DETERMINATION OF NATURAL RADIOACTIVITY IN SOILS AND THEIR TRANSFER RATIO FROM SOIL TO PLANT IN ONDO AND EKITI STATES, SOUTHWESTERN, NIGERIA

Abstract

The activity concentrations of 40K, and 238U and 232Th have been analyzed in a total of forty-five (45) soil and plant samples collected from selected areas in Ondo and Ekiti States namely: Akure, Ondo, Okitipupa, Owo and Ado-Ekiti. The samples were prepared for analysis according to the International Atomic Energy Agency (IAEA) procedures and then analyzed using gamma ray spectrometry technique with NaI(Tl) scintillation detector (Canberra Industries Inc.). The energy resolution of the detector was 8% at 0.662 MeV of 137Cs, a value good enough to distinguish the gamma ray energies of most environmental radionuclides present in the samples. The counting assembly was housed in a cylindrical lead castle to shield against environmental background radiation that might interfere with the result. The detector was coupled with a Multichannel Analyzer (MCA) via a 50 Ω coaxial cable. Calibration of the spectrometer system was done by detecting the efficiencies of radionuclides 40K, 232Th and 238U at fixed sample geometry and matrix achievable by keeping the detector-sample arrangement constant throughout the period of assay of the samples over the same counting time. Each sample was counted for 7 hours (25200 seconds) because of the low gamma counts in the samples. The soil 40K, 238U and 232Th activity concentrations ranged from 123.19 ± 28.32Bq kg-1 in Ayeka in Okitipupa to 521.00 ± 98.69 Bqkg-1 in Kajola in Akure, 8.87 ± 3.01Bqkg-1 in Itanla in Ondo to 31.13 ± 7.17 Bq kg-1 in Ureje in Ado-Ekiti and 9.87 ± 3.98 Bq kg-1 in Itanla in Ondo to 34.12 ± 8.73 Bq kg-1 in Ureje in Ado-Ekiti respectively for soil. Also in plant samples 40K, 238U and 232Th activity concentrations ranged from 77.65 ± 19.78 Bq kg-1 in Sabo in Ondo to 209.81 ± 42.30 Bq kg-1 in Ayeka in Okitipupa, 4.95 ± 1.78 Bq kg-1 in Sabo Ondo to 13.12 ± 4.17 Bq kg-1 in Oke-ogun in Owo and 6.28 ± 2.18 Bq kg-1 in Oda in Akure to 16.28 ± 5.96 Bq kg-1 in Oke-ogun in Owo respectively. The mean absorbed dose rates due to the presence of 40K, 238U and 232Th in the soil samples was 30.09 nGyh-1, the result obtained fell within the values (30 nGy h-1-70 nGy h-1) recommended by United Nations Scientific Committee on the Effects of Atomic Radiation for areas with normal background radiation and the annual effective dose in the farms due to soil radioactivity in the selected areas is 55.39mSv y-1. The mean effective doses for soils were considerably higher than the world mean value for soil of 1.0 mSv y-1.While the annual committed effective dose for ingestion of these plant samples as medicinal plants is 0.0060 ± 0.0018 mSv a-1.The average annual committed efffective dose calculated due to the ingestion of radionuclides in these plant samples is far below the average radiation dose of 0.3 mSv a-1 recommended byUnited Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). This indicated that there is no high exposure for either farmers or workers dealing with transportation of soil and plants in these study areas.