| Popis: |
Water samples from saline seepages in the south-western Yilgarn Block of Western Australia contain high activities of the four naturally-occurring radium isotopes. Activities of up to 310 pCi l for 226Ra and 1720 pCi l for 228Ra were measured and the 228 Ra 226 Ra ratio averaged 6.1. Activities of the two short-lived radium isotopes were also high. 223Ra activities of up to 94 pCi l were found with an average 226 Ra 223 Ra ratio of 3.3, considerably lower than the natural abundance ratio of 21.4. Activities of up to 23 pCi l 227Ac, the long-lived ( t 1 2 = 22 years ) grandparent of 223Ra, were also measured. The analysis of surface granite samples, the probable source rocks of the radium, gave Th U activity ratios of around 1.5. The higher 228 Ra 226 Ra ratios of the waters were attributed to readily leached 228Ra in the weathered granites as a result of thorium remaining after weathering. Leach experiments on U-Th ore by NaCl solutions showed that all four radium isotopes were equally leached. Sulphate anions reduced the 226Ra and 228Ra leaching to a greater extent than for 223Ra and 224Ra, suggesting that the latter isotopes were being supported in solution by parent isotopes. In particular this suggested 227Ac was leached into the sulphate solution but this does not fully account for the amount of 227Ac seen in the seepage waters. |
