Digital positron annihilation lifetime spectrometer capable of automatically optimizing energy window

Abstract

The invention provides a digital positron annihilation life spectrometer capable of automatically optimizing an energy window. The digital positron annihilation life spectrometer comprises an energy window optimizing device and a digital positron annihilation life measuring device, the energy window optimization device is used for automatically acquiring an optimal energy window threshold value, so that the digital positron annihilation lifetime measurement device performs measurement under the energy window threshold value; the digital positron annihilation life measuring device is used for measuring the positron annihilation life under the optimal energy window threshold value; the optimal energy window threshold value is automatically searched through the energy window optimization device under set parameters and algorithms, and the digital positron annihilation lifetime measurement device carries out measurement under the threshold value. According to the invention, the optimal energy window configuration is automatically searched according to specific parameters and algorithms instead of manual selection and debugging through manual experience in the past, the operation is tedious and personal errors exist, so that the positron annihilation life measurement implementation is simpler, more convenient and more accurate.

Description

Technical Field

[0001] The invention relates to the field of positron annihilation lifetime, and in particular to a digital positron annihilation lifetime spectrometer capable of automatically optimizing an energy window. Background Art

[0002] Positron annihilation lifetime spectroscopy is a technique that measures the 22 The time interval between the start signal (1275 keV companion gamma ray) and the end signal (one of the two 511 keV gamma rays produced by positron annihilation) of the decay of Na. Positron annihilation lifetime spectroscopy is a powerful tool used to study and characterize a wide variety of materials, with applications in many areas of physics, biology and chemistry. Today, many positron annihilation lifetime spectrometers are in widespread use.

[0003] In order to improve the quality of the positron annihilation lifetime spectrum data, a positron annihilation lifetime spectrum with high counts and good time resolution is required. The count rate and time resolution of the positron annihilation lifetime spectrometer used are dependent on the energy window settings of the start and end detectors. Therefore, experimentally determining the optimal value of the energy window width is a key point in this type of measurement. The selection of these values ​​usually depends on the experimenter's experience and may vary from person to person.

[0004] A conventional positron annihilation lifetime spectrometer consists of a scintillator, a photomultiplier tube, a high-voltage power supply, a Nuclear Instrumentation Module (NIM) plug-in (constant ratio timing discriminator, delay, time amplitude converter, multi-channel analyzer), and a computer, such as Figure 1 As shown. Two gamma detectors are used as the start detector and the end detector to measure the start signal and the end signal respectively. The output voltage signal is converted into a square wave signal with fixed width and amplitude after energy discrimination and constant ratio timing by the constant ratio timing discriminator. The time difference between the two square wave signals is converted into a square wave signal with fixed width and linear correlation between amplitude and time by the time amplitude converter. After analog-to-digital conversion by the multi-channel analyzer and collection and transmission to the computer for data processing, the final positron annihilation lifetime spectrum can be obtained. The traditional positron annihilation lifetime spectrometer cannot directly obtain the gamma detector pulse data, and only performs amplitude discrimination on the voltage pulse. Therefore, the energy window threshold is inaccurate and the lifetime spectrum is easily distorted (the rising front will be obviously distorted), which affects the spectrum interpretation results. Summary of the invention

[0005] The object of the present invention is to experimentally determine the optimal value of the energy window setting of the positron annihilation lifetime spectrometer used by mathematical methods. The positron annihilation lifetime spectrometer used is equipped with a multi-parameter acquisition system. The determination of the optimal value is based on a product-type fuzzy decision method.

[0006] The technical solution of the present invention is as follows: a digital positron annihilation lifetime spectrometer for automatically optimizing energy windows, comprising: an energy window optimization device and a digital positron annihilation lifetime measurement device; the energy window optimization device is used to automatically obtain an optimal energy window threshold, and the digital positron annihilation lifetime measurement device is used to measure the annihilation lifetime of positrons under the optimal energy window threshold; the energy window optimization device automatically searches for the optimal energy window threshold under set parameters and algorithms.

