A type of β + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer

Abstract

This invention discloses a β + This invention relates to a γ-γ coincidence positron annihilation lifetime-momentum correlation spectrometer, belonging to the field of nuclear detection technology. The spectrometer includes a positron annihilation lifetime detection module, a positron annihilation γ-photon energy detection module, a signal acquisition module, and a control and analysis module. The positron annihilation lifetime detection module is used to obtain the lifetime component of the AMOC spectrum; the positron annihilation γ-photon energy detection module is used to obtain the momentum component of the AMOC spectrum by detecting γ-photons generated by positron annihilation; the signal acquisition module is used to acquire the signals generated by the positron annihilation lifetime detection module and the positron annihilation γ-photon energy detection module; and the control and analysis module is used to control the signal acquisition module and obtain the AMOC spectrum based on the signals acquired by the signal acquisition module. This invention can effectively improve the coincidence count rate, has good coincidence time resolution and extremely low source composition, and helps to reduce the construction cost of the spectrometer.

Description

Technical Field

[0001] This invention belongs to the field of nuclear detection technology, specifically to the field of positron annihilation spectroscopy, and particularly relates to a β-type positron annihilation spectroscopy. + -γ-γ coincident positron annihilation lifetime-momentum correlation (AMOC) spectrometer. Background Technology

[0002] Positron annihilation spectroscopy is a unique method for characterizing material properties, particularly for studying the microstructure of various types of defects and free volumes in solid materials, demonstrating extremely high sensitivity. Generally, high-energy positrons, upon entering a material, undergo three main processes: thermalization, diffusion, and annihilation. Because positrons carry a positive charge, they automatically seek out negatively charged microstructures as they move within the material, ultimately annihilating at these locations. The positron annihilation lifetime (the time from generation to annihilation into a photon) depends on the distribution of electron density within the material, which is determined by the material's microstructure. Therefore, by detecting the lifetime of positrons within a material, information about its microstructure can be inferred. Based on this principle, positron annihilation lifetime spectrometers have been developed to measure the annihilation lifetime of positrons within materials.

[0003] Furthermore, when a positron annihilates with an electron within matter, it typically releases a pair of gamma photons emitted approximately back-to-back, with an energy close to 0.511 MeV. However, the actual energy of this pair of gamma photons depends on the momentum of the positron-electron annihilation pair. Since the positron usually loses almost all of its kinetic energy during thermalization when forming the annihilation pair, the momentum of the annihilation pair is mainly contributed by the electrons involved in the annihilation. Therefore, the actual energy of the gamma photon pair produced by annihilation carries the momentum information of the electron at the annihilation site. Based on this principle, a positron annihilation (coincidence) Doppler broadening spectrometer has been developed to measure the energy of gamma photons produced by positron annihilation within matter.

[0004] Building upon positron annihilation lifetime spectrometry and positron annihilation Doppler broadening spectrometry, AMOC spectrometry has been developed by measuring the annihilation lifetime of positrons and the energy of the γ-photons produced by annihilation, and is used to obtain the variation of electron momentum distribution with positron annihilation lifetime. AMOC spectrometry is mainly used to study the chemical environment and structure of pores in porous materials, the formation and behavior of electron dipoles, etc., and has irreplaceable advantages.

[0005] Early AMOC spectrometer data acquisition systems were based on complex electronic modules, typically employing the γ-γ-γ coincidence principle and using... 22Sodium isotopes are used as positron sources. AMOC spectra are obtained by statistically analyzing the positron annihilation lifetimes and corresponding annihilated gamma photon energies from a large number of coincidence events. To obtain accurate results, the total count for each spectrum typically needs to be greater than 10 million. However, conventional AMOC spectrometers usually have low coincidence count rates, generally only measuring a few coincidence events per second. Testing a spectrum with a sufficient total count takes about 10 days, and there is also significant room for improvement in coincidence time resolution. Furthermore, for AMOC spectrometers based on the gamma-gamma coincidence principle, the positron source needs to be sandwiched between two sample sheets during testing. When the positron source is wrapped with two 7.5-micrometer-thick Kapton films, approximately 13% of the source component is present in the obtained positron annihilation lifetime-momentum correlation spectrum, which cannot be subtracted, thus affecting the accuracy of the test results to some extent. Existing technologies use... 68 Ge isotopes as β-positron sources + -γ-γ coincident AMOC spectrometers use PMTs as a component of the positron detector and also employ complex electronic components. With the development of slow positron beam technology, beam-based β-positron beams have gradually emerged. + The -γ-γ coincidence AMOC spectrometer significantly improves the detection efficiency of the initiation signal by directly detecting positrons as the starting signal of positron annihilation lifetime, thereby significantly improving the coincidence count rate. However, this beam-based β... + -γ-γ coincidence AMOC spectrometers typically have poor coincidence time resolution and are very expensive to build and maintain, so they are not widely used. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the present invention provides a β + -γ-γ coincident positron annihilation lifetime-momentum correlation (AMOC) spectrometer. Using β + The -γ-γ coincidence principle is used, and a combination of silicon photomultiplier tubes (SiPMs) and scintillators is employed as the positron detector; positron emission energies with lower energy are also used. 22 Using sodium isotopes as a positron source, the spectrometer was optimized with particle detection system simulation software to obtain spectrometer structural parameters suitable for this low-energy positron source. A semi-digital approach combining electronic modules and a fast digital oscilloscope was employed for signal acquisition and processing. The AMOC spectrometer of this invention can effectively improve the coincidence count rate, while also possessing good coincidence time resolution and extremely low source composition, and helps to reduce the spectrometer's construction cost.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A type of β +The -γ-γ coincidence positron annihilation lifetime-momentum correlation spectrometer includes a positron annihilation lifetime detection module, a positron annihilation γ-photon energy detection module, a signal acquisition module, and a control and analysis module.

