空气衰减对自由空气电离室是最大的修正项,对总的修正因子不确定度影响也大.本文采用钼靶X射线、真空管测试系统和圆柱型自由空气电离室测量电离电流,用EGSnrc (Electron Gamma Shower)程序模拟恒压管对X射线束额外损失的影响;用抽真空法对圆柱型自由空气电离室空气衰减修正因子的测量,修正了实际测量中圆柱型自由空气电离室限束光阑入射面到收集极这段距离的空气衰减,得到准确的电离电流测量值.结果表明:自由空气电离室的电离电流与恒压管的压强成正比,恒压管对于X射线束额外损失小于0.5%;在钼靶X射线基准的4个辐射质条件下测得的圆柱型自由空气电离室空气衰减修正因子与德国物理技术研究院(The Physikalisch-Technische Bundesanstalt, PTB)的测量结果比较,呈现相同的变化趋势,符合实验要求和国际标准.圆柱型自由空气电离室空气衰减修正因子的实验测量对于钼靶X射线空气比释动能的绝对测量具有科学参考价值.%Background:Air attenuation of free-air ionization chamber is one of the biggest correction terms and also has an influence on the total uncertainty of correction factor. Purpose:This study aims to measure the air attenuation correction factor for cylindrical free-air ionization chamber, and the correction of attenuation length between the diaphragm and the collecting region of cylindrical free-air ionization chamber in actual for the exact ionization current measurements. Methods:Molybdenum target X-ray vacuum tube test system and cylindrical free-air ionization chamber were used to measure the ionization current by means of vacuum. Electron Gamma Shower (EGSnrc) program was used to simulate constant pressure pipe for X-ray beam of the influence of additional losses.Results:The results showed that the ionization current of free-air ionization chamber was proportional to the pressure of the constant pressure pipe; and the extra loss of constant pressure pipe for X-ray beam was less than 0.5%; the design of the vacuum tube test system conformed to the requirements of experiment. Conclusion:For the air kerma absolute measurement of molybdenum target X-ray, the method of experimental measurement of air attenuation correction factor of cylindrical free air ionization chamber is of scientific reference value.
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