Waste barrel sgs efficiency calibration method based on monte carlo simulation and functional model

Abstract

The application discloses a waste barrel SGS efficiency calibration method based on Monte Carlo simulation and a function model, and can solve the technical problems of a small transmission measurement density range and low activity reconstruction accuracy of a high-density waste barrel in the process of SGS technology detection of a 200L waste barrel with a transmission source. The method comprises the following steps: firstly, a relationship curve between a medium linear attenuation coefficient and density in high-energy gamma-ray transmission is established, and the average density of each fault of the waste barrel is calculated through SGS transmission measurement; secondly, an efficiency function of single-energy gamma-ray is established based on a function model; thirdly, the fault efficiency of a current layer, a neighboring layer and a neighboring two layers of a detector under a single gamma energy is calculated; and finally, a fault efficiency calibration curve is established, the fault efficiency is calculated according to the outgoing gamma-ray energy in the barrel, an efficiency matrix is established, and efficiency calibration is realized. The method can effectively realize accurate efficiency calibration of the SGS technology detection of the high-density waste barrel with the transmission source.

Description

Technical Field

[0001] This invention relates to a method for SGS efficiency calibration of waste bins based on Monte Carlo simulation and function model, and belongs to the field of gamma-ray nondestructive testing of radioactive waste bins. Background Technology

[0002] Nuclear energy, as an important component of my country's new energy sources, plays a vital role in optimizing the existing energy structure, ensuring energy supply and security, and achieving "dual carbon" goals. Utilizing nuclear energy will help balance my country's energy shortages, optimize energy efficiency, and thus protect the country's ecological environment. Under the current global carbon dioxide emission reduction framework, developing nuclear power technology is more conducive to enhancing my country's comprehensive national strength globally. Vigorously developing nuclear power and increasing its proportion in my country's energy mix is ​​imperative on the path to achieving sustainable energy development.

[0003] Nuclear power has driven the development of the nuclear energy industry chain. The operation and production processes of nuclear fuel plants, nuclear power plants, and nuclear waste treatment plants generate a large amount of 200L drums of radioactive waste. During radioactive waste management, in accordance with national regulations and standards, it is necessary to measure parameters such as the gamma dose rate on the surface of the radioactive waste drums, surface contamination, types of nuclei within the drum, and the radioactivity within the drum. These waste drums contain... 238 U、 241 Am、 137 Cs、 60 For nuclides such as Co, segmented gamma scanning (SGS) is the most ideal detection technology compared to conventional sampling and analysis methods. SGS technology can directly obtain the type and radioactivity of nuclides without destructive sampling of waste containers, reducing the risk of radiation exposure to personnel. It is particularly suitable for the detection of nuclear waste containers with low and medium levels of radioactivity. SGS technology has been widely used in the detection of radioactive waste containers in nuclear power plants, nuclear waste treatment plants and other places.

[0004] Compared to SGS technology without a transmission source, SGS technology with a transmission source divides the waste bin longitudinally, assuming that the average medium density and nuclide activity differ within different sections, resulting in more accurate nuclide activity measurements within the bin. SGS technology with a transmission source measures waste bins in two stages: transmission measurement and emission measurement.

[0005] Phase 1: The waste bin is divided longitudinally into several sections. A transmission source and detector are used to perform transmission measurements on each section. The detector acquires the transmitted gamma-ray energy spectrum of each section, and the linear attenuation coefficient of gamma rays for each section is calculated. The linear attenuation coefficient is calculated as follows:

[0006] ;

[0007] In the formula, I0(E) is the intensity of the incident γ-ray with energy E, I i (E) represents the intensity of gamma rays after penetrating the fault, μ i (E) is the γ-ray attenuation coefficient of the i-th layer of the waste bin, and d is the diameter of the waste bin.

