Tomographic gamma scanning voxel efficiency scaling method based on spatial properties of a radioactive source

Abstract

The application discloses a tomography gamma scanning voxel efficiency calibration method based on radioactive source spatial properties, which can reduce workload, improve work efficiency and has good universality. The tomography gamma scanning voxel efficiency calibration method based on radioactive source spatial properties comprises the following steps: S1, using Monte Carlo method related software to establish a TGS system 1:1 simulation model; S2, determining the radioactive source placement position for spatial efficiency measurement; S3, spatial efficiency calculation; S4, spatial efficiency correction coefficient calculation; S5, detection efficiency factor calculation; and S6, detection efficiency calculation. The tomography gamma scanning voxel efficiency calibration method based on radioactive source spatial properties can share a set of spatial efficiencies epsilon for all energy gamma rays, can reduce workload, improve calibration efficiency, guarantee data reliability, has good universality and migration.

Description

Technical Field

[0001] This invention relates to the field of nuclear waste chromatography gamma-ray scanning measurement, and more specifically to a method for calibration of voxel efficiency in chromatography gamma-ray scanning based on the spatial properties of radioactive sources. Background Technology

[0002] As is well known, regulations concerning the management and disposal of nuclear waste require the use of standard containers to encapsulate radioactive waste. Before the temporary storage, transportation, and disposal of radioactive waste, the composition, content, and distribution of radionuclides within the waste containers must be accurately measured.

[0003] Given the radioactivity of waste bins, non-destructive testing methods are often used for measurement. Tomographic Gamma Scanning (TGS) is one of the mature non-destructive testing methods currently available. It uses a gamma detector to scan the waste bin from multiple angles and locations, reconstructing images of the attenuation coefficient distribution (transmission image) and the radionuclide activity distribution (emission image) within the bin. This allows for accurate analysis of the types, amounts, and spatial distribution of radionuclides and their activity in a sample without altering its physical or chemical morphology.

[0004] The TGS inspection process consists of two parts: transmission scanning and emission scanning. Before transmission scanning, the waste bin is vertically divided into several layers, and each layer is further divided into voxel blocks using various methods such as meshing or polar coordinate partitioning. During scanning, an external transmission source emits a narrow beam of gamma rays to scan the waste bin. The height of the transmission source and detector is adjusted, and the waste bin is rotated and moved horizontally to complete the transmission scan. The attenuation law of the narrow beam of gamma rays in a homogeneous medium is as follows:

[0005] I = I0e -μx (1)

[0006] In the formula, I represents the intensity of the gamma rays after attenuation by the medium, I0 represents the initial intensity of the emitted gamma rays, μ represents the attenuation coefficient of the medium for the gamma rays, and x represents the thickness of the medium layer through which the gamma rays pass. When gamma rays pass through a non-homogeneous medium, the medium is sliced ​​into thin slices of thickness Δx. When Δx is small, the slice can be considered as a homogeneous medium, and the attenuation law of the gamma ray intensity can be expressed as:

[0007]

[0008] In the formula, μ n Let be the attenuation coefficient of the medium in the nth slice, and N be the number of medium slices. During TGS scanning, each waste bin is divided into multiple voxel blocks, and after multiple gamma-ray scans at multiple angles and positions, the transmission equation in each scan can be described as:

[0009]

[0010] In the formula, I i (E) represents the intensity of the γ-rays after attenuation by the medium during the i-th scan, X i,j Let X be the track length of the γ-ray traveling through voxel j during the i-th scan. If the ray does not pass through voxel j, then X is denoted as X. i,j =0. J is the number of voxel blocks in the waste bin, μ j (E) represents the attenuation coefficient of the j-th voxel for a gamma ray with energy E. By using projection data from multiple transmission scans and solving the transmission equation, the linear attenuation coefficient of the medium within each voxel can be obtained, thus reconstructing the transmission image of gamma rays with different energies.

