Positron annihilation lifetime spectrometer system with three-dimensional position resolution capability

Abstract

The invention discloses a positron annihilation lifetime spectrometer system with three-dimensional position resolution capability, which belongs to the technical field of nuclear detection and comprises a positron annihilation lifetime measuring device and a high-precision X-Y plane moving device. The positron annihilation life measuring device is used for acquiring annihilation characteristics of positron in different depth intervals within a millimeter depth range from the surface of a material, and comprises a life starting module and a life ending module which are respectively connected with an information acquisition module, and the information acquisition module is connected to a control and processing module; the high-precision X-Y plane moving device is used for moving the detector part of the positron annihilation life measuring device so as to realize scanning test of an X-Y plane, and comprises three servo motors and three sliding rails respectively controlled by the servo motors. According to the invention, three-dimensional space characterization can be carried out on microstructure characteristics of different depth intervals in a millimeter-level depth range from different areas of the surface of the material to the surface.

Description

Technical Field

[0001] This invention belongs to the field of nuclear detection technology, specifically relating to a positron annihilation lifetime spectrometer system with three-dimensional position resolution capability. Background Technology

[0002] Positron annihilation spectroscopy is a materials characterization technique based on the annihilation of positrons with electrons in matter. By analyzing parameters such as the time and energy of gamma photons released during the annihilation process, it studies the defect characteristics, microstructure, and electron distribution within the material. Positron annihilation spectroscopy mainly includes positron annihilation lifetime spectroscopy, Doppler broadening spectroscopy, angular correlation spectroscopy, and slow positron beam spectroscopy. Among these, positron annihilation lifetime spectroscopy is the most widely used due to its advantages of providing rich information and relatively low equipment setup and maintenance costs.

[0003] A conventional positron annihilation lifetime spectrometer mainly consists of a scintillator, a photomultiplier tube (PMT), a constant ratio timing discriminator, a delay unit, a time-amplitude converter, a multichannel analyzer, and a high-voltage power supply. 22 Sodium isotopes are used as the positron source. Conventional electron annihilation lifetime spectrometers employ gamma-gamma coincidence, obtaining the initiation signal by detecting gamma photons with an energy of 1.275 MeV produced almost simultaneously with the positron, and the termination signal by detecting gamma photons with an energy of approximately 0.511 MeV produced during positron annihilation. By statistically analyzing the time difference between the termination and initiation signals across a large number of events, the positron annihilation lifetime spectrum can be obtained. Then, specific software can be used to interpret the spectrum to determine the annihilation lifetime of the positron. Since the annihilation lifetime of positrons in a material is related to the electron density distribution, which in turn depends on the material's microstructure, various microstructures within the material can be studied by measuring the annihilation lifetime of positrons.

[0004] Conventional positron annihilation lifetime spectrometry typically requires two samples, employing a "sample-positron source-sample" sandwich structure. In 2011, researchers developed a positron annihilation lifetime spectrometer capable of measuring a single sample by adding a detector for anti-coincidence to a conventional positron annihilation lifetime spectrometer. However, due to the use of γ-γ coincidence, conventional positron annihilation spectrometers, whether used for single-sample or dual-sample characterization, suffer from low count rates. At the end of the last century, researchers proposed and constructed a β-positron source-sample-based detector... + -γ coincident positron annihilation lifetime spectrometers offer the potential advantage of high count rates and require only a single sample. In recent years, novel detectors and data acquisition systems, along with simulations using Geant4 (a particle transport simulation toolkit), have been employed to study the effects of conventional β-compliance positron annihilation lifetime spectrometry. + By optimizing the structure of the -γ coincidence positron annihilation lifetime spectrometer, researchers have developed a high-performance β-... +-γ coincident positron annihilation lifetime spectrometers increase the count rate by approximately two orders of magnitude compared to conventional spectrometers. The high count rate and single-sample-requirement nature of positron annihilation lifetime spectrometers enable two-dimensional scanning in-situ measurement of positron annihilation lifetimes.