[0008] Phase Two: The transmission source is turned off, and the emission gamma spectrum of each fault is obtained using a detector. Combined with the efficiency matrix, a nuclide activity reconstruction equation is established to calculate the nuclide activity. The activity reconstruction equation is as follows:

[0009] ;

[0010] ;

[0011] In the formula, ε ij (E) represents the attenuation efficiency of the detector at layer i for the fault at layer j, N is the number of segments in the waste bin, and A j (E) represents the fault activity of the j-th layer, n i (E) represents the total peak count of the detector at the i-th layer position, and A(E) represents the activity of the entire waste bin.

[0012] As can be seen from the above activity reconstruction equation, the efficiency matrix is ​​the core part of the equation and directly affects the accuracy of activity reconstruction. The decay efficiency ε in the activity reconstruction equation... ij (E) is calculated based on the linear attenuation coefficient of the fault medium and the emitted γ energy.

[0013] In active SGS testing systems, commonly used 152 Eu sources are used as transmission sources because they have multiple discrete points of gamma-ray energy and a wide energy range (0.122 MeV~1.408 MeV), which can basically cover the gamma-ray energy emitted from within the barrel, especially for fault media with an average density of less than 1.0 g / cm³. 3 At that time, the transmitted gamma rays emitted by the transmission source can completely penetrate a 200L waste container, but when the average density of the tomographic medium is greater than 1.0 g / cm³... 3 Especially when the average density of the fault medium is greater than 1.8 g / cm³ 3 Subsequently, the 0.122 MeV, 0.244 MeV, and 0.344 MeV gamma rays emitted from the transmission source cannot penetrate the waste container. Therefore, the linear attenuation coefficients of these three energies within the container's tomographic medium cannot be obtained; only the linear attenuation coefficients for rays with energies ≥0.779 MeV can be obtained. When the energy of the gamma rays emitted from the container is in the range of 0.122 MeV to 0.779 MeV, attenuation correction cannot be achieved, and an accurate efficiency matrix cannot be obtained through extrapolation interpolation, affecting the accuracy of nuclide activity reconstruction.

[0014] In summary, SGS technology with a transmission source provides more accurate measurements of 200L waste bins than SGS technology without a transmission source. However, when using a transmission source to perform transmission measurements on high-density waste bins, the transmitted gamma rays in the medium and low energy range cannot penetrate the waste bin. Consequently, accurate attenuation correction of the gamma rays emitted from inside the bin cannot be performed in the medium and low energy range, ultimately affecting the accuracy of reconstructing the activity of nuclear elements within the bin. This limits the practicality of current SGS technology with a transmission source. Summary of the Invention

[0015] The purpose of this invention is to provide a waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model, which solves the problems of small transmission measurement density range and low accuracy of activity reconstruction for high-density waste bins in the current SGS technology with transmission sources during the inspection of 200L waste bins.

[0016] The technical solution adopted by this invention to solve its technical problem is: a waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model, using an SGS system for transmission measurement; the SGS system includes a transmission source, a waste bin, and a detector;

[0017] The transmission source and the detector are located on opposite sides of the waste bin; the transmission source is equipped with a transmission source collimator; the detector is equipped with a detector collimator and a detector shield.

[0018] Divide the waste bin into N sections;

[0019] It also includes the following steps:

[0020] S1. Establish the relationship curve between the linear attenuation coefficient and density of the fault medium during high-energy gamma-ray transmission.

[0021] ;

[0022] In the formula, ρ is the density of the fault medium, μ is the linear attenuation coefficient of the fault medium for γ-rays, and a1 and a2 are curve parameters.

[0023] By using high-energy gamma rays to penetrate media of different densities, the above relationship can be established and the curve parameters can be determined.

[0024] After establishing the relationship curve between the dielectric linear attenuation coefficient and density, using... 152 Eu performed SGS transmission measurements on N sections of the waste bin; calculated the linear attenuation coefficient of 1.408 MeV high-energy gamma rays in the N sections; substituted this attenuation coefficient into the above formula to calculate the average density of each section of the waste bin.

