居室墙体中天然γ核素就地测量技术研究

本文通过理论计算和实验测量的方法,研究墙体中天然γ核素比活度就地测量问题。理论分析了不平衡状态下226 Ra、232 Th、40 K的测量方法,由实验室测量238 U的第1代子体234 Th及通过235 U和238 U的天然同位素丰度比值关系,获得典型居室墙体中235 U对226 Ra就地测量的干扰系数;利用ISOCS无源效率模拟研究了测量时墙体之间的干扰;并研究了砖块形状和墙材原料配比等因素对无源效率刻度的影响。结果表明,235 U对226 Ra就地测量的干扰系数约为0.46;Falcon 5000在居室几何条件下测量单面墙体时,经8mm钨屏蔽后,40 K的干扰扣除系数为0.33;最后分析获得居室墙体中226 Ra、232 Th和40 K就地测量时总不确定度分别为26%、17%和15%,可用于室内环境调查。

Study on In-situ Measurement Technique of Natural γ Nuclide in Indoor Wall

The in-situ measurement of natural γ nuclide specific activity in walls was studied through theoretical calculation and experimental measurement. The method for measuring ²²⁶Ra, ²³²Th and ⁴⁰K in non-equilibrium state was analyzed theoretically, and the interference coefficient from ²³⁵U when measuring ²²⁶Ra in situ was estimated by laboratory measuring ²³⁴Th which is the first generation daughter of ²³⁸U and according to the natural isotope abundance ratio between ²³⁵U and ²³⁸U. The interference in measurement between the walls in the same room was studied using ISOCS passive efficiency simulation. And the effects on the passive efficiency calibration from the brick shape and raw material ratio were also studied. The results show that the interference coefficient from ²³⁵U when measuring ²²⁶Ra in-situ is about 0.46, and the deduction coefficient of ⁴⁰K when measuring the single wall by Falcon 5000 is about 0.33 under the indoor geometrical condition with 8 mm tungsten shield. The total uncertainties of in-situ measurement were analyzed, which are respectively 26%, 17% and 15% for ²²⁶Ra, ²³²Th and ⁴⁰K in indoor walls. As a conclusion, the in-situ measurement can be used for indoor environment survey.