A method for investigating source terms of retired medical cyclotrons

Abstract

The invention discloses a method for investigating source items of medical cyclotron decommissioning, which comprises the following steps: using a Monte Carlo method to calculate neutron distribution and obtain neutron distribution conditions; setting the positions of sampling points according to the neutron distribution conditions; inputting the positions of various sampling points into Monte Carlo software to obtain the radioactive nuclide types and specific activities of various sampling points calculated theoretically; sampling is performed according to the positions of various sampling points; and radioactive nuclide chemical analysis is performed according to the sampling results to obtain the radioactive nuclide types and specific activities of the actual sampling points; the radioactive nuclide types and specific activities obtained by the two methods are compared, and the positions and specific activities of the actual sampling points are continuously corrected with the theoretical calculation results; the invention finally analyzes special key points and cutting ranges by combining theoretical calculation with actual measurement, so that the high-level radioactive waste finally cut out of an accelerator weighing hundreds of tons is only tens of kilograms or even a few kilograms.

Description

Technical Field

[0001] The present invention belongs to the field of medical cyclotron decommissioning, and in particular relates to a medical cyclotron decommissioning source item investigation method. Background Art

[0002] With the development of nuclear energy and nuclear science, radioactive isotopes are increasingly used in various fields such as medical care, industrial and agricultural production, scientific research, and national defense construction. Accelerator production of radioactive isotopes has great advantages over nuclear reactors. Accelerators occupy a small area, and the construction and operating costs are relatively low. The public acceptance is relatively high. Accelerators can be installed and used in hospitals, which greatly reduces the time and transportation distance restrictions on the use of short-lived radioactive nuclides. Therefore, in the past decade, the technology of accelerator production of medical radioactive isotopes has developed rapidly, especially the technology of using cyclotron accelerators to produce medical isotopes.

[0003] With the continuous promotion of accelerator applications and the continuous increase in the number of years of application, many early medical cyclotron accelerators are facing retirement. Accelerator retirement is a new thing in China. An accelerator weighs tens to hundreds of tons. After years of operation, some parts of the accelerator are activated and radioactive. Foreign countries take the approach of burying the accelerator in an uninhabited area and covering it with a certain thickness of concrete. The entire accelerator is buried underground for hundreds of years. Since land resources are limited, it is definitely unrealistic to bury the entire accelerator. Summary of the invention

[0004] In order to solve the problems raised by the prior art, the present invention proposes a method for investigating the source items of retired medical cyclotron accelerators, aiming to solve the problem of burying the entire accelerator and occupying too much land resources.

[0005] The present invention proposes the following technical solutions to solve the technical problems:

[0006] A method for investigating source items of medical cyclotron decommissioning, characterized in that it comprises the following steps:

[0007] Step 1: Calculate the neutron distribution using the Monte Carlo method to obtain the neutron distribution along the way from the neutron target outside the accelerator to the main magnet inside the accelerator; the specific process is as follows:

[0008] 1) Input various parameters excluding the sampling point location into the Monte Carlo software: particle type, beam energy, beam intensity, and the main components of the accelerator including the shape, size, material chemical element composition and proportion of the yoke, magnet, coil, neutron target material, neutron target location, and neutron target shape;

[0009] 2) Monte Carlo software outputs neutron distribution: First, the neutron distribution outside the accelerator is output: based on the input neutron target material, neutron target position, neutron target shape, and particle type, beam energy, and beam intensity of the isotope production, the neutron energy spectrum generated by the target and the distribution of neutrons to the accelerator that radiates them are calculated; then the neutron distribution along the neutron path inside the accelerator is output: based on the input particle type, beam energy, beam intensity, and the shape, size, material chemical element composition and proportion of the accelerator yoke, magnet, coil, and vacuum chamber, the neutron distribution is calculated.

[0010] Step 2: Set the position of the sampling point according to the neutron distribution. The position of the sampling point is divided according to the neutron distribution density. The specific process is as follows:

[0011] 1) Set the sampling point location in the Monte Carlo software according to the neutron distribution;

[0012] 2) When the sampling point position set in the software is suitable for on-site measurement, the sampling position set in the software is used as the final sampling position;

[0013] 3) When the sampling point position set in the software is not suitable for on-site measurement, a sampling point position suitable for on-site measurement is found at the measurement site to replace the sampling point position set in the software. The neutron density of the sampling point position suitable for on-site measurement should meet the sampling requirements, that is, the neutron density of the sampling point is medium-high density;

[0014] 4) The sampling point locations suitable for field measurement are inferred to the Monte Carlo software as the sampling point location input parameters of the Monte Carlo software.

