Positron annihilation lifetime-momentum correlation spectrometer for measuring film sample

Abstract

The invention provides a positron annihilation lifetime-momentum correlation spectrometer for measuring a film sample, which comprises a detector module, an NIM electronics module, a data acquisition module and a data processing module, and is characterized in that the detector module comprises a gamma detector, a high-purity germanium detector and an anticoincidence module; the anticoincidence module, the high-purity germanium detector and the gamma detector are respectively connected with the NIM electronics module, and the NIM electronics module is sequentially connected with the data acquisition module and the data processing module. The positron annihilation life-momentum correlation measurement of the micron-sized film sample is realized, and the application of the technology in the field of material science is expanded.

Description

Technical Field

[0001] The invention relates to the technical field of positron annihilation nuclear spectrometers, and in particular to a positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples. Background Art

[0002] Positron annihilation spectroscopy has long been recognized as an important technique for characterizing the microstructure of materials, especially for atomic-scale defects and ultra-microporous structures in solids. It uses positrons and positronium as probes to study electronic structure, atomic defects, surface properties, chemical environment, etc. Positron annihilation lifetime and positron annihilation radiation Doppler broadening techniques measure the positron annihilation lifetime and electron momentum in materials, respectively, and are widely used to characterize the microstructure of materials. In order to distinguish different positron annihilation states, based on these two techniques, positron annihilation lifetime-momentum correlation measurement technology was developed in 1976 to simultaneously measure the positron annihilation lifetime and electron momentum distribution. Since then, positron researchers have successfully applied positron annihilation lifetime-momentum correlation measurement to various studies, including the chemical environment of open and closed pores in porous materials, crystal structure, and the formation and annihilation mechanism of positronium. In addition, the sensitivity of positronium decay to metabolism makes it a promising biomarker that can provide information about disease progression. Therefore, this technology has great application potential in biology and medicine.

[0003] Currently, the positron annihilation lifetime-momentum correlation technique has and Two coincidence modes. In the positron annihilation lifetime-momentum correlation measurement system based on slow positron beam, a In coincidence mode, positrons are directly detected as the starting signal. Since the starting detector has almost 100% detection efficiency for positrons, the coincidence count rate of the beam-based positron annihilation lifetime-momentum correlation measurement system can reach hundreds of counts per second. However, due to the high cost of building a slow positron beam based on an accelerator, its technical implementation is difficult. In contrast, traditional positron annihilation lifetime-momentum correlation measurement systems that are not based on slow positron beams are easier to build. This system uses a 1275 keV positron beam as the starting signal. Photons are the starting signal for positron emission. After entering the sample, positrons usually annihilate with electrons to form two oppositely directed electrons with an energy of about 511 keV. Photon. A photon with an energy of about 511 keV The photon is collected by the termination gamma detector as the end signal of the positron lifetime, and another 511 keV Photons are detected by high-purity germanium detectors to measure the Doppler broadening of electron-positron pairs. The system consists of three parts: positron lifetime measurement (including two gamma detectors (one for detecting the start signal and the other for detecting the end signal), a time-amplitude converter and two differential constant ratio timing discriminators), annihilation radiation Doppler broadening measurement system (including a high-purity germanium detector and a shaped amplifier), and a multi-channel data acquisition system (including a multi-channel analyzer and a computer). In terms of hardware structure, unlike traditional measurement systems, we use two start detectors to receive 1275 keV Photons are used to generate the starting signal (more detectors or detector arrays can also be used), and a digital oscilloscope is used instead of a traditional multi-channel analyzer. This not only improves the detection efficiency of the starting signal and the reconfigurability of the system, but also doubles the coincidence count rate of the system.

[0004] In the conventional non-beam positron annihilation lifetime-momentum correlation measurement, due to 22 The positrons emitted by the Na radiation source have a kinetic energy distribution in the range of 0-545 keV. The samples to be tested are often required to be in blocks with a thickness of more than 1 mm and are wrapped in a sandwich form of sample-positron source-sample for testing. This ensures that more than 99% of the positrons are annihilated in the sample (about 10% in the carrier). 22 Therefore, the traditional positron annihilation lifetime-momentum correlation spectrometer cannot measure the new inorganic functional films, polymer films, metal films, semiconductor films and other thin film samples with thickness at the micron level.

