Design And Optimization Of The Energy Degarder System For A Proton Therapy Facility

Compared to other radiation therapy methods such as X-rays and electrons,proton therapy has higher treatment accuracy and smaller penumbra due to the Bragg peak characteristics.The cyclotron-based proton therapy facility that has the advantages of small floor area,continuous output beam,and so on is widely used in the world.The energy degrader system as the energy modulation unit is one of the essential core components in the cyclotron-based proton therapy facility because of the fixed energy beam extracted from the cyclotron.The restricting of the emittance and divergence is necessary because the energy degrader system will increase the emittance and the energy spread.However,this will cause a large beam loss,because of which the therapeutic dose rate decreases,and the radiation increases,and the temperature of the degrader system goes up,and the vacuum is deteriorated.Therefore,the physical and engineering design of the energy degrader is one of the technical difficulties in the development of proton therapy facility.Based on Monte-Carlo principle,the beam tracking simulation is carried out.The theoretical basis of the influence of structure and material on the performance of the energy degrader is systematically analyzed.The variable angle multi-wedge degrader and the hierarchical combined degrader are proposed respectively.In addition,the possibility of applying boron carbide graphite mixed material(BGC)to the energy degrader is explored.Finally,the modeling and calculation analysis of the radiation protection,water cooling and vacuum system of the energy degrader are completed in this thesis,and the experimental research on material processing technology and gas release rate is carried out.In the physical design of the energy degrader,the design index and basic layout of the degrader system are determined.The scheme of symmetrical multi-wedge graphite degrader and multi-aperture separated collimators are adopted.The principle of beam emittance suppression for collimators with tilted holes is analyzed.Meanwhile,the monte-carlo beam tracking simulation based on Geant4 is carried out and the goal of precise continuous rapid adjustment and beam quality recovery is achieved.The beam energy adjustment range,emittance,and transmission efficiency meet the design specifications.Finally,the error sources of the energy degrader are analyzed and the calibration scheme is put forward.The performance parameters of the degrader such as the beam transmission efficiency,energy mudulation range and response speed are optimized by structure design and material selection in the optimization design of the energy degrader.This thesis systematically discusses the influence of the wedge angle,the number of wedges and the length of the degrader on the performance of the degrader.The variable angle multi-wedge degrader and the hierarchical combined degrader are proposed respectively.The variable angle multi-wedge degrader ensures a higher response speed and improving the energy modulating range,while the hierarchical combined degrader has a 36.7% higher transmission efficiency than the symmetric multi-wedge graphite energy degrader.At the same time,the influence of materials on the beam transmission efficiency is studied,and the possibility of applying boron carbide graphite mixed material(BGC)to the energy degrader is explored.BGC degrader increases the beam transmission efficiency by 15.9% compared with graphite degrader.The manufacturing and processing technologies of the isotropic high-purity high-density graphite material are detailedly introduced.In the engineering design of the energy degrader subsystem,the modeling and calculation analysis of the radiation protection,water cooling and vacuum system of the energy degrader are completed in this thesis.Firstly,according to the basic theory of radiation protection of components,the radiation protection design of the motor is accomplished.Based on FLUKA Monte-Carlo simulation software,the radiation-shielding scheme of PbSn alloy and polyethylene is determined,so that the working life of the motor meets the design requirements.As for the water-cooling system,the fluid-solid-thermal coupling simulation of the degrader is completed and then the water-cooling parameters are obtained so that the final temperature rise and thermal deformation meet the design requirements.As for the vacuum system,the experimental research on coating and gas venting rate of high-density graphite materials is carried out,and the vacuum simulation based on the test results and the selection of vacuum components are completed.