| As a new non-thermal processing technology,electron beam irradiation has the advantages of high power,high beam energy utilization and short irradiation time.In recent years,the electron beam irradiation technology at home and abroad has become increasingly improving,and its application capabilities and socio-economic benefits have been continuously highlighted.However,the complexity of the radiation field distribution of electron irradiation accelerators during operation,as well as the diversity of the output beam and processed products,all reflect the urgency of further research of dose distribution characteristics and shielding optimization for specific energy electron irradiation accelerators.This paper focused on the 5 Me V electron irradiation accelerator,relying on the relevant project of CIAE,and based on the Geant4 framework,has carried out three aspects of work.The results provide a reference for the study of dose and shielding of specific energy electron irradiation accelerators.Firstly the dose rate of the shielding wall,points in the maze,the sky backscatter,and the ozone concentration during accelerator operation were calculated by using theoretical formulas:the dose rates were all less than 2.5μSv/h;the ozone mass volume concentration reached 68.2 mg/m~3per hour.When the air exchange rate was once per minute,the workers could enter the irradiation room after about 8 minutes of shutdown of the accelerator.Subsequently,the energy spectrum and angular distribution of bremsstrahlung generated by 5Me V electron shot the iron target were simulated using Geant4.Based on this,the dose rates at various detection points during the operation of the accelerator were simulated,with a deviation of less than 10%from the theoretical calculation value,verifying the accuracy of the simulation calculation.Secondly,theoretical calculations,simulation validation,and optimization of shielding materials were conducted for the existing shielding scheme of the accelerator.Theoretical calculations have been conducted on the required shielding thickness at different locations,and the calculated thickness was smaller than the existing design thickness.The simulation calculation of X-ray emissivity parameters for electron beam shooting showed that the X-ray emissivity of 5 Me V electron shooting in 90°and 0°directions was in good agreement with the values given in NCRP No.51 report;under the same target radius,the optimal target thickness to meet the maximum X-ray emissivity varied with the direction.The simulation results of different optimization schemes for the material combination of the shield showed that the use of low Z materials as the first shielding layer has effects on reducing the interaction between electrons and the shielding material nucleus;using a combination of aluminum clad polyethylene and steel clad lead as the shielding material for the same volume,the shielding effect remained unchanged and saved more than 50%of the construction cost.After exploring the radiation characteristics of the accelerator itself,based on the electron beam emitted by the accelerator,the internal dose distribution of two typical irradiation processing materials,water and polyethylene,was studied.From the results,the higher the incident electron energy,the greater the maximum dose depth and optimal thickness it can reach in the irradiated material;the absorbed dose value and distribution range within the irradiated material showed a trend of first increasing and then decreasing with the increase of electron incident depth.Under single-sided irradiation condition,there was an approximate linear proportional relationship between the electron incident energy and the optimal thickness of irradiated products.The presence or absence of substrates can affect the depth dose distribution of irradiated products,and the influence of high Z substrates on the depth dose distribution of products was much greater than that of low Z substrates.As for polyethylene materials,the different areas of the electron beam have no significant boundary effect on their depth dose distribution.Finally,based on the X-rays emitted by the electron irradiation accelerator,in order to improve the uniformity of its dose distribution,the simulation design and physical testing of the filter were carried out using Geant4.The results showed that it can meet the uniformity requirement,and the X-ray dose rate can reach over90%of the central point dose rate within the required±7.5°range,the relative error between the measured value and the simulated value is less than 2.8%,achieving the design goal. |
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