[0011] During emission scanning, the external transmission source was turned off, and the energy spectrum of spontaneously emitted gamma rays inside the container was measured, consistent with the transmission scanning sequence. The energy spectrum was analyzed to obtain the types of radionuclides and characteristic gamma ray energies within the container, and the characteristic gamma ray counts were obtained by spectrum decomposition. The linear attenuation coefficient distribution of the characteristic gamma rays was obtained by fitting the transmission image, and the emission image could be solved by combining the characteristic gamma ray counts. Spontaneously emitted gamma rays in the container also obey the attenuation law of gamma rays in the medium. During emission image reconstruction, it was assumed that the radioactivity was concentrated at the geometric center of each voxel. At the i-th measurement, the activity of radioactivity in the j-th voxel in the waste container can be expressed as…

[0012]

[0013] In the formula A j (E) represents the radioactivity in the j-th voxel during the i-th measurement, N i,j (E) represents the number of γ-photons with energy E emitted by the j-th voxel detected by the detector per unit time, α(E) represents the characteristic γ-ray branching ratio of the radionuclide emitting energy E, and ρ i,j (E) represents the detector efficiency for a γ photon with energy E emitted from voxel j, η i,j (E) is the attenuation coefficient correction factor for a γ photon with energy E emitted from voxel j before it reaches the detector, and can be described as:

[0014]

[0015] In the formula, K represents the number of elements in the waste bin, μ k (E) is the attenuation coefficient of the k-th voxel for this ray, x i,k It is the track length of the γ-ray traveling through voxel k during the i-th emission measurement.

[0016] According to equation (5), the total number of γ photons with energy E emitted by the detector in voxel J during the i-th emission measurement is:

[0017]

[0018] Let h i (E)=N i (E) / α(E) and w i,j (E)=ρ i,j η i,j Then we can obtain the matrix equation.

[0019] A·W=H (7)

[0020] In the formula, A is the radioactivity matrix of voxels J, A=[A1(E),A2(E),…,A j [E], where w is a J×I order attenuation correction matrix. H = [h1(E),h2(E),…,h i (E)]. By combining emission measurement data and transmission images to solve for matrix A, a distribution image of radionuclides in the waste bin can be established. To solve for matrix A, it is necessary to first construct matrix W, where W and η... i,j (E) and ρ i,j (E) related, η i,j (E) can be obtained from the track of the voxel-emitted gamma rays reaching the detector and the transmission image.

[0021] And ρ i,j (E) is related to the relative position of voxels, the detector's resolution for gamma rays, the count rate, and the collimator size. It is typically obtained through experimental efficiency calibration, using standard radiation sources of different energies placed at different locations, and the detector's efficiency at those locations is calibrated by measurement. This method is labor-intensive and only applicable to a single instrument; recalibration is required when any condition changes. Currently, passive efficiency calibration can also be performed entirely using the Monte Carlo method. This method uses Monte Carlo software to build a TGS model for simulation calibration, which is faster than experimental calibration. However, building the model requires continuous adjustment of the structural dimensions to approximate experimental results before it can be used, and it requires fitting experimental data from multiple locations and various energies of gamma rays, also requiring numerous experiments. Summary of the Invention

[0022] The technical problem to be solved by the present invention is to provide a tomographic gamma-ray scanning voxel efficiency calibration method based on the spatial properties of radioactive sources that can reduce workload, improve work efficiency, and has good versatility compared to the conventional TGS efficiency calibration method.

[0023] The technical solution adopted by this invention to solve its technical problem is: a tomographic gamma-ray scanning voxel efficiency calibration method based on the spatial properties of a radioactive source, comprising the following steps:

[0024] S1. Use Monte Carlo method-related software to establish a 1:1 simulation model of the TGS system;

[0025] S2. Determine the location of the radioactive source for space efficiency measurement;

[0026] Based on the voxel division method within the waste bin during measurement, the coordinates of the voxel centers are determined.