[0025] S2. Monte Carlo simulation calculation of the detector efficiency for monoenergetic gamma rays in the fault medium under different density conditions.

[0026] An SGS efficiency calculation model was established using the Monte Carlo program MCNP. Under the condition that each fault in the barrel has multiple densities or a single density, the detection efficiency of the detector for monoenergetic gamma rays in the fault was calculated using the MCNP model.

[0027] S3. Establish the efficiency function of monoenergetic gamma rays based on the function model;

[0028] Based on the collimator opening and detection area of ​​the SGS system detector; the fault currently detected by the detector is the current layer; the first fault above and below the current layer is the adjacent layer; the second fault above the current layer and the second fault below the current layer are the adjacent layers.

[0029] It can be seen that: when the height difference between the detector and the measured fault is greater than the height of two fault layers, the fault is not within the detection range; when the detector detects the current layer, only gamma photons emitted by nuclides in the current layer enter the detector; when the detector detects the adjacent layer, only gamma photons emitted by nuclides in the current layer and the adjacent layer enter the detector; when the detector detects the two adjacent layers, only gamma photons emitted by nuclides in the two adjacent layers enter the detector.

[0030] Therefore, the efficiency function models for the detector at the current layer, the next one layer, and the next two layers are as follows:

[0031] ① When the detector probes the current layer, the efficiency function model is:

[0032] ;

[0033] In the formula, ρ0 is the density of the current layer, b i (i=1,2,…,5) are function parameters;

[0034] ② When the detector probes a neighboring layer, the efficiency function model is:

[0035] ;

[0036] In the formula, ρ0 is the density of the current layer, ρ1 is the density of the adjacent layer, and b i (i=1,2,…,6) are function parameters;

[0037] ③ When the detector probes the two adjacent layers, the efficiency function model is:

[0038] ;

[0039] In the formula, ρ1 is the density of the adjacent layer, ρ2 is the density of the two adjacent layers, and b i(i=1,2,…,6) are function parameters;

[0040] Based on the efficiency function models of the current layer, the first neighboring layer, and the second neighboring layer, efficiency functions for different fault density distributions under a single γ energy were established.

[0041] By substituting the average density of each fault obtained by SGS transmission measurement in the waste bin in step S1 into the above efficiency function, the fault efficiency of the detector at a single γ energy for the current layer, the next one layer, and the next two layers can be calculated respectively.

[0042] When the height difference between the detector and the measured fault is greater than the height of two layers of sample, the fault is not within the detection range, so the detection efficiency is zero.

[0043] S4. Establish the efficiency calibration curve of the fault;

[0044] Different energies of gamma rays are selected; steps S2 and S3 are repeated once for each monoenergetic gamma ray, and the tomographic efficiency of the detector at multiple gamma energies for the current layer, the next adjacent layer, and the next adjacent layer is calculated; the detection efficiency of the detector at the detection position of layer i for the tomographic fault of layer j is calculated, and the calibration curve of the tomographic efficiency as a function of gamma energy is established as follows:

[0045] ;

[0046] In the formula, ε i,j Let E be the detection efficiency of the detector at the i-th layer detection location for the j-th layer fault, and let E be the γ-ray energy. i (i=1,2,…,5) are curve parameters;

[0047] Substitute the energy E0 of the emitted γ-rays from the barrel obtained from the SGS emission measurement into the above formula to calculate the detection efficiency of the detector at the i-th layer detection position for the γ-rays with energy E0 in the j-th layer fault.

[0048] S5. Establish the efficiency matrix;

[0049] Using the detector's detection efficiency at different detection positions i (1≤i≤N) for γ-rays with energy E0 in each fault j (1≤j≤N) at different detection positions in step S4, the fault efficiencies are arranged according to their corresponding positions (i, j) to establish an efficiency matrix ε. i,j [N×N].

[0050] Specifically, the waste bin is divided into N vertically equidistant layers, with each layer having the same height.

[0051] Specifically, the waste bin is filled with a medium; the medium is a mixture of aluminum silicate board, wood fiberboard, and polyvinyl chloride board.