[0005] Since plastic scintillators are sensitive to 511 and 1275 keV The photoelectric cross-section of photons is small and the light yield is very low. Therefore, a plastic scintillator with a thickness of 1 mm can be used to receive the signal generated by the kinetic energy of the positron as an anti-coincidence logic signal, which can screen out positron annihilation events that do not occur in the thin film sample. Summary of the invention

[0006] The present invention provides a positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples, which is used for micron-level thin film measurement, and realizes the overall upgrade of the traditional positron annihilation lifetime-momentum correlation spectrometer, so that the measurement of the test sample is upgraded from the previous millimeter-level measurement to the micron-level measurement, and the coincidence count rate is doubled. The present invention realizes the positron annihilation lifetime-momentum correlation measurement of a single micron-level thin film sample, and develops a high-performance reconfigurable multifunctional semi-digital positron annihilation spectrometer by integrating a multifunctional module of a positron annihilation lifetime spectrum, a Doppler broadening spectrum, and a lifetime-momentum correlation spectrum.

[0007] The present invention provides a positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples, comprising a detector module, a NIM electronics module, a data acquisition module and a data processing module, wherein the detector module comprises a gamma detector, a high-purity germanium detector and an anti-coincidence module, wherein the anti-coincidence module, the high-purity germanium detector and the gamma detector are respectively connected to the NIM electronics module, and the NIM electronics module is sequentially connected to the data acquisition module and the data processing module;

[0008] Gamma detectors are used to receive 22 The 1275 keV Na source decays The 511 keV gamma photons produced by photon and positron annihilation serve as the start and end signals of the positron lifetime, respectively;

[0009] High purity germanium detector is used to receive 511 keV generated by positron annihilation. photons, measuring their Doppler broadening;

[0010] The anti-coincidence module is used to detect the signal generated by the kinetic energy of positrons;

[0011] The NIM electronics module is used to pre-process the detector signal;

[0012] The data acquisition module is used to collect the pulse signal processed by the NIM electronics module;

[0013] The data processing module is used to acquire and process the collected pulse data. The pulse data comes from the anti-coincidence signal of the anti-coincidence module, the energy spectrum measurement signal of the high-purity germanium detector, the start signal of the positron annihilation lifetime spectrum of the start gamma detector, and the end signal of the positron annihilation lifetime spectrum of the end gamma detector (if a constant ratio timing discriminator is used to pre-process the signal, the start signal and the end signal are first coincident into a time-amplitude square wave signal, and then collected by the acquisition card or oscilloscope and enter the data processing module).

[0014] The present invention has the following beneficial effects:

[0015] 1. Through the positron signal of the anti-coincidence module, the positron annihilation lifetime signal and the energy signal are triple coincident, realizing the positron annihilation lifetime-momentum correlation measurement of micron-scale thin film samples, expanding the application of this technology in the field of materials science.

[0016] 2. Through the combination of the detector system and the NIM electronics module, as well as the self-programming technology of the digital acquisition and processing module based on the acquisition device software development kit, the rapid acquisition and processing of pulse signals is realized. The positron annihilation lifetime spectrum, positron annihilation Doppler broadening, and lifetime-momentum correlation measurement technology are integrated, and the detector geometry and acquisition performance are further optimized. The system has excellent reconfigurability. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 , one Schematic diagram of the structure of a positron annihilation lifetime-momentum correlation spectrometer used to measure thin film samples;

[0018] Figure 2 , a positron annihilation lifetime-momentum correlation measurement device for micron-scale thin film measurements;

[0019] Figure 3 , schematic diagram of the anti-coincidence module structure;

[0020] Figure 4 , the lifetime-momentum correlation two-dimensional spectrum after anti-coincidence screening. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in each embodiment of the present invention described below can be combined with each other as long as they do not conflict with each other. To achieve the above-mentioned purpose, the present invention adopts the following technical scheme.

