AMS-measurements of stable platinum isotopes in presolar nanodiamonds with the help of a negative-ion-injector for VERA

The Vienna Environmental Research Accelerator (VERA) is a universal facility for Accelerator Mass Spectrometry (AMS). VERA is based on a 3-MV tandemaccelerator and enables one to investigate isotope ratios down to 10-16 over the whole mass range of the elements. The main application is the measurement of long-lived radioisotopes of cosmogenic or anthropogenic origin (e.g. 10Be, 14C, 26Al, 36Cl, 41Ca, 129I, 236U, 244Pu). In principle it is also possible to detect stable isotopes of trace elements (Trace Element AMS: TEAMS). TEAMS is of particular interest with regard to so-called presolar grains in meteorites. Presolar grains contain trace elements whose isotope compositions differ substantially from those of our solar system. This points to a presolar origin of these grains. Among presolar grains, nanodiamonds are still poorly understood. They are composed of only a few thousand carbon atoms and contain platinum as one of the trace elements. There are still questions on their origin. E.g. were they 'doped' with the trace elements in the environment of a supernova? Which nucleosynthetic processes have taken place? Here the isotope signatures could deliver additional information. The aim of the present work was to establish conditions at VERA for the measurement of stable platinum isotopes in presolar nanodiamonds. The difficulty of such a measurement lies in the fact that trace analyses of stable Pt isotopes are only possible if the platinum background of the ion source is low. Hence, a new negative ion injector with a new sputtersource in which platinum was never used as a test material was build up at VERA. It was then possible to carry out Pt-isotope measurements in nanodiamonds for the first time. The sample material originates from the Allende-meteorite and was purified for nanodiamonds by the cosmochemistry group of the Max Planck Institute of Chemistry in Mainz. As a main result of the measurements it was possible to detect an access of the heaviest Pt isotope, 198Pt. This deviation is expected from certain nucleosynthetic models of supernova explosions. Starting with an overview on the presolar grain issue, the construction of the new negative ion injector and the realisation and the results of the measurement of stable Pt isotopes in nanodiamonds is described. Finally the astrophysical significance of the results is discussed. The experiences with this TEAMS pilot experiment allows one to conclude that such measurements at suitable AMS facilities can make an important contribution to the future understanding of the origin and history of presolar stardust. (author)