Semi-physical simulation method for three-dimensional radiation dose field containing strong radiation source

Abstract

A semi-physical simulation method for a three-dimensional radiation dose field containing a strong radioactive source, using a solid structural structure to perform geometric modeling and material modeling on the radiation source and the surrounding coating structure, and setting the radiation source according to the physical properties of the radioactive nuclide; then by reducing the density of the solid structure in the model, the global reference distribution of the three-dimensional radiation dose field is obtained, and the importance parameters are calculated with the global reference distribution; then the density of the solid structure is gradually increased, and a full variance reduction simulation is performed based on the importance parameters to obtain the global asymptotic distribution of the three-dimensional radiation dose field; finally, the global approximate distribution of the three-dimensional radiation dose field under the real density condition is calculated by combining the global reference distribution and the global asymptotic distribution, and then the importance parameters are set and Monte Carlo calculation is performed to obtain the three-dimensional radiation dose field distribution under the real density condition. The present invention can effectively improve the efficiency of Monte Carlo shielding calculation, realize efficient and accurate semi-physical simulation of the real physical model, thereby improving the safety and controllability of radioactive source processing.

Description

Technical Field

[0001] The present invention relates to a technology in the field of radiation source disposal and radiation protection, specifically a semi-physical simulation method for a three-dimensional radiation dose field containing a strong radioactive source based on a real physical model and Monte Carlo transport simulation. Background Art

[0002] When coating a strong radioactive source, it is necessary to know the three-dimensional radiation dose field distribution around the radiation source in order to achieve safe and controllable radiation source processing. However, it is impossible to place a detector inside the physical structure, and it is impossible to obtain the radiation dose value at that location through measurement. At this time, it is necessary to use semi-physical simulation technology to obtain the radiation dose rate at that location. Semi-physical simulation technology needs to ensure high accuracy and high efficiency of the simulation process at the same time, but traditional simulation methods cannot guarantee both accuracy and efficiency at the same time. Summary of the invention

[0003] In view of the problem that the existing technology cannot measure the three-dimensional radiation dose field distribution around the radiation source by placing a detector when coating the radiation source, and the problem that the traditional method cannot ensure both accuracy and efficiency, the present invention proposes a semi-physical simulation method of the three-dimensional radiation dose field containing a strong radiation source, simulates the three-dimensional radiation dose field based on a real physical model and Monte Carlo transport calculation, and proposes a "benchmark + asymptotic + approximate + real" simulation technology, which effectively improves the efficiency of Monte Carlo shielding calculation and realizes efficient and accurate semi-physical simulation of the real physical model, thereby improving the safety and controllability of radiation source processing.

[0004] The present invention is achieved through the following technical solutions:

[0005] The invention relates to a semi-physical simulation method of a three-dimensional radiation dose field containing a strong radioactive source. The method adopts a solid structural structure to perform geometric modeling and material modeling on the radiation source and the surrounding covering structure, and sets the radiation source according to the physical characteristics of the radioactive nuclide; then, by reducing the density of the solid structure in the model, a global reference distribution of the three-dimensional radiation dose field is obtained, and importance parameters are calculated with the global reference distribution; then, the density of the solid structure is gradually increased, and a full variance reduction simulation is performed based on the importance parameters to obtain a global asymptotic distribution of the three-dimensional radiation dose field; finally, the global approximate distribution of the three-dimensional radiation dose field under the condition of real density is calculated by combining the global reference distribution and the global asymptotic distribution, and then the importance parameters are set and Monte Carlo calculation is performed to obtain the three-dimensional radiation dose field distribution under the condition of real density.

[0006] The entity structure performs geometric modeling and material modeling on the radiation source and the surrounding covering structure, which is specifically achieved in the following ways: using mathematical expressions to describe basic surfaces, using surfaces to construct basic geometric bodies through "intersection-union-complement" operations, filling various basic geometric bodies with corresponding materials to achieve material modeling, and splicing multiple geometric bodies to achieve geometric modeling.

