A digital positron annihilation lifetime spectrometer for automatic optimization of energy windows

Abstract

The application provides a kind of automatic optimization energy window digital positron annihilation lifetime spectrometer, comprising: energy window optimization device and digital positron annihilation lifetime measurement device;Energy window optimization device is used to automatically obtain optimal energy window threshold, so that digital positron annihilation lifetime measurement device is measured at the energy window threshold;Digital positron annihilation lifetime measurement device is used to measure the annihilation lifetime of positron at the optimal energy window threshold;By energy window optimization device, optimal energy window threshold is automatically searched under the set parameters and algorithm, and digital positron annihilation lifetime measurement device is measured at the threshold.The application automatically searches the optimal energy window configuration according to specific parameters and algorithm, rather than manually selecting and debugging by artificial experience as before, which is tedious and has human error, so that the implementation of positron annihilation lifetime measurement is more simple, convenient and accurate.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of positron annihilation lifetime, in particular to a digital positron annihilation lifetime spectrometer with automatic optimization of energy window. BACKGROUND

[0002] The positron annihilation lifetime spectroscopy measures the time interval between the start signal (1275 keV accompanying gamma ray) and the end signal (one of the two 511 keV gamma rays produced by positron annihilation) generated from the decay of Na. 22 The positron annihilation lifetime spectrum is a powerful tool used to study and characterize various materials, and has applications in many fields of physics, biology and chemistry. Today, many positron annihilation lifetime spectrometers are widely used.

[0003] In order to improve the quality of the obtained positron annihilation lifetime spectrum data, a positron annihilation lifetime spectrometer with high counting and good time resolution is needed. The counting rate and time resolution of the positron annihilation lifetime spectrometer used depend on the energy window settings of the start and end detectors. Therefore, it is a key point in such measurements to experimentally determine the optimal values of the energy window width, and the selection of these values usually depends on the experimenter's experience, and therefore may vary from person to person.

[0004] A conventional positron annihilation lifetime spectrometer is composed of a scintillator, a photomultiplier tube, a high-voltage power supply, a nuclear instrumentation modular (NIM, Nuclear Instrumentation Module) plug-in (constant fraction discriminator, delay timer, time-to-amplitude converter, multichannel analyzer), and a computer, as shown in Figure 1 Two gamma detectors are used as start and end detectors to measure the start and end signals, and the output voltage signals are subjected to energy discrimination and constant fraction timing by the constant fraction discriminator, and then converted into a square wave signal with fixed width and amplitude. The time difference between the two square wave signals is converted into a square wave signal with fixed width and linear amplitude-time relationship by the time-to-amplitude converter, and the analog-to-digital conversion and data acquisition by the multichannel analyzer and transmission to the computer for data processing can obtain the final positron annihilation lifetime spectrum. The conventional positron annihilation lifetime spectrometer cannot directly obtain the pulse data of the gamma detector, and only performs amplitude discrimination on the voltage pulse, which may result in inaccurate energy window threshold and distorted lifetime spectrum (significant distortion of the rising front), thereby affecting the spectrum results. SUMMARY

[0005] The present application aims to experimentally determine the optimal values of the energy window settings of the positron annihilation lifetime spectrometer used using mathematical methods. The positron annihilation lifetime spectrometer used is equipped with a multi-parameter acquisition system, and the determination of the optimal values is based on the fuzzy decision method of the product type.

[0006] The technical scheme of the present application is as follows: an automatic optimization energy window digital positron annihilation lifetime spectrometer, comprising: an energy window optimization device and a digital positron annihilation lifetime measurement device; the energy window optimization device is used for automatically obtaining an optimal energy window threshold value, and the digital positron annihilation lifetime measurement device is used for measuring the positron annihilation lifetime under the optimal energy window threshold value; the energy window optimization device automatically searches for the optimal energy window threshold value under set parameters and algorithms.

[0007] Compared with the prior art, the present application has the following advantages:

[0008] 1. The present application automatically searches for the optimal energy window configuration according to specific parameters and algorithms, instead of manually selecting and debugging by experience as in the prior art, which is tedious and has human errors, so that the positron annihilation lifetime measurement is more simple, convenient and accurate.

