A method of energy resolution calibration for organic scintillation detectors

Abstract

The present application is directed to the problem that the full energy peak is absent in the gamma ray response spectrum of an organic scintillation detector, which leads to the difficulty in calibrating the energy resolution. Based on the Compton coincidence measurement experiment, the influence of accidental coincidence events and annihilation coincidence events is eliminated, and a quasi-Gaussian peak is formed at the Compton edge. The energy deposition spectrum of true coincidence events is obtained by Monte Carlo simulation, and then the energy resolution at the Compton edge is calculated. Then the undetermined parameters of the calculation expression are fitted by using these energy resolutions, and the calibration of the energy resolution is realized. The energy resolution calibration of an organic scintillation detector is carried out by using this method, and after the calibration result is added to the Monte Carlo simulation, the simulated spectrum of the detector for gamma rays is in good agreement with the measured spectrum, which verifies the accuracy of the method. The organic scintillation detector energy resolution calibration method disclosed by the present application is simple to operate, good in accuracy and strong in universality, and is suitable for the energy resolution calibration of various plastic scintillators, liquid scintillators and organic crystals.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of nuclear radiation detection, and particularly relates to a method for calibrating energy resolution of an organic scintillation detector. BACKGROUND

[0002] An organic scintillation detector is composed of an organic scintillator and a photoelectric conversion device such as a photomultiplier tube (PMT), and is commonly used for neutron and gamma ray detection and energy spectrum measurement. The organic scintillator includes a plastic scintillator, a liquid scintillator and an organic crystal scintillator. When measuring the neutron and gamma ray energy spectrum using an organic scintillation detector, it is necessary to carry out spectrum calculation according to the measured spectrum and a response matrix. The response matrix is usually established by Monte Carlo simulation. Influenced by factors such as scintillation light quantity fluctuation, scintillation light transport and absorption, photoelectric random conversion, electron multiplication coefficient fluctuation and electronic noise, the energy spectrum generated by a certain light yield has a certain broadening, and the broadening size is related to the light yield. This phenomenon can be characterized by energy resolution. Since Monte Carlo simulation does not involve scintillation light quantity fluctuation and electronic noise, etc., to ensure that the simulated spectrum is consistent with the measured spectrum, it is necessary to calibrate the energy resolution of the organic scintillation detector, and manually broaden the light yield in the simulation calculation.

[0003] For NaI and other detectors, the full energy peak of gamma rays is generally used to calibrate the energy resolution thereof. The organic scintillation detector is sensitive to both gamma rays and neutrons, and the energy resolution thereof can be calibrated by using these two types of radiation respectively. The light yield of the organic scintillator for electrons with a energy of 125 keV and above is linearly related to the electron energy, and the light yield of the organic scintillator for protons and other heavy charged particles is nonlinearly related to the energy thereof, so the equivalent electron energy is commonly used to represent the light yield of the organic scintillator. The average atomic number of the organic scintillator is low, and the probability of photoelectric effect of gamma rays in the organic scintillator is very low, and Compton scattering mainly occurs. This results in that the energy spectrum of gamma rays detected by the organic scintillation detector is mainly composed of a Compton plateau, and it is difficult to identify the full energy peak. When using gamma rays to calibrate the energy resolution of the organic scintillation detector in the past, it is necessary to repeatedly adjust the parameters of the expression so as to make the simulated spectra of multiple energy gamma rays consistent with the measured spectra, to determine the parameter values. However, the energy resolution expression has three parameters to be determined which influence each other, and it is very difficult to manually adjust and determine the parameter values. When using neutrons to calibrate the energy resolution of the organic scintillation detector, it is necessary to use the time-of-flight method to measure the wide-spectrum neutrons, to obtain the measured spectra of the detector for multiple energy regions of quasi-monochromatic neutrons, and to perform first-order differentiation on the measured spectra, to determine the energy resolution according to the characteristic peaks of the differentiated spectra. However, this method is relatively complex, time-consuming, and requires the equivalent electron energy expression of protons to be determined in advance, which is difficult and has a large amount of work. It can be seen that the problem of calibrating the energy resolution of the organic scintillation detector has not been well solved. SUMMARY

[0004] (I) Invention purpose

[0005] The purpose of the present application is to design an organic scintillation detector energy resolution calibration method, aiming at quickly and accurately calibrating the energy resolution of the organic scintillation detector, so as to use the detector for neutron and gamma ray energy spectrum measurement.

