Activity concentrations and dose assessment of 226Ra, 228Ra, 232Th, 40K , 222Rn and 220Rn in soil samples from Newmont-Akyem gold mine using gamma-ray spectrometry

Abstract

In this study 14 soil samples were measured for natural radioactivity levels including radon-222 (222Rn) and radon-220 (220Rn) concentration at Akyem-Gold Mine premises, surrounding communities in Ghana. Both radon and radioactivity concentrations of radium-226 (226Ra), thorium-232 (232Th) and potassium-40 (40K) were determined by means of gamma spectrometry system equipped with high purity germanium detector. The studied samples gave natural radioactivity levels of 28, 12 and 11 Bq/kg, respectively compared to global 226Ra, 232Th, and 40K concentrations of 37, 33 and 400 Bq/kg, respectively, according to UNSCEAR (2000) report. The annual effective dose rate (AED) due to external and internal gamma exposure ranged from 0.060 to 0.18 mSvy-1 with a mean value of 0.11 ± 0.03 mSvy-1 compared to the recommended value of 1 mSvy-1. There is a correlation between 226Ra and 222Rn in soil gas with a good linear coefficient of (R2 =1). The availability of 226Ra and 222Rn shows that there is a source of uranium-238 (238U) and thorium-232 (232Th) bearing minerals within the adjacent geologic units of Akyem. This implies that most of the radon in the soil gas comes from 226Ra. The assessment of radium equivalent activity varied from 19.71 to 69.88 Bq/kg with mean value of 37.53 ± 15.51 Bq/kg lower than the global limit of 370 Bq/kg. The internal hazard index ranged from 0.07 to 0.25 Bq/Kg with a mean value of 0.13 ± 0.05 Bq/Kg, also lower than the accepted value of unity, while external hazard index ranged from 0.05 to 0.19 Bq/Kg with a mean value of 0.10 ± 0.04 Bq/Kg. Key words: Radon, thoron, natural radioactivity, annual effective dose, radium equivalent index, external and internal hazard index.