[0052] Specifically, a gamma point source is installed inside the waste bin.

[0053] Beneficial effects of the present invention

[0054] The waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model described in this invention has the following advantages and beneficial effects compared with the prior art:

[0055] 1) By using high-energy gamma rays to penetrate the waste bin, the average density of the medium in different faults can be accurately obtained based on the established relationship curve between the medium attenuation coefficient and density. High-energy gamma rays have a strong ability to penetrate the waste bin, which improves the high-density detection range of the waste bin.

[0056] 2) Based on the different densities of each fault in the barrel and the different γ-ray energies emitted from the barrel, the efficiency database is calculated using the Monte Carlo simulation method. As the range of measurable waste barrel densities increases, the range of fault efficiency data is thus improved.

[0057] 3) An efficiency function for monoenergetic gamma rays under different density distributions of faults was established, which can quickly calculate the fault efficiency of the detector for monoenergetic gamma rays in the current layer, the next adjacent layer, and the next adjacent layer.

[0058] 4) An efficiency calibration curve of fault efficiency as a function of γ energy was established to accurately calibrate fault efficiency, establish an efficiency matrix, and thus improve the accuracy of reconstructing cell activity in barrels.

[0059] In summary, the waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model proposed in this invention can solve the problems of small transmission measurement density range and low accuracy of activity reconstruction for high-density waste bins in the current SGS technology with transmission source for detecting 200L waste bins. It can conveniently and effectively realize accurate efficiency calibration when using SGS technology with transmission source to detect high-density waste bins. Attached Figure Description

[0060] Figure 1 This is a schematic diagram of the segmented γ measurement principle in an embodiment of the present invention;

[0061] Figure 2 This is a schematic diagram of the medium filling inside the waste bin in an embodiment of the present invention;

[0062] Figure 3 This is a schematic diagram showing the distribution of nuclear elements in the waste bin according to an embodiment of the present invention;

[0063] The image shows:

[0064] 1-Transmission source, 2-Transmission source collimator, 3-Waste bin, 4-Detector, 5-Detector collimator, 6-Detector shield, 7-Point source, 8-Fault, A-Current layer, B-Adjacent layer 1, C-Adjacent layer 2, D-Aluminum silicate board, E-Wood fiberboard, F-Polyvinyl chloride board. Detailed Implementation

[0065] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0066] This invention provides a method for SGS efficiency calibration of waste bins based on Monte Carlo simulation and function models, using the SGS system for transmission measurement; such as Figures 1 to 3 As shown, the SGS system includes a transmission source 1, a waste bin 3, and a detector 4;

[0067] The transmission source 1 and the detector 4 are located on opposite sides of the waste bin 3; the transmission source 1 is equipped with a transmission source collimator 2; the detector 4 is equipped with a detector collimator 5 and a detector shield 6.

[0068] The waste bin is divided into N vertically equidistant layers, with each layer having the same height.

[0069] The waste bin 3 is filled with a medium; the medium is a mixture of aluminum silicate board D, wood fiberboard E, and polyvinyl chloride board F. A γ point source is installed inside the waste bin 3.

[0070] It also includes the following steps:

[0071] S1. Establish the relationship curve between the linear attenuation coefficient and density of the fault medium during high-energy gamma-ray transmission.

[0072] ;

[0073] In the formula, ρ is the density of the fault medium, μ is the linear attenuation coefficient of the fault medium for γ-rays, and a1 and a2 are curve parameters.

[0074] The curve parameters were determined by transmitting high-energy gamma rays through media of different densities.

[0075] use 152 Eu performed SGS transmission measurements on N sections of the waste bin; calculated the linear attenuation coefficient of 1.408 MeV high-energy gamma rays in the N sections; substituted this attenuation coefficient into the above formula to calculate the average density of each section of the waste bin.

[0076] S2. Monte Carlo simulation calculation of the detector efficiency for monoenergetic gamma rays in the fault medium under different density conditions.

