粒子源水吸收剂量绝对测量装置内部电场模拟

Simulation of internal electric field of particle source water absorbed dose absolute measurement device

拟研制125I粒子源水吸收剂量绝对测量电离室,通过Maxwell软件建立电离室内部模型,模拟得出电离室无保护极、不同保护极环宽度,不同绝缘环宽度、不同栅极个数、不同栅极形状和不同栅极厚度下的电场强度分布,并进行了定性和定量的分析。结果表明:绝缘环宽度的增加会使收集极边缘处的边缘效应增强,应尽量使绝缘环宽度减小;在设计电离室时,应当使保护极宽度和收集极半径比值不低于2。当栅极个数为15个时,电场强度的变化幅度可减少为1%左右,表明通过增加栅极个数能有效改善电场的均匀性。相较于栅极截面为圆形和三角形,当栅极截面为矩形时,电场强度的变化幅度较小。栅极厚度越大,栅极边缘处的边缘效应越严重,应尽量减少栅极厚度。本次研究结果将为后续粒子源水吸收剂量绝对测量标准装置的优化设计提供支撑。

[Background]The physical quantity concerned in the clinical application of 125I particle source brachytherapy is the 1 cm water absorbed dose rate Ḋw,1cm.However,there is no corresponding standard for this physical quantity in China.It is planned to develop an ionization chamber for the absolute measurement of the water absorbed dose rate of the 125I particle source as a standard device,hence the internal electric field of the ionization chamber must first be analyzed to obtain the most reasonable design scheme to satisfy the condition that the electric field intensity is uniformly distributed in the ionization chamber.[Purpose]This study aims to simulate the internal electric field of the ionization chamber for the design of particle source water absorbed dose absolute measurement device.[Methods]Firstly,the internal model of the ionization chamber was established by Maxwell software to simulate the distribution of electric field intensity under the six variables of the ionization chamber:with or without a protective electrode,different protective electrode ring width,different insulation ring width,different grid number,different grid shape and different grid thickness.Then,qualitative and quantitative analysis were carried out using finite element method.Finally,the influence of different variables on the distribution of electric field intensity in the ionization chamber of the absolute measurement of water absorbed dose of 125I particle source was obtained.[Results]Analysis results show the ratio of the width of the guard electrode to the radius of the collector electrode must be not less than 2 for design of the ionization chamber.The edge effect at the edge of the collector is increased with width increment of the insulator ring,hence the width of the insulator ring should be reduced as much as possible.When the number of grid electrodes is 15,the variation of the electric field intensity can be reduced to about 1%.When the grid section is rectangular,the variation in electric field intensity is small compared with the circular and triangular grid sections.The larger the thickness of the grid,the more severe the edge effect at the edge of the grid,and the thickness of the grid should be reduced as much as possible.[Conclusions]The uniformity of the electric field can be effectively improved by increasing the number of grid electrodes.Results of this study is helpful to optimal design of the standard device for measuring the absolute water absorbed dose of 125I particle source.