DETERMINATION OF NATURAL RADIOACTIVITY OF SOME SELECTED ECONOMIC MINERALS FROM QUARRY SITES IN IGARRA AREA, EDO STATE, NIGERIA

Abstract

Activity concentrations of radionuclides in rocks and minerals are of vital importance in geosciences and health physics especially that primordial radionuclides constitute the main source of radiation exposure risks externally in soil, rocks and minerals. In this study, samples of four (4) economic minerals, namely dolomite, feldspar, limestone and calcite, were collected from quarry sites in Igarra area, Edo State, Nigeria and analysed using gamma ray spectrometric technique. From the analytical results, the specific activity concentrations of 226Ra, 232Th and 40K for dolomite ranges from 0.79±0.1 to 4.3±0.24 Bq/kg, 0.27±0.09 to 3.65±0.30 Bq/kg and 27.4±1.91 to 505.19±8.19 Bq/kg with respective mean of 2.88 Bq/kg, 1.61Bq/kg and 227.33 Bq/kg. In feldspar, the activity values range from 0.69±0.09 to 2.01±0.15 Bq/kg for 226Ra, 0.52±0.11 to 1.06±0.15 Bq/kg for 232Th and 49.29±2.30 to 248.42±4.99 Bq/kg for 40K with respective mean of 1.40, 0.76 and 152.28 Bq/kg. Also in calcite, the activity concentration ranges from 0.54±0.09 to 10.57±0.41 Bq/kg for 226Ra, 1.28±0.19 to 6.12±0.44 Bq/kg for 232Th and 81.37±3.32 to 795.16±11.02 Bq/kg for 40K with respective mean of 5.54, 3.81 and 466.62 Bq/kg. Similarly, in limestone, the activity concentrations range from 0.63±0.09 to 3.91±0.22 Bq/kg for 226Ra, 0.67±0.13 to 2.67±0.26 Bq/kg for 232Th and 57.49±2.60 to 421.39±7.17 Bq/kg for 40K with respective mean of 2.05, 1.59 and 220.02 Bq/kg. Mean specific activities of 226Ra, 232Th and 40K in the four economic minerals were below the world average values of 30, 35 and 400 Bq/kg respectively as stated by the United Nation Scientific Committee on the Effect of Atomic Radiation except calcite that exceeded slightly the value of 40K. Mean absorbed dose computed for dolomite, feldspar, calcite and limestone are 9.99, 7.08, 23.58 and 10.70 nGy/h respectively, which are below the world average of 58 nGy/h. Computed mean annual effective dose equivalent outdoor (occupational) for dolomite, feldspar, calcite and limestone are 0.02, 0.01, 0.04 and 0.02 mSv/y respectively, with corresponding average indoor (residential) values of 0.05, 0.04, 0.12 and 0.05 mSv/y in sequence. Mean external hazard index computed for dolomite, feldspar, calcite and limestone are 0.06, 0.03, 0.12 and 0.05 respectively, which are below unity. Similarly calculated excess lifetime cancer risk for both residential and occupational exposures to dolomite, feldspar, calcite and limestone are (0.06, 0.14, 0.43, 0.21)×10-3 and (0.22, 0.04, 0.13, 0.06)×10-3 respectively. These values agree with the worldwide average value of 0.29×10-3. The usage of these economic minerals as aggregate of building materials or for any domestic purposes does not pose any risk to the public from the point of radiation protection. Continuous radiological check of the economic minerals is however recommended to always ensure that the radiation doses are as low as reasonably achievable.