  The use of building materials containing naturally occurring
radionuclides,such as K-40, U-238 and Th-232 and their progeny, could lead to
external exposures of the residents of such buildings. In this paper, a set of
models are set up to calculate the specific effective dose rates(the effective
dose rate per Bq/kg of K-40, U-238 series, and Th-232 series) imposed to
residents by building materials with MCNPX code. Effect of chemical
composition, position concerned in the room and thickness as well as density of
material is analyzed. In order to facilitate more precise assessment of indoor
external dose due to gamma emitting radionuclides in building materials, three
regressive expressions are proposed and validated by measured data to calculate
specific effective rate for K-40, U-238 series and Th-232 series, respectively.
