324 E . Srinivasa et al .: Radioprotection 2024 , 59 ( 4 ), 317 – 326
Fig . 5 . Distribution of radionuclides as a function of grain size .
Fig . 3 . Correlation between measured and estimated annual effective dose rate in soil . must be less than 1 to keep the radiation hazard negligible for the general population . Except for the soil samples collected from Mysore Mineral Industries Limited ( MMIL ) of Hassan city , the calculated I g values are well below the criterion limit of unity as per European Commission of Radiation Protection reports ( European Commission , 1999 ).
3.4 Variation of activity of radionuclides in soil with depth and grain size
Fig . 4 . Distribution of radionuclides as a function of depth profile .
For safe use of the material in housing construction , H ex and H in should be less than 1 so that the radiation risk is negligible . The calculated internal ( H in ) and external ( H ex ) radiation hazard index values of the soil samples range from 0.24 to 1.27 with an average value of 0.57 and 0.18 to 1.07 with an average value of 0.46 . The average values of external and internal health risk indices due to the presence of natural radionuclides in soil samples are less than unity , which is highly acceptable and within the safe limit in the studied area ( Srinivasa et al ., 2015 ; Farai and Vincent , 2006 ; Mahur et al ., 2008 ; Beretka and Mathew , 1985 ; Akkurt et al ., 2009 ; Nambi et al ., 1986 ; UNSCEAR , 1993 ). Therefore , these areas may not pose a radiological risk to residents due to the harmful effects of ionizing radiation from natural radionuclides in the soil .
The estimated gamma index ( I g ) values of the soil samples vary from 0.25 to 1.42 , with an average of 0.63 . The value of I g
To study the variation of activity concentration with different depths , soil samples were collected at the same location from different depths , viz . 0 – 15 , 15 – 30 , and 30 – 45 cm . The activity of radionuclides with different depth profile is shown in Figure 4 . Various processes such as weathering , water action , water transport , chemical and geochemical reactions , among others , cause the uneven distribution of natural radionuclide activity in soil ( Volchok and de Planque , 1983 ). The highest activity was found in the topsoil , and activity values decrease as depth increases .
The activity concentration also varies with the grain size of the soil matrix . To quantify variation , soil samples were collected at a depth of 15 cm , crushed , and sieved through 150 mm , 250 mm , 400 mm , and 500 mm sieves to obtain different soil grain sizes . The activity of radionuclides with different grain sizes in the soil samples is shown in Figure 5 . The lowest activity was observed for coarser particles ( 400 – 500 mm ). The activity increases in the next fraction ( 400 – 250 mm and 250 – 150 mm ). The highest activity of all three radionuclides was found to be in the (< 150 mm ) particle size fraction . This means that the activity is inversely proportional to the grain size . Higher activity was associated with the finest particle size . This is due to the weathering of minerals and a high degree of absorption ( Kerur et al ., 2010 ).
4 Conclusion
From the study , it was observed that the average activity concentration of 238 U , 226 Ra , 232 Th , and 40 K in soil samples