Cartographie de la radioactivité naturelle de la région de l’extreme-nord du Cameroun

Abstract

The current work focuses on the mapping of natural radioactivity in the Far North region, Cameroon. To achieve this, the effective dose from internal irradiation by measuring the concentrations of 222Rn in soil and 222Rn, 220Rn and their progeny in dwellings. RADTRAK2 and RadonEye detectors were used to measure radon concentrations in confined air in 276 and 33 homes respectively. RADUET detectors were used to measure radon, thoron and thoron progeny monitors were used to measure thoron progeny in 40 homes. In order to better understand the variations of radon in different localities of the study area, using a Markus 10 type detector, radon measurements were also made at 101 points in the ground at 90 cm depth. To measure the ambient equivalent dose rate, a RadEye PRD-ER, Thermo Scientific was used. These different measurements allowed to determine the effective doses by external and internal irradiation in 4 Divisions of the Region. In soil, the radon concentrations varied from 1.2 to 138.3 kBq.m-3 with an average value of 32.7 kBq.m-3 ; more than half of the measurement points had concentrations higher than 25 kBq.m-3. This value is below 40 kBq.m-3, the action value according to Swedish criteria. In dwellings, the concentrations varied between 62 ± 26 and 310 ± 50 Bq.m-3, with a geometric mean of 151(42) Bq.m-3 and, from 1 to 1557 Bq.m-3 with a mean value of 74(3) Bq.m-3 respectively with RADTRAK2 and RadonEye. With RADTRAK2, half of the houses had a concentration greater than or equal to 160 Bq.m-3, a value higher than the reference level of 100 Bq.m-3 recommended by the World Health Organization (WHO). In 40 dwellings of the Division of Mayo-Tsanaga, the concentration of radon, thoron and EETC varied from 13 ± 1 to 125 ± 1 Bq.m-3, from 14 ± 1 to 357 ± 4 Bq.m-3, from 1 ± 1 to 35 ± 9 Bq.m-3 with the average value of 40(1) Bq.m-3, 111 ± 3 Bq.m-3 and 12(4) Bq.m-3 respectively. The ambient equivalent dose rate varied between 0.03 and 0.13 mSv.h-1 with an effective value by inhalation due to radon and its associated progeny varied from 1.17 to 5.86 mSv.y-1 with a mean value of 2.86 mSv.y-1. The effective dose by external irradiation varied from 0.23 to 1.14 mSv.y-1 with a mean value of 0.6 mSv.y-1. These different mean values are all above their world-wide counterparts set by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). In addition, the results obtained in the current study reveal a good correlation between the level of radioactivity of the area, the geological structure and the mineralogical composition of the soil. Although these different results in general do not present a very high risk of radiological exposure to the public, nevertheless, it is necessary that the rules of radioprotection are respected in order to reduce it.