[0015] Step 3: Input the location of each sampling point into the Monte Carlo software to obtain the theoretically calculated radionuclide type and specific activity of each sampling point, where the specific activity is the degree of radiation contamination. The specific process is as follows:

[0016] 1) Input various parameters including sampling point location into the Monte Carlo software: sampling point location, particle type, beam energy, beam intensity, main components of the accelerator including yoke, magnet, coil shape, size, material chemical element composition and proportion, neutron target material, neutron target location, neutron target shape;

[0017] 2) The Monte Carlo software output includes the location of each sampling point and a list of the specific activity of the radionuclide at each sampling point.

[0018] Step 4: Sampling is performed according to the location of each sampling point, and then radionuclide chemical analysis is performed according to the sampling results to obtain the type and specific activity of the radionuclide of the actual sampling point; the radionuclide chemical analysis is to use chemical analysis methods to analyze the radionuclide and its specific activity of the sampled sample at each actual sampling point. The types of radionuclides mainly include Fe55, Ni63,, Mn54, and Co60; different types have different chemical analysis methods; the specific process is as follows:

[0019] 1) Number each sampling point and record its location and depth;

[0020] 2) Drill holes at the target location according to the recorded depth. The metal powder generated during drilling needs to be properly collected and preserved. In addition, when taking samples from deeper locations, first clean up the powder remaining in the hole and on the sampler from the shallower locations to avoid affecting the measurement results of the deep samples.

[0021] 3) Take samples; clean up the superficial powder to avoid affecting the results.

[0022] Step 5: Compare the radionuclide types and specific activity results obtained by the two methods, and continuously use the theoretical calculation results to correct the actual sampling point position and specific activity until the statistical results of the two methods are close to the same; the two methods are the Monte Carlo algorithm for theoretical simulation calculation and sampling measurement methods to respectively determine the radioactive isotope type and specific activity for the same sampling point. The specific process is as follows:

[0023] 1) Perform statistical analysis on the theoretical calculated specific activity and the actual calculated specific activity. If there is no statistical difference in the results obtained by the two methods, the current positions and specific activity results of each sampling point are recognized and the process is transferred to process 6); if the statistical analysis results show that there is a statistical difference in the results obtained by the two methods, continue with process 2) for cause analysis:

[0024] 2) Analyze the reasons for the statistical differences. First, there is inaccuracy in the location of the sampling point during sampling, which is too different from the sampling point location determined for the current specific activity during the Monte Carlo software simulation calculation; second, the sample at the sampling point may be affected by powder at other locations, and the influence of the powder at other locations is that the powder at the deep position of the sampling point is affected by the powder at the shallow position, and the powder at the shallow position is mixed into the powder at the deep position; third, when the specific activity analysis of the radionuclides of the samples at the sampling point is actually calculated, the uncertainty of the chemical analysis method is high, resulting in a large difference in the results of the two methods;

[0025] The analysis of the reasons for the statistical differences also includes a fourth reason, the difference in specific activity between the two methods caused by different material compositions: the types of material compositions actually sampled may be more than the types of material compositions input into the software. When this happens, the specific activity of the extra material compositions is calculated by chemical analysis methods. When the percentage of this specific activity is lower than a set value, it can be ignored. When the percentage of this specific activity is higher than the set value, this part of the specific activity should be calculated in the total specific activity of the sampling point, and the newly discovered material type should be input into the Monte Carlo software, and the software will recalculate the total specific activity of the sampling point, so as to achieve consistency between the theoretical calculation and the actual measurement results.

[0026] 3) If the first reason causes a statistical difference between the two methods, the sampling point position is adjusted and re-sampled, the re-sampling results are subjected to radionuclide chemical analysis to obtain the radionuclide type and specific activity of the sampling point after re-sampling, and then the specific activity of the sampling point obtained after re-sampling is compared with the radionuclide type and specific activity of each sampling point calculated theoretically; if there is no statistical difference in the results obtained by the two methods, the current position and specific activity results of each sampling point are recognized and transferred to process 6); if there is still a difference and it exceeds the set number of repetitions, continue with process 4); if there is still a difference but it does not exceed the set number of repetitions, repeat process 3);

