A beta + A gamma-gamma coincidence positron annihilation coincidence Doppler broadening spectrometer

Abstract

The application discloses a kind of based on β + The coincidence signal generating module is used to detect the positron generated by the decay of positron source, and generate coincidence signals for event selection. The annihilation gamma photon detection module is used to detect the gamma photon generated by positron annihilation, and generate pulse signals containing energy information. The signal acquisition module is used to acquire coincidence signals and pulse signals. The control and analysis module is used to control the signal acquisition module and obtain the positron annihilation coincidence Doppler broadening spectrum from the acquired coincidence signals and pulse signals. The positron annihilation coincidence Doppler broadening spectrometer of the application can achieve a very low source component ratio, thereby improving the accuracy of test results.

Description

TECHNICAL FIELD

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

[0002] A positron is the antiparticle of an electron, and it can be used as a sensitive probe for non-destructive investigation of the microscopic structure of materials. The annihilation characteristics of a positron in matter are related to the electron density distribution, which is determined by the microscopic structure. Positron annihilation spectroscopy, which is based on the principle of positron annihilation, has been widely used in the fields of solid state physics, materials science, physical metallurgy, chemistry, national defense technology, and biomedical science. Positron annihilation spectroscopy includes a variety of measurement methods and means, one of which is positron annihilation coincidence Doppler broadening, which can provide information about the momentum of the electron involved in the annihilation and the chemical environment at the annihilation site.

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

[0004] The conventional positron annihilation coincidence Doppler broadening spectrometer adopts various electronic components, mainly composed of a high-purity germanium detector, a preamplifier, a main amplifier, a single-channel analyzer, a coincidence unit, an analog-to-digital converter, a dual-channel multichannel analyzer, a computer, and the like. For the conventional positron annihilation coincidence Doppler broadening spectrometer, two identical samples are usually used to sandwich the positron source during testing, forming a "sample-positron source-sample" sandwich structure. The positron source is wrapped with a polyimide Kapton film or a metal film. The thicker the film used to wrap the radioactive material, the better the mechanical properties of the positron source, and thus the lower the risk of damage to the positron source and leakage of the radioactive material during use, and the longer the service life. However, a thicker film will result in a higher proportion of source components in the positron annihilation coincidence Doppler broadening test results, i.e., a higher proportion of positrons detected in the coincidence events that are annihilated in the non-sample. For example, when a Na positron source wrapped with two layers of 7.5-micron-thick Kapton film is used, the proportion of source components is about 13%; when a Na positron source wrapped with two layers of 15-micron-thick Kapton film is used, the proportion of source components is about 23%. Obviously, the lower the proportion of source components, the more accurate the test results of the positron annihilation coincidence Doppler broadening spectrometer. It is worth noting that there is currently no effective method to subtract the source components from the test results for the positron annihilation coincidence Doppler broadening technology. 22 22 SUMMARY

[0005] To solve the above technical problems, the present application provides a positron annihilation coincidence Doppler broadening spectrometer based on β + -γ-γ coincidence. 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 to perform event selection, and the β + -γ-γ coincidence method is used to obtain the positron annihilation coincidence Doppler broadening spectrum. On this basis, the high-purity germanium detector is placed at a specific position, and the proportion of source components in the obtained positron annihilation coincidence Doppler broadening spectrum can be effectively reduced. In addition, the new spectrometer uses a semi-digital method combining a digital oscilloscope with electronic components 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 application is as follows:

[0007] A positron annihilation coincidence Doppler broadening spectrometer based on β + ​​A gamma-gamma coincidence positron annihilation coincidence Doppler broadening spectrometer, comprising a coincidence signal generation module, an annihilation gamma photon detection module, a signal acquisition module, a control and analysis module, the coincidence signal generation module and the annihilation gamma photon detection module being connected to the signal acquisition module, and the signal acquisition module being connected to the control and analysis module, wherein,

[0008] The coincidence signal generation module is used for detecting positrons generated by decay of a positron source and generating coincidence signals for event selection.

[0009] The annihilation gamma photon detection module is used for detecting gamma photons generated by positron annihilation and generating pulse signals containing energy information.

[0010] The signal acquisition module is used for acquiring the coincidence signals generated by the coincidence signal generation module and the pulse signals generated by the annihilation gamma photon detection module.

