Nuclear signal-noise discrimination method based on pulse width

Abstract

The invention discloses a nuclear signal-noise discrimination method based on pulse width, relates to the technical field of discrimination of flicker signals and photomultiplier dark noise, and can effectively distinguish photomultiplier dark noise signals and flicker signals and discriminate electronic noise at the same time. The method comprises the following steps: obtaining a plurality of pulse signals which are automatically triggered without a threshold value; carrying out peak searching on the signal, and judging the time t0 corresponding to a peak value; carrying out leading edge timing on the peak value t0 of the pulse signal to obtain the time t1 of the pulse leading edge; carrying out trailing edge timing on the peak value t0 of the pulse signal, and setting a time threshold value of the trailing edge timing; judging the data of the trailing edge timing, judging whether a plurality of peak superposition events exist in the trailing edge timing, and determining the time t2 of the pulse trailing edge; performing pulse width calculation according to the difference between the leading edge timing t1 and the trailing edge timing t2 judged from the peak time t0; and judging the types of PMT dark noise and nuclear signals according to the calculated pulse width.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of scintillation signal and photomultiplier tube dark noise discrimination, in particular to a nuclear signal-noise discrimination method based on pulse width. BACKGROUND

[0002] With the development of nuclear radiation detection technology, nuclear radiation detection has great application value in the fields of medical treatment, industry, aerospace, scientific research and the like, and is one of the main directions of China's strategic development, and it is particularly important to accurately and quickly detect the corresponding nuclear radiation signal and give the result without losing information. With the progress of nuclear technology, the research and manufacturing level of nuclear radiation detectors is also increasing. The liquid scintillator probe detector has become one of the widely used nuclear radiation detectors due to its high detection efficiency and low manufacturing cost. The liquid scintillator probe nuclear radiation detection technology is an analysis technology which measures the activity of radionuclides from the light photon rate emitted by the liquid scintillator probe sample, and has high detection efficiency for α and β. The liquid scintillator probe detector is composed of a photomultiplier tube (PMT), a liquid scintillator probe and back-end electronics, and the basic working principle is to collect the fluorescent photons generated by the interaction of the liquid scintillator and the radiation emitted by the radionuclide by the PMT, and then the back-end electronics collects the pulse signal to obtain the activity information of the radionuclide.

[0003] In the process of measuring the sample mixed with liquid scintillator probe and radionuclide, there are two kinds of noises, the dark noise of PMT and the single photon noise due to chemical excitation. The two kinds of noises will cause the accuracy of the measurement result to be reduced. The dark noise of PMT is mainly generated from the photocathode and dynode thermocurrent noise, the anode and the contact point of other elements inside the tube leakage current, the photoelectric current generated by the glass shell and the welding point, the field emission current, the ionization current generated by the residual gas, the cosmic rays, the radioactive isotopes in the glass and the gamma rays in the environment. The single photon noise generated in the sample mixed with liquid scintillator probe and radionuclide is due to the analyte or the solute in the analyte can react with each other, or react with the scintillation mixture, and emit single photons due to chemical excitation. Another reason is that a large amount of single photons are generated due to the heat generated by the chemical reaction of the oxidizing agent existing in the liquid scintillator probe. The three-tube two-tube coincidence technique (TDCR) relies on the coincidence circuit of 2-3 photomultiplier tubes to eliminate the noise of PMT and the single photon noise due to chemical excitation. The system has good ability to suppress single photon noise, but the overall circuit design is complex, the cost is high, and the overall volume is large due to the large number of photomultiplier tubes. By means of single PMT card address, the low-energy part of the liquid scintillator probe detector spectrum is carded, which plays a role in eliminating single photon noise. Taking the beta spectrum as an example, in the low-energy part, the beta rays exist and the single photons have a certain coincidence, which will cause the inaccuracy of the beta activity measurement to increase. In the analysis process of the pulse signal, the pulse shape discrimination is one of the important methods for discriminating nuclear signals, including the rise time method, the charge comparison method, the frequency domain gradient analysis method and the like. The fast and slow components of the pulse signal are amplified and discriminated, and the electronic circuit plays a good discriminating effect on the signal. The fast response photomultiplier tube (PMT) which can accurately capture the pulse time characteristic and the pulse width method are used to discriminate the noise of PMT, the single photon noise due to chemical excitation and the nuclear signal at a low threshold. The fast response PMT can better preserve the time information of the incident photon, and then the rise time and the fall time of each pulse are analyzed, the pulse width of each signal is extracted by the algorithm, and the single photon and the nuclear signal are discriminated. SUMMARY