[0006] (II) Technical solutions

[0007] In order to achieve the above purpose and solve the above technical problems, the present application provides an organic scintillation detector energy resolution calibration method. The method can make the characteristic peak at the Compton edge, calculate the undetermined parameter of the energy resolution expression according to the half-height width of multiple characteristic peaks, and does not need manual parameter adjustment and neutron spectrum data processing. The key of the method is to eliminate the interference factors of the Compton coincidence measurement spectrum and the characteristic peak spectrum, and ensure the accuracy of the undetermined parameter. The method comprises the following steps:

[0008] S1, place another gamma detector (such as BaF2 detector) opposite to the organic scintillation detector, fix the gamma source along the central axis of the two detectors close to the organic scintillation detector, and obtain the total coincidence spectrum Q of the organic scintillation detector to the gamma source through Compton coincidence measurement experiment t . Q t contains the true coincidence spectrum Q tc and the accidental coincidence spectrum Q cc (to 22 Na, also contains the annihilation coincidence spectrum Q ac ), the former has a characteristic distribution, and the latter is a continuous spectrum composed of Compton plateau. Existing researches use the peak position of Q t to complete the energy calibration of the organic scintillation detector, but to carry out energy resolution calibration, a key step is to eliminate Q cc . This step is completed by S2-S4 as follows.

[0009] S2, use a digitizer to measure the no coincidence spectrum Q d of the organic scintillation detector to the gamma source.

[0010] S3, the two pulses of accidental coincidence events have no correlation, so Q cc and Q d have the same distribution. Calculate the accidental coincidence spectrum Q cc according to the following formula.

[0011]

[0012] Where C t and C d are the total counts of the continuous plateau in the spectrum with and without coincidence conditions.

[0013] S4, eliminate Q cc according to the following formula, and calculate the spectrum Q temp .

[0014] Q temp = Q t - Q cc (2)

[0015] S5, for the γ source other than Na, Q 22 = Q temp = Q tc ; for Na, on the basis of the Q 22 calculated in step S4, calculate the annihilation coincidence spectrum Q ac according to formula (1), and the true coincidence spectrum is Q tc = Q temp - Q ac . The scattering angle of the true coincidence event forming Q tc is about 180°, and the energy deposition of the γ ray in the organic scintillation detector is basically fixed, so Q tc contains a characteristic peak at the Compton edge.

[0016] S6, fit Q tc , and calculate the full width at half maximum FWHM tc from the fitting parameters.

[0017] S7, since the γ source and the detector form a certain opening angle, the Compton scattering slightly smaller than 180° can also constitute a true coincidence event, which causes the energy deposition spectrum to have a certain broadening, and this factor needs to be eliminated when calculating the energy resolution. The energy deposition spectrum Q dep of the true coincidence event in the organic scintillation detector is calculated by Monte Carlo simulation.

[0018] S8, fit Q dep , and calculate the full width at half maximum FWHM dep from the fitting parameters.

[0019] S9, according to the synthesis rule of the Gaussian function, calculate the full width at half maximum FWHM reso of the energy resolution at the Compton edge according to the following formula.

[0020]

[0021] S10, fit the FWHM reso at different Compton edges with the following formula to determine the parameters a1, a2 and a3.

[0022]

[0023] where L is the light yield of the organic scintillator, and a1, a2 and a3 are undetermined parameters.