[0011] The control and analysis module is used for controlling the signal acquisition module and obtaining a positron annihilation coincidence Doppler broadening spectrum from the acquired coincidence signals and pulse signals.

[0012] Further, the coincidence signal generation module comprises a silicon photomultiplier, a scintillator, a light guide, a positron source, an in-phase amplifier, an inverse amplifier, and a time stretcher. The positrons generated by decay of the positron source deposit energy in the scintillator after entering the scintillator, causing the scintillator to generate scintillation photons. When the silicon photomultiplier detects the scintillation photons, it generates an electrical pulse signal. The electrical pulse signal is processed by the in-phase amplifier, the inverse amplifier, and the time stretcher, and then connected to the signal acquisition module.

[0013] Further, the thickness of the scintillator is changed, and the energy window of the silicon photomultiplier detector is set within a predetermined range on both sides of a single peak appearing in the energy spectrum, so that the detected events are mainly events of positrons annihilating in the sample.

[0014] Further, the annihilation gamma photon detection module comprises 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 gamma photons generated by positron annihilation deposit energy in the sensitive regions of the first and second high-purity germanium detectors, the first and second high-purity germanium detectors generate first and second pulse signals. The first and second pulse signals are amplified by the first and second preamplifiers, respectively, 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] Further, the height is adjusted so that the side surfaces of the cylindrical sensitive regions in the first and second high-purity germanium detectors are tangent to the upper surface of the scintillator in the coincidence signal generation module.

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

[0017] Further, the signal acquisition module comprises a digital oscilloscope, and the digital oscilloscope comprises four channels, one of which is used to acquire the coincidence signal of the coincidence signal generation module, and two of which are used to acquire the output signals of the first main amplifier and the second main amplifier respectively.

[0018] Further, the control and analysis module comprises a computer and a C++ program, the C++ program is used for realizing real-time interaction between the digital oscilloscope and the computer, controlling the working mode of the digital oscilloscope, extracting effective signals for positron annihilation coincidence Doppler broadening spectrum analysis, and performing 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 positron annihilation coincidence Doppler broadening spectrometer has the advantages that:

[0020] (1) The positron annihilation coincidence Doppler broadening spectrometer increases a coincidence signal generated by directly detecting positrons, which is used for screening non-sample annihilation cases. In addition, by adjusting the geometric positions of the two high-purity germanium detectors, non-sample annihilation cases can be further removed. Under the two case screening strategies, the positron annihilation coincidence Doppler broadening spectrometer can realize an extremely low source component ratio, thereby improving the accuracy of the test results.

[0021] (2) The positron annihilation coincidence Doppler broadening spectrometer uses a semi-digital data acquisition system, especially can obtain complete output signals of the high-purity germanium detectors, allows flexible use of various algorithms for de-piling processing of the signals, and is also conducive to improving the accuracy of the test.

[0022] (3) The new spectrometer allows two samples to be placed on the same side of the positron source, the sample replacement process is simple, and the radiation time of the tester during the experiment is reduced, so that the new spectrometer has the characteristics of being friendly to the tester. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The positron annihilation coincidence Doppler broadening spectrometer is based on a β + A structure diagram of the positron annihilation coincidence Doppler broadening spectrometer based on a β

[0024] Figure 2 A distribution of scintillator photons detected by the SiPM is simulated.

[0025] Figure 3This is a graph showing the change in the proportion of annihilated events in the sample, as determined by the coincidence signal generation module, under different scintillator thicknesses, obtained through simulation. Detailed Implementation

[0026] The present application will be further described below with reference to the accompanying drawings and embodiments.

[0027] The specific technical solution of the present invention is as follows: a method based on β + A positron annihilation coincidence Doppler broadening spectrometer based on -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, which is connected to the control and analysis module. The spectrometer device provided by this invention is as follows: Figure 1 As shown, the composition and functions of each module are as follows:

[0028] Coincidence Signal Generation Module: This module detects positrons decaying from the positron source and generates a coincidence signal for event selection. It includes a silicon photomultiplier tube (SiPM) and its signal readout circuit board, a scintillator, a light guide, a positron source, a non-inverting amplifier, an inverting amplifier, a time extender, and a constant low-voltage power supply. The decaying positrons deposit energy in the scintillator, causing it to produce scintillating photons. The electrical pulse signal generated when the SiPM detects scintillating photons is processed by the non-inverting amplifier, inverting amplifier, and time extender before being input 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 made of two-component optical adhesive with a thickness in the micrometer range; the scintillator can be hot-pressed from a thicker scintillator with a thickness in the tens of micrometer range. The appropriate thickness of the light guide and scintillator can be determined by Geant4 simulation results. The thickness of the scintillator is crucial for reducing the source composition and depends on the positron source (commonly...). 22 Na、 68 Selection of the positron emission source (Ge source). A source with a lower positron emission energy is chosen. 22 Taking the Na positron source as an example, when the thickness of the photoconductor is 7.1 micrometers and the thickness of the scintillator is 83 micrometers, the simulated SiPM detector scintillator photon number distribution spectrum (approximate integrated energy spectrum) is as follows: Figure 2 As shown. It can be seen that when the energy window of the SiPM detector is set... Figure 2 When the two dashed lines are in the middle, the detected events are mainly those that are buried in the sample. Figure 3 This demonstrates the variation in the proportion of annihilated events in the sample, selected by the coincidence signal generation module, under different scintillator thicknesses by setting an appropriate energy window for the SiPM detector (considering both count rate and source component ratio). Figure 3The fitting line of the middle data points can be inferred that when the thickness of the scintillator is about 20 microns, about 95% of the detection events are the events of positron annihilation in the sample. When a positron source with a higher positron emission energy is used 68 Ge, the same source component ratio can be achieved, and a thicker scintillator can be used, which is beneficial to reduce the difficulty of preparing a thin scintillator. 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 the 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, which is the first strategy of the present application to reduce the source component of the positron annihilation coincidence Doppler broadening spectrum.

[0029] Annihilation gamma photon detection module: This module is used to detect the gamma photons generated by positron annihilation, and includes 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 power supply, and a second high-voltage constant power supply. The first and second high-voltage constant power supplies are used to power the first and second high-purity germanium detectors. When the gamma photons generated by positron annihilation deposit energy in the sensitive region (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, respectively. The first and second pulse signals are amplified by the first and second preamplifiers and then transmitted to the first and second main amplifiers, respectively. 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 very high energy resolution, and the amplitude of the generated pulse signal is proportional to the energy deposited by the gamma photon, accurate annihilation gamma photon energy information can be obtained by recording the generated signal. When building this module, the height is adjusted so that the side of the cylindrical sensitive region in the first and second high-purity germanium detectors is tangent to the upper surface of the scintillator in the coincidence signal generation module (the dashed line connecting the first and second high-purity germanium detectors in the figure is the corresponding tangent line). Figure 1 Since most positron annihilation events will produce a pair of approximately back-to-back gamma photons, placing the first and second high-purity germanium detectors in the above manner will enable them to detect gamma photons mainly from the pair of gamma photons generated by positron annihilation in the sample in a coincidence event, thereby facilitating the reduction of the source component ratio in the test results. This is the second strategy of the present application to reduce the source component of the positron annihilation coincidence Doppler broadening spectrum.

[0030] Signal Acquisition Module: This module acquires signals from the coincidence signal generation module and the annihilation gamma photon detection module, and includes a digital oscilloscope. Considering that the signal width of the coincidence signal generation module is in the microsecond range, and the signal width of the amplified high-purity germanium detector is in the tens of microsecond range, the digital oscilloscope used in this module should have high vertical resolution and a low sampling rate. The digital oscilloscope typically has four channels: A, B, C, and D. Channel B is used to acquire the output signal of the coincidence signal generation module, channels A and C are used to acquire the output signals of the first and second main amplifiers, respectively, and channel D is not used. The digital oscilloscope uses an "OR trigger" mode. When the signal in channel A or channel C meets the trigger condition, it transmits the analog signals from 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 controls the signal acquisition module and extracts the positron annihilation coincidence Doppler broadening spectrum from the acquired signals. It includes a computer and a C++ program. The C++ program enables real-time interaction between the digital oscilloscope and the computer, controlling the oscilloscope to operate in specific modes and complete data transmission. It also extracts effective signals for positron annihilation coincidence Doppler broadening spectrum analysis from a large number of acquired signals. The main steps of signal extraction are as follows: Taking channel A as an example, first, it checks whether the signal in channel C meets the set threshold condition. If not, the current case is discarded; otherwise, it checks whether there is significant signal accumulation in channels A and C. If so, the current case is also discarded; otherwise, it checks whether the signal in channel B in the current case meets the set threshold condition. If not, the current case is also discarded; otherwise, it records the baseline-corrected amplitude values ​​of the signals in channels A and C in the current case, i.e., a set of positron annihilation coincidence Doppler broadening spectrum data. By performing two-dimensional frequency statistics on a large number of these data sets, a positron annihilation coincidence Doppler broadening spectrum with an extremely low source component ratio can be obtained.

