Method for calibrating energy resolution of organic scintillation detector

Abstract

Aiming at the problem that an organic scintillation detector lacks a full-energy peak to a gamma ray response energy spectrum, so that the energy resolution is difficult to scale, the Compton coincidence measurement experiment is taken as a basis, the influence of an accidental coincidence event and an annihilation coincidence event is eliminated, and a quasi-Gaussian peak is formed at the Compton edge. An energy deposition spectrum of the true coincidence event is obtained through Monte Carlo simulation, and then the energy resolution at the Compton edge is calculated. Then, undetermined parameters of an expression are calculated by means of fitting of the energy resolutions, and the scale of the energy resolutions is achieved. By adopting the method, energy resolution calibration is carried out on the organic scintillation detector, and after the calibration result is added into Monte Carlo simulation, the simulation spectrum of the obtained detector to gamma rays is well matched with the actual measurement spectrum, and the accuracy of the method is verified. The organic scintillation detector energy resolution calibration method disclosed by the invention is simple to operate, good in accuracy, high in universality and suitable for energy resolution calibration of various plastic scintillation detectors, liquid scintillation detectors, organic crystal detectors and the like.

Description

Technical Field

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

[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 γ-ray detection and energy spectrum measurement. Organic scintillators include plastic scintillators, liquid scintillators, and organic crystal scintillators. When using an organic scintillation detector to measure neutron and γ-ray energy spectra, de-spectroscopy calculations need to be carried out based on the measured spectrum and the response matrix. The response matrix is often established through Monte Carlo simulation. Affected by factors such as the fluctuation of the number of scintillation photons, the transport and absorption of scintillation photons, photoelectric random conversion, the fluctuation of the electron multiplication coefficient, and electronic noise, the energy spectrum generated by a certain determined 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 the fluctuation of the number of scintillation photons 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 detectors such as NaI, the full-energy peak of γ-rays is generally used to calibrate their energy resolution. Organic scintillation detectors are sensitive to both γ-rays and neutrons, and the energy resolution can be calibrated using these two types of radiation respectively. The light yield of organic scintillators for electrons above 125 keV has a linear relationship with the electron energy, while the light yield for heavy charged particles such as protons is non-linear. Therefore, the equivalent electron energy is commonly used to represent the light yield of organic scintillators. The average atomic number of organic scintillators is low, and the probability of the occurrence of the photoelectric effect of γ-rays in them is very low, and Compton scattering mainly occurs. This results in the energy spectrum of the organic scintillation detector for γ-rays mainly consisting of a Compton plateau, and it is difficult to identify the full-energy peak. In the past, when calibrating the energy resolution of an organic scintillation detector using γ-rays, it was necessary to repeatedly adjust the expression parameters to make the simulated spectra of γ-rays with multiple energies coincide with the measured spectra to determine the parameter values. However, there are 3 mutually influencing undetermined parameters in the energy resolution expression, and it is very difficult to manually adjust and determine the parameter values. When calibrating the energy resolution of an organic scintillation detector using neutrons, it is necessary to measure the broad-spectrum neutrons using the time-of-flight method, obtain the measured spectra of the detector for quasi-monoenergetic neutrons in multiple energy regions, and perform a first-order difference on the measured spectra, and determine the energy resolution based on the characteristic peaks of the difference spectra. However, this method is relatively complex, time-consuming, and it is necessary to determine the equivalent electron energy expression of protons in advance, with great difficulty and workload. It can be seen that the problem of calibrating the energy resolution of organic scintillation detectors has not been well solved. Summary of the Invention

[0004] (I) Object of the Invention

[0005] The objective of the present invention is to design a method for calibrating the energy resolution of an organic scintillation detector, aiming to quickly and accurately calibrate the energy resolution of the organic scintillation detector so that the detector can be used for neutron and gamma-ray energy spectrum measurements.

[0006] (II) Technical Solution

[0007] To achieve the above objective and solve the above technical problems, the present invention provides a method for calibrating the energy resolution of an organic scintillation detector. This method can generate characteristic peaks at the Compton edge, calculate the undetermined parameters of the energy resolution expression based on the full width at half maximum of multiple characteristic peaks, and eliminates the need for manual parameter adjustment and neutron energy spectrum data processing. The key to this method is to eliminate the interference factors of the Compton coincidence measurement energy spectrum and the characteristic peak energy spectrum to ensure the accuracy of the undetermined parameters. The method includes the following steps:

[0008] S1. Place another gamma detector (such as a BaF 2 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 energy spectrum Q of the organic scintillation detector for the gamma source through a Compton coincidence measurement experiment. t . Q t includes the true coincidence energy spectrum Q tc and the accidental coincidence energy spectrum Q cc (for 22 Na, it also includes the annihilation coincidence energy spectrum Q ac ). The former has a characteristic distribution, while the latter is a continuous spectrum composed of the Compton plateau. Existing research has used the peak position of Q t to complete the energy calibration of the organic scintillation detector. However, to carry out the energy resolution calibration, a key step is to eliminate Q cc . This step is completed by the following S2 - S4.