[0024] Further, the organic scintillation detector for detecting Compton scattering is a pulse type scintillation detector or a semiconductor detector, and the two detectors are placed opposite to each other, and the gamma source is arranged along the central axis of the two detectors and close to the organic scintillation detector.

[0025] Further, the continuous flat range of the annihilation coincidence spectrum is 240keV-350keV.

[0026] Further, the function for fitting the true coincidence spectrum and the energy deposition spectrum is a Gaussian distribution.

[0027] Further, the step S7 utilizes the Monte Carlo software to model and simulate according to the actual layout of the Compton coincidence measurement experiment, and records the energy deposition spectrum of the true coincidence event in the organic scintillation detector.

[0028] (Three) effective benefits

[0029] Compared with the prior art, the effective benefits of the present application are as follows:

[0030] 1. The method disclosed by the present application adopts gamma ray calibration of the energy resolution of the organic scintillation detector, through the Compton coincidence measurement experiment, and eliminates the influence of accidental coincidence events and annihilation coincidence events, forms a characteristic peak at the Compton edge, and quickly and accurately calculates the energy resolution according to the half width of the characteristic peak, without manual repeated adjustment of parameters.

[0031] 2. The method disclosed by the present application does not need to determine the equivalent electron energy expression of protons in advance, and the experiment time is shorter and the data processing is simpler. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 A Compton coincidence measurement experiment schematic diagram of the organic scintillation detector energy resolution calibration method described in the embodiment of the present application;

[0033] Figure 2 A total coincidence spectrum decomposition diagram of the organic scintillation detector energy resolution calibration method described in the embodiment of the present application;

[0034] Figure 3 A simulation spectrum and measured spectrum comparison diagram of the organic scintillation detector energy resolution calibration method described in the embodiment of the present application. DETAILED DESCRIPTION

[0035] The present application will be further described below in combination with the drawings and embodiments.

[0036] Embodiment 1

[0037] The organic scintillation detector energy resolution calibration method disclosed by the present application comprises the following steps:

[0038] S1, according to the attached Figure 1 The layout shown includes a detector, a gamma source, and a digitizer. A Compton coincidence measurement experiment is conducted using the digitizer's built-in coincidence function to obtain the total coincidence energy spectrum Q of the organic scintillation detector at the gamma source. t The γ sources were respectively employed 137 Cs、 60 Co、 152 Eu and 22 Na.

[0039] S2. Disable the digitizer's compatibility function, and sequentially measure the organic scintillation detector pair. 137 Cs、 60 Co、 152 Eu and 22 The non-matching energy spectra of the four γ sources of Na are Q. d ;

[0040] S3. True coincidence events have a filtering effect on pulse amplitude, and the true coincidence energy spectrum of the organic scintillation detector forms a characteristic peak. In accidental coincidence events, the output pulses of the two detectors are uncorrelated, and the accidental coincidence energy spectrum of the organic scintillation detector has the same spectral shape as the measured spectrum when there is no coincidence. Therefore, the continuous spectrum in the total coincidence energy spectrum is composed of the accidental coincidence energy spectrum. The accidental coincidence energy spectrum Q can be calculated according to equation (1). cc .

[0041] S4. Subtract the random coincidence energy spectrum from the total coincidence energy spectrum, and calculate the energy spectrum Q according to equation (2). temp .

[0042] S5, to 137 Cs、 60 Co and 152 Eu, Q temp That is, the true coincidence energy spectrum Q tc .right 22 Na, due to β + The positrons produced during decay release paired, counter-current 511 keV gamma rays upon annihilation, which triggers an annihilation coincidence event. Therefore, for 22 Based on step S4, the annihilation coincidence spectrum Q of the Naγ source is calculated according to equation (1). ac This truly matches the energy spectrum as Q. tc =Q temp -Q ac The results are attached. Figure 2 As shown.

[0043] S6, attached Figure 2 It can be seen that it truly matches the energy spectrum Q. tc The characteristic peaks exhibit a certain degree of broadening. A Gaussian distribution is used to fit the peaks, and the full width at half maximum (FWHM) is calculated based on the fitting parameters. tc .

[0044] S7. Because the two detectors in the Compton coincidence measurement experiment are at a certain solid angle to each other, the deposited energy of a true coincidence event is not a fixed value, but rather has a broadened spectral distribution. The Monte Carlo method is used to simulate and calculate the energy deposition spectrum Q of a true coincidence event in an organic scintillation detector. dep .

[0045] S8. Use Gaussian distribution on Q. dep Fitting, calculating the full width at half maximum (FWHM) based on the fitted parameters. dep .

[0046] S9, True coincidence energy spectrum full width at half maximum (FWHM) tc It is the energy resolution and the full width at half maximum (FWHM) of the energy deposition spectrum. dep The combined effect of these factors. The full width at half maximum (FWHM) of the energy resolution at the Compton edge is calculated according to equation (3). reso .

[0047] S10. Use equation (4) to fit the FWHM at different Compton edges. reso Determine the parameters a1, a2, and a3.

[0048] S11. Incorporate energy resolution into the Monte Carlo model and calculate the organic scintillation detector pair under no-matching conditions. 137 Cs、 60 Co and 152 The simulated spectrum of the Eu source is shown in the appendix. Figure 3 As shown, the simulated spectrum agrees well with the measured spectrum, verifying the accuracy of the method.

[0049] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A method for calibrating the energy resolution of an organic scintillation detector, the method comprising the following steps: S1. Obtain the total coincidence energy spectrum Q of the organic scintillation detector for the γ source through the Compton coincidence measurement experiment. t ; S2. Measure the non-coincidence energy spectrum Q of the organic scintillation detector for the γ source using a digitizer. d ; S3. Calculate the random coincidence energy spectrum Q according to the following formula. cc ; Where C t and C d These represent the total counts of the continuum in the energy spectrum under conditions of conformity and non-consistency. S4. Calculate the energy spectrum Q according to the following formula. temp ; Q temp =Q t -Q cc (2) S5, for the division 22 γ sources other than Na, Q temp That is, the true coincidence energy spectrum Q tc ;right 22 Na, based on step S4, calculates the annihilation coincidence spectrum Q according to equation (1). ac This truly matches the energy spectrum as Q. tc =Q temp -Q ac ; S6, Q tc Fitting, calculating the full width at half maximum (FWHM) based on the fitted parameters. tc ; S7. Simulation calculation of the energy deposition spectrum Q of true coincidence events in an organic scintillation detector. dep ; S8, Q dep Fitting, calculating the full width at half maximum (FWHM) based on the fitted parameters. dep ; S9. Calculate the full width at half maximum (FWHM) of the energy resolution at the Compton edge using the following formula. reso ; S10. Fit the FWHM at different Compton edges using the following formula. reso Determine the parameters a1, a2, and a3; Where L represents the light yield of the organic scintillator.

2. The method for calibrating the energy resolution of an organic scintillation detector according to claim 1, characterized in that: The detector used to detect Compton scattering from an organic scintillation detector should be a pulsed scintillation detector or a semiconductor detector. The two detectors should be placed facing each other, and the γ source should be positioned close to the organic scintillation detector along the central axis of the two detectors.

3. The method for calibrating the energy resolution of an organic scintillation detector according to claim 1, characterized in that: The calculated continuum plateau range of the annihilation coincidence energy spectrum is 240 keV to 350 keV.

4. The method for calibrating the energy resolution of an organic scintillation detector according to claim 1, characterized in that: The function used to fit the true coincidence energy spectrum and the energy deposition spectrum is a Gaussian distribution.

5. The method for calibrating the energy resolution of an organic scintillation detector according to claim 1, characterized in that: Step S7 uses Monte Carlo software to model the actual layout of the Compton coincidence measurement experiment and performs simulation calculations to record the energy deposition spectrum generated by the true coincidence event in the organic scintillation detector.

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