[0077] An SGS efficiency calculation model was established using the Monte Carlo program MCNP. Under the condition that each fault in the bucket has multiple densities or a single density, the detection efficiency of the detector for monoenergetic gamma rays in the fault was calculated using the MCNP model.

[0078] In steps S1 and S2, high-energy gamma rays are used to pierce the waste bin. Based on the established relationship curve between the dielectric attenuation coefficient and density, the average density of the medium in different faults can be accurately obtained. High-energy gamma rays have a strong ability to penetrate the waste bin, which improves the high-density detection range of the waste bin. Based on the different densities of each fault in the bin and the different energies of gamma rays emitted from the bin, the efficiency database is calculated using the Monte Carlo simulation method. Since the range of measurable waste bin density is increased, the range of fault efficiency data is further improved.

[0079] S3. Establish the efficiency function of monoenergetic gamma rays based on the function model;

[0080] Based on the collimator opening and detection area of ​​the SGS system detector; the fault currently detected by the detector is the current layer; the first fault above and below the current layer is the adjacent layer; the second fault above the current layer and the second fault below the current layer are the adjacent layers.

[0081] The efficiency function models for the detector at the current layer, the next one layer, and the next two layers are as follows:

[0082] ① When the detector probes the current layer, the efficiency function model is:

[0083] ;

[0084] In the formula, ρ0 is the density of the current layer, b i (i=1,2,…,5) are function parameters;

[0085] ② When the detector probes a neighboring layer, the efficiency function model is:

[0086] ;

[0087] In the formula, ρ0 is the density of the current layer, ρ1 is the density of the adjacent layer, and b i (i=1,2,…,6) are function parameters;

[0088] ③ When the detector probes the two adjacent layers, the efficiency function model is:

[0089] ;

[0090] In the formula, ρ1 is the density of the adjacent layer, ρ2 is the density of the two adjacent layers, and b i (i=1,2,…,6) are function parameters;

[0091] Based on the efficiency function models of the current layer, the first neighboring layer, and the second neighboring layer, efficiency functions for different fault density distributions under a single γ energy were established.

[0092] By substituting the average density of each fault obtained by SGS transmission measurement of waste bin 3 in step S1 into the above efficiency function, the fault efficiency of the detector at a single γ energy for the current layer, the next one layer, and the next two layers can be calculated respectively.

[0093] When the height difference between the detector and the measured fault is greater than the height of two layers of sample, the fault is not within the detection range, so the detection efficiency is zero.

[0094] In step S3, an efficiency function for monoenergetic gamma rays under different density distributions of the fault is established, which can quickly realize the calculation of the fault efficiency of the detector for monoenergetic gamma rays in the current layer, the next adjacent layer, and the next adjacent layer.

[0095] S4. Establish the efficiency calibration curve of the fault;

[0096] Different energies of gamma rays are selected; steps S2 and S3 are repeated once for each monoenergetic gamma ray, and the tomographic efficiency of the detector at multiple gamma energies for the current layer, the next adjacent layer, and the next adjacent layer is calculated; the detection efficiency of the detector at the detection position of layer i for the tomographic fault of layer j is calculated, and the calibration curve of the tomographic efficiency as a function of gamma energy is established as follows:

[0097] ;

[0098] In the formula, ε i,j Let E be the detection efficiency of the detector at the i-th layer detection location for the j-th layer fault, and let E be the γ-ray energy. i (i=1,2,…,5) are curve parameters;

[0099] Substitute the energy E0 of the emitted γ-rays from the barrel obtained from the SGS emission measurement into the above formula to calculate the detection efficiency of the detector at the i-th layer detection position for the γ-rays with energy E0 in the j-th layer fault.

[0100] In step S4, an efficiency scale curve of fault efficiency as a function of γ energy was established to accurately calibrate the fault efficiency and establish an efficiency matrix, thereby improving the accuracy of reconstructing the activity of cinnamic cells in the barrel and conveniently and effectively achieving accurate efficiency calibration when using SGS technology with a transmission source to detect high-density waste barrels.