[0005] Conventional positron annihilation lifetime spectrometers based on radioactive sources characterize the average properties of a sample surface within a millimeter depth range, and typically require two samples to clamp the radioactive source during testing. Slow positron beam spectrometers generally only have the ability to characterize properties at different depth ranges within a micrometer depth range from the sample surface, and their setup and maintenance costs are very high. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A positron annihilation lifetime spectrometer system with three-dimensional position resolution includes: a positron annihilation lifetime measurement device and a high-precision XY plane moving device;

[0008] The positron annihilation lifetime measurement device is used to acquire the annihilation characteristics of positrons at different depths within a millimeter range from the material surface. It comprises a lifetime initiation module, a lifetime termination module, an information acquisition module, and a control and processing module. The lifetime initiation module and lifetime termination module are respectively connected to the information acquisition module, which is connected to the control and processing module. The lifetime initiation module includes... 68 Ge positron source, first scintillator, utilizing 68 Positrons of different energies emitted by the Ge positron source deposit different energies when passing through the first scintillator. By setting different energy window ranges of silicon photomultiplier tubes, different incident depths of positrons within a millimeter depth range from the material surface can be distinguished, thus achieving positron annihilation spectroscopy characterization of microstructure characteristics in different depth ranges. The silicon photomultiplier tubes included in the lifetime initiation module and the photomultiplier tubes included in the lifetime termination module are overlapped and set on the same side of the sample to achieve non-contact characterization of the sample.

[0009] A high-precision XY plane moving device is used to move the detector part of the positron annihilation lifetime measurement device to achieve XY plane scanning test.

[0010] The present invention has the following beneficial effects:

[0011] This invention utilizes 68The positron annihilation lifetime measurement device in this patent possesses depth-resolution capabilities by utilizing the characteristic that positrons of different energies emitted from a Ge positron source deposit different energies when passing through the plastic scintillator (first scintillator) in the system. This is achieved by setting different energy window ranges for silicon photomultiplier tubes, distinguishing different incident depths of positrons within a millimeter depth range from the material surface. Combined with a high-precision XY-plane movement device, the final positron annihilation lifetime spectrometer system exhibits three-dimensional position resolution. This invention expands the application fields of positron annihilation lifetime spectrometers, particularly in areas such as in-situ flaw detection of metal devices and in-situ monitoring of surface pathological conditions in biological organisms.

[0012] (1) This invention utilizes the correspondence between the initial signal energy spectrum slice and the positron incident depth in a positron annihilation lifetime spectrometer to characterize the properties of different depth ranges within a millimeter depth range from the sample surface. Furthermore, by combining the improved positron annihilation lifetime spectrometer with a high-precision XY-plane moving device, the spectrometer system possesses two-dimensional scanning testing capabilities. Combined with depth resolution, this invention can characterize the microstructural properties of different regions on the material surface within a millimeter depth range from the surface.

[0013] (2) By placing the start signal and end signal detectors of the positron annihilation lifetime spectrometer on the same side of the sample, the present invention can perform non-contact characterization of the sample with only one sample, which is beneficial for studying unconventional samples such as those that are not easy to disassemble, have sticky surfaces, or are liquid.

[0014] (3) The present invention uses low strength 68 Ge-based radioactive sources and positron annihilation lifetime spectrometers that do not require electromagnetic field control of positron velocity are used to obtain the annihilation characteristics of positrons in materials. While possessing depth resolution capabilities, their construction and maintenance costs are far lower than those of slow positron beam spectrometers, and the detection depth is extended from the micrometer level to the millimeter level. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the positron annihilation lifetime spectrometer system with three-dimensional position resolution capability of the present invention;

[0016] Figure 2 The energy spectrum of the simulated positron detector;

