Improvements in the application of a tandem Van-de-Graaff accelerator for ultra-sensitive mass spectrometry

In 1979, when this project was started, several other laboratories had already demonstrated that 14C and other rare radioisotopes present in concentrations of 10-10 - 10-16 can be detected with existing tandem accelerator facilities. These experiments have shown that the existing facilities are neither suited for these high precision measurements nor efficient in routine applications. Therefore, specifically designed facilities for highly sensitive mass spectrometry were proposed at the First Conference on Radiocarbon Dating with accelerators and are now under construction. A modification of the EN-Tandem facility existing at the ETH was undertaken applying a concept similar to that used for the dedicated facilities. First an isotope analyzer system consisting of an electrostatic deflector, a magnetic spectrometer and a ΔE-E counter was set up on a separate beam line at the high energy end of the accelerator. After replacement of some unstabilized power supplies, first measurements were performed in 1980 using the existing ion sputter source with cone geometry. These measurements gave reproducible results within 5 to 10%. Subsequently, a new ion source arrangement was installed including a vacuum lock for sample loading and a new high resolution 900-inflection magnet. For measuring isotopic ratios accurately, a fast cycling system is now under development. Furthermore an electronic control system has been designed which allows to tune the ion beam in a reproducible way and continously monitor it during operation