[0027] 4) If the statistical difference between the two methods is due to the second reason, the sampling position is adjusted and re-sampled. The possibility of mixing shallow powder into the sample at the sampling point is eliminated by adjusting the sampling position. The re-sampling results are subjected to radionuclide chemical analysis to obtain the type and specific activity of the radionuclide at the sampling point after resampling. The specific activity of the sampling point obtained after resampling is then compared with the type and specific activity of the radionuclide at each sampling point calculated theoretically. If there is no statistical difference in the results obtained by the two methods, the position and specific activity results of the current sampling point are recognized and transferred to process 6); if there is still a difference and it exceeds the set number of repetitions, continue process 5); if there is still a difference but it does not exceed the set number of repetitions, repeat process 4);

[0028] 5) If the third reason causes a statistical difference between the two methods, the uncertainty of the chemical analysis method of each radionuclide Fe55, Ni63, Mn54, and Co60 is analyzed. If the uncertainty of the chemical analysis method is high and does not meet the set standard, the position and specific activity of the current sampling point with the difference will be used as the position and specific activity of the final sampling point, and the process is transferred to process 6); if the uncertainty of the chemical analysis method is low and meets the set standard, but there is still a difference between theory and practice, return to process 3) until the difference between the theoretical calculated specific activity and the actual calculated specific activity is within the allowable range;

[0029] The uncertainty of the chemical analysis method is calculated as follows:

[0030] 1) Divide the samples from the same sampling point into several parts;

[0031] 2) Analyze each sample using radionuclide chemical analysis methods;

[0032] 3) If the analysis results of several samples from the same sampling point are close to the same, the uncertainty of the chemical analysis method is considered to be acceptable; if the analysis results of several samples from the same sampling point are quite different, the uncertainty of the chemical analysis method is considered to be high; the close to the same means that if the analysis results of several samples are statistically analyzed and P>0.05, it is considered that there is no difference in the analysis results; the large difference means that if the analysis results of several samples are statistically analyzed and P<0.05, it is considered that there is a difference in the analysis results;

[0033] 6) Make source survey results based on the location and specific activity of the current sampling point.

[0034] Advantages and effects of the present invention

[0035] The present invention uses a method combining theoretical calculation and actual measurement, as well as an iterative optimization method and a mutual verification method, to finally analyze the special key points and cutting range, so that the high-level radioactive waste cut out from the accelerator weighing hundreds of tons is only tens of kilograms or even a few kilograms, and the area occupied by the high-level radioactive waste is only one ten-thousandth of the area occupied by the entire machine. This solves the problem that the existing technology cannot implement accelerator decommissioning due to the large area occupied by the entire machine and the long burial period. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a schematic diagram of the distribution of neutrons along the route;

[0037] Figure 2 The present invention is a flow chart of a method for investigating source items of a medical cyclotron decommissioning device. DETAILED DESCRIPTION

[0038] Design principle of the present invention:

[0039] 1. The necessity of studying the decommissioning methods of accelerators. An accelerator weighs tens or even hundreds of tons, but the real radioactive waste only accounts for one percent of the total weight at most. If radioactive waste and non-radioactive waste are mixed together and buried deep, then for an accelerator, 99% of the land resources will be wasted.

[0040] 2. Mutual verification design principle: Use theoretical calculation methods to guide the selection of actual measurement sampling point locations, thereby verifying the accuracy of the actual sampling locations; then use field measurement methods to find the material composition of each sampling point, thereby verifying the accuracy of the theoretically calculated sampling point specific activity. First, use theoretical calculation methods to guide the selection of actual measurement sampling point locations: Set the sampling point locations in the Monte Carlo software according to the neutron distribution. After the sampling point locations are set, the software returns a list that calculates the nuclear element composition and specific activity of each component at each set sampling point. After the sampling point position is set, the sampling point is measured manually on site. Since there will always be an error of several centimeters between the sampling point position measured on site and the position set in the software interface, and the sampling point may be contaminated by powder in the sample (shallow samples mixed into deep samples), there will always be an error between the specific activity of the sampling point measured on site and the specific activity calculated in the software list. There are three reasons for the error: For the first reason, the current sampling point position should be adjusted to make the specific activity of the current sampling point consistent with the theoretically calculated specific activity; for the second reason, the current sampling point position should also be adjusted to prevent sample contamination. Each time the position is adjusted, the new sampling point position should be entered into the Monte Carlo software for recalculation. For the third reason, if the conclusions drawn by the analysis methods of Ni63, Mn54, and Co60 are acceptable, if there are still differences between theoretical calculations and actual measurements, the first and second reasons should be re-searched until the specific activities of theoretical calculations and actual sampling are close to each other; if the conclusions drawn by the analysis method of Fe55 are unacceptable, because the analysis method has a high uncertainty, if there are still differences between theoretical calculations and actual measurements, this difference should be recognized, and the location and specific activity of the final sampling point should be used as the final conclusion to make the source survey results. Second, the accuracy of the theoretically calculated specific activity of the sampling point is verified by the method of field measurement of the material composition of the sampling point. The materials that make up the accelerator are analyzed by actual measurement methods. The ultimate goal of analyzing the material composition is to calculate the radioactive specific activity of various materials. If the material composition input into the Monte Carlo software is incomplete, the specific activity will be inaccurate. Since the material composition initially input into the Monte Carlo software is based on several conventional compositions, in fact, some accelerators are not made by ourselves but by third parties. The material composition of the accelerators made by third-party manufacturers may be different from the conventional compositions. If it is input according to the conventional types, the material composition will be missed and the specific activity will be undercalculated.Therefore, it is necessary to use actual measurement methods to find out all the material components, but it is not enough to just find out the material components. Monte Carlo software is also needed to calculate the radioactive specific activity produced by the component after activation and calculate its percentage at the sampling point. When the percentage is lower than a set value, it can be ignored. When the specific activity percentage is higher than the set value, the specific activity is calculated in the total specific activity, and then the level of radioactive waste is determined based on this specific activity.