[0007] The physical properties of the radionuclides are: the type of particles released when the radionuclides decay, usually neutrons, photons, electrons and alpha particles; the energy spectrum of the particles released when the radionuclides decay, that is, the initial kinetic energy of the particles; the half-life of the radionuclide, that is, the number of rays released per unit time.

[0008] The global benchmark distribution of the three-dimensional radiation dose field is specifically obtained by the following method: reducing the density of all material areas by M times, performing Monte Carlo fixed source calculation to obtain the global benchmark distribution of the three-dimensional radiation dose field. The global benchmark distribution can describe the overall distribution trend of the three-dimensional radiation dose field, but due to the reduction in model density, the radiation dose rate in various places in the space is overestimated.

[0009] The global asymptotic distribution of the three-dimensional radiation dose field is obtained by the following method: on the basis of obtaining the global reference distribution, the density of all material regions is increased by 2 times, and the global asymptotic distribution of the three-dimensional radiation dose field is obtained by performing Monte Carlo fixed source calculation. The global asymptotic distribution can still describe the overall distribution trend of the three-dimensional radiation dose field, and because of the increase in density, it is more accurate than the global reference distribution. However, because the model density is still lower than the true density, the global asymptotic distribution is still overestimated.

[0010] The global approximate distribution of the three-dimensional radiation dose field under the true density condition is obtained by: Where: the global approximate distribution is G(r), the global reference distribution is F(r), the global asymptotic distribution is H(r), r is the spatial position, and M is the multiple of the density reduction in step 1.

[0011] The importance parameter setting mentioned above refers to: the importance parameter at r Where: G(r) is the global approximate distribution of the three-dimensional radiation dose field, and max[G(r)] is the maximum value of the global approximate distribution of the three-dimensional radiation dose field.

[0012] The three-dimensional radiation dose field distribution under the real density condition is used to evaluate the safety of the coating structure or to guide the adjustment of the geometry and material parameters of the coating structure.

[0013] Technical Effects

[0014] The present invention proposes a simulation technology of "benchmark + asymptotic + approximation + reality", that is, the global benchmark distribution and global asymptotic distribution of the three-dimensional radiation dose field are obtained by adjusting the model density, and the global approximate distribution is constructed based on the global benchmark distribution and the global asymptotic distribution, and finally the global real distribution of the three-dimensional radiation dose field is quickly obtained based on the global approximate distribution, thereby effectively improving the efficiency and accuracy of Monte Carlo shielding calculations. The present invention solves the problem that traditional simulation methods cannot guarantee accuracy and efficiency at the same time, that is, it guarantees the accuracy of semi-physical simulation and also guarantees the efficiency of semi-physical simulation. At the same time, the simulation method solves the problem that detectors cannot be placed in physical structures to measure dose rates, and realizes efficient and accurate semi-physical simulation of real physical models, thereby improving the safety and controllability of radiation source processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a flow chart of the present invention;

[0016] Figure 2 The following is a schematic diagram of a coating scheme for a cobalt-60 radioactive source;

[0017] Figure 3 is a global reference distribution diagram of the three-dimensional radiation dose field of the embodiment;

[0018] Figure 4 is a global asymptotic distribution diagram of a three-dimensional radiation dose field of an embodiment;

[0019] Figure 5 is a global approximate distribution diagram of the three-dimensional radiation dose field of the embodiment;

[0020] Figure 6 is a global true distribution diagram of the three-dimensional radiation dose field of the embodiment;

[0021] Figure 7 FIG. 4 is a standard deviation distribution diagram of a semi-physical simulation of an embodiment. DETAILED DESCRIPTION

