The role of chemistry in setting-up a new AMS facility
Résumé
Medium-energy AMS facilities such as the British 5 MV-NEC machine at "SUERC", the French 5 MVHVEE- machine "ASTER" and the two German 6 MV-HVEE-machines "DREAMS" at Dresden [1] and "Cologne AMS" have been recently installed in Central Europe. Of course, these machines need physicists to get them running but also scientists to establish AMS chemistry on-site. As it is not advisable to change simultaneously two "things", i.e. machine and chemistry, a close cooperation with the teams of ASTER and VERA helped to check the new sample preparation laboratories of DREAMS. Generally, a "good" AMS sample has two features: high stable isotope current and low isobar concentration. High chemical yields and low concentrations of other elements originating from the matrix or chemical products used are less important, but may play a role if e.g. a matrix contains Ti being introduced into BeO-targets as shown by μ-XRF [2] and recent μ-PIXE analyses of final AMS-targets at HZDR. A processing blank with low radionuclide/stable nuclide ratio is, however, essential for projects near the detection limit. For high sample throughput and reasonable costs a fast, easy and cheap chemical separation is also favourable. The results from first samples processed at DREAMS are excellent: For 10Be-AMS-targets isolated from quartz-rich river sediments (Himalaya, erosion rate study) 9Be-currents had been as high as ASTER standards and machine blanks. Corresponding processing blanks were in the same order as the machine blank (1x10-15), thus, more than one order of magnitude lower than the lowest sample ratio. Chlorine-36 targets originating from calcite-rich boulders from a medieval rockfall had been well-measurable with blank-corrections lower than 8 %. Finally, 36Cl prepared from river terraces (Anatolian Plateau, Turkey, uplift rate study) showed also perfect performance at VERA.