[0017] Figure 3 The diagram shows three different energy window selections for silicon photomultiplier tubes and the positron injection depth distribution in the material under different energy window selections. (a) is a schematic diagram of three different energy window selections for silicon photomultiplier tubes, and (b) is a simulated diagram of the positron injection depth distribution in the material under different energy window selections. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0019] This invention provides a technical solution for a positron annihilation lifetime spectrometer system (hereinafter referred to as the spectrometer system) with three-dimensional position resolution. This spectrometer system, through an improved positron annihilation lifetime spectrometer, can characterize the properties of different depth ranges within a millimeter depth range from the sample surface. The improvement lies in overlapping the positron detector and the annihilation gamma photon detector of the positron annihilation lifetime spectrometer on the same side of a single sample, and using the correspondence between the energy spectrum range of the positron detector and the initial kinetic energy of the positrons to resolve the incident depth of the positrons in the material. Furthermore, this spectrometer system combines the improved positron annihilation lifetime spectrometer with a high-precision XY-plane moving device, allowing the detector section to move on the sample surface in a preset direction with millimeter-level steps. The depth resolution combined with XY-plane movement enables the spectrometer system to possess three-dimensional position resolution capabilities, which is of great significance for conducting defect distribution analysis of semiconductor materials, fatigue damage assessment of metal devices, and in-situ monitoring of surface pathology in biological organisms.

[0020] The specific technical solution of the present invention is as follows: a positron annihilation lifetime spectrometer system with three-dimensional position resolution, such as... Figure 1 As shown, it mainly consists of two parts: a positron annihilation lifetime measurement device and a high-precision XY plane moving device. The corresponding composition and function of each part are as follows:

[0021] Positron Annihilation Lifetime Measurement Device: This device is used to acquire the annihilation characteristics of positrons at different depths within a millimeter range from the material surface. It consists of a lifetime initiation module, a lifetime termination module, an information acquisition module, and a control and processing module. The lifetime initiation module and lifetime termination module are respectively connected to the information acquisition module, which in turn is connected to the control and processing module.

[0022] Lifetime Initiation Module: This module is used to acquire the initiation signal of the positron annihilation lifetime, including the first scintillator, photoconductive film, 68 Ge positron source, silicon photomultiplier tube (SiPM) and silicon photomultiplier tube signal readout circuit board, signal amplifier, first low-voltage power supply, and second low-voltage power supply. 68A positron source is dropped onto the central region of the optical window of a silicon photomultiplier tube (SMT). Then, a photoconductive film and a first scintillator are sequentially attached to the SMT's optical window. The SMT is connected to a SMT signal readout circuit board, and a signal amplifier is connected between the SMT signal readout circuit board and the information acquisition module. First and second low-voltage power supplies power the SMT signal readout circuit board and the signal amplifier, respectively. 68 The positrons produced by Ge decay are... 68 When a positron source enters the first scintillator, the scintillator generates scintillating photons. When the silicon photomultiplier tube detects these scintillating photons, its signal readout circuit board outputs an electrical pulse signal. This electrical pulse signal is amplified by a signal amplifier and then transmitted to the information acquisition module. 68 Positrons of different energies emitted by the Ge positron source deposit different energies when passing through the first scintillator. By setting different energy window ranges of silicon photomultiplier tubes, different incident depths of positrons within a millimeter depth range from the material surface can be distinguished, thus achieving positron annihilation spectroscopic characterization of microstructure properties in different depth ranges.

[0023] The control and processing module uses a constant-ratio timing method to obtain the start time of the positron annihilation lifetime from the electrical pulse signal. The simulated energy spectrum of the silicon photomultiplier tube (approximately determined by the distribution of detected scintillation photons) is as follows: Figure 2 As shown, when the energy window of the detector is set to the area between the two dashed lines, most of the detected coincidence events are annihilated events from the sample, thus effectively reducing the source component of the test results. A small number of positrons entering the first scintillator are backscattered into the photoconductive film, some are directly annihilated in the first scintillator, and most pass through the plastic scintillator to reach the sample. Since the energy deposited by positrons with different initial kinetic energies after entering the first scintillator varies, different ranges of the silicon photomultiplier tube's energy spectrum correspond to energy deposition events of positrons with different initial kinetic energies. Because positrons with different initial kinetic energies penetrate the sample to different depths, theoretically, by setting different energy windows of the silicon photomultiplier tube, the annihilation characteristics of positrons at different depths from the sample surface can be obtained. Figure 3 This diagram illustrates three different energy window selections for silicon photomultiplier tubes and the positron injection depth distribution within the material under different energy window selections. Figure 3 (a) is a schematic diagram of three different energy window selections for silicon photomultiplier tubes. Figure 3 (b) is a simulation of the positron injection depth distribution in the material under different energy window selections. Figure 3The paper shows the distribution of the distances between the annihilation sites of positrons in a sample (aluminum) and the sample surface after setting three different silicon photomultiplier tube energy windows, as obtained through simulation. It can be seen that by using the method provided by the present invention, the improved positron annihilation lifetime spectrometer has depth position resolution capability.