[0007] The advantages of the present invention compared with the prior art are:

[0008] 1. The present invention automatically searches for the optimal energy window configuration based on specific parameters and algorithms, rather than manually selecting and debugging through human experience in the past, which is cumbersome and prone to human errors. This makes the positron annihilation lifetime measurement more concise, convenient and accurate.

[0009] 2. Digital waveform technology makes data processing more concise and accurate, simplifies the hardware structure and greatly reduces costs.

[0010] 3. The modular design of this device has high integration and good reconfigurability, which facilitates system optimization and upgrading.

[0011] 4. The device of the present invention makes the signal-to-noise ratio of the positron annihilation lifetime spectrum higher, reduces the spectrum distortion, and greatly improves the quality and stability of spectrum interpretation. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a traditional positron annihilation lifetime spectrometer;

[0013] Figure 2 Energy window optimization for product-type fuzzy decision making method;

[0014] Figure 3 A schematic diagram of a digital positron annihilation lifetime measurement device of the present invention;

[0015] Figure 4 A schematic diagram of an embodiment of the present invention;

[0016] Figure 5 is the distribution of the quality factor (FOM) relative to the start and end energy window thresholds;

[0017] Figure 6 is the measured positron annihilation lifetime spectrum. DETAILED DESCRIPTION

[0018] 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.

[0019] The technical solution of the present invention is: a digital positron annihilation lifetime spectrometer that automatically optimizes the energy window, using a product-type fuzzy decision-making method, including: an energy window optimization device and a digital positron annihilation lifetime measurement device; the energy window optimization device is used to automatically obtain an optimal energy window threshold, so that the digital positron annihilation lifetime measurement device performs measurement under the energy window threshold; the digital positron annihilation lifetime measurement device is used to measure the annihilation lifetime of the positron under the optimal energy window threshold; the energy window optimization device automatically searches for the optimal energy window threshold under set parameters and algorithms, and the digital positron annihilation lifetime measurement device performs measurement under the threshold, so that the front distortion of the positron annihilation lifetime spectrum is reduced, and the signal-to-noise ratio of the signal is higher, which greatly improves the quality and stability of the spectrum interpretation.

[0020] The energy window optimization device includes a first detector module, a first data acquisition module, and a processing terminal; the first detector module includes a start detector and a stop detector, both of which are gamma detectors, respectively receiving 1275 keV nuclear photon signals and 511 keV positron annihilation photon signals, and the start detector and the stop detector are photomultiplier devices with scintillators; according to chemical composition, scintillators can generally be divided into organic scintillators (such as plastics, anthracene, stilbene, liquid scintillators, etc.) and inorganic scintillators (such as alkali metal halides, barium fluoride, bismuth germanate, lutetium silicate, yttrium lutetium silicate, lanthanum bromide, etc.). In most organic crystals, fluorescence is the main luminescence process, so its light decay time is shorter; inorganic crystals are characterized by high density and high atomic number elements, so they have large ionization losses and high detection efficiency for charged particles or gamma rays, and good energy resolution. Users can choose the crystal type according to specific experimental needs. The function of a photomultiplier device is to convert optical signals into electrical signals and amplify their signals. Commonly used ones include photomultiplier tubes, silicon photomultipliers, microchannel plate photomultipliers, metal-oxide-semiconductor photomultipliers, etc.

[0021] The first data acquisition module generally uses a digital oscilloscope or a data acquisition card to sample analog signals and convert continuous voltage or current signals into discrete digital pulse signals. It is necessary to set a suitable sampling rate and vertical accuracy to facilitate subsequent data processing.