[0004] To solve the above technical problems, the present application is realized by the following technical solutions:

[0005] The present application is a nuclear signal-noise discrimination method based on pulse width, comprising the following steps:

[0006] Step S1: obtaining a plurality of pulse signals triggered automatically without threshold;

[0007] Step S2: performing peak searching on the signals to determine the peak value corresponding time t0;

[0008] Step S3: front edge timing is performed on the peak value t0 of the pulse signal to obtain the time t1 of the pulse front edge;

[0009] Step S4: back edge timing is performed on the peak value t0 of the pulse signal to set the time threshold of the back edge timing;

[0010] Step S5: the data of the back edge timing is judged to determine whether there are multiple peak superposition events in the back edge timing, and the time t2 of the pulse back edge is determined;

[0011] Step S6: the pulse width is calculated according to the difference between the front edge timing t1 and the back edge timing t2 determined from the peak value time t0;

[0012] Step S7: the dark noise of the photomultiplier tube and the category of the nuclear signal are determined according to the calculated pulse width.

[0013] Further, in the step S1, obtaining a plurality of scintillation signals and photomultiplier tube dark noise pulse signals specifically includes:

[0014] The noise signals generated by the photomultiplier tube (PMT) include:

[0015] Dark current noise: This is the current generated by the photomultiplier tube without light irradiation; This noise is mainly caused by the thermal electron emission in the photocathode and the multiplier, and is the main source of PMT dark noise;

[0016] Thermal noise: caused by the thermal motion of the internal components of the photomultiplier tube, usually proportional to temperature; The higher the temperature, the greater the thermal noise;

[0017] Shot noise (quantum noise): Due to the quantum nature of photons, the number of incident photons fluctuates over time, resulting in irregular current pulsation in the output signal; This noise is related to the statistical properties of incident photons;

[0018] Power supply noise: noise caused by unstable power supply or external electromagnetic interference, including power supply ripple, external electromagnetic interference, etc.;

[0019] Noise in the multiplication process: the movement of electrons between the multiplication stages will produce noise in the photomultiplier process; This noise is related to the structure and material of the photomultiplier tube;

[0020] 1 / f noise (flicker noise): a low-frequency noise, usually significant at low frequencies; The power spectral density of this noise is inversely proportional to frequency;

[0021] Mechanical vibration noise: mechanical vibration can also cause small displacement of the internal structure of the PMT, which will produce noise, especially in high-sensitivity applications;

[0022] The single photon signal generated by the liquid scintillator probe, the natural radioactive radiation in the environment, such as cosmic rays or radioactive substances in the environment, may generate weak background signals; these signals are usually random, but may be recorded in a very high sensitivity detection system;

[0023] The PMT here is a PMT with fast time characteristics (transit time: 550 ps; anode rise time: 1.6 ns; electron transit time: 17 ns), for example but not limited to R9420 of Hamamatsu; the PMT with low transit time spread can respond to the incident light signal more quickly, the signal delay is smaller, the overall performance and measurement consistency between channels are better, and it is more sensitive to rapid signal changes;

[0024] Preparation 14 C, 210 pb and 22 Na source;

[0025] The PMT (R9420) is measured respectively, and the measurement is carried out after matching the scintillator probe (EJ301);

[0026] The liquid scintillator probe selected for the liquid scintillator probe is matched with the PMT model, and EJ301 is taken as an example here, but not limited to EJ301;

[0027] The 14 C, 210 pb and 22 Na source is detected;

[0028] The pulse signal of the PMT (R9420) and the liquid scintillator probe (EJ301) is obtained by a digital oscilloscope; 22 Na source;

[0029] The pulse signal of the PMT (R9420) and the liquid scintillator probe (EJ301) is obtained by a digital oscilloscope; 14 C and 210 pb.

