Oceans circulation and electron cyclotron resonance sources: Measurement of the AR-39 isotopic ratio in Seawater

Abstract

The radionuclide Ar39 is produced in the atmosphere by cosmic rays and has an isotopic abundance of 8.1×10−16. Because its half life (T1/2=269 years) is well matched to the time periods involved in the oceanic currents around the Earth, the measurement of the Ar39 isotopic ratio is an ideal tool to date ocean water from different depths. It would complement the information gained by the C14 measurements (T1/2=5730 years). However, the measurement of the isotopic ratio Ar39/Ar40 is a technical challenge: 1 L of modern ocean water contains ∼6500 atoms of Ar39, and produces ∼17 decays per year. Although it has been possible to detect the Ar39 decays in large volumes of sea water by using the low level counting technique, the possibility of measuring the number of Ar39 atoms faster and in smaller samples using the accelerator mass spectrometry (AMS) technique would be a major breakthrough for this type of measurement. The development of a viable AMS method for Ar39 has been underway for several years at Argonne National Laboratory, and is presently hampered by the presence of stable K39 ions coming from the ion source. Although the intensity of this isobaric contaminant is low (∼pA extracted from the source), it has to be compared with the Ar39 beam intensity (atoms per minutes). In order to separate these two beams (whose mass difference is only 1.6×10−5), the intensity of the K39 beam coming from the ion source has to be reduced by several orders of magnitude. This reduction has been investigated both at Argonne National Laboratory and at Louvain-la-Neuve. Two techniques have been tried out. In the first, a quartz liner is used to provide a clean surface, while in the second these impurities are buried in a SiO2 layer formed in situ by running the source with a mixture of silane and oxygen. The K39 background has been reduced by a factor of 100 with these treatments. These techniques and their results obtained both at Argonne and Louvain-la-Neuve will be presented. The ion source specific requirements for this type of application will also be discussed.