[0032] Example

[0033] according to Figure 1 The diagram illustrates the construction of the novel spectrometer involved in this invention. The SiPM used is Onsemi's MicroFJ-60035, and the corresponding signal readout circuit board can be fabricated according to the circuit diagram provided in the product manual. Subsequently, small quantities of... 22 NaCl liquid (or use) 68 A positron source (Ge positrons) is dropped onto the optical window of the SiPM detector; the intensity of the positron source is typically around 30 microcurves. 22After the NaCl liquid is completely dried, the micron-sized light guide and the tens-of-micron-sized scintillator (the specific thicknesses of both can be determined according to simulation results) are placed on the positron source spot in sequence. Among them, the extremely thin light guide can be scraped from SL600 double-component optical glue, and the relatively thin scintillator (preferably a plastic scintillator such as EJ-228) can be prepared by hot pressing. The light guide just prepared from SL600 has certain viscosity, and one side thereof can be closely adhered to the optical window glass of the 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 a Mini-circuits ZFL-1000LN+ low-noise amplifier, the anti-phase amplifier can use an OPA855, and the time stretcher can use an Ortec 460 delay line amplifier. Two high-purity germanium detectors and corresponding main amplifiers can use Ortec GEM35-76-PL and 672 type amplifiers respectively, and when placed, the side surface of the cylindrical sensitive region of the high-purity germanium detector is tangent to the upper surface of the scintillator (the dashed line connected with the first and second high-purity germanium detectors in the figure is the corresponding tangent line). Figure 1 The digital acquisition card can use Pico 5444D, and the sampling rate is set to 1 / 24 GHz and the vertical precision is set to 14 bit. When testing, two samples are placed above the scintillator in the manner shown in FIG. 6. 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 extremely low source component ratio.

[0034] The above-described specific embodiments further illustrate the purposes, technical solutions and beneficial effects of the present application, and it should be understood that the above-described specific embodiments are only specific embodiments of the present application and are not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A method based on β + -γ-γ coincidence positron annihilation coincidence Doppler broadening spectrometer, characterized in that... The spectrometer includes 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 and the annihilation gamma photon detection module are respectively connected to the signal acquisition module, which is connected to the control and analysis module. The coincidence signal generation module is used to detect positrons generated by the decay of the 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 acquire 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 broadened spectrum from the acquired coincidence signal and pulse signal; The coincidence signal generation module includes a silicon photomultiplier tube, a scintillator, a light guide, a positron source, a non-inverting 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 scintillating photons. When the silicon photomultiplier tube detects the scintillating photons, it generates an electrical pulse signal, which is processed by the non-inverting amplifier, the inverting amplifier, and the time extender before being connected to the signal acquisition module. 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 examples of positrons annihilated in the sample. The annihilation gamma 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 gamma photons generated by positron annihilation deposit energy in the sensitive regions of the first and second high-purity germanium detectors, the first and second high-purity germanium detectors generate first and second pulse signals. The first and second pulse signals 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. The height is adjusted so that the side of the cylindrical sensitive area in the first and second high-purity germanium detectors is tangent to the upper surface of the scintillator in the coincidence signal generation module; The gamma photons detected by the first and second high-purity germanium detectors in a coincidence event originated from a pair of gamma photons generated by the annihilation of positrons in the sample.

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

3. A method based on β 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 enables real-time interaction between the digital oscilloscope and the computer, controls the working mode of the digital oscilloscope, extracts the effective signal for positron annihilation coincidence Doppler broadening spectrum analysis, performs two-dimensional frequency statistics on the extracted effective signal, and obtains a positron annihilation coincidence Doppler broadening spectrum with an extremely low source component ratio.

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