[0041] 3. Iterative optimization and design difficulties: The original intention of the present invention is to cut the whole accelerator weighing hundreds of tons into small pieces of radioactive waste with a single volume and weight of only tens of kilograms or even several kilograms according to different radiation doses. The volume and weight of the small pieces of radioactive waste are only one ten-thousandth of the total volume of the accelerator. The difficulty lies in the accuracy of cutting, which is to cut according to four conditions: high-level radioactive waste, medium-level radioactive waste, low-level radioactive waste, and very low-level radioactive waste. Since the treatment method for each type of waste is different, if the cutting position is inaccurate, it will lead to the wrong treatment method, which may cause nuclear radiation leakage and harm to humans due to insufficient burial depth, insufficient concrete thickness, or insufficient storage years. The present invention adopts an iterative optimization method to ultimately achieve relatively accurate cutting. The iterative optimization is to perform iterative optimization by combining theoretical calculation with actual measurement. The initial theoretical calculation only gives the distribution of neutrons, and then the collection point position is further set in the software interface according to the neutron distribution, and then the difference between the theoretical calculation and the actual measurement is further analyzed through actual measurement, and then the cause of the difference is further analyzed, and then the theoretical calculation and the actual measurement are repeatedly verified with each other according to the analyzed cause, so as to finally find relatively accurate collection points or cutting point positions of four types of wastes.

[0042] Based on the above invention principle, the present invention designs a method for investigating the source term of medical cyclotron decommissioning, such as Figure 2 As shown, it is characterized in that it includes the following steps:

[0043] Step 1: Calculate the neutron distribution using the Monte Carlo method to obtain the neutron distribution along the way from the neutron target outside the accelerator to the main magnet inside the accelerator; the specific process is as follows:

[0044] 1) Input various parameters excluding the sampling point location into the Monte Carlo software: particle type, beam energy, beam intensity, and the main components of the accelerator including the shape, size, material chemical element composition and proportion of the yoke, magnet, coil, neutron target material, neutron target location, and neutron target shape;

[0045] 2) Monte Carlo software outputs neutron distribution: First, the neutron distribution outside the accelerator is output: based on the input neutron target material, neutron target position, neutron target shape, and particle type, beam energy, and beam intensity of the isotope production, the neutron energy spectrum generated by the target and the distribution of neutrons to the accelerator that radiates them are calculated; then the neutron distribution along the neutron path inside the accelerator is output: based on the input particle type, beam energy, beam intensity, and the shape, size, material chemical element composition and proportion of the accelerator yoke, magnet, coil, and vacuum chamber, the neutron distribution is calculated.

[0046] Explanation on the source of cyclotron radioactivity:

[0047] Neutron distribution is Figure 1 As shown, the neutron target is outside the accelerator and close to the accelerator. The neutron distribution density is highest around the neutron target and decreases toward the center of the accelerator.

