Possibilites of the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA, Seville, Spain) to analyze 233U in natural samples
Dr Elena Chamizo1, Dr. Mercedes López-Lora2, Dr. Marcus Christl3
1Centro Nacional de Aceleradores (CNA) (Universidad de Sevilla. Junta de Andalucía. Consejo Superior de Investigaciones Científicas), Seville, Spain, 2Department of Health, Medicine and Caring Sciences (HMV), Linköping University, Linköping, Sweden, 3Laboratory of Ion Beam Physics, Institute for Particle Physics and Astrophysics, ETH Zürich, Zürich, Switzerland
²³³U has recently been put in the limelight of interesting AMS radionuclides. The reason is the potential of the ²³³U/²³⁶U atom ratio to unravel anthropogenic U sources. The measurement of ²³³U by AMS is challenging due to its extremely low abundance in the environment. For example, the reported ²³³U/²³⁸U atom ratios in surface seawater are at the level of 1E-11 (i.e. 10⁵ atoms/L), and published ²³³U/²³⁸U values in sediments are above 1E-10. The achievement of such low ratios by AMS is usually hampered by: i) chemical contamination of the samples with ²³³U, and ii) background effects caused by the presence of either ²³²Th and/or the naturally occurring U isotopes ²³⁴U and ²³⁵U. In this work we explore the possibilities that the 1 MV compact AMS system at the Centro Nacional de Aceleradores (CNA, Seville, Spain), offers to analyse ²³³U in natural samples. In contrast to other AMS facilities used for ²³³U determinations, the CNA AMS system is limited by its ion-beam design. On the low energy side, neighbouring masses are suppressed solely by a sector magnet. On the high energy side, scattered molecular fragments are filtered out making use of a sector magnet and an electrostatic analyser. He is used as a stripper gas, causing additional scattering process on the acceleration tubes. The achievement of ²³³U/²³⁸U atom ratios at the level of 1E-11 entails a thorough set up of the instrument in connection with a deep knowledge of the ²³²Th, ²³⁴U and ²³⁵U background effects, and the use of different proxies to correct them. Another key point is the radiochemistry applied to the samples, which must be aimed at suppressing ²³²Th from the U fraction specially in the case of soils and sediments. At the CNA, existing radiochemistry methods have been adapted to measure ²³³U in ²³²Th-rich matrixes, and the background effects caused by ²³²Th, ²³⁴U and ²³⁵U have been evaluated. A series of IAEA reference materials, soils and sediments, have been studied at CNA, and the obtained ²³³U/²³⁸U and ²³⁶U/²³⁸U ratios were compared to the results of duplicate sample analysis performed at the 600 kV Tandy AMS facility at ETH Zurich that offers significantly lower abundance sensitivity for actinide analyses. As we will demonstrate, the overall technique applied at CNA (radiochemistry plus AMS method) provides a ²³³U/²³⁸U detection limit of roughly 2E-11 in sediment samples, which is mostly conditioned by ²³⁴U and ²³⁵U scattered molecular fragments.
My name is Elena Chamizo. I work at the Centro Nacional de Aceleradores (CNA), in Seville, Spain, that hostes a 1 MV AMS system designed by HVEE. My field of expertise is actinides AMS measurements (Pu isotopes, U236, U233 and Np237), including the development of radiochemical procedures and the study of natural samples.