Monday, November 24, 2008
Saline Soil possesses no Radionuclide hazards for humans. (st rpt)
The gamma ray spectrometer, activity concentrations of the natural radionuclides of the uranium and thorium series, 40K and a fission product 137Cs were investigated in the soil samples from saline patch of Rakh Dera Chal near the city of Lahore.
The average values of specific gamma ray activities due to 137Cs, 40K, 232Th and 226Ra and the activity levels were found to fallow the normal distribution. Three most important primordial radionuclides investigated in the area of interest were 40K, 226Ra and 232Th. Natural potassium has three isotopes; 39K, 40K and 41K, among them only 40K (T1/2 = 1.3x109 yr) possesses natural gamma radioactivity and its
abundance in nature is 0.012 % of all Potassium.
During decay 40K produces two daughter products with the emission of beta and gamma radiation they are 40Ca and 40Ar. Testing the use of fertilizers in large extent have affected radio nuclides concentration, specially potassium containing fertilizers are the one of the cause of presence of high activity of 40K in soils. The high value of 40K may be also due to their soil origin. The concentration range of 40K in soil was 524.8–601.6 Bq kg-1 with and average value of 561.6 Bq kg-1.
The other naturally occurring radionuclides measured were 226Ra and 232Th. Radium-226 (a member of 238U series) is considered as 'the highly radiotoxic natural radionuclide'. The range of measured activity
of 226Ra was 24.7Å|28.2 Bq kg-1 with an average of 25.8 Bq kg-1. The range of measured specific activity of 232Th (T1/2 = 1.4 x 1010 yr) was 45.5Å|52.6 Bq kg-1 with an average of 49.2 Bq kg-1. The average activity value of 232Th was two times higher than that of 226Ra. The activity concentration of 40K in soil is order of magnitude higher than that of 226Ra and 232Th.
The activity of 137Cs in all the samples was found below the lowest limit of detection. Im suggesting the reasons of non existence of 137Cs in the soil of Rakh Dera Chal may be that the trees and grass in the land might have provided shield to the land for the reach of 137Cs (from the nuclear fall) to the land. The grass contaminated with 137Cs might have been grazed by the cattles and most of the remaining 137Cs would have been eroded by rain, wind, etc. The decay of naturally occurring radionuclides in soil produces a gamma–beta radiation fields in soil that also crosses the soil–air interface to produce exposures to humans. External exposures outdoors arise from terrestrial radionuclides present at trace levels in all soils. The external gamma dose rate in air is calculated from measurement of concentration of the relevant radionuclide in soil.