[0048] The medical cyclotron is a complex system, mainly including the main magnet system, main magnet coil system, ion source system, central area system, high frequency system, extraction system, lifting system, main vacuum system, water cooling system, pneumatic system, beam diagnostic system, control system, power supply system, target system and more than a dozen other systems. The medical cyclotrons currently used all accelerate H- ions, and use carbon stripping membranes at the extraction outlets. The H- accelerated to the required energy is stripped of two electrons to become positively charged protons, which are then extracted and used for target shooting to produce the required radioactive isotopes. Depending on whether each system will come into contact with accelerated particles and whether it will be activated by neutrons generated by beam target shooting, each system is divided into radioactive waste and non-radioactive waste. Non-radioactive waste is treated according to the clean clearance level according to regulatory requirements, including ion source system, main vacuum system, water cooling system, pneumatic system, control system, power supply system, etc. Radioactive waste is further classified according to weight. The radioactivity of light-weight waste can be directly measured, and the radioactive waste type is classified and treated according to regulatory requirements; the heavier-weight waste is analyzed in detail.

[0049] There are two main sources of radioactivity: one is the components that are in direct contact with the accelerated particles. During the particle acceleration process, beam loss will occur due to residual gas stripping and Lorentz stripping, and the particles will react with the cyclotron components to produce radioactivity; the other is that the accelerated particles are extracted and bombarded with the target material to produce (p, n) reactions. The neutrons produced react with the accelerator components to produce radioactivity. The location of the first radioactive source is mainly inside the vacuum chamber and the target system, including the main magnet system, the central area system, the high-frequency system, the vacuum chamber lining, the stripping film, the stripping film bracket and the target. The location of the second radioactive source is mainly outside the vacuum chamber, including the magnet coil, the yoke, and the vacuum chamber. Due to the strong penetrability of neutrons, the main magnet inside the vacuum chamber will also produce radioactivity due to neutron activation. Since the radioactivity generated by the first reason is very small compared to the second reason, the second reason is mainly considered. Among all the radioactive components, except for the main magnet, the magnet coil, the yoke, and the vacuum chamber, the other components are radioactive wastes with small volume or weight, which can be directly classified and processed. The present invention aims at radioactive components with large volume or weight, such as main magnet, magnet coil, magnetic yoke, vacuum chamber, etc., and adopts Monte Carlo algorithm to perform theoretical simulation calculation and sampling measurement to respectively determine the type of radioactive isotope and specific activity of each part.

[0050] Step 2: Set the position of the sampling point according to the neutron distribution. The position of the sampling point is divided according to the neutron distribution density. The specific process is as follows:

[0051] 1) Set the sampling point location in the Monte Carlo software according to the neutron distribution;

[0052] 2) When the sampling point position set in the software is suitable for on-site measurement, the sampling position set in the software is used as the final sampling position;

[0053] 3) When the sampling point position set in the software is not suitable for on-site measurement, a sampling point position suitable for on-site measurement is found at the measurement site to replace the sampling point position set in the software. The neutron density of the sampling point position suitable for on-site measurement should meet the sampling requirements, that is, the neutron density of the sampling point is medium-high density;

[0054] 4) The sampling point locations suitable for field measurement are inferred to the Monte Carlo software as the sampling point location input parameters of the Monte Carlo software.

[0055] Step 3: Input the location of each sampling point into the Monte Carlo software to obtain the theoretically calculated radionuclide type and specific activity of each sampling point, where the specific activity is the degree of radiation contamination. The specific process is as follows:

[0056] 1) Input various parameters including sampling point location into the Monte Carlo software: sampling point location, particle type, beam energy, beam intensity, main components of the accelerator including yoke, magnet, coil shape, size, material chemical element composition and proportion, neutron target material, neutron target location, neutron target shape;

[0057] 2) The Monte Carlo software output includes the location of each sampling point and a list of the specific activity of the radionuclide at each sampling point.

[0058] Step 4: Sampling is performed according to the location of each sampling point, and then radionuclide chemical analysis is performed according to the sampling results to obtain the type and specific activity of the radionuclide of the actual sampling point; the radionuclide chemical analysis is to use chemical analysis methods to analyze the radionuclide and its specific activity of the sampled sample at each actual sampling point. The types of radionuclides mainly include Fe55, Ni63,, Mn54, and Co60; different types have different chemical analysis methods; the specific process is as follows:

[0059] 1) Number each sampling point and record its location and depth;

[0060] 2) Drill holes at the target location according to the recorded depth. The metal powder generated during drilling needs to be properly collected and preserved. In addition, when taking samples from deeper locations, first clean up the powder remaining in the hole and on the sampler from the shallower locations to avoid affecting the measurement results of the deep samples.

[0061] 3) Take samples; clean up the superficial powder to avoid affecting the results.