[0101] S5. Establish the efficiency matrix;

[0102] Using the detector's detection efficiency at different detection positions i (1≤i≤N) for γ-rays with energy E0 in each fault j (1≤j≤N) at different detection positions in step S4, the fault efficiencies are arranged according to their corresponding positions (i, j) to establish an efficiency matrix ε. i,j [N×N].

[0103] Example

[0104] Using a transmission source 152 Eu (activity 2.568 × 10⁻⁶) 8 The 1.408 MeV high-energy gamma rays emitted by Bq were transmitted through aluminum silicate (density 0.33 g / cm³). 3 Wood-based panels (density 0.64 g / cm³) 3 ), polyethylene (density 0.98 g / cm³) 3 ), polyvinyl chloride (density 1.84 g / cm³) 3 The calculated linear attenuation coefficients are 1.68 × 10⁻⁶. -2 cm -1 3.66×10 -2 cm -1 5.38×10 -2 cm -1 9.42×10 -2 cm -1 Linear fitting was performed to establish the relationship curve between the dielectric linear attenuation coefficient and density, as shown below:

[0105] ;

[0106] The existing SGS system mainly includes a transmission source, a transmission source collimator, a waste bin, a detector, a detector collimator, and a detector shield, such as... Figure 1 As shown. The collimator of the transmission source is 15 cm long, with a collimation thickness of 9.5 cm and a collimation aperture diameter of 1 cm. The collimator of the detector is 21.5 cm long, with a collimation thickness of 5 cm and a collimation aperture cross-section of 16 cm × 7 cm. The detector shield is 19.5 cm long and has a shielding thickness of 6 cm. The distance from the front face of the transmission source collimator to the waste bin is 57.5 cm, the diameter of the waste bin is 56 cm, and the distance from the waste bin to the front face of the detector collimator is 35.5 cm. Detector structural parameters: detector diameter 8.2 cm, length 15 cm, crystal diameter 6.86 cm, crystal length 7.55 cm.

[0107] In the simulation calculation of detection efficiency, the waste bin was divided into 9 longitudinal segments, each with a height of 9 cm. The cross-sectional density of the waste bin was chosen to be 0.2 g / cm³. 3 0.7 g / cm 31.2 g / cm 3 1.7 g / cm 3 2.2 g / cm 3 2.7 g / cm 3 3.2 g / cm 3 3.7g / cm 3 The gamma-ray energies were selected as 0.1 MeV, 0.5 MeV, 0.9 MeV, 1.3 MeV, 1.7 MeV, and 2.1 MeV. The elemental composition and mass fraction of the fault medium were: O (20%), C (10%), H (10%), Fe (10%), N (5%), S (5%), Si (5%), Na (5%), Mg (5%), Al (5%), K (5%), Ca (5%), Cu (5%), and Pb (5%). Based on these parameters, a physical model was established using the Monte Carlo program MCNP to calculate the detector efficiency for monoenergetic gamma rays in the fault under different density conditions.

[0108] Based on step S3 of the waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model of the present invention, an efficiency function model for monoenergetic gamma rays is established. The Matlab program is used to fit the detection efficiency data of the detector for monoenergetic gamma rays in the fault under different density conditions, obtain the function parameters, and establish the detection efficiency function of the detector for the current layer, the next layer, and the next second layer.