[0027] S3, Space efficiency calculation;

[0028] The ratio of the number of characteristic γ-photons emitted at each measurement position detected by the detector at different detection positions to the set activity is calculated to obtain the proportion of γ-photons emitted at different measurement positions that reach the detector, i.e., the spatial efficiency ε, and a spatial efficiency database is formed; or, for a selected voxel division method, the spatial efficiency of γ-photons emitted at the center position of different voxels is obtained.

[0029] S4. Calculation of space efficiency correction coefficient;

[0030] The external transmission source is configured with gamma-ray energies matching the characteristic gamma-ray energies of all radionuclides. The attenuation coefficients of gamma rays with different energies in air are simulated and calculated, and a database is established. If simulation experiments are conducted targeting the voxel center and specific energy gamma rays, the obtained spatial efficiency does not require correction; let τ be the denoted τ. j (E) = 1;

[0031] S5. Calculation of detection efficiency factor;

[0032] In the TGS physical system and simulation model, multiple locations are selected in front of the detector to place standard gamma-ray sources. The types of standard sources include all nuclides that may appear in the waste bins or nuclides of interest. Simulation and physical measurements are performed, and the gamma-ray photon counts detected in the simulation and physical experiments are compared to obtain the detector's ability to detect gamma rays of different energies entering the detector, i.e., the detection efficiency factor f(E), and a detection efficiency factor library is established. Alternatively, without establishing a detection efficiency factor library, the radionuclides present in the waste bins are determined through emission measurements, and targeted simulations and experiments are performed to obtain only the detector's detection efficiency factor f(E) for radionuclides in the waste bins.

[0033] S6. Detection efficiency calculation;

[0034] The spatial efficiency ε of the detector for the voxel center coordinates is obtained by interpolation or by fitting the data in the spatial efficiency database in step S3.

[0035] Based on the types of radionuclides and their characteristic gamma-ray energies in the waste bin obtained from emission measurements, the space efficiency correction coefficient τ(E) determined in step S4 and the detection efficiency factor f(E) determined in step S5 are then used.

[0036] The detector efficiency for radionuclides at different voxel centers in the waste bin was calculated as follows:

[0037] ρ i,j =ε i,j τ i,j (E)f(E)

[0038] In the formula ρ i,j ε represents the detector efficiency for detecting the radionuclide at the center of voxel j during the i-th emission scan. i,j (E) represents the spatial efficiency of the detector for the γ-ray with energy E at the center of voxel j, τ i,j (E) is the spatial efficiency correction coefficient for γ-rays with energy E at the center of voxel j, and f(E) is the detection efficiency factor of the detector for γ-rays with energy E. The detection efficiency of the detector for γ-rays of different energies in each voxel can be obtained by calculating according to the scanning order and voxel number. The voxel efficiency calibration is completed.

[0039] Furthermore, in step S1, the Monte Carlo method software used is Geant4 or MCNP5; the simulation model is consistent with the size of the TGS system without any adjustments; the detector size in the simulation model is consistent with the TGS detector, the detector can be made of any material, and the detection efficiency is set to 100%.

[0040] Furthermore, determining the placement location of the radiation source for space efficiency measurement in step S2 includes the following steps:

[0041] m×n×k measurement positions are uniformly set within the space of the covered waste bin; an isotropic monoenergetic γ point source is set at each measurement position in sequence, the energy of the γ rays can be set to any energy, the radioactivity can be set to any suitable value, and the environment is set to vacuum for simulation; or, as needed, the center position of each voxel can be determined by a specific voxel division method; m, n, and k are integers.

[0042] Furthermore, in step S2, the center position of each voxel is determined by a specific voxel division method; isotropic monoenergetic γ point sources are sequentially set at the center positions of the voxels, the γ-ray energy is consistent with the characteristic γ-ray energy detected in the emission measurement, the radioactivity can be set to any suitable value, the environment is set to air, and simulation is performed.

[0043] The voxel division method includes two methods: in a rectangular coordinate system, voxels are divided into cube shapes; in a polar coordinate system, voxels are divided into sector shapes.