[0009] The positron annihilation lifetime detection module is used to obtain the lifetime portion of the AMOC spectrum;

[0010] The positron annihilation gamma photon energy detection module is used to obtain the momentum component in the AMOC spectrum by detecting gamma photons generated by positron annihilation;

[0011] The signal acquisition module is used to acquire signals generated by the positron annihilation lifetime detection module and the positron annihilation gamma photon energy detection module;

[0012] The control and analysis module is used to control the signal acquisition module and obtain the AMOC spectrum based on the signal acquired by the signal acquisition module.

[0013] The beneficial effects of this invention are as follows:

[0014] (1) This invention utilizes a novel SiPM to replace PMT as β + -γ-γ matches the composition of the positron detector in an AMOC spectrometer. Compared to PMTs, SiPMs offer advantages such as compact structure, high photon detection efficiency, and excellent time resolution, making them more suitable for acquiring energy and timing information of signals under weak light conditions and with small emitting surfaces, thus improving the spectrometer's coincidence time resolution. Furthermore, SiPMs are significantly cheaper than PMTs, reducing the spectrometer's setup costs.

[0015] (2) This invention utilizes a semi-digital data acquisition system to replace purely digital and complex electronic plug-in-based data acquisition systems, allowing the simultaneous acquisition of positron annihilation lifetime and positron annihilation gamma photon energy signals using a high vertical accuracy but low sampling rate fast digital oscilloscope. Furthermore, it allows for stacking judgment and destacking processing of high-purity germanium signals using various algorithms, and effectively reduces the proportion of random coincidence events by analyzing the relative positions of the two signals within a time window. These features all contribute to improving the accuracy of the test.

[0016] (3) The present invention uses 22 Na isotope substitution 68 Using Ge isotopes as a positron source, the spectrometer was optimized using particle detection system simulation software, resulting in spectrometer structural parameters suitable for this low-energy positron source. Compared to 68 Ge isotope, 22 Na isotopes have a longer half-life, approximately 68The half-life of Ge isotopes is 3.5 times that of Ge, which helps the spectrometer maintain a stable coincidence count rate over a longer period. Furthermore, the low-energy positron source requires less sample thickness. Attached Figure Description

[0017] Figure 1 For the present invention, a β + Schematic diagram of the -γ-γ conformal positron annihilation lifetime-momentum correlation spectrometer;

[0018] Figure 2 The positron annihilation Doppler broadening spectrum and positron annihilation lifetime spectrum of amorphous SiO2 are based on the actual measurements of this invention;

[0019] Figure 3 The positron annihilation lifetime-momentum correlation spectrum of amorphous SiO2 is based on the actual measurements of this invention. Detailed Implementation

[0020] The present application will be further described below with reference to the accompanying drawings and embodiments.

[0021] like Figure 1 As shown, the specific technical solution of the present invention is as follows: a β + A semi-digital positron annihilation lifetime-momentum correlation spectrometer with -γ-γ coincidence includes a positron annihilation lifetime detection module, a positron annihilation gamma photon energy detection module, a signal acquisition module, and a control and analysis module. The positron annihilation lifetime detection module and the positron annihilation gamma photon energy detection module are respectively connected to the signal acquisition module, which in turn is connected to the control and analysis module. Specifically:

[0022] Positron annihilation lifetime detection module: This module is used to obtain the lifetime portion of the AMOC spectrum. It consists of three sub-modules: a start signal module, a stop signal module, and an electronics plug-in module. The start signal module and the stop signal module are connected to the electronics plug-in module, respectively.

[0023] Start signal module: This module obtains the start signal of the positron annihilation lifetime by directly detecting the positrons generated by the decay of the positron source. This includes the SiPM and the SiPM circuit board used for signal readout. 22 The system consists of a NaCl solution, a light guide film, a first scintillator, a non-inverting amplifier, an inverting amplifier, and a constant-voltage power supply. The constant-voltage power supply powers the SiPM circuit board. The light guide film can be fabricated using a two-component optical adhesive. Simulation results using Genat4 particle detection system simulation software indicate that the optimal thicknesses for the light guide film and the first scintillator are in the 10-micrometer and 100-micrometer range, respectively. The fabrication process of the starting signal module is as follows: First, the light guide film (the freshly prepared light guide film has a certain degree of adhesion) is attached to the upper surface of the first scintillator; then... 22The NaCl solution was transferred to the center of the light guide film drop by drop and dried, with the source spot size controlled at approximately 2 mm; finally, ... 22 After the NaCl solution has dried completely, apply a small amount of optical adhesive to the edge of the optical window of the SiPM, and then drop on... 22 The light-guiding film of the NaCl radioactive source and the first scintillator are attached as a single unit to the optical window of the SiPM, with the light-guiding film serving as the contact surface. In this module, the light source... 22 NaCl radioactive source 22 Na isotopes serve as a positron source. Some of the positrons produced by their decay will enter the first scintillator and deposit energy therein. When energy is deposited in the first scintillator by a particle, the first scintillator will generate scintillating photons. When SiPM detects these scintillating photons, it will generate an electrical signal. After being amplified by an in-phase amplifier and an out-of-phase amplifier in sequence, the starting signal of the positron annihilation lifetime is obtained.

[0024] Termination Signal Module: This module detects gamma photons generated by positron annihilation to obtain the termination signal of the positron annihilation lifetime. It includes a PMT (Position Targeting Mechanism), a second scintillator (cylindrical, cuboid, etc.), reflective material (Teflon tape or other), black electrical tape, silicone grease, and a first constant-voltage power supply for the PMT. The module's fabrication process is as follows: Taking a cylinder as an example, first, reflective material is applied to the top and sides of the second scintillator; then, a small amount of silicone grease is dropped into the center of the PMT's optical window, and the bottom surface of the second scintillator is attached to the PMT's optical window. The second scintillator is slowly rotated in the same direction for about 10 minutes to ensure even distribution of the silicone grease at the interface; finally, black electrical tape is used to fix the second scintillator covered with reflective material to the PMT's optical window, further serving as a light shield. When placing the detector, the lower tangent of the second scintillator should be tangent to the upper surface of the first scintillator in the start signal module (e.g., ...). Figure 1 As shown (on the same extended dashed line), this helps reduce the source composition. In this module, when annihilated γ photons deposit energy in the second scintillator, the second scintillator will generate scintillating photons. When the PMT detects these scintillating photons, it will generate an electrical signal, thereby obtaining the termination signal of the positron annihilation lifetime.

[0025] Electronics plug-in module: This module is used to process the signals generated by the start signal module and the stop signal module, including two constant ratio timing discriminators (first and second constant ratio timing discriminators), a delay unit, and a time-amplitude converter. The first constant-ratio timing discriminator is used to extract the start time of the positron annihilation lifetime from the signal output by the start signal module using a constant-ratio timing method. The lower threshold of its energy window is slightly larger than the amplitude of the background signal, and the upper threshold of the energy window can theoretically be set to include the entire energy spectrum of the start signal. The second constant-ratio timing discriminator is used to extract the end time of the positron annihilation lifetime from the signal output by the stop signal module using a constant-ratio timing method. The upper and lower thresholds of its energy window are set on both sides of the full-energy peak of the annihilated γ photon. The delay unit is connected after the second constant-ratio timing discriminator that processes the stop signal and is used to add a certain delay to the end time of the positron annihilation lifetime. The time-amplitude converter is used to receive the signals output by the first constant-ratio timing discriminator and the delay unit that process the start signal and convert the time difference between the start time of the positron annihilation lifetime and the delayed end time into a pulse signal with an amplitude proportional to it. That is, the amplitude of this signal reflects the length of the positron annihilation lifetime.

[0026] Positron Annihilation Gamma Photon Energy Detection Module: This module detects the gamma photons generated by positron annihilation to obtain the momentum component of the AMOC spectrum. It includes a high-purity germanium detector, a main amplifier, and a second constant-voltage power supply to power the high-purity germanium detector. When the gamma photons generated by positron annihilation enter the sensitive region of the high-purity germanium detector and deposit energy, the detector generates a pulse signal. The preamplifier inside the detector and the external main amplifier amplify this signal sequentially. When positioning the high-purity germanium detector, the side of the high-purity germanium in the detector is tangent to the upper surface of the first scintillator in the starting signal module (e.g., ...). Figure 1 As shown (on the extended line of the same dashed line), this helps reduce the source component. The signal generated by this module will carry high-precision energy information of the annihilated γ photons, from which the momentum component in the AMOC spectrum can be calculated.

[0027] Signal Acquisition Module: This module acquires signals generated by the electronics module and the positron annihilation gamma photon energy detection module, including a fast digital oscilloscope. The fast digital oscilloscope should feature high vertical accuracy and a low sampling rate. It typically has four channels: A, B, C, and D. Channel A acquires the electrical pulse signal output from the time-amplitude converter, channel C acquires the electrical pulse signal output from the main amplifier of the high-purity germanium detector, and channels B and D are unused. When the signal in channel A of the digital oscilloscope meets the trigger condition, the pulse signals from channels A and C within a certain time window are digitized and transmitted to the computer in the control and analysis module.

[0028] Control and Analysis Module: This module is used to control the signal acquisition module and obtain the AMOC spectrum from the acquired signal. It includes a computer and a C++ program, and consists of two sub-modules: an acquisition control module and a data analysis and recording module.

[0029] Acquisition and Control Module: This module controls the signal acquisition module. It uses a C++ program to implement the interaction between the high-speed digital oscilloscope and the computer, enabling real-time signal transmission while controlling the oscilloscope.

[0030] Data Analysis and Recording Module: This module extracts and records the AMOC spectrum from the acquired signals. First, it determines whether the signal in channel C meets a threshold condition, i.e., whether the amplitude of the signal in this channel falls within the full-energy peak range of annihilated gamma photons in the energy spectrum at the termination signal end. If the threshold condition is not met, the current event is discarded; otherwise, it checks for severe signal accumulation in channel C. If severe accumulation exists, the current event is also discarded. Otherwise, it checks whether the relative positions of the signals in channels A and C within the corresponding time window of the current event meet a set relative position range, which effectively reduces the proportion of random coincidence events. If the set relative position range is not met, the current event is also discarded; otherwise, it records the baseline-corrected amplitude values ​​of the signals in channels A and C of the current event, i.e., a set of positron annihilation lifetime-momentum data. By performing two-dimensional frequency statistics on a large number of paired positron annihilation lifetime-momentum data, the AMOC spectrum can be obtained.

[0031] Example

[0032] according to Figure 1 The diagram illustrates the actual construction of the AMOC spectrometer involved in this invention, and preliminary experimental results were obtained. During the spectrometer construction, a 100-micron-sized plastic scintillator, EJ-228, was used as the first scintillator; a 10-micron-sized light-guiding film was prepared using SL600 optical adhesive; a cylindrical BaF2 scintillator was used as the second scintillator; a PicoScope 5444D fast digital oscilloscope was used; an Onsemi MICROFJ-60035 SiPM was used; a Hamamatsu H6610 PMT was used; a ZFL-1000LN inverting amplifier was used; and an OPA855 inverting amplifier was used. 22 The Na positron source intensity was approximately 40 microcurves, and the high-purity germanium detector was approximately 45 cm away from the positron source. An amorphous SiO2 sample was tested using a pre-built AMOC spectrometer. The positron annihilation Doppler broadening spectrum and positron annihilation lifetime spectrum are shown below. Figure 2 As shown, the corresponding AMOC spectrum (preliminary experiments only measured approximately 450,000 total counts) is as follows: Figure 3As shown in the figure, the spectral results of the positron annihilation lifetime spectrum and the St curve calculated from the AMOC spectrum are basically consistent with the results reported in the literature, indicating that the AMOC spectrometer of the present invention can achieve the expected experimental objectives.

[0033] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are 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 within the protection scope of the present invention.

