Natural radiation exposure to the public in the uranium bearing region of Poli, Cameroon: From radioactivity measurements to external and inhalation dose assessment

The present work reports measurements of activity concentrations of U-238, Th-232, and K-40 in soil, Rn-222 and Rn-220 in dwellings, and air kerma rates followed by external and inhalation dose assessment to the public in the uranium bearing region of Poli. These measurements were carried out using a 3-in x 3-in NaI(Tl) scintillation spectrometer for radioactivity in soil, car-borne survey method for air kerma rates, and Passive integrated radon-thoron discriminative detectors (RADUET) and thoron progeny monitors for radon, thoron, and thoron progeny indoors respectively. In-situ activity concentrations of U-238, Th-232, and K-40 range between 13-52 Bq kg(-1), 10-67 Bq kg(-1), and 242-777 Bq kg(-1) respectively with respective average values of 32 Bq kg(-1), 31 Bq kg(-1), and 510 Bq kg(-1). Air kerma rates range between 25 and 102 nGy h(-1) with the mean value of 57 nGy h(-1). The annual effective doses due to public external exposure range between 0.20 and 0.83 mSv y(-1) with the mean value of 0.46 mSv y(-1). Taking into account of the number of inhabitants and the level of external dose, some inhabited areas were selected for radon, thoron and thoron progeny measurements in 100 houses. Radon and thoron concentrations range between 46-137 Bq.m(-3) and 24-238 Bq.m(-3) with the arithmetic mean values of 82 Bq m(-3) and 94 Bq m(-3) respectively. The equilibrium equivalent thoron concentrations (EETC) and the equilibrium factor for thoron range respectively between 4-29.6 Bq m(-3) and 0.008-0.3 with the average values of 8.3 Bq m(-3) and 0.08. The equilibrium factor found to be higher than the world average value of 0.02 given by the United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR). The inhalation doses due to radon progeny and thoron progeny range between 0.87-2.6 mSv y(-1) and 0.34-6.2 mSv y(-1) respectively. The contribution of thoron to the total inhalation dose ranges between 12-67% with the average value of 49%, showing that thoron cannot be neglected in dose assessment as done in the past.