Study On Key Question Of Radiation Protection For 15MV Medical Electron Linear Accelerator Based On Monte Carlo

Objective:With the widespread application of high-energy medical electron linear accelerator in the tumor radiotherapy,its radiation protection has become the focus of attention,especially in photoneutron contamination.Due to the photon radiation field mixed with neutron inside high-energy electron accelerator room,using the conventional detectors to measurement the neutron radiation has great limitation,particularly in the maze.The measurement deviation is so big that it is not conducive to the radiation protection of the accelerators.In order to deeply study the characteristics of the radiation field of the accelerator rooms,and optimize calculating method for radiation field in maze,this research has used Monte Carlo to simulate radiation dose field of photoneutron and capture gamma in maze and tube.New estimation formula for calculating the dose equivalent of photo-neutrons and capture gamma in maze was established,to analyze the radiation dose field in the linac room more accurately,and promote the radiation shielding for medical electron accelerator.Methods: For studying medical electron linear accelerator radiation field inside the room,15 MV X-ray was used as the source term of Monte Carlo simulation,which is constructed to research X-ray beam from the electrons' collision with aim.PDD curve and OAR curve in standard radiation field were compared with the measured results,to verify the head model is reliable.Moreover,some parameters in irradiation field was also simulated,such as neutron energy spectrum,dose equivalent of the isocenter,neutron quantum,which were compared with the related literature to verify the reliability of simulation results for photoneutron radiation field,ensuring the accuracy of the photo-neutron radiation field data in maze next.Then,by simulating radiation field within the maze,energy spectrum feature and attenuation law for scattering X-ray,neutron and capture gamma dose were analyzed.Characteristics of neutron radiation field inside the inclined pipe was analyzed also,to put forward radiation protection suggestions.In addition,the calculation differences of radiation dose between formula recommended by NCRP and MC simulation was studied.Coupled with radiation field simulation data within the mazes of the different size rooms,more accurate formula of neutron and capture gamma radiation dose equivalent in maze was established on the basis of the formula recommended by NCRP.Finally,through studying door's amplification for capture gamma radiation intensity at the entrance,and radiation shielding effect of the doors with different material set orders,the reasonable arrangement order of the door materials was determined and shielding design of the door will be optimized.Results: With different exposure radiation fields,scattering X-ray dose equivalent variation is great in the maze of 15 MV accelerator room,but the difference of average energy is very small.Moreover,the photoneutron and capture gamma dose equivalent in proportion to the maximum and minimum values are less than 1.35.After the collimation is entiredly closed,average energy of the maze capture gamma is 1.56 MeV,far more than the scattering X-ray,and the contribution of neutron fluence is given priority to with thermal neutron at the entrance.The attenuation speed for photoneutron and scattering X-ray dose equivalent is significantly faster than the capture gamma.Three attenuation TVD values are 1.9 m,5.4 m,and 8.7 m respectively.Compared to the state with no door,capture gamma fluences in energy zone near 2.2 MeV and low-energy zone are increased significantly in the entrane of the door.Energy spectrum of duct neutron is changed in the same rule with the maze.However,under the influence of transmission neutron from pipe wall the attenuation speed for dose equivalent is very slow at first,after a few centimeters grew quickly.Under maximum exposure conditions,dose rate outside the mouth of pipe 20 cm in diameter(with 60 degrees Angle)is 4.7 ?Sv/h.The difference between the neutrons and capture gamma dose equivalent caculated by new formula and the MC simulation value is less than 25% in the maze,which is far less than the calculation deviation of original formula.Finally,the door shielding effect with different material arrangement orders was validated.Results show that the lead plate of the door should be all set on the outside of neutron shielding material,door shielding effect is the best.Conclusions: The radiation spectrum and dose distribution in the maze of the medical electron accelerator room are closely related to the attenuation of the radiation and the design of shielding protection.This research results,equivalent formula for calculating radiation dose of the internal mazde,which was as establish on MC simulation data and the shielding effect of different materials order of shielding door is performance in this study,provide radiation protection personnel with important scientific reference on optimizing design of linac shielding.