[0022] like Figure 2 As shown in the figure, this embodiment is specifically implemented in a coating scheme of a cobalt-60 radioactive source: cobalt-60 is a β - Decaying nuclides will emit gamma rays with energies of 1.1732MeV and 1.3325MeV, with a half-life of 5.272 years and 8.85×10 -4 A milligram of cobalt-60 will generate 3.7×10 7The radiation rate of cobalt-60 is 13.2 roentgens / hour at a distance of 1 cm from the cobalt-60 point source. Therefore, cobalt-60 has extremely strong radioactivity and affects the human body. It is a highly toxic nuclide. The application of cobalt-60 radioactive sources is very extensive. However, since strong radioactive sources such as cobalt-60 are very harmful to the human body, radiation protection against strong radioactive sources is very important. Figure 2 In the coating scheme shown, 316 stainless steel and water at room temperature and pressure are used as the coating material of cobalt-60, wherein the cobalt-60 radiation source is placed inside the coating material, and the size of the entire coating entity structure is 10 (m) × 8 (m) × 10 (m) = 800 (m 3 ).

[0023] like Figure 1 As shown, it is a flow chart of the semi-physical simulation method of the three-dimensional radiation dose field containing a strong radiation source involved in this embodiment. Based on this embodiment, the three-dimensional radiation dose field distribution inside the entire model is obtained to evaluate the safety of the coating scheme. When the coating scheme cannot meet the safety limit, the coating scheme is adjusted.

[0024] This embodiment specifically includes:

[0025] Step 1: Use physical structure to construct the structure pair Figure 2 The geometric modeling and material modeling of the cladding structure shown in the figure are carried out, where the density of 316 stainless steel is 8.0g / cm 3 The density of water at room temperature and pressure is 1.0 g / cm 3 The physical properties of the radioactive source are described. Cobalt-60 releases photons with an energy of 1.1732 MeV, so the source particles simulated by Monte Carlo simulation are monoenergetic photons with an energy of 1.1732 MeV. The number of particles simulated each time is N = 10,000,000.

[0026] The coating structure is meshed, with the X-axis direction divided into 200 equal parts, the Y-axis direction divided into 160 equal parts, and the Z-axis direction not meshed. Therefore, a total of 200×160=32,000 grids are formed, and the size of each grid is 5cm×5cm×1000cm. The three-dimensional radiation dose field distribution is constructed by counting the radiation dose values ​​of these 32,000 grids.

[0027] Step 2: Reduce the density of 316 stainless steel and water at room temperature and pressure by 32 times at the same time. At this time, the density of 316 stainless steel is 0.25g / cm 3 The density of water at room temperature and pressure is 0.03125 g / cm 3 The radiation dose values ​​of the 32,000 grids were obtained by performing Monte Carlo fixed source calculations, and the global reference distribution of the three-dimensional radiation dose field was constructed based on the radiation dose values ​​of the 32,000 grids.

[0028] Step 3: Reduce the density of 316 stainless steel and water at room temperature and pressure by 16 times at the same time. At this time, the density of 316 stainless steel is 0.5g / cm 3 The density of water at room temperature and pressure is 0.0625 g / cm 3 The importance parameter is calculated using the global reference distribution obtained in step 1, and a global variance reduction simulation is performed based on the importance parameter to obtain the radiation dose values ​​of the 32,000 grids. The global asymptotic distribution of the three-dimensional radiation dose field is constructed using the radiation dose values ​​of the 32,000 grids.

[0029] Step 4: Combine the global reference distribution F(r) obtained in step 2 and the global asymptotic distribution H(r) obtained in step 3 to calculate the approximate distribution of the three-dimensional radiation dose field under the real density condition G(r) = [H(r)] 31 ×[F(r)] -30 Because the radiation dose values ​​of 32,000 grids are counted in total, the global approximate distribution also contains the radiation dose values ​​of 32,000 grids.

