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.