[0022] The processing terminal includes a first data processing module, a first data timing module, a first coincidence module, and an energy window optimization module; the energy window optimization module includes an energy window classification module and a target parameter module. The first data processing module performs distorted pulse discrimination on the pulse and extracts amplitude and energy information; the first data timing module extracts pulse time information; the first coincidence module performs coincidence on the start signal and the end signal within a certain time window; the energy window classification module classifies and combines the four thresholds of the start and end detector energy spectra, and a positron annihilation lifetime spectrum can be obtained under each energy window combination; the target parameter module calculates the parameters of the counting rate, time resolution and goodness of fit for the positron annihilation lifetime spectrum corresponding to each energy window, and selects appropriate target parameters as the quality factor to measure the quality of the positron annihilation lifetime spectrum, traverses and calculates all quality factors, obtains the optimal energy window, and uses it for the positron annihilation lifetime measurement device.

[0023] The digital positron annihilation lifetime measurement device includes a second detector module, a second data acquisition module, a second data processing module, a second data timing module, an optimal energy window module, a second coincidence module, and a data analysis module. The second detector module has the same structure as the first detector module in the energy window optimization device, including a start detector and a stop detector. 22 Na radioactive source and test sample. The second data acquisition module, the second data processing module, the second data timing module, and the second coincidence module have the same structure as the corresponding modules in the energy window optimization device. The optimal energy window module uses the optimal energy window threshold obtained in the energy window optimization device to perform data measurement, and the coincidence cases outside this energy window range will be screened out. In the final data analysis module, a large number of coincidence cases are subjected to time difference frequency statistics to obtain the positron annihilation lifetime spectrum.

[0024] like Figure 2 As shown, it is a schematic diagram of the energy window optimization using the product type fuzzy decision method adopted by the energy window optimization device above.

[0025] Firstly, the start detector and the end detector of the first detector module are both gamma detectors, which are composed of a photomultiplier tube and a scintillator. 22The decay of the Na radioactive source produces 1275 keV accompanying gamma rays, which are used as the starting signal for the generation of positrons. A positron is emitted about 3 ps later. After entering the material, the positron annihilates with the electron, generating two 511 keV gamma rays in opposite directions, one of which is used as the termination signal of the positron's lifetime. When the gamma ray enters the gamma detector, it transfers all its energy to the inner electrons through the photoelectric effect inside the scintillator. The initial kinetic energy of the generated photoelectrons is very large, and they can continue to ionize or excite the surrounding electrons. The excited electrons form fluorescent centers, and within picoseconds, the excited electrons de-excite to emit fluorescent photons. The fluorescent photons decay exponentially and reach the cathode window of the photomultiplier tube (generally composed of alkali metal materials with small work function), where the photoelectric effect occurs to generate photoelectrons. Accelerated by the electric field of the photomultiplier tube, the photoelectrons reach the anode, forming a 50Ω DC matching voltage signal.

[0026] An oscilloscope is used to collect signals and convert analog voltage signals into digital waveform signals.

[0027] The first data processing module and the first data timing module perform pulse shape identification on the digital signal to filter out distorted pulses, obtain information such as pulse amplitude and energy, and use a high-precision timing method to obtain the pulse time.

[0028] The first coincidence module matches the start signal pulse and the end signal pulse within a 100 nanosecond time window. If the two signals exist simultaneously within the coincidence time window, it is considered to be a positron annihilation event.

[0029] The classification parameters of the energy window classification module include the lower threshold and upper threshold of the starting energy window, and the lower threshold and upper threshold of the ending energy window. The corresponding quantities are set as ,constitute Next, calculate the count rate of the positron annihilation lifetime spectrum corresponding to each energy window. (Benefit type), goodness of fit (Cost type). In order to make the optimal value of the parameter transformation 1 and the worst value 0, it is necessary to normalize the attributes of the two optimization parameters. Let the normalized parameters be recorded as and The normalization method can use the standard 0-1 transformation:

[0030] For the benefit attribute, ;

[0031] For cost attributes, ;

[0032] Generally, the quality factor ( )= , calculate the maximum The value of can obtain the optimal energy window.

[0033] Schematic diagram of the digital positron annihilation lifetime measurement device, such as Figure 3 It is composed of a start detector, a stop detector, a second data acquisition module, a second data processing module, an optimal energy window module, a second data timing module, a second coincidence module, and a data analysis module.