[0024] Lifetime Termination Module: This module is used to acquire the termination signal of positron annihilation lifetime and includes a photomultiplier tube (PMT), a second scintillator, PTFE raw rubber tape, and a high-voltage power supply. The second scintillator is coupled to the optical window of the PMT via silicone oil. The PTFE raw rubber tape covers the outside of the second scintillator, serving to reflect and shield light. The high-voltage power supply powers the PMT. When γ photons generated by positron annihilation deposit energy in the second scintillator, the scintillator produces scintillating photons. When the PMT detects the scintillating photons, its anode generates an electrical pulse signal, which is directly transmitted to the information acquisition module. The control and processing module uses a constant-ratio timing method to obtain the termination time of the positron annihilation lifetime from this electrical pulse signal. The silicon PMT in the lifetime initiation module and the PMT in the lifetime termination module are overlapped and arranged on the same side of the sample, making sample characterization extremely simple and enabling non-contact characterization, while also providing great convenience for XY plane scanning tests.

[0025] Information Acquisition Module: This module acquires signals from the lifespan start-up and lifespan end-up modules, and includes a digital oscilloscope. The digital oscilloscope features high vertical resolution and a high sampling rate to improve the accuracy of test results. It has four channels: A, B, C, and D. Channel A acquires signals from the lifespan end-up module, and channel C acquires signals from the lifespan start-up module. When the signal from channel A meets the trigger condition, the digital oscilloscope transmits the signals from channels A and C within a certain time window to the control and processing module.

[0026] Control and Processing Module: This module controls the information acquisition module to acquire signals according to preset working modes and conditions, and performs signal analysis to obtain the positron annihilation lifetime spectrum at different regions. This is implemented by a computer program. The program (e.g., C++) controls the A channel of the digital oscilloscope to operate in window-triggered mode, with the corresponding energy window range set to the region where the photoelectric peak of the annihilated gamma photon is located. The acquired signals are transmitted to the computer in real time. For the acquired signals from channels A and C, it is first determined whether the signal from channel C is within the set energy window range of the silicon photomultiplier tube detector. If it is not within the set energy window range, it is discarded; otherwise, a constant ratio timing method is used to extract time information from the signals from channels A and C. Frequency statistics are performed on the time differences of a large number of coincident events in channels A and C to obtain the positron annihilation lifetime spectrum. While extracting time information, the amplitude or area information of the C channel signal in each event is recorded. Combined with the energy spectrum of the silicon photomultiplier tube detector, the events are segmented to obtain the lifetime spectrum of positron annihilation at different depths from the sample surface.

[0027] High-precision XY plane moving device: This device is used to move the detector part of the positron annihilation lifetime measurement device. Figure 1 The detector section (located at the lower end of the third slide rail) is used for scanning tests in the XY plane (moving the detector section to measure positron annihilation lifetimes at different locations). It includes a first servo motor, a second servo motor, a third servo motor, a first slide rail, a second slide rail, a third slide rail, and a stainless steel frame. During testing, the sample is placed at the lower end of the detector section. The control and processing module controls the operation of the three servo motors: the first servo motor controls the detector section to move along the first slide rail, achieving millimeter-level movement in the X direction; the second servo motor controls the detector section to move along the second slide rail, achieving millimeter-level movement in the Y direction; and the third servo motor controls the detector section to move along the third slide rail, bringing the detector as close as possible to but without contacting the sample surface. The stainless steel frame is used to fix the three slide rails and the three servo motors.