[0062] Step 5: Compare the radionuclide types and specific activity results obtained by the two methods, and continuously use the theoretical calculation results to correct the actual sampling point position and specific activity until the statistical results of the two methods are close to the same; the two methods are the Monte Carlo algorithm for theoretical simulation calculation and sampling measurement methods to respectively determine the radioactive isotope type and specific activity for the same sampling point. The specific process is as follows:

[0063] 1) Perform statistical analysis on the theoretical calculated specific activity and the actual calculated specific activity. If there is no statistical difference in the results obtained by the two methods, the current positions and specific activity results of each sampling point are recognized and the process is transferred to process 6); if the statistical analysis results show that there is a statistical difference in the results obtained by the two methods, continue with process 2) for cause analysis:

[0064] 2) Analyze the reasons for the statistical differences. First, there is inaccuracy in the location of the sampling point during sampling, which is too different from the sampling point location determined for the current specific activity during the Monte Carlo software simulation calculation; second, the sample at the sampling point may be affected by powder at other locations, and the influence of the powder at other locations is that the powder at the deep position of the sampling point is affected by the powder at the shallow position, and the powder at the shallow position is mixed into the powder at the deep position; third, when the specific activity analysis of the radionuclides of the samples at the sampling point is actually calculated, the uncertainty of the chemical analysis method is high, resulting in a large difference in the results of the two methods;

[0065] The analysis of the reasons for the statistical differences also includes a fourth reason, the difference in specific activity between the two methods caused by different material compositions: the types of material compositions actually sampled may be more than the types of material compositions input into the software. When this happens, the specific activity of the extra material compositions is calculated by chemical analysis methods. When the percentage of this specific activity is lower than a set value, it can be ignored. When the percentage of this specific activity is higher than the set value, this part of the specific activity should be calculated in the total specific activity of the sampling point, and the newly discovered material type should be input into the Monte Carlo software, and the software will recalculate the total specific activity of the sampling point, so as to achieve consistency between the theoretical calculation and the actual measurement results.

[0066] 3) If the first reason causes a statistical difference between the two methods, the sampling point position is adjusted and re-sampled, the re-sampling results are subjected to radionuclide chemical analysis to obtain the radionuclide type and specific activity of the sampling point after re-sampling, and then the specific activity of the sampling point obtained after re-sampling is compared with the radionuclide type and specific activity of each sampling point calculated theoretically; if there is no statistical difference in the results obtained by the two methods, the current position and specific activity results of each sampling point are recognized and transferred to process 6); if there is still a difference and it exceeds the set number of repetitions, continue with process 4); if there is still a difference but it does not exceed the set number of repetitions, repeat process 3);

[0067] 4) If the statistical difference between the two methods is due to the second reason, the sampling position is adjusted and re-sampled. The possibility of mixing shallow powder into the sample at the sampling point is eliminated by adjusting the sampling position. The re-sampling results are subjected to radionuclide chemical analysis to obtain the type and specific activity of the radionuclide at the sampling point after resampling. The specific activity of the sampling point obtained after resampling is then compared with the type and specific activity of the radionuclide at each sampling point calculated theoretically. If there is no statistical difference in the results obtained by the two methods, the position and specific activity results of the current sampling point are recognized and transferred to process 6); if there is still a difference and it exceeds the set number of repetitions, continue process 5); if there is still a difference but it does not exceed the set number of repetitions, repeat process 4);

[0068] 5) If the third reason causes a statistical difference between the two methods, the uncertainty of the chemical analysis method of each radionuclide Fe55, Ni63, Mn54, and Co60 is analyzed. If the uncertainty of the chemical analysis method is high and does not meet the set standard, the position and specific activity of the current sampling point with the difference will be used as the position and specific activity of the final sampling point, and the process is transferred to process 6); if the uncertainty of the chemical analysis method is low and meets the set standard, but there is still a difference between theory and practice, return to process 3) until the difference between the theoretical calculated specific activity and the actual calculated specific activity is within the allowable range;

[0069] The uncertainty of the chemical analysis method is calculated as follows:

[0070] 1) Divide the samples from the same sampling point into several parts;

[0071] 2) Analyze each sample using radionuclide chemical analysis methods;

[0072] 3) If the analysis results of several samples from the same sampling point are close to the same, the uncertainty of the chemical analysis method is considered to be acceptable; if the analysis results of several samples from the same sampling point are quite different, the uncertainty of the chemical analysis method is considered to be high; the close to the same means that if the analysis results of several samples are statistically analyzed and P>0.05, it is considered that there is no difference in the analysis results; the large difference means that if the analysis results of several samples are statistically analyzed and P<0.05, it is considered that there is a difference in the analysis results;

[0073] 6) Make source survey results based on the location and specific activity of the current sampling point.