[0109] In SGS experimental analysis, the media inside the container was filled with a mixture of aluminum silicate board, wood fiberboard, and polyvinyl chloride board, and point sources were placed at different locations inside the container. 137 Cs (activity: 3.06 × 10⁻⁶) 5 Bq), 60 Co (activity: 1.24 × 10⁻⁶) 5 Bq), build a 200L waste bin, fill the waste bin as follows Figure 2 As shown. The point sources are distributed within the bucket as follows. Figure 3As shown, there are a total of 16 positions, where h is the height and r is the eccentric distance. The coordinates (h, r) of the point source position are (13.5cm, 0cm), (13.5cm, 6.5cm), (13.5cm, 14.5cm), (13.5cm, 17.5cm), (31.5cm, 0cm), (31.5cm, 6.5cm), (31.5cm, 14.5cm), (31.5cm, 17.5cm), (49.5cm, 0cm), (49.5cm, 6.5cm), (49.5cm, 14.5cm), (49.5cm, 17.5cm), (67.5cm, 0cm), (67.5cm, 6.5cm), (67.5cm, 14.5cm), and (67.5cm, 17.5cm).

[0110] First, SGS transmission measurement revealed that the linear attenuation coefficients of the nine sections inside the barrel at 1.408 MeV were 2.96 × 10⁻⁶. -2 cm -1 3.60×10 -2 cm -1 1.43×10 -2 cm -1 1.33×10 -2 cm -1 3.03×10 -2 cm -1 1.22×10 -2 cm -1 8.18×10 -2 cm -1 1.32×10 -2 cm -1 1.30×10 -2 cm -1 The densities of the nine faults calculated using the curve formula in step 1) are 0.54 g / cm³. 3 0.66 g / cm 3 0.23 g / cm 3 0.22 g / cm 3 0.55 g / cm 3 0.19 g / cm 3 1.57 g / cm 3 0.21 g / cm 3 0.21 g / cm 3 .

[0111] Secondly, these fault densities are substituted into the efficiency function in step S3 of the waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model described in this invention to calculate the detector's detection efficiency for different energy rays of 0.1 MeV, 0.5 MeV, 0.9 MeV, 1.3 MeV, 1.7 MeV, and 2.1 MeV in the current layer, the next adjacent layer, and the next adjacent layer.

[0112] Furthermore, in step S4 of the waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model described in this invention, the tomographic efficiency calibration curve is established by substituting the emitted γ-energy (0.662 MeV, 1.173 MeV, 1.332 MeV) from inside the bin into the calibration curve to calculate the detector's tomographic efficiency for the emitted γ-energy from inside the bin. The tomographic efficiencies are then arranged according to the detection position (i, j) to establish the efficiency matrix ε. i,j [9×9].

[0113] Finally, the SGS emission measurement projection data and efficiency matrix were substituted into the nuclide activity reconstruction equation set and solved using the maximum likelihood expectation maximization (MLEM) algorithm to obtain the single-point nuclide activity results at 16 locations within the bucket: (1) For 0.662 MeV ( 137 The activity of Cs is: 1.89 × 10 5 Bq~3.19×10 5 Bq, reconstruction error is: -38.25%~+4.45%; (2) for 1.173MeV ( 60 The activity of Co is: 1.06 × 10⁻⁶ 5 Bq ~ 1.51 × 10 5 Bq, reconstruction error is: -14.90%~+21.67%; (3) for 1.332MeV ( 60 The activity of Co is: 1.07 × 10⁻⁶ 5 Bq ~ 1.52 × 10 5 The reconstruction error is -14.28% to +22.50%. The reconstruction results and relative error analysis of single-point nuclide activity at different locations within the bin meet the accuracy requirements of SGS testing for waste bins, demonstrating the feasibility, accuracy, and reliability of this invention.