[0030] Further, in the step S2, the peak value judgment is carried out on the scintillation signal and the photoelectric multiplier dark noise pulse signal, and the time t0 corresponding to the maximum pulse value is obtained, which specifically includes:

[0031] No test voltage threshold is set, and the signal is collected automatically by the digital oscilloscope, and the purpose is to collect all PMT output signals;

[0032] The maximum amplitude value Vmax of the signal pulse is found by the data comparison method, and the time t0 corresponding to the maximum amplitude value Vmax is found.

[0033] Further, the step S3 of performing front edge timing on the scintillation signal and the photomultiplier tube dark noise pulse signal to obtain the time t1 of the pulse rear edge specifically includes:

[0034] Calculate the mean value of the maximum amplitude value of each group of pulses And variance σ;

[0035] Set a dynamic threshold VP1;

[0036] When the front edge height of the scintillation signal and the photomultiplier tube dark noise pulse signal exceeds the threshold VP1, record the time t1 < t0 of the pulse front edge.

[0037] Further, in steps S4 and S5, the rear edge timing of the scintillation signal and the photomultiplier tube dark noise pulse signal is performed to obtain the time t2 of the pulse rear edge, specifically including:

[0038] Pre-set a dynamic threshold VP1;

[0039] Monitor the rear edge height of the scintillation signal and the photomultiplier tube dark noise pulse signal;

[0040] When the rear edge height of the scintillation signal and the photomultiplier tube dark noise pulse signal intersects with the dynamic threshold VP1, record the time t2 > t0 of the pulse rear edge;

[0041] Perform logical judgment on the rear edge signal, calculate whether there are multiple peaks in the rear edge, and find the time t corresponding to the last peak position in the confidence interval i (i = 1, 2…) to replace t2, at which time the nuclear signal needs to be analyzed and the maximum time t max .

[0042] Further, the dynamic threshold Wherein is the mean value of the pulse signal front baseline data, σ is the corresponding variance, and the deviation multiple is generally 1-3 times, and a = 2 is commonly used; a is a correction coefficient ranging from 2 to 4.

[0043] Further, in step S6, the difference between the front edge timing t1 and the rear edge timing t2 determined according to the peak time t0 is used to calculate the pulse width:

[0044] According to the observed peak position, judge the accuracy of t2, estimate and set the confidence interval of t2;

[0045] Calculate the difference between the time t2 of the pulse rear edge and the time t1 of the pulse front edge, which is the pulse width Δt.

[0046] Further, the step S6, the step of judging the category of the scintillation signal and the photomultiplier tube dark noise pulse signal according to the pulse width specifically comprises:

[0047] A histogram is drawn according to the pulse width Δt calculated above, and first, the PMT and the single photon signal generated by the liquid scintillator probe are distinguished;

[0048] Then, the nuclear signal and the single photon noise signal are distinguished, and the different signals are processed through the different pulse widths.

[0049] The present application has the following beneficial effects:

[0050] The method of the present application acquires the pulse signal without threshold value cutting and automatic triggering of the digital oscilloscope. The PMT without scintillation, the PMT with liquid scintillator and the PMT with liquid scintillator and radioactive source are respectively detected. The peak time t0 is acquired by analyzing the noise and single photon signal of the liquid scintillator and the PMT; the front edge time t1 is acquired by front edge timing of the signal from t0; the back edge time t2 is acquired by back edge timing of the signal from t0; the characteristics of the noise and single photon signal are judged according to the difference Δt of the front edge and back edge time. The signals with radioactive source are analyzed in the same way, and the nuclear signal, noise and single photon signal are distinguished through different pulse widths.