[0044] Each division method uses the following steps to determine the location of the voxel center: first, the bucket is divided longitudinally into I faults, and each fault is further divided into several voxels, totaling J voxels. The voxel center is the geometric center of each voxel.

[0045] Furthermore, in step S4, the spatial efficiency correction coefficients for γ-rays with different energy characteristics at different measurement locations are analytically obtained using the following formula:

[0046] τ j (E)=μ air (E)·x j

[0047] In the formula τ j (E) is the spatial efficiency correction factor for γ-rays with energy E at the j-th position, x j Let be the distance from the point source at position j where the γ-rays emitted reach the detector.

[0048] The beneficial effects of this invention are as follows: The tomographic gamma-ray scanning voxel efficiency calibration method based on the spatial properties of radioactive sources described in this invention has the following advantages:

[0049] 1. All energy gamma rays share a single set of space efficiency ε, which can reduce the workload; the space efficiency ε at different locations is only related to the collimator size and detector area. In a vacuum environment, an isotropic monoenergetic point source of any energy can be used for simulation. After correction by the space efficiency correction coefficient, the simulation data can be applied to all energy gamma rays.

[0050] 2. The detector only needs to conduct a small number of experiments on gamma rays of different energies, which improves calibration efficiency while ensuring data reliability;

[0051] Different detectors have different resolution capabilities for gamma rays. Using a small number of standard source-calibrated detectors to detect gamma rays greatly improves the efficiency factor compared to calibrating the efficiency at all positions.

[0052] 3. The universality of efficiency scale;

[0053] The tomographic gamma-ray scanning voxel efficiency calibration method based on the spatial properties of the radioactive source described in this invention is not affected by the voxel division method. It only requires the actual voxel center coordinates and gamma-ray energy to be substituted into the simulation experiment for simulation or to use the spatial efficiency ε database to fit the spatial efficiency of radioactivity at that location, correct it with the spatial efficiency correction coefficient, and then find the detector's detection efficiency factor for gamma rays of that energy from the detection efficiency factor database to obtain the detection efficiency at that location. The actual calculation is very simple and has good versatility.

[0054] 4. Portability: When different detectors need to be replaced, only a small amount of calibration of the gamma rays with the new detector is needed to obtain the detection efficiency factor. The spatial efficiency ε does not change. Compared with the passive efficiency calibration method, which requires modification of the model and re-simulation of the calculation function, it has higher portability. Attached Figure Description

[0055] Figure 1 The flowchart illustrates the steps of the tomographic gamma-ray scanning voxel efficiency calibration method based on the spatial properties of a radioactive source provided in Embodiment 1 of the present invention.

[0056] Figure 2 This is a schematic diagram of the TGS system provided in Embodiment 1 of the present invention;

[0057] Figure 3 This is a schematic diagram of the measurement location of the spatial efficiency database provided in Embodiment 1 of the present invention;

[0058] Figure 4 This is a schematic diagram of the polar coordinate voxel division method and targeted measurement position provided in Embodiment 1 of the present invention;

[0059] Figure 5 This is a schematic diagram of the rectangular grid voxel division method and targeted measurement positions provided in Embodiment 1 of the present invention;

[0060] Figure 6 This is a schematic diagram of the detection efficiency factor measurement location provided in Embodiment 1 of the present invention;

[0061] The diagram shows: 10-vertical mechanical transmission device, 20-transmission source, 30-detector system, 40-horizontal and rotary mechanical transmission device, 50-waste bin, 60-control and analysis system, 1-detector collimator, 2-detector; I is the number of voxel division layers; J is the number of voxels per layer; k, m, n are the number of measurement positions; 3-detection efficiency factor measurement position. Detailed Implementation

[0062] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0063] Different radionuclides are known to spontaneously emit characteristic gamma rays of varying energies, with the emitted gamma rays being isotropic. The proportion of characteristic gamma rays emitted from a radionuclide at different locations reaches the detector after passing through the collimator varies, resulting in differences in the detection efficiency of different detectors. By combining the radionuclide's location, collimator size, and the detector's efficiency in detecting incident gamma rays, the detector's detection efficiency for radionuclides at different spatial locations can be obtained.

