New fluoride target matrix preparation procedure for determination of ²³⁶U with accelerator mass spectrometry

In recent years, radionuclide U-236 has become a valuable analytical tool covering a wide area of applications, including overall monitoring of environmental radioactive contamination for nuclear safeguards purposes, forensic analyses, tracing of various natural processes, e.g. ocean currents, soil erosion and others. Due to low concentrations of U-236 in environmental samples, highly sensitive mass spectrometric methods as ICP-MS and AMS require to be applied in related analyses. AMS measurements usually utilize target samples with uranium converted to a form of oxide incorporated in iron(III) oxide (Fe2O3) carrier matrix. Alternatively, fluoride target matrices, uranium tetrafluoride (UF4) in particular, have been studied as a promising alternative, providing significantly higher ion currents at a simultaneous reduction of occurring interferences by replacement of oxygen with monoisotopic fluorine F-19. Despite these advantages, application of fluoride target matrices in routine analytical process still faces several major problems regarding the preparation of pure, anhydrous uranium tetrafluoride as well as the continuity of preparation procedures with preceding sample treatment, which demand further modifications. In this respect, a new fluoride target matrix preparation method has been proposed, more suitable for determination of U-236 in natural samples. Applicability of the procedure has been demonstrated on a series of target samples analysed at VERA AMS laboratory, University of Vienna.