[0074] It should be emphasized that the above specific embodiments are merely explanations of the present invention, and they are not limitations of the present invention. After reading this specification, those skilled in the art can make modifications to the above embodiments without any creative contribution as needed, but as long as they are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A method for investigating source items of medical cyclotron decommissioning, characterized by: The following steps are involved: Step 1: Calculate the neutron distribution using the Monte Carlo method to obtain the neutron distribution along the way from the neutron target outside the accelerator to the main magnet inside the accelerator; Step 2: setting the position of the sampling point according to the neutron distribution, wherein the position of the sampling point is divided according to the neutron distribution density; Step 3: Input the location of each sampling point into the Monte Carlo software to obtain the theoretically calculated type and specific activity of the radioactive nuclides at each sampling point, where the specific activity is the degree of radiation contamination; Step 4: Sampling is performed according to the location of each sampling point, and then radionuclide chemical analysis is performed according to the sampling results to obtain the type and specific activity of the radionuclide of the actual sampling point; the radionuclide chemical analysis is to use chemical analysis methods to analyze the radionuclide and specific activity of the sampled samples at each actual sampling point. The types of radionuclides mainly include Fe55, Ni63, Mn54, and Co60; different types have different chemical analysis methods; Step 5: Compare the radionuclide types and specific activity results obtained by the two methods, and continuously correct the position and specific activity of the actual sampling point with the theoretical calculation results until the statistical results of the two methods are close to the same; the two methods are both methods of using Monte Carlo algorithm for theoretical simulation calculation and sampling measurement to respectively determine the radioactive isotope type and specific activity for the same sampling point; The specific process of step five is as follows: 1) Perform statistical analysis on the theoretical calculated specific activity and the actual calculated specific activity. If there is no statistical difference in the results obtained by the two methods, the current positions and specific activity results of each sampling point are recognized and the process is transferred to process 6); if the statistical analysis results show that there is a statistical difference in the results obtained by the two methods, continue to process 2) for cause analysis: 2) Analyze the reasons for the statistical differences. First, there is inaccuracy in the location of the sampling point during sampling, which is too different from the sampling point location determined for the current specific activity during the Monte Carlo software simulation calculation; second, the sample at the sampling point may be affected by powder at other locations, and the influence of the powder at other locations is that the powder at the deep position of the sampling point is affected by the powder at the shallow position, and the powder at the shallow position is mixed with the powder at the deep position; third, when the specific activity analysis of the radionuclides of the samples at the sampling point is actually calculated, the uncertainty of the chemical analysis method is high, resulting in a large difference in the results of the two methods; 3) If the first reason causes a statistical difference between the two methods, adjust the sampling point position and resample, perform radionuclide chemical analysis on the resampled results, obtain the radionuclide type and specific activity of the sampling point after resampling, and then compare the specific activity of the sampling point obtained after resampling with the radionuclide type and specific activity of each sampling point calculated theoretically; if there is no statistical difference in the results obtained by the two methods, recognize the current position and specific activity results of each sampling point and proceed to process 6); if there is still a difference and it exceeds the set number of repetitions, continue with process 4); if there is still a difference but it does not exceed the set number of repetitions, repeat process 3); 4) If the statistical difference between the two methods is due to the second reason, the sampling position is adjusted and re-sampled. The possibility of shallow powder being mixed into the sample at the sampling point is eliminated by adjusting the sampling position. The re-sampling results are subjected to radionuclide chemical analysis to obtain the type and specific activity of the radionuclide at the sampling point after resampling. The specific activity of the sampling point obtained after resampling is then compared with the type and specific activity of the radionuclide at each sampling point calculated theoretically. If there is no statistical difference in the results obtained by the two methods, the position and specific activity results of the current sampling point are recognized and transferred to process 6); if there is still a difference and it exceeds the set number of repetitions, continue with process 5); if there is still a difference but it does not exceed the set number of repetitions, repeat process 4); 5) If the third reason causes a statistical difference between the two methods, the uncertainty of the chemical analysis method of each radionuclide Fe55, Ni63, Mn54, and Co60 is analyzed. If the uncertainty of the chemical analysis method is high and does not meet the set standard, the position and specific activity of the current sampling point with the difference will be used as the position and specific activity of the final sampling point, and the process will be transferred to process 6); if the uncertainty of the chemical analysis method is low and meets the set standard, but there is still a difference between theory and practice, return to process 3) until the difference between the theoretical calculated specific activity and the actual calculated specific activity is within the allowable range; 6) Make source survey results based on the location and specific activity of the current sampling point.