[0064] Based on this, the present invention provides a tomographic gamma-ray scanning voxel efficiency calibration method based on the spatial properties of the radioactive source, including simulation calculation of spatial efficiency ε, simulation calculation of spatial efficiency correction coefficient τ(E), and calculation of detector detection efficiency factor f(E).

[0065] Specifically, taking a laboratory-level gamma-ray scanning system as an example, such as Figure 2 As shown, the TGS system includes a vertical mechanical transmission device 10, a transmission source 20, a detector system 30, a horizontal and rotary mechanical transmission device 40, a waste bin 50, a control and analysis system 60, a detector collimator 1, a detector 2, and other main components.

[0066] During TGS operation, the voxel partitioning method is first established. Using polar coordinates or rectangular grids, the waste bin is divided into I layers, and each layer is further divided into J voxel blocks. Based on the voxel partitioning method, the scanning position, scanning angle, and number of scans are set, and transmission and emission measurements are performed. In the emission measurements, the types of radionuclides and the energy of characteristic gamma rays in the bin are determined by the measured energy spectrum. Combined with the transmission measurement data, the linear attenuation coefficient of each voxel for characteristic gamma rays is calculated. Finally, using the tomographic gamma scanning voxel efficiency calibration method based on the spatial properties of the radioactive source provided in this invention, the detector efficiency for characteristic gamma rays in each voxel is obtained, the radioactivity distribution in the waste bin is calculated, and the entire waste bin tomographic gamma scanning process is completed.

[0067] Combined with appendix Figures 1 to 6 As shown, the method for calibration of voxel efficiency in tomographic gamma scanning based on the spatial properties of a radioactive source according to the present invention includes the following steps:

[0068] 1) Use Monte Carlo method-related software such as Geant4 to build a simulation model based on the TGS system dimensions;

[0069] The simulation model maintains the same dimensions as the TGS system without any adjustments; the detector dimensions in the simulation model are the same as the TGS detector, and the detector can be made of any material, with the detection efficiency set to 100%.

[0070] 2) Determine the placement location of the radioactive source for space efficiency measurement;

[0071] Within the space covered by the waste bin, m×n×k measurement locations are uniformly set (m, n, and k are integers; larger values ​​result in more accurate calculations, and can be determined as needed). An isotropic monoenergetic gamma point source is sequentially set at each measurement location. The gamma ray energy can be set to any arbitrarily high energy, and the radioactivity activity can be set to any suitable value. The environment is set to a vacuum for simulation. Alternatively, the center position of each voxel can be determined according to the specific voxel division method as needed. An isotropic monoenergetic gamma point source is then sequentially set at the voxel center position. The gamma ray energy is consistent with the characteristic gamma ray energy detected in the emission measurement. The radioactivity activity can be set to any suitable value, and the environment is set to air for simulation.

[0072] 3) Space efficiency calculation;

[0073] The ratio of the number of characteristic γ-photons emitted at each measurement position detected by the detector at different detection positions to the set activity is calculated to obtain the proportion of γ-photons emitted at different measurement positions that reach the detector, i.e., the spatial efficiency ε, and a spatial efficiency database is formed; or, for a selected voxel division method, the spatial efficiency of γ-photons emitted at the center position of different voxels is obtained.

[0074] 4) Calculation of space efficiency correction coefficient;

[0075] In the physics experiment, the waste bin is placed in the air. During efficiency calibration, gamma rays emitted from different positions in front of the detector must pass through air media of varying thicknesses before reaching the detector. Since the linear attenuation coefficient of air media varies for gamma rays of different energies, the spatial detection efficiency of the detector for gamma rays of different energies should be corrected using a spatial efficiency correction factor.