[0028] When setting up the spectrometer system involved in this invention, it is recommended to use Onsemi's MicroFJ-60035 silicon photomultiplier tube. Its signal readout circuit board can be made according to the circuit schematic in the product manual. 68A positron source of Ge is directly dropped onto the central region of the optical window of the silicon photomultiplier tube (SMT), with a source intensity of approximately 30 microclusters, and the source spot diameter is controlled to be as small as possible. A photoconductive film with a thickness of approximately 200 micrometers (which can be fabricated using optical adhesive) and a scintillator 1 with a thickness of approximately 400 micrometers are sequentially attached to the optical window of the SMT. The recommended signal amplifier is the Mini-Circuits ZFL-1000LN+ (product model) low-noise amplifier. A frustum-shaped scintillator 2 (preferably inorganic barium fluoride scintillator) is coupled to the optical window of the SMT via silicone oil. The recommended digital oscilloscope is the PicoScope 6424E (product model) from Pico Technology, set to a sampling rate of 2.5 GHz and a vertical accuracy of 10 bits. A servo motor, which can be precisely controlled by a computer program, is used to control the movement of the detector section. After the spectrometer system is set up, the detector section is first adjusted to the test starting point, and the positron annihilation lifetime spectrum at that point is measured. Different initial signal energy spectrum slices are selected to divide the spectrum into positron annihilation lifetime spectra at different depth intervals. Then, the detector section is continuously moved to a preset position, and the corresponding positron annihilation lifetime spectra are measured. From the above test results, combined with the detector section's position in the XY plane and the different incident depth intervals given by the different energy spectrum ranges of the silicon photomultiplier tube, the three-dimensional positron annihilation characteristic distribution of the sample under test can be obtained (the spectrometer system proposed in this invention, built using other models of equipment with the same functions as the equipment mentioned above, is included within the scope of protection of this invention).

[0029] The above description is merely an embodiment of the present invention and does not limit the scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification and drawings, or direct or indirect applications in other related system fields, are similarly included within the protection scope of the present invention.

[0030] The contents not described in detail in this specification are existing technologies known to those skilled in the art.

Claims

1. A positron annihilation lifetime spectrometer system with three-dimensional position resolution, characterized in that, include: Positron annihilation lifetime measurement device and high-precision XY plane moving device; The positron annihilation lifetime measurement device is used to acquire the annihilation characteristics of positrons at different depths within a millimeter range from the material surface. It comprises a lifetime initiation module, a lifetime termination module, an information acquisition module, and a control and processing module. The lifetime initiation module and lifetime termination module are respectively connected to the information acquisition module, which is connected to the control and processing module. The lifetime initiation module includes... 68 Ge positron source, first scintillator, utilizing 68 Positrons of different energies emitted by the Ge positron source deposit different energies when passing through the first scintillator. By setting different energy window ranges of silicon photomultiplier tubes, different incident depths of positrons within a millimeter depth range from the material surface can be distinguished, thus achieving positron annihilation spectroscopy characterization of microstructure characteristics in different depth ranges. The silicon photomultiplier tubes included in the lifetime initiation module and the photomultiplier tubes included in the lifetime termination module are overlapped and placed on the same side of the sample to achieve non-contact characterization of the sample. A high-precision XY plane moving device is used to move the detector part of the positron annihilation lifetime measurement device to achieve XY plane scanning test.

2. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 1, characterized in that, The high-precision XY plane moving device includes a first servo motor, a second servo motor, a third servo motor, a first slide rail, a second slide rail, a third slide rail, and a stainless steel frame. The control and processing module controls the operation of the three servo motors. The first servo motor controls the detector part to move along the first slide rail; the second servo motor controls the detector part to move along the second slide rail; and the third servo motor controls the detector part to move along the third slide rail. The stainless steel frame is used to fix the first slide rail, the second slide rail, the third slide rail, the first servo motor, the second servo motor, and the third servo motor.

3. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 2, characterized in that, The lifetime initiation module is used to acquire the initiation signal of the positron annihilation lifetime, and includes a first scintillator, a photoconductive thin film, and 68 Ge positron source, silicon photomultiplier tube and silicon photomultiplier tube signal readout circuit board, signal amplifier, first low-voltage power supply, second low-voltage power supply; 68 A positron source is dropped onto the central region of the optical window of a silicon photomultiplier tube (SMT). Then, a light guide film and a first scintillator are sequentially attached to the optical window of the SMT. The SMT is connected to the SMT signal readout circuit board, and a signal amplifier is connected between the SMT signal readout circuit board and the information acquisition module. The first and second low-voltage power supplies are used to power the SMT signal readout circuit board and the signal amplifier, respectively.

4. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 3, characterized in that, when 68 The positrons produced by Ge decay are... 68 When the positron source enters the first scintillator, the first scintillator will generate scintillating photons; when the silicon photomultiplier tube detects the scintillating photons, the silicon photomultiplier tube signal readout circuit board will output an electrical pulse signal, which is amplified by a signal amplifier and then transmitted to the information acquisition module.

5. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 4, characterized in that, The control and processing module uses a constant ratio timing method to obtain the start time of the positron annihilation lifetime from the electrical pulse signal.

6. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 5, characterized in that, A small number of positrons entering the first scintillator are backscattered into the photoconductive film, some are directly annihilated in the first scintillator, and most will pass through the plastic scintillator to reach the sample; different ranges of the energy spectrum of the silicon photomultiplier tube correspond to energy deposition events of positrons with different initial kinetic energies.

7. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 6, characterized in that, The lifetime termination module is used to obtain the termination signal of the positron annihilation lifetime, and includes a photomultiplier tube, a second scintillator, PTFE raw rubber tape, and a high-voltage power supply. The second scintillator is coupled to the optical window of the photomultiplier tube through silicone oil. The PTFE raw rubber tape covers the outside of the second scintillator for reflection and light shielding. The high-voltage power supply is used to power the photomultiplier tube.

8. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 7, characterized in that, When the γ photons generated by positron annihilation deposit energy in the second scintillator, the second scintillator will generate scintillating photons; when the photomultiplier tube detects the scintillating photons, the anode of the photomultiplier tube generates an electrical pulse signal, which is directly transmitted to the information acquisition module; the control and processing module obtains the termination time of the positron annihilation lifetime from the electrical pulse signal.

9. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 8, characterized in that, The information acquisition module is used to acquire the signals output by the lifespan start module and the lifespan end module, including a digital oscilloscope. The digital oscilloscope has four channels: A, B, C, and D. Channel A is used to acquire the signal from the lifespan end module, and channel C is used to acquire the signal from the lifespan start module. When the signal from channel A meets the trigger condition, the digital oscilloscope transmits the signals from channels A and C within a certain time window to the control and processing module.

10. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 9, characterized in that, The control and processing module is used to control the information acquisition module to acquire signals according to preset working modes and conditions, and to perform signal analysis to obtain the positron annihilation lifetime spectrum in different regions. This is implemented by a computer program.

11. The positron annihilation lifetime spectrometer system with three-dimensional position resolution according to claim 10, characterized in that, The control and processing module uses a program to control the A channel of the digital oscilloscope to operate in window-triggered mode, with the corresponding threshold range set to the region where the photoelectric peak of annihilated gamma photons is located, and transmits the acquired signals to the computer in real time. For the acquired signals from the A and C channels, it first determines whether the signal from the C channel is within the set energy window range of the silicon photomultiplier tube detector. If it is not within the set energy window range of the silicon photomultiplier tube detector, it is discarded; otherwise, a constant ratio timing method is used to extract time information from the signals from the A and C channels. Frequency statistics are performed on the time differences of the A and C channel signals of a large number of coincident events to obtain the positron annihilation lifetime spectrum. While extracting time information, the amplitude or area information of the C channel signal in each event is recorded. Combined with the energy spectrum of the silicon photomultiplier tube detector, the events are segmented to obtain the lifetime spectrum of positron annihilation at different depths from the sample surface.

Images