ortec's GEM35-76-PL and 672 amplifiers respectively. When placing them, try to make the side of the cylindrical sensitive area of ​​the high-purity germanium detector tangent to the upper surface of the scintillator ( Figure 1 The dotted line connecting the first and second high purity germanium detectors in the figure is the corresponding tangent line). The digital acquisition card can use Pico 5444D, set the sampling rate to 1 / 24 GHz, and the vertical accuracy to 14 bit. During the test, place the two samples in Figure 1 After the spectrometer is built, the C++ program in the control and analysis module can be used to collect the positron annihilation coincidence Doppler broadening spectrum with a very low source component ratio.

[0034] 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 protection scope of the present invention.

Claims

1. A β-based + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, characterized in that, The spectrometer includes a coincidence signal generation module, an annihilation γ photon detection module, a signal acquisition module, and a control and analysis module. The coincidence signal generation module and the annihilation γ photon detection module are respectively connected to the signal acquisition module, and the signal acquisition module is connected to the control and analysis module, wherein: The coincidence signal generating module is used to detect positrons generated by decay of a positron source and generate coincidence signals for event selection; The annihilation gamma photon detection module is used to detect gamma photons generated by positron annihilation and generate a pulse signal containing energy information; The signal acquisition module is used to collect the coincidence signal generated by the coincidence signal generation module and the pulse signal generated by the annihilation gamma photon detection module; The control and analysis module is used to control the signal acquisition module and obtain the positron annihilation coincidence Doppler broadening spectrum from the collected coincidence signal and pulse signal.

2. A β-based method according to claim 1. + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, characterized in that, The coincidence signal generating module comprises a silicon photomultiplier tube, a scintillator, a photoconductor, a positron source, a common-phase amplifier, an inverting amplifier, and a time extender. The positrons generated by the decay of the positron source enter the scintillator and deposit energy, causing the scintillator to generate scintillation photons. When the silicon photomultiplier tube detects the scintillation photons, it generates an electrical pulse signal, which is processed by the common-phase amplifier, the inverting amplifier, and the time extender and then connected to the signal acquisition module.

3. A β-based method according to claim 2. + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, characterized in that, By changing the thickness of the scintillator and setting the energy window of the silicon photomultiplier tube detector within a preset range on both sides of the single peak appearing in the energy spectrum, the detected event is an event of positron annihilation in the sample.

4. A β-based method according to claim 1. + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, characterized in that, The annihilation γ photon detection module includes a first high-purity germanium detector, a second high-purity germanium detector, a first preamplifier, a second preamplifier, a first main amplifier, and a second main amplifier. When the γ photons generated by positron annihilation deposit energy in the sensitive areas of the first and second high-purity germanium detectors, the first and second high-purity germanium detectors generate first and second pulse signals, which are amplified by the first and second preamplifiers and then transmitted to the first and second main amplifiers. The first and second main amplifiers amplify and shape the received signals and transmit them to the signal acquisition module.

5. A β-based method according to claim 4. + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, characterized in that, The height is adjusted so that the side surfaces of the cylindrical sensitive areas in the first high purity germanium detector and the second high purity germanium detector are tangent to the upper surface of the scintillator in the coincidence signal generating module.

6. A β-based method according to claim 4. + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, characterized in that, The gamma photons detected by the first high-purity germanium detector and the second high-purity germanium detector in a coincidence event come from a pair of gamma photons generated by the annihilation of positrons in the sample.

7. A β-based method according to claim 1. + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, characterized in that, The signal acquisition module includes a digital oscilloscope, and the digital oscilloscope includes four channels, wherein one channel is used to acquire the coincidence signal of the coincidence signal generating module, and two channels are used to acquire the output signals of the first main amplifier and the second main amplifier respectively.

8. A β-based method according to claim 1. + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, characterized in that, The control and analysis module includes a computer and a C++ program. The C++ program is used to realize real-time interaction between the digital oscilloscope and the computer, and to control the working mode of the digital oscilloscope, extract effective signals for positron annihilation coincidence Doppler broadening spectrum analysis, perform two-dimensional frequency statistics on the extracted effective signals, and obtain a positron annihilation coincidence Doppler broadening spectrum with an extremely low source component ratio.

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