[0030] Step 5: Based on the approximate distribution of the three-dimensional radiation dose field obtained in step 4, calculate the importance parameters again. Ensure that the 316 stainless steel and water at room temperature and pressure return to their true density, that is, the density of 316 stainless steel returns to 8.0 g / cm 3 , the density of water at room temperature and pressure returns to 1.0g / cm 3 Based on the updated importance parameters, the global variance reduction simulation is performed again to obtain the true distribution R(r) of the three-dimensional radiation dose field under the real density condition.

[0031] Step 6: Evaluate the safety of the encapsulation structure based on the true distribution of the three-dimensional radiation dose field obtained in step 5, such as determining whether the radiation dose rate at a certain point in space meets the safety threshold. If not, adjust the geometry and material parameters of the encapsulation structure based on the true distribution of the three-dimensional radiation dose field.

[0032] To evaluate the above effects: As a comparison, the traditional simulation method was used to model the coating scheme of the cobalt-60 radiation source, simulating a total of 1 trillion photons, and still constructing the three-dimensional radiation dose field distribution by counting the radiation dose values ​​of these 32,000 grids. By comparing the average quality factor Quantify the technical effect of this embodiment. At the same time, because the size of this model is very large, the photons are transported from the source area to the boundary area, and there will be 10 80 The flux attenuation is so that the traditional simulation method cannot complete the calculation of the entire model, that is, the traditional simulation method cannot obtain the radiation dose value of these 32,000 grids, and only some grids will have statistical values. Therefore, the counting rate per unit time η = r / T is also used to quantify the technical effect of this embodiment, where: the subscript "i" represents different grid areas; T represents the calculation time, Rei is the standard deviation of the radiation dose value in the ith region. When there is no statistical value in the region, the standard deviation of the region is considered to be 1.0; r is the percentage of grids with statistical radiation dose rates in the 32,000 grids. The comparison of quantitative data indicators is shown in Table 1, and the variance distribution of radiation dose rates is shown in Figure 7 .

[0033] Table 1. Acceleration effect of this embodiment compared with the traditional method

[0034]

[0035] As can be seen from Table 1, even if the traditional simulation method simulates 1 trillion photons, after 3544.8 minutes of simulation, it can only obtain the radiation dose rate of 3191 grids, which is only 9.9% of the total number of grids. Therefore, it is difficult for the traditional simulation method to complete the semi-physical simulation of the three-dimensional radiation dose field containing strong radiation sources. Compared with the traditional simulation method, this method achieves a good acceleration effect, in which the average quality factor is increased by 1247.6 times and the counting rate per unit simulation time is increased by 582.2 times. Therefore, this method can quickly obtain global information and effectively improve the computational efficiency of Monte Carlo shielding calculations.

[0036] like Figure 7 As shown, the method only takes 60.7 minutes to obtain the radiation dose values ​​of 31777 grids, with an effective counting rate of 99.3%. Among the 31777 radiation dose values, the standard deviation of the radiation dose values ​​of 12104 grids is less than 5%, and the standard deviation of the radiation dose values ​​of 21497 grids is less than 10%. That is, while ensuring that the calculation efficiency is 1247.6 times higher than that of the traditional method, the embodiment also ensures that the standard deviation of 38.1% of the radiation dose values ​​is less than 5%, and the standard deviation of 66.9% of the radiation dose values ​​is less than 10%. Therefore, the method realizes high-fidelity semi-physical simulation of the three-dimensional radiation dose field containing a strong radiation source.

[0037] Compared with the existing technology, this method can quickly obtain global information through the simulation technology of "benchmark + asymptotic + approximate + real", solve the problem that traditional methods cannot guarantee accuracy and efficiency at the same time, and effectively improve the efficiency and accuracy of Monte Carlo shielding calculations; at the same time, this method solves the problem that detectors cannot be placed in the physical structure to measure the dose rate by semi-physical simulation of the three-dimensional radiation dose field of the coated strong radioactive source, and realizes efficient and accurate semi-physical simulation of the real physical model, thereby improving the safety and controllability of radioactive source processing and providing technical support for radiation protection of strong radioactive sources.