[0051] Of course, implementing any product of the present application does not necessarily need to achieve all the advantages described above at the same time. BRIEF DESCRIPTION OF DRAWINGS

[0052] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiment description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0053] Figure 1 It is a flowchart of the nuclear signal-noise distinguishing method based on pulse width of the present application;

[0054] Figure 2 It is a noise (single photon signal) pulse waveform diagram of the present application;

[0055] Figure 3 It is a nuclear signal waveform diagram of the present application;

[0056] Figure 4 It is a nuclear signal multi-peak sample waveform diagram of the present application;

[0057] Figure 5 It is a noise pulse width histogram of the present application;

[0058] Figure 6 is a noise and nuclear signal discrimination pulse width histogram of the present application. DETAILED DESCRIPTION

[0059] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0060] Please refer to Figures 1-6 The present application is a nuclear signal-noise discrimination method based on pulse width, which comprises the following steps:

[0061] Step (1), obtaining a plurality of scintillation signals and photomultiplier tube dark noise pulse signals; specifically, in step (1), a plurality of scintillation signals and photomultiplier tube dark noise pulse signals triggered automatically without threshold are obtained.

[0062] Step (2), determining the peak position corresponding to the scintillation signal and the photomultiplier tube dark noise pulse signal, determining the peak position Vmax and recording the time t corresponding to each peak position i (i = 0, 1, 2…).

[0063] Step (3), setting the threshold value of Vmax through analysis and discrimination of Vmax, and filtering out electronic noise.

[0064] Step (4), determining the peak position corresponding to the scintillation signal and the photomultiplier tube dark noise pulse signal, and recording the time t0 corresponding to the peak position, and performing front edge timing on the scintillation signal and the photomultiplier tube dark noise pulse signal from the time t0; in step (4), the front edge timing is performed on the scintillation signal and the photomultiplier tube dark noise pulse signal to obtain the time t1 of the pulse front edge, which specifically comprises: setting the threshold value VP1 according to the mean value and the standard deviation of the baseline; monitoring the front edge height of the scintillation signal and the photomultiplier tube dark noise pulse signal from t0; when the front edge height of the scintillation signal and the photomultiplier tube dark noise pulse signal exceeds the threshold value VP1 (since the signal is negative, the judgment of passing the threshold value here is that when the previous amplitude is less than the dynamic threshold value VP1 and the next point is greater than the dynamic threshold value VP1, the front edge time is recorded), the time t1 of the pulse front edge is recorded.

[0065] Step (5), the trailing edge timing of the scintillation signal and the photomultiplier tube dark noise pulse signal, the trailing edge timing of the scintillation signal and the photomultiplier tube dark noise pulse signal from t0 time; Specifically, in step (5), the trailing edge timing of the scintillation signal and the photomultiplier tube dark noise pulse signal specifically includes: first, judge the number of peak values existing from the trailing edge starting from t0 time and record the time t corresponding to each peak position i (i = 0, 1, 2…), and determine the time corresponding to each peak; second, determine the trailing edge confidence interval, that is, the upper limit of the trailing edge time of the nuclear signal; Step (6), when the trailing edge height of the scintillation signal and the photomultiplier tube dark noise pulse signal intersects with the dynamic threshold VP1 when there are multiple peak positions in the trailing edge of the scintillation signal and the photomultiplier tube dark noise pulse signal based on step (5), record the time t2 of the trailing edge of the pulse; if there are multiple peaks in the trailing edge, judge the time when the trailing edge of the last peak in the confidence interval intersects with the dynamic threshold VP1, and record the trailing edge time as the trailing edge time t2 of the pulse.

[0066] Step (7), the difference between the leading edge time t1 and the trailing edge time t2 is the pulse width Δt. Step (8) is to distinguish different nuclear signals (neutron, a, β, γ, etc.) and noise and single photon signals according to the different pulse widths.