2. The method for investigating source items of medical cyclotron decommissioning according to claim 1, characterized in that: The uncertainty of the chemical analysis method is calculated as follows: 1) Divide the samples from the same sampling point into several parts; 2) Analyze each sample using radionuclide chemical analysis methods; 3) If the analysis results of several samples from the same sampling point are close to the same, the uncertainty of the chemical analysis method is considered acceptable; If the analysis results of several samples from the same sampling point differ greatly, it is considered that the uncertainty of the chemical analysis method is high; the said approach to the same means that when the analysis results of several samples are statistically analyzed, P>0.05, it is considered that there is no difference in the analysis results; the said large difference means that when the analysis results of several samples are statistically analyzed, P<0.05, it is considered that there is a difference in the analysis results.

3. A method for investigating source terms of medical cyclotron decommissioning according to claim 1, characterized in that: The specific process of step one is as follows: 1) Input various parameters excluding the sampling point location into the Monte Carlo software: particle type, beam energy, beam intensity, and the main components of the accelerator including the shape, size, material chemical element composition and proportion of the yoke, magnet, coil, neutron target material, neutron target location, and neutron target shape; 2) Monte Carlo software outputs neutron distribution: First, the neutron distribution outside the accelerator is output: based on the input neutron target material, neutron target position, neutron target shape, and particle type, beam energy, and beam intensity of the isotope production, the neutron energy spectrum generated by the target and the distribution of neutrons to the accelerator that it radiates are calculated; then the neutron distribution along the neutron path inside the accelerator is output: based on the input particle type, beam energy, beam intensity, and the shape, size, material chemical element composition and proportion of the accelerator yoke, magnet, coil, and vacuum chamber, the neutron distribution is calculated.

4. The method for investigating source items of medical cyclotron decommissioning according to claim 1, characterized in that: The specific process of step three is as follows: 1) Input various parameters including sampling point location into the Monte Carlo software: sampling point location, particle type, beam energy, beam intensity, main components of the accelerator including yoke, magnet, coil shape, size, material chemical element composition and proportion, neutron target material, neutron target location, neutron target shape; 2) The Monte Carlo software output includes the location of each sampling point and a list of the specific activity of the radionuclides at each sampling point.

5. The method for investigating source terms of medical cyclotron decommissioning according to claim 1, characterized in that: The step 2 is to set the position of the sampling point according to the neutron distribution, and the specific process is as follows: 1) Set the sampling point location in the Monte Carlo software according to the neutron distribution; 2) When the sampling point position set in the software is suitable for on-site measurement, the sampling position set in the software is used as the final sampling position; 3) When the sampling point position set in the software is not suitable for on-site measurement, a sampling point position suitable for on-site measurement is found at the measurement site to replace the sampling point position set in the software. The neutron density of the sampling point position suitable for on-site measurement should meet the sampling requirements, that is, the neutron density of the sampling point is medium-high density; 4) The sampling point locations suitable for field measurements are inferred to the Monte Carlo software as the sampling point location input parameters of the Monte Carlo software.

6. The method for investigating source terms of medical cyclotron decommissioning according to claim 1, characterized in that: The step 4 is to perform sampling according to the position of each sampling point, and the specific process is as follows: 1) Number each sampling point and record its location and depth; 2) Drill holes at the target location according to the recorded depth. The metal powder generated during drilling needs to be properly collected and stored. In addition, when taking samples from deeper locations, first clean the powder remaining in the hole and on the sampler from the shallower locations to avoid affecting the measurement results of the deep samples. 3) Sampling: clean up the superficial powder to avoid affecting the results.

7. The method for investigating source items of medical cyclotron decommissioning according to claim 1, characterized in that: The analysis of the causes of statistical differences in process 2) also includes a fourth cause, the difference in specific activity between the two methods caused by different material compositions: the types of material compositions actually sampled may be more than the types of material compositions input into the software. When this happens, the specific activity of the extra material compositions is calculated by chemical analysis. When the percentage of this specific activity is lower than a set value, it can be ignored. When the percentage of this specific activity is higher than the set value, this part of the specific activity is calculated in the total specific activity of the sampling point, and the newly discovered material type is input into the Monte Carlo software, which recalculates the total specific activity of the sampling point, so as to achieve consistency between the theoretical calculation and the actual measurement results.

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