Estimation of absorbed gamma dose rate from granite by Monte Carlo simulation approach

Apart from the continuous exposure of humans to background ionising radiation, an increased level of radiation may also originate from the use of building materials with an enhanced level of radioactivity. Thus, it is necessary to examine the content of radionuclides present in building materials, as well as the corresponding dose which may be received by residents from these materials. In this paper, particular attention was dedicated to finding the absorbed dose rate and annual effective dose caused by the presence of naturally occurring radioisotopes ²²⁶Ra, ²³²Th, and ⁴⁰K in granite, a widely used building material, by means of Monte Carlo simulations. In addition, the obtained dose rate simulation results were compared with values estimated from commonly used simple equations, relating to the dose rate emitted by granite plates, covering the interior of a standard room. In the simulation, a room was constructed with standard dimensions (4 m × 5 m × 2.8 m), and with floor and walls covered with 3 cm thick granite. A water cylinder (approximate mass 65 kg) was positioned in the center of the room, representing a human body. The emission of the most intense gamma rays from ²²⁶Ra and ²³²Th progenies, as well as from ⁴⁰K, emanating from the granite matrix, was simulated. The number of generated photons in each simulation (typically it was an order of magnitude of ~10⁶) precisely represented actual activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in granite samples. All processes playing a role in the interactions of gamma photons with the granite matrix itself, the outer concrete shell, the air within the room, and the water cylinder, were taken into account by GEANT4 simulation software, after which the spectra of deposited energy inside of the water cylinder were obtained. Based on the deposited energy, the absorbed dose rate and annual effective dose were calculated for 6 analysed granite samples, each with different ²²⁶Ra, ²³²Th, and ⁴⁰K contents. Furthermore, the effect of the position of the water cylinder in the simulated room on the absorbed dose rate was considered, as well as the distribution of the deposited energy within the water cylinder. The absorbed dose rates, and consequently annual effective dose, obtained in the simulations were found to be 30%-40% higher than the values obtained from using a standard formula.