[0067] As shown in Figure 2 , it is a single photon pulse signal. Figure 3 As shown in Figure 3 , it is an electronic noise. When there is no threshold automatic trigger, both kinds of pulse signals will be recorded, and again we will refer to these two signals as noise signals. The method is to distinguish the above signals from nuclear signals (neutron, a, β, γ, etc.). In this embodiment, the electronic noise is filtered by judging the peak value Vmax Figure 2 As shown in Figure 6 , there is only one pulse peak, so the leading edge timing and the trailing edge timing are determined from the peak value corresponding to t0, the pulse leading edge time t1 and the pulse trailing edge time t2 are determined, and the difference Δt is obtained. Figure 4 and Figure 5 The nuclear signal is shown in Figure 4 , which is a pulse nuclear signal with higher energy collected. Figure 5 Then it is a pulse signal with multiple peak positions superimposed, which is the object of the method for processing nuclear signals proposed in this patent.

[0068] In this embodiment, Na22 source is measured, and it can be seen from Figure 6 that the pulse width of the (noise) single photon signal is 7-8 ns, and the pulse width of the neutron signal is 20-30 ns.Figure 6 It can be seen that the pulse width of the nuclear signal is 10-50ns, so the single photon can be distinguished from the nuclear signal according to the pulse width.

[0069] Step (8) is to distinguish the scintillation signal from the photomultiplier dark noise pulse signal according to the pulse width. In step (8), the distinguishing of the scintillation signal from the photomultiplier dark noise pulse signal according to the pulse width specifically includes: selecting an appropriate threshold value as the discrimination threshold value TH of the pulse width according to the pulse width Δt; judging the noise and the nuclear signal by comparing the size of TH and Δt; when the pulse width Δt is less than TH, the pulse signal is considered as the noise signal; when the pulse width Δt is greater than TH, the pulse signal is considered as the nuclear signal. In the embodiment, the threshold value TH is 8.5ns, when the pulse width Δt<8.5ns, we consider that the noise signal is obtained, and when the pulse width Δt>8.5ns, we consider that the nuclear signal is obtained. By using this method, the nuclear signal and the noise signal can be distinguished obviously.

[0070] In the description of the present specification, the description of the terms "one embodiment", "an example", "a specific example" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily mean the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. The above disclosed preferred embodiments of the present application are only used to help explain the present application. The preferred embodiments do not describe all the details and limit the present application to the specific embodiments described. Obviously, many modifications and changes can be made according to the content of the present specification. The present specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present application, so that those skilled in the art can well understand and utilize the present application. The present application is limited only by the claims and their full scope and equivalents.

Claims

1. A pulse width based nuclear signal-noise discrimination method, comprising: The method comprises the following steps: Step S1: obtaining a threshold-free automatic trigger pulse signal; Step S2: peak searching of the signal to determine the peak value corresponding time t0; Step S3: front edge timing of the peak value t0 of the pulse signal to obtain the time t1 of the pulse front edge; Step S4: back edge timing of the peak value t0 of the pulse signal to set the time threshold of the back edge timing; Step S5: judging the back edge timing data to determine whether there are multiple peak superposition events in the back edge timing, and determining the time t2 of the pulse back edge; Step S6: calculating the pulse width according to the difference between the front edge timing t1 and the back edge timing t2 determined from the peak value time t0; Step S7: judging the photoelectric multiplier dark noise and the nuclear signal category according to the calculated pulse width.

