Effect of the grain size of the soil on the measured activity and variation in activity in surface and subsurface soil samples

Correlation between grain size and activity concentrations of soils and concentrations of various radionuclides in surface and subsurface soils has been measured for samples taken in the state of qatar by gamma-spectroscopy using a high purity germanium detector. from the obtained gamma-ray spectra,the activity concentrations of the 238u (226ra) and 232th (228ac) natural decay series,the long-lived naturally occurring radionuclide 40k and the fission product radionuclide 137cs have been determined. gamma dose rate,radium equivalent,radiation hazard index and annual effective dose rates have also been estimated from these data. in order to observe the effect of grain size on the radioactivity of soil,three grain sizes were used i.e.,smaller than 0.5 mm; smaller than 1 mm and greater than 0.5 mm; and smaller than 2 mm and greater than 1 mm. the weighted activity concentrations of the 238u series nuclides in 0.5-2 mm grain size of sample numbers was found to vary from 2.5±0.2 to 28.5±0.5 bq/kg,whereas,the weighted activity concentration of 40 k varied from 21±4 to 188±10 bq/kg. the weighted activity concentrations of 238u series and 40 k have been found to be higher in the finest grain size. however,for the 232th series,the activity concentrations in the 1-2 mm grain size of one sample were found to be higher than in the 0.5-1 mm grain size. in the study of surface and subsurface soil samples,the activity concentration levels of 238 u series have been found to range from 15.9±0.3 to 24.1±0.9 bq/kg,in the surface soil samples (0-5 cm) and 14.5±0.3 to 23.6±0.5 bq/kg in the subsurface soil samples (5-25 cm). the activity concen- trations of 232th series have been found to lie in the range 5.7±0.2 to 13.7±0.5 bq/kg,in the surface soil samples (0-5 cm) and 4.1±0.2 to 15.6±0.3 bq/kg in the subsurface soil samples (5-25 cm). the activity concentrations of 40k were in the range 150±8 to 290±17 bq/kg,in the surface soil samples (0-5 cm) and 129±7 to 299±14 bq/kg,in the subsurface soil samples (5-25 cm). the activity concentrations of 238u series,232th series and 40k in the surface and deep soil samples are approximately same. the 137cs activity concentration levels in surface soil samples in four sites were found to be higher than those observed for the soil samples that been collected at a depth of 5-25 cm. they ranged from 1.65±0.22 to 19.0±0.9 bq/kg,in the surface soil samples (0-5 cm) and 0.5±0.2 to 15.4±0.7 bq/kg,in the subsurface soil samples (5-25 cm).