Claims

1. A waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model, using an SGS system for transmission measurement; the SGS system includes a transmission source (1), a waste bin (3), and a detector (4). The transmission source (1) and the detector (4) are located on opposite sides of the waste bin (3); the transmission source (1) is provided with a transmission source collimator (2); the detector (4) is provided with a detector collimator (5) and a detector shield (6). Its features are: Divide the waste bin into N sections; It also includes the following steps: S1. Establish the relationship curve between the linear attenuation coefficient and density of the fault medium during high-energy gamma-ray transmission. ； In the formula, ρ is the density of the fault medium, μ is the linear attenuation coefficient of the fault medium for γ-rays, and a1 and a2 are curve parameters. The curve parameters were determined by transmitting high-energy gamma rays through media of different densities. use 152 Eu performed SGS transmission measurements on N sections of the waste bin; calculated the linear attenuation coefficient of 1.408 MeV high-energy gamma rays in the N sections; substituted this attenuation coefficient into the above formula to calculate the average density of each section of the waste bin. S2. Monte Carlo simulation calculation of the detector efficiency for monoenergetic gamma rays in the fault medium under different density conditions. An SGS efficiency calculation model was established using the Monte Carlo program MCNP. Under the condition that each fault in the bucket has multiple densities or a single density, the detection efficiency of the detector for monoenergetic gamma rays in the fault was calculated using the MCNP model. S3. Establish the efficiency function of monoenergetic gamma rays based on the function model; Based on the collimator opening and detection area of ​​the SGS system detector; the fault currently detected by the detector is the current layer; the first fault above and below the current layer is the adjacent layer; the second fault above the current layer and the second fault below the current layer are the adjacent layers. The efficiency function models for the detector at the current layer, the next one layer, and the next two layers are as follows: ① When the detector probes the current layer, the efficiency function model is: ； In the formula, ρ0 is the density of the current layer, b i (i=1,2,…,5) are function parameters; ② When the detector probes a neighboring layer, the efficiency function model is: ； In the formula, ρ0 is the density of the current layer, ρ1 is the density of the adjacent layer, and b i (i=1,2,…,6) are function parameters; ③ When the detector probes the two adjacent layers, the efficiency function model is: ； In the formula, ρ1 is the density of the adjacent layer, ρ2 is the density of the two adjacent layers, and b i (i=1,2,…,6) are function parameters; Based on the efficiency function models of the current layer, the first neighboring layer, and the second neighboring layer, efficiency functions for different fault density distributions under a single γ energy were established. By substituting the average density of each fault obtained in step S1 by measuring the waste bin (3) through SGS transmission into the above efficiency function, the fault efficiency of the detector at a single γ energy for the current layer, the next one layer, and the next two layers can be calculated respectively. When the height difference between the detector and the measured fault is greater than the height of two layers of sample, the fault is not within the detection range, so the detection efficiency is zero. S4. Establish the efficiency calibration curve of the fault; Different energies of gamma rays are selected; steps S2 and S3 are repeated once for each monoenergetic gamma ray, and the tomographic efficiency of the detector at multiple gamma energies for the current layer, the next adjacent layer, and the next adjacent layer is calculated; the detection efficiency of the detector at the detection position of layer i for the tomographic fault of layer j is calculated, and the calibration curve of the tomographic efficiency as a function of gamma energy is established as follows: ； In the formula, ε i,j Let E be the detection efficiency of the detector at the i-th layer detection location for the j-th layer fault, and let E be the γ-ray energy. i (i=1,2,…,5) are curve parameters; Substitute the energy E0 of the emitted gamma rays from the barrel obtained from the SGS emission measurement into the above formula to calculate the detection efficiency of the detector at the i-th layer detection position for the gamma rays with energy E0 in the j-th layer fault. S5. Establish the efficiency matrix; Using the detector's detection efficiency at different detection positions i (1≤i≤N) for γ-rays with energy E0 in each fault j (1≤j≤N) at different detection positions in step S4, the fault efficiencies are arranged according to their corresponding positions (i, j) to establish an efficiency matrix ε. i,j [N×N].

2. The waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model as described in claim 1, characterized in that: The waste bin is divided into N vertically equidistant layers, with each layer having the same height.

3. The waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model as described in claim 1, characterized in that: The waste bin (3) is filled with a medium; the medium is a mixture of aluminum silicate board (D), wood fiberboard (E), and polyvinyl chloride board (F).

4. The waste bin SGS efficiency calibration method based on Monte Carlo simulation and function model as described in claim 1, characterized in that: A γ point source is installed inside the waste bin (3).

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