2. The pulse width based nuclear signal-noise discrimination method according to claim 1, wherein, In the step S1, the obtained scintillation signal and photoelectric multiplier dark noise pulse signal specifically comprises: The noise signal generated by the photoelectric multiplier (PMT), including: Dark current noise: This is the current generated by the photoelectric multiplier without light irradiation; this noise is mainly caused by the thermal electron emission in the photocathode and the multiplier, and is the main source of PMT dark noise; Thermal noise: caused by the thermal motion of internal components of the photoelectric multiplier, usually proportional to temperature; the higher the temperature, the greater the thermal noise; Shot noise: due to the quantum nature of photons, the number of incident photons fluctuates over time, resulting in irregular current pulsation in the output signal; this noise is related to the statistical properties of incident photons; Power supply noise: noise caused by unstable power supply or external electromagnetic interference, including power supply ripple and external electromagnetic interference; Noise in the multiplication process: the movement of electrons between the multiplication stages will produce noise; this noise is related to the structure and material of the multiplier; 1 / f noise: a low-frequency noise, usually significant at low frequencies; the power spectral density of this noise is inversely proportional to frequency; Mechanical vibration noise: mechanical vibration causes slight displacement of the internal structure of the PMT, which also produces noise, especially in high-sensitivity applications; Single photon signal generated by liquid scintillator probe, natural radioactive radiation in the environment, such as cosmic rays or radioactive substances in the environment, may produce weak background signals; these signals are usually random, but may be recorded in very high sensitivity detection systems; Here, the PMT is a PMT with fast time characteristics, a low transit time spread PMT, which can respond to incident light signals more quickly, has less signal delay, better overall performance and measurement consistency between channels, and is more sensitive to rapid signal changes; Preparation 14 C, 210 pb and 22 Na source; The PMT is measured separately, and the measurement is performed after matching the scintillator probe; For liquid scintillator probes, the liquid scintillator probe matching the PMT model is selected; Regarding the 14 C 210 pb and 22 Na source for detection; The PMT is detected by a digital oscilloscope and the liquid scintillator probe is carried behind 22 The pulse signal of the Na source; The pulse signal of the PMT is acquired by a digital oscilloscope 14 C and 210 pb 3. The pulse width based nuclear signal-noise discrimination method according to claim 1, wherein, In the step S2, the peak value of the scintillation signal and the photoelectric multiplier dark noise pulse signal is determined to obtain the time t0 corresponding to the maximum value of the pulse, which specifically comprises: Without setting the test voltage threshold, the signal is automatically triggered and collected by the digital oscilloscope, and the purpose is to collect all PMT output signals; The maximum amplitude value Vmax of the signal pulse is found by a data comparison method, and the time t0 corresponding to the maximum amplitude value Vmax is found.

4. The pulse width based nuclear signal-noise discrimination method of claim 1, wherein, The step S3 includes: calculating the mean value of the maximum amplitude values of each group of pulses and the variance σ; A dynamic threshold VP1 is set. When the front edge height of the scintillation signal and the photomultiplier tube dark noise pulse signal exceeds the threshold VP1, the time t1 of the pulse front edge is recorded, and t1 < t0.

5. The pulse width based nuclear signal-noise discrimination method according to claim 1, wherein, The step S4 and the step S5 include: A dynamic threshold VP1 is set. The back edge height of the scintillation signal and the photomultiplier tube dark noise pulse signal is monitored. When the back edge height of the scintillation signal and the photomultiplier tube dark noise pulse signal intersects with the dynamic threshold VP1, the time t2 of the pulse back edge is recorded, and t2 > t0. Logical judgment is made on the trailing edge signal to calculate whether multiple peaks exist in the trailing edge and to find the time t corresponding to the last peak position in the confidence interval i (i = 1, 2…), replace t2, at this time, the core signal needs to be analyzed and the maximum time t is set max .

6. The pulse width based nuclear signal-noise discrimination method according to claim 1, wherein, The dynamic threshold Where X is the mean of the baseline data preceding the pulse signal, and σ is the corresponding variance. The multiple of the standard deviation is typically 1-3, with a=2 being commonly used.

7. The pulse width based nuclear signal-noise discrimination method according to claim 1, wherein, The step S6 includes: The difference between the front edge timing t1 and the back edge timing t2 is calculated according to the peak value time t0. The accuracy of t2 is judged according to the observed peak position, and the confidence interval of t2 is estimated and set.

8. The pulse width based nuclear signal-noise discrimination method according to claim 1, wherein, The difference between the time t2 of the pulse back edge and the time t1 of the pulse front edge is calculated, and the pulse width Δt is obtained. The step S6 includes: A histogram is drawn according to the pulse width Δt obtained by the above calculation. First, the PMT and the single-photon signal generated by the liquid scintillator probe are distinguished. Then, the nuclear signal and the single-photon noise signal are distinguished, and different signals are processed by the difference in pulse width.

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