[0009] S2. Use a digitizer to measure the non-coincidence energy spectrum Q d of the organic scintillation detector for the gamma source.

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

[0011]

[0012] where C t and C d are respectively the total counts of the continuous plateau in the energy spectrum with and without the coincidence condition.

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

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

[0015] S5. For γ - sources other than 22 Na, Q temp is the true - coincidence spectrum Q tc ; for 22 Na, based on the S4 step, calculate the annihilation - coincidence spectrum Q ac according to Equation (1), and the true - coincidence spectrum is Q tc = Q temp -Q ac . The scattering angle of the true - coincidence events forming Q tc is about 180°, and the energy deposition of γ - rays in the organic scintillation detector is basically fixed. Therefore, Q tc contains characteristic peaks at the Compton edge.

[0016] S6. Fit Q tc and calculate the full - width at half - maximum FWHM according to the fitting parameters tc .

[0017] S7. Since there is a certain opening angle between the γ - source and the detector, Compton scattering with an angle slightly less than 180° can also form true - coincidence events, which leads to a certain broadening of the energy - deposition spectrum. When calculating the energy resolution, the influence of this factor needs to be eliminated. Use Monte - Carlo simulation to calculate the energy - deposition spectrum Q dep of true - coincidence events in the organic scintillation detector.

[0018] S8. Fit Q dep and calculate the full - width at half - maximum FWHM according to the fitting parameters dep .

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

[0020]

[0021] S10. Fit the FWHM at different Compton edges with the following formula reso to determine the parameters a 1 , a 2 and a 3 .

[0022]

[0023] where L is the light yield of the organic scintillator, a 1 , a 2 and a 3are parameters to be determined.

[0024] Furthermore, for detecting Compton scattering of an organic scintillation detector, a pulse-type scintillation detector or a semiconductor detector should be used. The two detectors are placed facing each other, and the γ source is arranged close to the organic scintillation detector along the central axis direction of the two detectors.

[0025] Furthermore, the continuous plateau range for calculating the annihilation coincidence spectrum is 240 keV to 350 keV.

[0026] Furthermore, the function used to fit the true coincidence spectrum and the energy deposition spectrum is a Gaussian distribution.

[0027] Furthermore, in step S7, the Monte Carlo software is used to model according to the actual layout of the Compton coincidence measurement experiment, and simulation calculations are performed to record the energy deposition spectrum generated by true coincidence events in the organic scintillation detector.

[0028] (III) Effective benefits

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

[0030] 1. The method proposed by the present invention uses γ rays to calibrate the energy resolution of an organic scintillation detector. Through the Compton coincidence measurement experiment, the influence of accidental coincidence events and annihilation coincidence events is eliminated, a characteristic peak is formed at the Compton edge, and the energy resolution is calculated quickly and accurately according to the full width at half maximum of the characteristic peak, without manually adjusting parameters repeatedly.

[0031] 2. The method proposed by the present invention does not need to determine the equivalent electron energy expression of protons in advance, has a shorter experimental time, and simpler data processing. Description of the drawings

[0032] Figure 1 is a schematic diagram of a Compton coincidence measurement experiment for a method for calibrating the energy resolution of an organic scintillation detector according to an embodiment of the present invention;

[0033] Figure 2 is a total coincidence spectrum decomposition diagram of a method for calibrating the energy resolution of an organic scintillation detector according to an embodiment of the present invention;

[0034] Figure 3 is a comparison diagram of a simulated spectrum and a measured spectrum of a method for calibrating the energy resolution of an organic scintillation detector according to an embodiment of the present invention. Detailed implementation manners

[0035] The present invention will be further described below with reference to the drawings and embodiments.

