Accelerator-SIMS for isotopic analysis of trace elements

Secondary ion mass spectrometry (SIMS) is one of the most extensively used methods to detect impurities and their isotopic signatures in materials. However, especially for heavier trace elements, the sensitivity of SIMS is severely limited by molecular and charge state interferences. This problem can be overcome using accelerator mass spectrometry (AMS). Here negative ions are analyzed by an electromagnetic mass spectrometer and then injected into a 6 MV EN-Tandem accelerator. At the terminal the ions are stripped to high charge states, which guarantees the destruction of all molecular ions. At the high energy side of the accelerator, 40 MeV ions are selected by an electrostatic deflector and analyzed by a magnetic spectrometer. Final detection of the ions is done with a position-sensitive gas ionization chamber that also measures dE/dx and the residual energy for isobar separation. A fast beam bouncing system and a wide detector entrance window allow for quasi simultaneously detection of multiple isotopes. Within the EuroGENESIS program we are planning to use Accelerator-SIMS to measure isotopic compositions of rare earth elements in pre-solar grains. The isotopic signatures in these grains carry the fingerprint of nucleosynthesis and provide valuable astrophysical information.