[0034] The starting detector and the ending detector, i.e. the gamma detector, have exactly the same structure and detect the 1275 keV nuclear photon signal and the 511 keV positron annihilation signal respectively.

[0035] The second data acquisition module generally uses a digital oscilloscope, which needs to set a suitable sampling rate, vertical accuracy, and bandwidth to receive the analog signal from the detector and obtain a digital waveform through analog-to-digital conversion to facilitate subsequent data processing.

[0036] The second data processing module is used to perform pulse identification, smoothing or fitting processing on the digital waveform, and filter out pulses with obvious distortion to facilitate further analysis in the next module.

[0037] The optimal energy window module measures data under the optimal energy window threshold obtained by the energy window optimization device, and the matching cases outside the energy window range will be screened out.

[0038] The second data timing module generally uses a constant ratio timing method to respectively extract the time when the start detector pulse and the end detector pulse arrive at the data acquisition device.

[0039] The second coincidence module sets a coincidence time window of 100 nanoseconds. If both the start detector pulse and the end detector pulse appear within the window and no third pulse appears, the trigger is recorded as a coincidence event.

[0040] The data analysis module counts the frequency of the timing differences between the starting detector pulse and the ending detector pulse of all the events that meet the requirements, and obtains the positron annihilation lifetime spectrum, which is convenient for further spectrum analysis.

[0041] like Figure 4 The following is a diagram of an example of the present invention. The sample is clamped between two pieces of 1-2 mm thick 22Na radioactive source, the radioactive source decays and emits 1275 keV gamma rays, which enter the starting detector and deposit energy to output a voltage signal. The positrons produced by the decay enter the material and annihilate with the electrons inside the material to produce two 511 keV gamma rays in opposite directions. One of the rays will be received by the termination detector, which also deposits energy and outputs a voltage signal. Both the starting and termination detectors are gamma detectors, which consist of a photomultiplier tube and a scintillator coupled with silicone oil. In general, a coaxial cable with a BNC (Bayonet Neill-Concelman) interface is used to input the analog voltage signal of the detector into a digital oscilloscope, and the oscilloscope uses a fast data line to transmit the digital pulse to the computer terminal. The written data processing program processes the transmitted digital pulses, performs pulse discrimination, smooth fitting, extracts time and amplitude information, optimizes the energy window, and calculates the quality factor ( ), determine the optimal energy window threshold. Figure 5 The quality factor ( ) with the two-dimensional distribution of the start and end energy window thresholds. Then, a coincidence time window of 100 ns is set. If the pulses of the end detector and the start detector appear simultaneously within this window, it can be considered as a positron decay event. By performing frequency statistics on the time differences of a large number of such coincidence events, the positron annihilation lifetime spectrum can be obtained. Figure 6 The positron annihilation lifetime spectra with and without energy window optimization are shown. It can be seen that the lifetime spectrum with energy window optimization has a narrower peak width, a faster rising edge, and a lower background. This shows that the lifetime spectrum with optimized energy window has better time resolution and signal-to-noise ratio, which is conducive to improving the accuracy of spectrum interpretation.

Claims

1. A digital positron annihilation lifetime spectrometer with automatic energy window optimization, characterized in that: include: Energy window optimization device and digital positron annihilation lifetime measurement device; The energy window optimization device is used to automatically obtain the optimal energy window threshold, and the digital positron annihilation lifetime measurement device is used to measure the annihilation lifetime of positrons under the optimal energy window threshold; the energy window optimization device automatically searches for the optimal energy window threshold under the set parameters and algorithms.

