Inequilibrium of Thorium Series Radionuclides in the Sediments around a River Mouth

Abstract

The distribution of ²²⁸Th and ²²⁸Ra concentrations in the sediments around a river mouth was studied. High resolution gamma-ray spectrometry with a Ge (Li) detector made it possible to analyze the radionuclides in the sediment samples without chemical treatment. The activities of ²²⁸Th and ²²⁸Ra were determined from the activities of their daughters, i.e., ²¹²Pb, ²⁰⁸Tl and ²²⁸Ac. Samples containing monazite sand of known activity were used as the standards in which the thorium series radionuclides were in secular equilibrium.
It was shown that the radionuclides were mostly accumulated in a narrow band stretching along the stream from the river.
The activity ratios of ²²⁸Th to ²²⁸Ra were large at the river mouth and at the places aside the band.
The activity ratios of ²²⁸Th to ²³²Th were also measured with alpha-ray spectrometry using a double grid ionization chamber.
These studies made clear the situation of the fractionation effect around a river mouth due to the difference of chemical properties between thorium and radium in the sea water.