[0022] The technical solution of the present invention is: a positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples, such as Figure 1 , Figure 2 As shown, it includes a detector module, a data acquisition module and a data processing module. The detector module includes a gamma detector, a high-purity germanium detector and an anti-coincidence module. The anti-coincidence module, the high-purity germanium detector and the gamma detector are respectively connected to the NIM electronics module, and the NIM electronics module is connected to the acquisition module and the data processing module in sequence.

[0023] like Figure 3 As shown, the anti-coincidence module includes two thin scintillators with low density and small atomic number, a photomultiplier tube, a thin film sample to be tested and a positron radiation source; the two thin scintillators sandwich the thin film sample to be tested and the positron radiation source. Among them, there are generally two setting modes for the packaging of the two thin scintillators. One is that the two thin scintillators can be coupled with photomultiplier tubes respectively, and the other is that one thin scintillator is coupled with a photomultiplier device, and the other thin scintillator is coated with a layer of reflective material. The function of the reflective material is to reflect the photons generated by the positrons in the thin scintillator to the photomultiplier tube of the other coupled thin scintillator.

[0024] like Figure 1 As shown in the figure, the gamma detector is composed of a scintillator coupled photomultiplier device; the function of the high-purity germanium detector is to receive the 511 keV generated by positron annihilation. The photons are measured by Doppler broadening, which contains information about the elemental composition, defect type, and concentration inside the material. The termination gamma detector opposite to the high-purity germanium detector receives another 511 keV photon. Photon, as the end signal of positron life. The initial gamma detector receives 22 The 1275keV Na radioactive source decays Photons are used as the starting signal of the positron lifetime (the number of detectors and analog input channels can be increased according to measurement requirements). The anti-coincidence module detects the signal generated by the kinetic energy of the positron to exclude the case where the positron does not decay in the thin film sample.

[0025] The data acquisition module includes an oscilloscope or acquisition card with at least four analog input channels for acquiring pulse signals. The analog bandwidth is generally not less than 200 MHz, the data sampling rate is generally not less than 40 MS / s, the vertical resolution is generally not less than 14 bits, and the capture memory is generally not less than 512 MS.

[0026] The data processing module includes: writing a data acquisition program on a computer to connect the data acquisition device, acquiring and processing the acquired pulse data, the data respectively come from the anti-coincidence signal of the anti-coincidence module, the energy spectrum measurement signal of the high-purity germanium detector, the start signal of the positron annihilation lifetime spectrum of the starting gamma detector, and the end signal of the positron annihilation lifetime spectrum of the ending gamma detector. First, select a suitable trigger source as the trigger signal. The selection standard is generally to use the signal with the lowest detection efficiency as the trigger source and trigger within a suitable time window. Then, set a suitable anti-coincidence threshold. If the highest amplitude of the pulse of the anti-coincidence module within the time window exceeds the threshold, it means that the triggering case is caused by the annihilation of the positron in the thin scintillator, and the case needs to be discarded. On the contrary, it means that this triggering case is caused by the annihilation of the positron in the thin film sample, then continue to remove the baseline, signal de-pile, and smooth fitting processing of the pulse signal of the high-purity germanium to obtain the maximum value point of the pulse data. And smooth and time the pulse of the starting gamma detector and the pulse of the ending gamma detector to obtain the size of the lifetime value. Finally, the Doppler broadening and positron lifetime of the positron-electron annihilation pair of this triggering event are obtained, which is a coincidence event of lifetime-momentum correlation. In order to meet the statistical accuracy, it is generally necessary to collect at least 10 million total counts.

[0027] like Figure 2 As shown in the figure, the high-purity germanium detector is from ORTEC, model GEM35-76-PL; the three gamma detectors use Hamamatsu photomultiplier tubes, model H3378, and the coupling crystal is a barium fluoride scintillator. #1 and #3 are used as the starting gamma detectors, and #2 is used as the ending gamma detector; the two setting modes of the anti-coincidence module are as follows Figure 3As shown, a Hamamatsu photomultiplier tube, model H6610, is used, and the coupling crystal is a 1 mm thick thin plastic scintillator.