[0009] 2. The digital waveform technology makes the data processing more simple and accurate, simplifies the hardware structure, and greatly reduces the cost.

[0010] 3. The modular design of the device has high integration and good reconfigurability, and is convenient for system optimization and upgrading.

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

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

[0013] Figure 2 is a product type fuzzy decision method for energy window optimization;

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

[0015] Figure 4 is a schematic diagram of an embodiment of the present application;

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

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

[0018] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and are not used to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other. In order to achieve the above-mentioned purpose, the technical scheme of the present application is as follows.

[0019] The technical scheme of the present application is: an automatic optimization energy window digital positron annihilation lifetime spectrometer, using a product type fuzzy decision method, comprising: an energy window optimization device and a digital positron annihilation lifetime measurement device; the energy window optimization device is used to automatically obtain the optimal energy window threshold, so that the digital positron annihilation lifetime measurement device measures at the energy window threshold; the digital positron annihilation lifetime measurement device is used to measure the positron annihilation lifetime at the optimal energy window threshold; the energy window optimization device automatically searches for the optimal energy window threshold under the set parameters and algorithm, and the digital positron annihilation lifetime measurement device measures at the threshold, so that the front edge distortion of the positron annihilation lifetime spectrum is reduced, and the signal-to-noise ratio of the signal is higher, greatly improving the quality and stability of the spectrum.

[0020] The energy window optimization device comprises a first detector module, a first data acquisition module and a processing terminal; the first detector module comprises a starting detector and a terminal detector, both of which are gamma detectors, respectively receiving 1275 keV nuclear photon signals and 511 keV positron annihilation photon signals, and both of which are photomultiplier devices provided with a scintillator. In terms of chemical composition, the scintillator can generally be divided into organic scintillators (such as plastic, anthracene, stilbene, liquid scintillator, etc.) and inorganic scintillators (such as alkali metal halide, barium fluoride, bismuth germanate, lutetium silicate, yttrium lutetium silicate, lanthanum bromide, etc.). In most organic crystals, fluorescence is the main light-emitting process, so the light attenuation time is shorter; the inorganic crystal has the characteristics of high density and contains elements with high atomic number, so it has large ionization loss and high detection efficiency for charged particles or gamma rays, and has good energy resolution. The user can select the crystal type according to the specific experimental requirements. The photomultiplier device converts the optical signal into an electrical signal and amplifies the signal, and commonly used 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 the analog signal, converts the continuous voltage or current signal into a discrete digital pulse signal, and needs to set appropriate sampling rate and vertical precision for subsequent data processing.

[0022] The processing terminal comprises 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 comprises an energy window classification module and a target parameter module. The first data processing module discriminates distorted pulses and extracts amplitude and energy information. The first data timing module extracts pulse time information. The first coincidence module performs coincidence on a start signal and an end signal within a certain time window. The energy window classification module classifies and combines four threshold values of start and end detector energy spectra. Each energy window combination can obtain a positron annihilation lifetime spectrum. The target parameter module calculates count rate, time resolution, and fitting goodness parameters of the positron annihilation lifetime spectrum corresponding to each energy window, selects appropriate target parameters as a quality factor for measuring the quality of the positron annihilation lifetime spectrum, calculates all quality factors, and obtains an optimal energy window for a positron annihilation lifetime measuring device.

[0023] The digital positron annihilation lifetime measuring device comprises 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 and comprises start and end detectors. 22 Na radioactive sources and test samples. 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 value obtained by the energy window optimization device to perform data measurement. Coincidence events outside the energy window range are excluded. In the final data analysis module, a positron annihilation lifetime spectrum is obtained by counting the time difference frequency of a large number of coincidence events.