[0076] The external transmission source gamma-ray energy is set to the characteristic gamma-ray energy of all radionuclides. The attenuation coefficients of gamma rays with different energy characteristics in air are simulated and calculated. A database is established, and the spatial efficiency correction coefficients of gamma rays with different energy characteristics at different measurement locations are obtained analytically using the following formula:

[0077] τ j (E)=μ air (E)·x j (8)

[0078] In the formula τ j (E) is the spatial efficiency correction factor for γ-rays with energy E at the j-th position, x j Let τ be the distance from the point source at position j to the detector where the gamma ray emitted reaches. If a simulation experiment is conducted targeting the voxel center and specific energy gamma rays, the obtained space efficiency does not need correction, and τ is denoted as τ. j (E) = 1;

[0079] 5) Calculation of detection efficiency factor

[0080] In the TGS physical system and simulation model, multiple locations are selected in front of the detector to place standard gamma radiation sources. The types of standard sources include all nuclides that may appear in the waste bin or nuclides that need to be of interest. Simulation and physical measurements are performed, and the gamma photon counts detected in the simulation and physical experiments are compared to obtain the detector's ability to detect gamma rays of different energies entering the detector, i.e., the detection efficiency factor f(E), and a detection efficiency factor library is established.

[0081] Preferably, the detector is moved to the middle height and middle horizontal position, and three positions are selected directly in front of the detector, covering the center and edge of the waste bin, to reduce the impact of errors such as collimator placement on the accuracy of the measurement results; or, a database is not established, and the radionuclides present in the waste bin are determined by emission measurement, and then targeted simulations and experiments are carried out to obtain only the detector's detection efficiency factor f(E) for radionuclides in the waste bin.

[0082] 6) Calculation of detection efficiency

[0083] Based on the voxel division method within the waste bin during measurement, the coordinates of the voxel centers are determined. The spatial efficiency ε of the detector at that location is obtained by interpolation using data from the spatial efficiency database or by fitting using other methods. Then, based on the types of radionuclides and their characteristic gamma-ray energies in the waste bin obtained from emission measurements, the spatial efficiency correction coefficient τ(E) and the detection efficiency factor f(E) are determined. The detector's detection efficiency for radionuclides at different voxel center locations within the waste bin is calculated as follows:

[0084] ρ i,j =ε i,j τ i,j (E)f(E) (9)

[0085] In the formula ρ i,j ε represents the detector efficiency for detecting the radionuclide at the center of voxel j during the i-th emission scan. i,j (E) represents the spatial efficiency of the detector for the γ-ray with energy E at the center of voxel j, τ i,j (E) is the spatial efficiency correction coefficient for γ-rays with energy E at the center of voxel j, and f(E) is the detector's detection efficiency factor for γ-rays with energy E. Calculated sequentially according to the scanning order and voxel number, the detector's detection efficiency for γ-rays of different energies in each voxel can be obtained. This completes the voxel efficiency calibration.