[0028] The NIM electronics module includes electronic plug-ins such as shaping amplifiers, constant ratio timing devices, delay devices, time amplitude converters, and linear delay shaping amplifiers. The shaping amplifier of the energy spectrum part uses ORTEC's 672 spectrum amplifier, which shapes the output signal of the high-purity germanium detector into a narrow quasi-Gaussian pulse, which can obtain a better signal-to-noise ratio. The top maintains a certain width so that the shape of the signal meets the requirements of subsequent measurement equipment and is conducive to working at a high count rate. The output signal of the anti-coincidence module is amplified by a 460-type linear delay shaping amplifier. The constant ratio timing device of the positron annihilation lifetime spectrum part uses ORTEC's 583B constant fraction differential discriminator, which performs constant ratio timing on the output pulse of the photomultiplier device and generates a clock for the arrival of the signal; the delay device is ORTEC's 425A nanosecond delay device, which ensures that the termination signal of the positron lifetime arrives at the time amplitude converter later than the start signal. The size of the delay determines the zero point of the final positron annihilation lifetime spectrum. The time-to-amplitude converter is ORTEC's 567 time-to-amplitude converter, which converts the time interval between the start signal and the end signal into a pulse signal whose height is proportional to the time interval, that is, the lifetime of the positron is proportional to the height of the pulse signal.

[0029] The data acquisition module uses an oscilloscope, which is a PicoScope 5444D fast digital oscilloscope with four analog input channels, an analog bandwidth of 200 MHz, a data sampling rate of 40 MS / s, a vertical resolution of 14 bits, and a capture memory of 512 MS. The four analog input channels are A, B, C, and D channels, and the acquisition program can be freely written using the software development kit of the oscilloscope.

[0030] Write a data acquisition program on the computer to connect the data acquisition equipment. The data processing module acquires and processes the acquired pulse data. Channel A acquires the Doppler broadening of the positron-electron pair measured by the high-purity germanium detector, channels B and D acquire the positron lifetime, and channel C acquires the positron kinetic energy signal of the anti-coincidence module. First, channels B and D are used as OR logic trigger signals. Channels B and D stipulate that there is only one channel with a positron lifetime signal. The trigger time window is set to 43.4 microseconds, medium trigger mode, front sampling points 1024, and total sampling points 1800. The anti-coincidence threshold is set to 40 mV. If the highest amplitude of the pulse of the anti-coincidence module in the time window exceeds the threshold, it means that this triggering event is caused by the annihilation of positrons in the thin scintillator, and the event needs to be discarded. Otherwise, it means that this triggering event is caused by the annihilation of positrons in the thin film sample. Then, the pulse signal of high-purity germanium is further subjected to baseline removal, signal de-pile-up, and smooth fitting processing to obtain the maximum amplitude value of the pulse data. Finally, the positron lifetime and the Doppler broadening of the positron-electron annihilation pair of this triggering event are obtained, which is a coincidence event of positron annihilation lifetime-momentum correlation. By collecting a large number of coincidence events and performing two-dimensional histogram frequency statistics on them, we can obtain the two-dimensional spectrum of positron annihilation lifetime-momentum correlation, such as Figure 4 shown.

Claims

1. A positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples, It is characterized in that It includes a detector module, a NIM electronics module, a data acquisition module and a data processing module. The detector module includes a gamma detector, a high-purity germanium detector and an anti-coincidence module. The anti-coincidence module, the high-purity germanium detector and the gamma detector are respectively connected to the NIM electronics module. The NIM electronics module is connected to the data acquisition module and the data processing module in sequence. Gamma detectors are used to receive 22 The 1275 keV Na source decays The 511 keV gamma photons produced by photon and positron annihilation serve as the start and end signals of the positron lifetime, respectively; High purity germanium detector is used to receive 511 keV generated by positron annihilation. photons, measuring their Doppler broadening; The anti-coincidence module is used to detect the signal generated by the kinetic energy of positrons; The NIM electronics module is used to pre-process the detector signal; The data acquisition module is used to collect the pulse signal processed by the NIM electronics module; The data processing module is used to acquire and process the collected pulse data. The pulse data comes from the anti-coincidence signal of the anti-coincidence module, the energy spectrum measurement signal of the high-purity germanium detector, the start signal of the positron annihilation lifetime spectrum of the start gamma detector, and the end signal of the positron annihilation lifetime spectrum of the end gamma detector (if a constant ratio timing discriminator is used to pre-process the signal, the start signal and the end signal are first coincident into a square wave signal with a time amplitude, and then collected by the acquisition card or oscilloscope and enter the data processing module).