Claims

1. A type of β + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... It includes a positron annihilation lifetime detection module, a positron annihilation gamma photon energy detection module, a signal acquisition module, and a control and analysis module. The positron annihilation lifetime detection module is used to obtain the lifetime portion of the AMOC spectrum; The positron annihilation gamma photon energy detection module is used to obtain the momentum component in the AMOC spectrum by detecting gamma photons generated by positron annihilation; wherein, the positron annihilation gamma photon energy detection module includes a high-purity germanium detector; The signal acquisition module is used to acquire signals generated by the positron annihilation lifetime detection module and the positron annihilation gamma photon energy detection module; The control and analysis module is used to control the signal acquisition module and obtain the AMOC spectrum based on the signal acquired by the signal acquisition module; The positron annihilation lifetime detection module includes a start signal module, a stop signal module, and an electronics module. When placing the high-purity germanium detector, the side of the high-purity germanium in the detector is tangent to the upper surface of the first scintillator in the start signal module. The stop signal module includes a PMT and a second scintillator. The second scintillator is covered with a reflective material and fixed to the optical window of the PMT. The lower cut surface of the second scintillator is tangent to the upper surface of the first scintillator in the start signal module. The electronics module and the fast digital oscilloscope in the signal acquisition module constitute a semi-digital data acquisition system.

2. A β according to claim 1 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... The start signal module and the stop signal module in the positron annihilation lifetime detection module are respectively connected to the electronics plug-in module.

3. A β according to claim 2 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... The start signal module is used to acquire the start signal of the positron annihilation lifetime. The start signal module also includes a SiPM and a SiPM circuit board for signal readout. 22 NaCl solution, light guide film, in-phase amplifier, inverting amplifier, and constant-stable low-voltage power supply, among which... First scintillator, droplet 22 The light-guiding film of NaCl solution and SiPM are stacked sequentially on the SiPM circuit board; The constant-voltage low-voltage power supply is used to power the SiPM circuit board; The 22 NaCl solution is used to provide 22 Na isotope is used as a positron source. When some of the positrons generated by the decay of the positron source enter the first scintillator and deposit energy, the first scintillator generates scintillating photons. The SiPM is used to detect scintillation photons and generate an electrical signal, which is then amplified sequentially by an in-phase amplifier and an out-of-phase amplifier to obtain the starting signal of the positron annihilation lifetime.

4. A β according to claim 2 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... The termination signal module obtains the termination signal of the positron annihilation lifetime by detecting the γ photons generated by positron annihilation. The termination signal module also includes a first constant high voltage power supply for powering the PMT. When annihilated γ photons deposit energy in the second scintillator, the second scintillator generates scintillating photons. When the PMT detects the scintillating photons, it generates an electrical signal, which is the termination signal of the positron annihilation lifetime.

5. A β according to claim 2 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... The electronics module is used to process the start signal and the termination signal generated by the start signal module and the termination signal module. It includes a first and a second constant-ratio timing discriminator, a delay unit, and a time-amplitude converter. The first constant-ratio timing discriminator is used to extract the start time of the positron annihilation lifetime from the start signal output by the start signal module using a constant-ratio timing method. The second constant-ratio timing discriminator is used to extract the end time of the positron annihilation lifetime from the termination signal output by the termination signal module using a constant-ratio timing method. The delay unit is connected after the second constant-ratio timing discriminator and is used to add a certain delay to the end time of the positron annihilation lifetime. The time-amplitude converter is used to receive the signals output by the first constant-ratio timing discriminator and the delay unit and convert the time difference between the start time of the positron annihilation lifetime and the delayed end time into a pulse signal.

6. A β according to claim 1 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... The positron annihilation gamma photon energy detection module also includes a main amplifier and a second constant-voltage power supply for powering the high-purity germanium detector. When the gamma photons generated by positron annihilation enter the sensitive region of the high-purity germanium detector and deposit energy, the high-purity germanium detector generates a pulse signal, which is amplified by the external main amplifier to calculate the momentum part in the AMOC spectrum.

7. A β according to claim 1 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... The fast digital oscilloscope in the signal acquisition module includes four channels, one of which is used to acquire the pulse signal output by the time-amplitude converter, and another channel is used to acquire the pulse signal output by the main amplifier connected to the high-purity germanium detector.

8. A β according to claim 1 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... The control and analysis module includes an acquisition and control module and a data analysis and recording module.

9. A β according to claim 8 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... The acquisition control module is used to control the signal acquisition module. It uses a C++ program to realize the interaction between the fast digital oscilloscope and the computer, and completes the real-time transmission of signals while controlling the fast digital oscilloscope.

10. A β according to claim 8 + -γ-γ coincident positron annihilation lifetime-momentum correlation spectrometer, characterized in that... The data analysis and recording module is used to extract and record the AMOC spectrum from the signal acquired by the signal acquisition module.

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