Claims

1. A method for calibration of voxel efficiency in tomographic gamma scanning based on the spatial properties of a radioactive source, characterized in that, Includes the following steps: S1. Use Monte Carlo method-related software to establish a 1:1 simulation model of the TGS system; S2. Determine the location of the radioactive source for space efficiency measurement; Based on the voxel division method within the waste bin during measurement, the coordinates of the voxel centers were determined. S3, Space efficiency calculation; The ratio of the number of characteristic γ-photons emitted at each measurement position detected by the detector at different detection positions to the set activity is calculated to obtain the proportion of γ-photons emitted at different measurement positions that reach the detector, i.e., the spatial efficiency ε, and a spatial efficiency database is formed; or, for a selected voxel division method, the spatial efficiency of γ-photons emitted at the center position of different voxels is obtained. S4. Calculation of space efficiency correction coefficient; The external transmission source gamma-ray energy is set to the characteristic gamma-ray energy of all radionuclides. The attenuation coefficients of gamma rays with different energies in air are simulated and calculated, and a database is established. If simulation experiments are conducted targeting the voxel center location and specific energy gamma rays, the obtained space efficiency does not need correction. ; S5. Calculation of detection efficiency factor; In the TGS physical system and simulation model, multiple locations are selected in front of the detector to place standard gamma-ray sources. The types of standard sources include all nuclides that may appear in the waste bins or nuclides of interest. Simulation and physical measurements are performed, and the gamma-ray photon counts detected in the simulation and physical experiments are compared to obtain the detector's ability to detect gamma rays of different energies entering the detector, i.e., the detection efficiency factor f(E), and a detection efficiency factor library is established. Alternatively, without establishing a detection efficiency factor library, the radionuclides present in the waste bins are determined through emission measurements, and targeted simulations and experiments are performed to obtain only the detector's detection efficiency factor f(E) for radionuclides in the waste bins. S6. Detection efficiency calculation; The spatial efficiency of the detector at the voxel center coordinates is obtained by interpolation using data from the spatial efficiency database in step S3 or by fitting using other methods. ; Based on the types of radionuclides and their characteristic gamma-ray energies in the waste bin obtained from emission measurements, the space efficiency correction coefficient determined in step S4 is then used. and the detection efficiency factor determined in step S5 ; The detector efficiency for detecting radionuclides at different voxel centers in the waste bin is calculated as follows: ； In the formula Let be the detector efficiency for detecting the radionuclide at the center of voxel j during the i-th emission scan. Let E be the spatial efficiency of the detector for gamma rays with energy E at the center of voxel j. This is the spatial efficiency correction factor for γ-rays with energy E at the center of voxel j. The detector efficiency factor for gamma rays with energy E is calculated sequentially according to the scanning order and voxel number to obtain the detector efficiency for gamma rays of different energies in each voxel; thus, the voxel efficiency calibration is completed.

2. The method for calibration of voxel efficiency in tomographic gamma scanning based on the spatial properties of a radioactive source as described in claim 1, characterized in that: In step S1, the Monte Carlo method is used, and the relevant software is Geant4 or MCNP5 software; the simulation model is consistent with the size of the TGS system without any adjustments; the detector size in the simulation model is consistent with the TGS detector, the detector can be made of any material, and the detection efficiency is set to 100%.

3. The method for calibration of voxel efficiency in tomographic gamma scanning based on the spatial properties of a radioactive source as described in claim 2, characterized in that: Determining the placement location of the radiation source for space efficiency measurement in step S2 includes the following steps: m×n×k measurement positions are uniformly set within the space of the covered waste bin; an isotropic monoenergetic γ point source is set at each measurement position in sequence, the energy of the γ rays can be set to any energy, the radioactivity can be set to any suitable value, the environment is set to vacuum, and simulation is performed; or, as needed, the center position of each voxel is determined by a specific voxel division method; m, n, and k are integers.

4. The method for calibration of voxel efficiency in tomographic gamma scanning based on the spatial properties of a radioactive source as described in claim 3, characterized in that: In step S2, the center position of each voxel is determined by a specific voxel division method; isotropic monoenergetic γ point sources are set at the center positions of the voxels in sequence, the γ-ray energy is consistent with the characteristic γ-ray energy detected in the emission measurement, the radioactivity can be set to any suitable value, the environment is set to air, and simulation is performed. The voxel division method includes two methods: in a rectangular coordinate system, voxels are divided into cube shapes; in a polar coordinate system, voxels are divided into sector shapes. Each division method uses the following steps to determine the location of the voxel center: first, the bucket is divided longitudinally into I faults, and each fault is further divided into several voxels, totaling J voxels. The voxel center is the geometric center of each voxel.

5. The method for calibration of voxel efficiency in tomographic gamma scanning based on the spatial properties of a radioactive source as described in claim 3, characterized in that: In step S4, the spatial efficiency correction coefficients for γ-rays of different energy characteristics at different measurement locations are obtained analytically using the following formula: ； In the formula This is the spatial efficiency correction factor for γ-rays with energy E at the j-th position. Let be the distance from the point source at position j where the γ-rays emitted reach the detector.

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