First measurements of 236U concentrations and 236U/239Pu isotopic ratios in a Southern Hemisphere soil far from nuclear test or reactor sites

The variation of the U-236 and Pu-239 concentrations as a function of depth has been studied in a soil profile at a site in the Southern Hemisphere well removed from nuclear weapon test sites. Total inventories of U-236 and Pu-239 as well as the U-236/Pu-239 isotopic ratio were derived. For this investigation a soil core from an undisturbed forest area in the Herbert River catchment (17 degrees 30' - 19 degrees S) which is located in northeastern Queensland (Australia) was chosen. The chemical separation of U and Pu was carried out with a double column which has the advantage of the extraction of both elements from a relatively large soil sample (similar to 20 g) within a day. The samples were measured by Accelerator Mass Spectrometry using the 14UD pelletron accelerator at the Australian National University. The highest atom concentrations of both U-236 and Pu-239 were found at a depth of 2-3 cm. The U-236/Pu-239 isotopic ratio in fallout at this site, as deduced from the ratio of the U-236 and Pu-239 inventories, is 0.085 +/- 0.003 which is clearly lower than the Northern Hemisphere value of similar to 0.2. The U-236 inventory of (8.4 +/- 0.3) x 10(11) at/m(2) was more than an order of magnitude lower than values reported for the Northern Hemisphere. The Pu-239 activity concentrations are in excellent agreement with a previous study and the Pu239+240 inventory was (13.85 +/- 0.29) Bq/m(2). The weighted mean Pu-240/Pu-239 isotopic ratio of 0.142 +/- 0.005 is slightly lower than the value for global fallout, but our results are consistent with the average ratio of 0.173 +/- 0.027 for the southern equatorial region (0-30 degrees S). (C) 2014 Elsevier Ltd. All rights reserved.