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1 测量β射线的吸收剂量 1.1 基本原理 外推电离室是其电极之间距离可变的平行板电离室。当电离室的电极逐渐接近而电极间距离逐渐缩短时,其体积和电离电流也将减小。 在讨论把空腔电离室理论运用到β射线而设计的外推电离室时,必须假定这个趋于零的小空腔的存在,不会扭曲β射线的注量。为了测量β粒子源产生的辐射场中某处的组织剂量率D_T,建议外推电离室及其7mg/cm~2的入射窗用低原子序数的材料(如石墨或塑料)制作。此外,电离室的体积应由足够厚的材料包围起来,相当于一个无限大的组织等效模体,也就是要求电离室后壁及其侧壁足够厚,至少能够全部吸收所存在的最大能量的β粒子。均匀辐射束的面积应至少是上面的最小模体的面积。在这些条件下,利用所熟悉的Bragg-Gray公式得到:
1 Measurement of β-ray absorbed dose 1.1 Basic principle Extrapolation ionization chamber is a parallel plate ionization chamber whose distance between electrodes is variable. When the ionization chamber electrode gradually approached and the distance between the electrodes gradually shortened, the volume and ionization current will also be reduced. When discussing extrapolated ionization chambers designed to apply the theory of cavity ionization to β-rays, it must be assumed that the existence of a small cavity that tends to zero does not distort the beta-ray fluence. In order to measure the tissue dose rate D_T somewhere in the radiation field generated by the beta particle source, it is recommended that the extrapolation chamber and its entrance window of 7 mg / cm ~ 2 be made of low atomic number material such as graphite or plastic. In addition, the volume of the ionization chamber should be surrounded by a sufficiently thick material, equivalent to an infinite tissue equivalent phantom, that is to say the back wall of the ionization chamber and its side walls are thick enough to at least fully absorb the maximum energy present Of beta particles. The area of the uniform radiation beam should be at least the area of the smallest phantom above. Under these conditions, using the familiar Bragg-Gray formula: