Evaluation de l’exposition du public à la radioactivité naturelle suivant l’axe Akongo-Lolodorf-Kribi au Cameroun par les mesures directes des descendants du Radon et du Thoron

Abstract

This study deals with natural radiation exposure to the public caused by external sources and inhalation of radon, thoron and their progeny in seven inhabited areas, following the Akongo Lolodorf-Kribi axis, Cameroon. The equilibrium (FT n) factor between thoron and its progeny was determined in order to show the importance of direct progeny measurements for correct estima tion of effective dose due to radon, thoron and their progenies. The level of natural radioactivity in the soil was evaluated by car-borne survey, in-situ and laboratory gamma spectrometry me thod using 3-in × 3-in NaI(Tl) scintillation spectrometer, and a distribution map of absorbed dose rate in air was performed. In order to assess internal exposure due to radon and thoron, passive type radon-thoron detectors and thoron progeny monitors were deployed for two months in 220 dwellings of the study area. The average absorbed dose rates in air, the external effective dose and the activity concentrations of 238U, 232T h and 40K are 50.4 nGy h−1, 0.33 mSv y−1, 33 Bq kg−1 53 Bq kg−1, 182 Bq kg−1 in soil, and 37 nGy h−1, 0.3 mSv y−1, 22 Bq kg−1, 37 Bq kg−198 Bq kg−1 in soil samples analyzed in laboratory. The materials used in the construction of most of buildings contain radionuclides. The average ratio of the indoor dose rate to outdoor dose rate of 1.02 estimated in the entire studied area was applied to take into account their contribution in the external dose estimation. Compared to UNSCEAR limits, these values are relatively high in some parts of the study area. Except for these points and their nearest neighborhoods, soils can be used as building material. Average geometric concentrations of radon, thoron and thoron progeny were respectively 89 Bq m−3, 118 Bq m−3, 7.4 Bq m−3. Total effective dose due to radon and thoron was estimated at 4.2 mSv y−1; thoron contribution to effective dose ranged between 3 and 80% with the average value of 53%. FT n varied according to seasons, the type of dwelling, building materials and localities. In addition, effective dose due to thoron varied greatly according to the different values taken by FT n and was different from that determined directly using the thoron progeny concentrations. Thus, effective dose due to thoron determined from the equilibrium factor is unreliable. Therefore, the risk of public exposure due to thoron (Tn and TnP) may therefore be higher than that of radon (Rn and RnP) in many parts of the world if FT n is no longer used in estimating total effective dose. This is not in contradiction with the UNSCEAR conclusions. It is therefore important to directly measure the radon and thoron progeny for a correct estimate of effective dose.