[0038] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. A semi-physical simulation method for a three-dimensional radiation dose field containing a strong radiation source, characterized in that: A solid structural structure is used to perform geometric modeling and material modeling on the radiation source and the surrounding covering structure, and the radiation source is set according to the physical properties of the radioactive nuclide; then, the global reference distribution of the three-dimensional radiation dose field is obtained by reducing the density of the solid structure in the model, and the importance parameters are calculated with the global reference distribution; then, the density of the solid structure is gradually increased, and a full variance reduction simulation is performed based on the importance parameters to obtain the global asymptotic distribution of the three-dimensional radiation dose field; finally, the global approximate distribution of the three-dimensional radiation dose field under the real density condition is calculated by combining the global reference distribution and the global asymptotic distribution, and then the importance parameters are set and the Monte Carlo calculation is performed to obtain the three-dimensional radiation dose field distribution under the real density condition; The global reference distribution of the three-dimensional radiation dose field is specifically obtained by the following method: reducing the density of all material areas by M times, performing Monte Carlo fixed source calculation to obtain the global reference distribution of the three-dimensional radiation dose field, the global reference distribution can describe the overall distribution trend of the three-dimensional radiation dose field, but due to the reduction of the model density, the radiation dose rate at various locations in the space is overestimated; The global asymptotic distribution of the three-dimensional radiation dose field is specifically obtained by the following method: on the basis of obtaining the global benchmark distribution, the importance parameters are calculated using the global benchmark distribution, the density of all material areas is increased by 2 times, and a global variance reduction simulation is performed to obtain the global asymptotic distribution of the three-dimensional radiation dose field. The global asymptotic distribution can still describe the overall distribution trend of the three-dimensional radiation dose field, and because of the increase in density, the accuracy is higher than that of the global benchmark distribution. However, because the model density is still lower than the true density, the global asymptotic distribution is still overestimated.

2. The semi-physical simulation method of a three-dimensional radiation dose field containing a strong radiation source according to claim 1 is characterized in that: The entity construction structure performs geometric modeling and material modeling on the radiation source and the surrounding covering structure, which is specifically achieved in the following ways: using mathematical expressions to describe basic surfaces, using surfaces to construct basic geometric bodies through "intersection-union-complement" operations, filling various basic geometric bodies with corresponding materials to achieve material modeling, and splicing multiple geometric bodies to achieve geometric modeling.

3. The semi-physical simulation method of a three-dimensional radiation dose field containing a strong radiation source according to claim 1 is characterized in that: The physical properties of the radionuclide refer to: the type of particles released when the radionuclide decays; the energy spectrum of the particles released when the radionuclide decays, that is, the initial kinetic energy of the particles; the half-life of the radionuclide, that is, the number of rays released per unit time.

4. The semi-physical simulation method of a three-dimensional radiation dose field containing a strong radiation source according to claim 1 is characterized in that: The global approximate distribution of the three-dimensional radiation dose field under the true density condition is obtained by: , where: the global approximate distribution is G(r), the global reference distribution is F(r), the global asymptotic distribution is H(r), r is the spatial position, and M is the multiple of density reduction.

5. The semi-physical simulation method of a three-dimensional radiation dose field containing a strong radiation source according to claim 1 is characterized in that: The importance parameter setting mentioned above refers to: the importance parameter at r , where: G(r) is the global approximate distribution of the three-dimensional radiation dose field, and max[G(r)] is the maximum value of the global approximate distribution of the three-dimensional radiation dose field.

6. The semi-physical simulation method of a three-dimensional radiation dose field containing a strong radiation source according to claim 1 is characterized in that: The three-dimensional radiation dose field distribution under the real density condition is used to evaluate the safety of the coating structure or to guide the adjustment of the geometry and material parameters of the coating structure.

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