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Abstract. The radioactivity of some structural building materials, rows, binders, and fi nal construction products, originating from Serbia or imported from other countries, was investigated in the current study by using the standard HPGe gamma spectrometry. The absorbed dose in the air was computed by the method of buildup factors for models of the room with the walls of concrete, gas-concrete, brick and stone. Using the conversion coeffi cients obtained by interpolation of the International Commission on Radiobiological Protection (ICRP) equivalent doses for isotropic irradiation, the corresponding average indoor effective dose from the radiation of building materials of 0.24 mSvy −1 was determined. The outdoor dose of 0.047 mSvy −1 was estimated on the basis of values of the specifi c absorbed dose rates calculated for the radiation of the series of 238 U, 232 Th and 40 K from the ground and covering materials. The literature values of the effective dose conversion coeffi cients for ground geometry were applied as well as the published data for content of the radionuclides in the soil.

The absorbed gamma dose rate in indoor air due to natural radionuclides in concrete as a building material was determined in this work. The dose rate conversion factors for (238)U, (232)Th and (40)K, for standard rooms as well as rooms with different sets of dimensions, were evaluated by the point kernel technique, using Harima (geometric progression) build-up factors. The values of the conversion factors, in units (nGy h(-1) (Bq kg(-1))(-1)) calculated for the standard room are: 0.76, 0.91 and 0.070, respectively for (238)U, (232)Th and (40)K. The fitting formula was obtained for dose rate conversion factors, enabling them to be conveniently calculated for a room with arbitrary dimensions. For concrete block samples collected in the area of Niš, Serbia, the measurement of the radionuclide activity concentrations was also carried out. The evaluated absorbed dose rate conversion factors were then applied in the assessment of corresponding indoor gamma dose rates, finding that all the concrete samples fulfilled the usage requirement.

The specific absorbed gamma dose rates, originating from natural radionuclides in concrete, were calculated at different positions of a detection point inside the standard room, as well as inside an example room. The specific absorbed dose rates corresponding to a wall with arbitrary dimensions and thickness were also evaluated, and appropriate fitting functions were developed, enabling dose rate calculation for most realistic rooms. In order to make calculation simpler, the expressions fitting the exposure build-up factors for whole (238)U and (232)Th radionuclide series and (40)K were derived in this work, as well as the specific absorbed dose rates from a point source in concrete. Calculated values of the specific absorbed dose rates at the centre point of the standard room for (238)U, (232)Th and (40)K are in the ranges of previously obtained data.