[0036] Embodiment 1

[0037] A method for calibrating the energy resolution of an organic scintillation detector disclosed by the present invention, the method comprising the following steps:

[0038] S1. Arrange equipment such as detectors, γ sources, and digitizers according to the layout shown in the appendix, and use the coincidence function of the digitizer itself to carry out Compton coincidence measurement experiments to obtain the total coincidence energy spectrum Q of the organic scintillation detector for the γ source. The γ sources are respectively Figure 1 Cs, t . 137 Co, 60 Eu, and 152 Na. 22

[0039] S2. Turn off the coincidence function of the digitizer and measure the non-coincidence energy spectra Q of the organic scintillation detector for a total of 4 γ sources, namely 137 Cs, 60 Co, 152 Eu, and 22 Na, in sequence; d

[0040] S3. True coincidence events have a screening effect on the 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 not correlated, and the accidental coincidence energy spectrum of the organic scintillation detector has the same spectral shape as the measured spectrum without 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 accidental coincidence energy spectrum from the total coincidence energy spectrum and calculate the energy spectrum Q according to Equation (2). temp

[0042] S5. For 137 Cs, 60 Co, and 152 Eu, Q temp is the true coincidence energy spectrum Q tc . For 22 Na, since the positrons generated by β + decay release pairs of back-to-back 511 keV γ rays during annihilation, this will cause annihilation coincidence events. Therefore, for the 22 Na γ source, on the basis of Step S4, calculate the annihilation coincidence energy spectrum Q ac , and the true coincidence energy spectrum is Q tc = Q temp - Q ac . The results are as shown in the appendix Figure 2 .

[0043] S6. As can be seen from the appendix Figure 2 , the true coincidence energy spectrum Q tc ​​​​The characteristic peak has a certain broadening. Use Gaussian distribution to fit it and calculate the full width at half maximum (FWHM) according to the fitting parameters. tc .

[0044] S7. Since the two detectors in the Compton coincidence measurement experiment form a certain solid angle with each other, the deposited energy of the true coincidence events is not a certain value, but a spectral distribution with a certain broadening. Use the Monte Carlo method to simulate and calculate the energy deposition spectrum Q of the true coincidence events in the organic scintillation detector. dep .

[0045] S8. Use Gaussian distribution to fit Q dep and calculate the full width at half maximum (FWHM) according to the fitting parameters. dep .

[0046] S9. The full width at half maximum (FWHM) of the true coincidence energy spectrum tc is the combined result of the energy resolution and the full width at half maximum (FWHM) of the energy deposition spectrum dep . Calculate the full width at half maximum (FWHM) of the energy resolution at the Compton edge according to Equation (3). reso .

[0047] S10. Fit the FWHM at different Compton edges with Equation (4) reso to determine the parameters a 1 , a 2 and a 3 .

[0048] S11. Incorporate the energy resolution into the Monte Carlo model and calculate the simulated spectra of the organic scintillation detector for 137 Cs, 60 Co and 152 Eu sources without coincidence conditions, and compare them with the measured spectra as shown in the appendix Figure 3 . The simulated spectra are in good agreement with the measured spectra, verifying the accuracy of the method.

[0049] The above content is a further detailed description of the present invention in combination with specific implementation manners. It cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention belongs, without departing from the concept of the present invention, several simple deductions or substitutions can still be made, and all should be regarded as belonging to the protection scope 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 to the γ source through the Compton coincidence measurement experiment t ; S2. Use a digitizer to measure the coincidence-free energy spectrum Q of the organic scintillation detector to the γ source d ; S3, calculate the accidental energy spectrum Q according to the following formula cc ; Among them C t and C d are the sum of counts of continuous plateaus in the energy spectrum with and without the conditions met; S4. Calculate the energy spectrum Q according to the following formula temp ; Q temp =Q t -Q cc (2) S5. Elimination 22 γ sources other than Na, Q temp That is, the true energy spectrum Q tc ;right 22 Na, based on step S4, calculate the annihilation spectrum Q according to formula (1) ac , the true energy spectrum is Q tc =Q temp -Q ac ; S6, Q tc Fitting, calculate the half-height width FWHM based on the fitting parameters tc ; S7. Simulation and calculation of energy deposition spectrum Q of true coincidence events in organic scintillation detectors dep ; S8, Q dep Fitting, calculate the half-height width FWHM based on the fitting parameters dep ; S9. Calculate the FWHM of the energy resolution at the Compton edge according to the following formula reso ; S10, use the following formula to fit the FWHM at different Compton edges reso , determine the parameters a1, a2 and a3; Where L is the light output 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 of organic scintillation detectors should be a pulsed scintillation detector or a semiconductor detector. The two detectors are placed opposite each other, and the gamma source is arranged 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 annihilation spectrum corresponds to a continuous plateau in the range of 240keV to 350keV.

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 energy deposition spectrum is 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 build a model according to 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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