[0024] As shown in Figure 2 Fig. 1 is a schematic diagram of an energy window optimization method using a product type fuzzy decision method for the energy window optimization device,

[0025] First, the start and end detectors of the first detector module are both gamma detectors, which are composed of a photomultiplier tube and a scintillator. 22Na radioactive source decay produces 1275 keV accompanying gamma rays, which are used as the starting signal of positron generation. About 3 ps later, a positron is emitted, which will annihilate with an electron after entering the material, generating two 511 keV gamma rays in opposite directions, one of which is used as the end signal of positron lifetime. When the gamma ray enters the gamma detector, it will transfer all the energy to the inner electrons through the photoelectric effect inside the scintillator. The initial kinetic energy of the photoelectron produced is large enough to ionize or excite the surrounding electrons. The excited state electrons form fluorescence centers, and the excited state electrons de-excitation emits fluorescence photons in the order of picoseconds. The fluorescence photons decay exponentially to the cathode window of the photomultiplier tube (usually composed of alkali metal materials with smaller work function) to produce photoelectrons through the photoelectric effect. The photoelectrons are accelerated to the anode under the electric field of the photomultiplier tube to form a 50Ω direct current matched voltage signal.

[0026] The oscilloscope is used for signal acquisition, and the analog voltage signal is converted into a digital waveform signal.

[0027] The first data processing module and the first data timing module perform pulse shape discrimination on the digital signal to screen out distorted pulses, obtain the amplitude, energy, etc. of the pulse, and use a high-precision timing method to obtain the pulse time.

[0028] The first coincidence module performs coincidence on the starting signal pulse and the end signal pulse within a 100 nanosecond time window. If the two signals exist in the coincidence time window at the same time, 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 end energy window. The corresponding quantities are respectively set as , constituting energy window combinations. Then, the count rate (benefit type) of the corresponding positron annihilation lifetime spectrum of each energy window is calculated (cost type). In order to make the optimal value of the parameter transformation 1 and the worst value 0, the two optimization parameters need to be normalized, and the normalized parameters are respectively denoted as and . The normalization method can use the standard 0-1 transformation:

[0030] For the benefit attribute, ;

[0031] For the cost attribute, ;

[0032] Generally, the quality factor ( ) = , the maximum value is calculated, and the optimal energy window can be obtained.

[0033] A schematic diagram of a digital positron annihilation lifetime measurement device, as shown below. Figure 3 As shown, it consists 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 start detector and the stop detector, also known as the gamma detector, have identical structures and detect nuclear photon signals of 1275 keV and positron annihilation signals of 511 keV, respectively.

[0035] The second data acquisition module typically uses a digital oscilloscope, which requires setting appropriate sampling rate, vertical accuracy, and bandwidth. It receives analog signals from the detector and obtains digital waveforms through analog-to-digital conversion, facilitating subsequent data processing.

[0036] The second data processing module is used to perform pulse identification, smoothing or fitting processing on digital waveforms, and to filter out pulses with obvious distortion, so as to facilitate further analysis by the following modules.

[0037] The optimal energy window module uses the optimal energy window threshold obtained from the energy window optimization device to perform data measurement, and conforming cases outside this energy window range will be filtered out.

[0038] The second data timing module typically uses a constant ratio timing method to extract the arrival times of the start detector pulse and the end detector pulse at the data acquisition device.

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

[0040] The data analysis module performs frequency statistics on the timing difference between the start and end detector pulses of all matching cases to obtain the positron annihilation lifetime spectrum, which facilitates further spectral analysis.

[0041] like Figure 4 The diagram shown is an example of an apparatus for this discovery. Two sample pieces, each approximately 1-2 mm thick, are clamped together. 22A Na radioactive source decays and emits 1275 keV gamma rays, which enter the initiation detector and deposit energy to output a voltage signal. The positrons produced by the decay enter the material and annihilate with electrons within the material, generating two 511 keV gamma rays in opposite directions. One of these rays is received by the termination detector, which similarly deposits energy to output a voltage signal. Both the initiation and termination detectors are gamma detectors, composed of photomultiplier tubes and scintillators coupled with silicone oil. Typically, a BNC (Bayonet-Neill-Concelman) coaxial cable is used to input the analog voltage signal from the detector to a digital oscilloscope. The oscilloscope uses a high-speed data line to transmit the digital pulses to a computer terminal. A data processing program processes the transmitted digital pulses, performing pulse discrimination, smoothing and fitting, extracting time and amplitude information, optimizing the energy window, and calculating the quality factor (…). ), determine the optimal energy window threshold. Figure 5 The quality factor was displayed. The two-dimensional distribution of the start and stop energy window thresholds is then determined. Subsequently, a coincidence time window of 100 ns is set. If the pulses from the stop detector and the start detector appear simultaneously within this window, it can be considered a positron decay event. Frequency statistics of the time differences of a large number of such coincidence events yield the positron annihilation lifetime spectrum. Figure 6 The positron annihilation lifetime spectra with and without energy window optimization are shown. It can be seen that the optimized lifetime spectrum has a narrower peak width, a faster rise time, and a lower background. This indicates that the optimized lifetime spectrum has better time resolution and signal-to-noise ratio, which is beneficial for improving spectral accuracy.