2. A digital positron annihilation lifetime spectrometer with automatic energy window optimization according to claim 1, characterized in that: The energy window optimization device includes a first detector module, a first data acquisition module, and a processing terminal; The first detector module includes a start detector and a stop detector, which receive 1275 keV nuclear photon signals and 511 keV positron annihilation photon signals respectively; The first data acquisition module uses a digital oscilloscope or a data acquisition card to sample analog signals and convert continuous voltage or current signals into discrete digital pulse signals; The processing terminal includes a first data processing module, a first data timing module, a first coincidence module, and an energy window optimization module; the energy window optimization module includes an energy window classification module and a target parameter module; the first data processing module performs distorted pulse discrimination on the pulse and extracts amplitude and energy information; the first timing module extracts pulse time information; the first coincidence module performs coincidence on the start signal and the end signal within a certain time window; the energy window classification module classifies and combines the four thresholds of the start and end detector energy spectra, and obtains a positron annihilation lifetime spectrum under each energy window; The target parameter module calculates the counting rate, time resolution and goodness of fit parameters of the positron annihilation lifetime spectrum corresponding to each energy window, selects appropriate target parameters as the quality factor to measure the quality of the positron annihilation lifetime spectrum, traverses and calculates all quality factors to obtain the optimal energy window.

3. A digital positron annihilation lifetime spectrometer with automatic energy window optimization according to claim 2, characterized in that: The digital positron annihilation lifetime measurement device includes a second detector module, a second data acquisition module, a second data processing module, a second data timing module, an optimal energy window module, a second coincidence module, and a data analysis module; the second detector module has the same structure as the first detector module in the energy window optimization device, including a start detector and a stop detector, which respectively receive a 1275 keV nuclear photon signal and a 511 keV positron annihilation photon signal; the second data acquisition module, the second data processing module, the second data timing module, and the second coincidence module have the same structure as the corresponding modules in the energy window optimization device; the optimal energy window module uses the obtained optimal energy window threshold to perform data measurement, and the coincidence events outside this energy window range will be screened out. In the final data analysis module, the coincidence events are subjected to time difference frequency statistics to obtain the positron annihilation lifetime spectrum.

4. A digital positron annihilation lifetime spectrometer with automatic energy window optimization according to claim 3, characterized in that: The classification parameters of the energy window classification module include the lower threshold and upper threshold of the starting energy window, as well as the lower threshold and upper threshold of the ending energy window. The corresponding quantities are set as ,constitute energy window combination; then, the target parameter module calculates the count rate of the corresponding positron annihilation lifetime spectrum under each energy window , goodness of fit , in order to make the optimal value of the parameter transformation 1 and the worst value 0, the two optimization parameters are normalized. Let the normalized parameters be recorded as and , the normalization method can use the standard 0-1 transformation: For the benefit attribute, ; For cost attributes, ; Quality Factor = , calculate the maximum The value of is used to obtain the optimal energy window.

5. The digital positron annihilation lifetime spectrometer with automatic energy window optimization according to claim 4, characterized in that: The starting detector and the ending detector of the second detector module are both gamma detectors. The starting detector and the ending detector have the same structure and detect 1275 keV nuclear photon signals and 511 keV positron annihilation signals respectively. The second data acquisition module uses a digital oscilloscope, sets appropriate sampling rate, vertical accuracy and bandwidth, receives analog signals from the detector, and obtains digital waveforms through analog-to-digital conversion to facilitate subsequent data processing; The second data processing module is used to perform pulse identification, smoothing or fitting processing on the digital waveform, and filter out pulses with obvious distortion to facilitate further analysis in the next module; The optimal energy window module measures data under the optimal energy window threshold obtained by the energy window optimization device, and the matching cases outside the energy window range will be screened out; The second data timing module uses a constant ratio timing method to respectively extract the time when the start detector pulse and the end detector pulse arrive at the data acquisition device; The second coincidence module sets a coincidence time window. If both the start detector pulse and the end detector pulse appear within the window and no third pulse appears, the trigger is recorded as a coincidence event. The data analysis module counts the frequency of the timing differences between the starting detector pulse and the ending detector pulse of all the events that meet the requirements, and obtains the positron annihilation lifetime spectrum, which is convenient for further spectrum analysis.

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