电离法测量~(60)Coγ射线在石墨体内吸收剂量的扰动修正

本文主要介绍用改变空腔体积的直线外推法测量指形石墨电离室在石墨模体内的扰动修正因子 k_p 的方法和结果。采用一组形状和外径(φ=13.5mm)完全相同、空腔有效体积υ分别为0.4923、0.6832、0.9060、1.2092、1.4713和1.8818cm~3的6个指形石墨电离室,放置在距离模体表面深度 h为5.13g/cm~2的石墨模体内,在准直~(60)Co γ射线束照射下,测量了空腔内单位质量空气中的电离电荷 J_(?)随υ的变化曲线;由该曲线外推得到υ=0时的 J_a 值,即(J_a)_0;再求得 k_p=(J_a)_0/(J_a)_v)值及其随υ的变化曲线。得到回归方程为 k_p=1.000—8.24×10~(-3)υ。所得 k_p 值,可用于指形石墨电离室测量石墨模体内吸收剂量时的扰动修正。

PERTURBATION CORRECTION FOR THE IONOMETRIC DETERMINATION OF ABSORBED DOSE IN A GRAPHIT PHANTOM FOR ~(60)Co GAMMA RAYS

This article mainly introduces a method for measuring the perturbation correction factor ofthe thumb chamber in a graphit phantom by means of varying the volume of cavity of the chamber andusing linear extrapolation method.A set of ionizing chambers,which have the same shape and outer di-ameter(the effective cavity volumes 0.4923,0.6832,0.9060,1.2092,1.4713 and 1.8818cm~3 re-spectively),were placed by turns in graphite phantom with mass depth of 5.13 g/cm~2 and irradiated bycollimated beam of ~(60)Co gamma rays.The charge per unit mass,J_a,in the cavity for chambers of dif-ferent volume was measured and used for extrapolation from volume v to zero so that to find the per-turbation factor k_p(?)as function of the volume and then the regression equation of k_p=a+bvThe obtained k_p can be used for the perturbation correction in the measurement of absorbed dose in thegraphit phantom with thumb chamber.