Research On Radiation Protection And Beam Measurement Of The Degrader For Proton Therapy

The proton therapy system is a large-scale medical device involving a variety of disciplines such as medical accelerators,medical physics,beam physics,medical imaging,and radiation protection.In the HUST-PTF proton therapy device beam transport system,the function of the energy reducer is to achieve rapid and continuous adjustment of the proton beam energy in the range of 70~240 MeV,while considering the requirements of radiation protection and beam measurement.On the one hand,it is necessary to evaluate the effects of the instantaneous radiation and induced radiation generated under the maximum energy modulation on the degrader material and the auxiliary electronic device.On the other hand,according to the functional requirements,it is required to design a faraday cup device used for the measurement and countercheck of the proton beam current with energy in 250MeV,and optimize collection efficiency and design corresponding suppression devices based on the secondary electron emission rate.Based on the requirements of the radiation resistance of the degrader and its components in the HUST-PTF beam transport system,the detailed radiation dose calculation and partial shielding design of the vacuum chamber seal and motor platform are carried out.In the calculation of the absorbed dose of the sealing ring,this paper compares the three schemes of natural rubber,NBR(Nitrile Butadiene Rubber)and EPDM(Ethylene-Propylene-Diene Monomer).Finally,combined with the calculation results,the radiation resistance of each material and the actuality of the HUST-PTF project are considered.For engineering requirements,EPDM material was selected as the sealing material for the vacuum chamber of the degrader.For the partial shielding design of the motor platform,the high radiation-resistant motor(radiation-tolerant cumulative dose 10~6Gy)is used as the research object.Based on the simplified model,the absorbed dose,neutron and photon flux of the motor are calculated,and the corresponding combined shielding body is designed to finally make the reliable working time of the motor is more than 25,000 hours.At the same time,this paper also analyzes and calculates the induced radiation field of the degrader.The degrader and the collimator are taken as the research object,analyzing the type,radioactivity and decay process of the radioisotope due to the activation effect after the equipment is stopped,to provided theoretical guidance for the later maintenance of the energy device.In order to meet the requirements of accurate measurement of 250MeV proton beam in HUST-PTF beam transport system,this paper designed a faraday cup for 250MeV proton beam current measurement,and studied the influence of faraday cup wall thickness,well depth and taper on collection efficiency.The collection efficiency is increased to 98%by optimizing the structure.Under the faraday cup structure designed and optimized in this paper,the escape rate of secondary electrons is less than 1%.Theoretically,it is not necessary to design a suppression scheme separately,but considering the material process level and working environment of the degrader,a high-voltage electrode at the cup interface is used to suppress the reverse escape of secondary electrons in the engineering scheme.