2. A positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples according to claim 1, It is characterized in that The anti-coincidence module includes two thin scintillators with low density and small atomic number, a photomultiplier tube, a thin film sample to be tested and a positron radiation source; the two thin scintillators sandwich the thin film sample to be tested and the positron radiation source in the middle; wherein, there are two setting modes for the packaging of the two thin scintillators, one is to couple the two thin scintillators respectively with photomultiplier tubes, and the other is to couple one thin scintillator with a photomultiplier device and coat the other thin scintillator with a layer of reflective material; the function of the reflective material is to reflect the photons generated by the positrons in the thin scintillator to the photomultiplier tube of the other coupled thin scintillator.

3. A positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples according to claim 1, It is characterized in that The gamma detector is composed of a scintillator coupled to a photomultiplier device; the termination gamma detector opposite to the high-purity germanium detector receives another 511 keV Photon, as the end signal of positron lifetime; the starting gamma detector receives 22 The 1275 keV Na radioactive source decays The photon serves as the starting signal for the positron's lifetime.

4. A positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples according to claim 1, It is characterized in that The data acquisition module includes an oscilloscope or acquisition card with at least four analog input channels, which is used to acquire pulse signals. The analog bandwidth is generally not less than 200 MHz, the data sampling rate is generally not less than 40 MS / s, the vertical resolution is generally not less than 14 bits, and the capture memory is generally not less than 512 MS.

5. The positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples according to claim 1, It is characterized in that The coincidence events associated with lifetime and momentum include: first, selecting a suitable trigger source as the trigger signal, generally selecting the signal with the lowest detection efficiency as the trigger source, and triggering within a suitable time window, then setting a suitable anti-coincidence threshold, if the highest amplitude of the pulse of the anti-coincidence module within the time window exceeds the threshold, it means that the trigger event is caused by the annihilation of positrons in the thin scintillator, and the event needs to be discarded, otherwise, it means that the trigger event is caused by the annihilation of positrons in the thin film sample, then continue to perform baseline removal, signal de-pile, and smooth fitting processing on the pulse signal of the high-purity germanium detector to obtain the maximum value point of the pulse data, and smooth and time the pulse of the starting gamma detector and the pulse of the ending gamma detector to obtain the size of the lifetime value, and finally obtain the Doppler broadening of the positron-electron annihilation pair and the positron lifetime of this trigger event, which is a coincidence event associated with lifetime and momentum.

6. A positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples according to claim 1, It is characterized in that The NIM electronics module includes a shaping amplifier, a constant ratio timing device, a delay device, a time amplitude converter, and a linear delay shaping amplifier; the shaping amplifier of the energy spectrum part is used to shape the output signal of the high-purity germanium detector into a narrower quasi-Gaussian pulse, the output signal of the anti-coincidence module is amplified by a linear delay shaping amplifier, and the constant ratio timing device of the positron annihilation lifetime spectrum part is used to perform constant ratio timing on the output pulse of the photomultiplier tube and generate a clock for the arrival of the signal; the delay device is a nanosecond delay device, which is used to make the end signal of the positron lifetime arrive at the time amplitude converter later than the start signal, and the time amplitude converter converts the time interval between the start signal and the end signal into a pulse signal whose height is proportional to it, that is, the lifetime of the positron is proportional to the height of the pulse signal.

7. A positron annihilation lifetime-momentum correlation spectrometer for measuring thin film samples according to claim 1, It is characterized in that A data acquisition program is written on the computer to connect the data acquisition device. The data processing module obtains and processes the collected pulse data. One analog input channel collects the Doppler broadening of the positron-electron pair measured by the high-purity germanium detector, the other two analog input channels collect the positron lifetime, and another analog input channel collects the positron kinetic energy signal of the anti-coincidence module.

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