Claims

1. A digital positron annihilation lifetime spectrometer with automatically optimized energy window, 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 algorithm. The energy window optimization device includes a processing terminal, which comprises 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 identification on the pulse and extracts amplitude and energy information. The first timing module extracts pulse time information. The first coincidence module performs coincidence between the start signal and the end signal within a certain time window. The energy window classification module classifies and combines four thresholds of the energy spectrum of the start and end detectors, obtaining a positron annihilation lifetime spectrum under each energy window. The target parameter module calculates the count rate, time resolution, and goodness of fit parameters for the positron annihilation lifetime spectrum corresponding to each energy window, and selects appropriate target parameters as quality factors to measure the quality of the positron annihilation lifetime spectrum. It iterates through all quality factors to obtain the optimal energy window. The classification parameters for the energy window classification module include the lower and upper thresholds of the starting energy window, and the lower and upper thresholds of the ending energy window, with the corresponding quantities set as follows: ,constitute The module then calculates the count rate of the corresponding positron annihilation lifetime spectrum for each energy window combination. Goodness of fit To ensure that the optimal value of the transformed parameters is 1 and the worst value is 0, attribute normalization is performed on the two optimization parameters. Let the normalized parameters be denoted as follows: and The normalization method uses a standard 0-1 transformation: Regarding the benefit attribute, ; For cost-type attributes, ; quality factor = Calculate the maximum The optimal energy window is obtained by finding the value of .

2. The digital positron annihilation lifetime spectrometer with automatically optimized energy window according to claim 1, characterized in that, The energy window optimization device also includes a first detector module and a first data acquisition module; The first detector module includes a start detector and a stop detector, which receive nuclear photon signals of 1275 keV and positron annihilation photon signals of 511 keV, respectively. The first data acquisition module uses a digital oscilloscope or data acquisition card to sample analog signals, converting continuous voltage or current signals into discrete digital pulse signals.

3. The digital positron annihilation lifetime spectrometer with automatically optimized energy window 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 receive nuclear photon signals of 1275 keV and positron annihilation photon signals of 511 keV, respectively. 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. Coincidence events outside this energy window range will be filtered out. In the final data analysis module, the coincidence events are statistically analyzed by time difference to obtain the positron annihilation lifetime spectrum.

4. The digital positron annihilation lifetime spectrometer with automatically optimized energy window according to claim 3, characterized in that, The start and stop detectors of the second detector module are both gamma detectors. The start and stop detectors have the same structure and detect nuclear photon signals of 1275 keV and positron annihilation signals of 511 keV, respectively. The second data acquisition module uses a digital oscilloscope, with appropriate sampling rate, vertical accuracy, and bandwidth set, to receive analog signals from the detector and obtain digital waveforms through analog-to-digital conversion, which facilitates subsequent data processing. The second data processing module is used to perform pulse identification, smoothing or fitting processing on digital waveforms, and to filter out pulses with obvious distortion, so as to facilitate further analysis by the following modules. The optimal energy window module uses the optimal energy window threshold obtained from the energy window optimization device to measure data, and conforming cases outside this energy window range will be filtered out. The second data timing module uses a constant ratio timing method to extract the arrival times of the start detector pulse and the end detector pulse at the data acquisition device. The second matching module sets a matching time window. If both the start detector pulse and the end detector pulse appear within this window and no third pulse appears, then this trigger is recorded as a matching event. The data analysis module performs frequency statistics on the timing difference between the start and end detector pulses of all matching cases to obtain the positron annihilation lifetime spectrum, which facilitates further spectral analysis.

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