P_05

(R)evolutionary developments towards high precision radiocarbon analyses

Synal H¹

¹ETH Zurich, Zurich, Switzerland

Since the discovery of accelerator mass spectrometry (AMS) measurement procedures have undergone a continuous evolution. Modern AMS instruments are still following design concepts of the pioneering experiments, but they do not have much in common with huge accelerator infrastructures. AMS is based on fundamental principles which made suppression of nuclear and molecular isobaric interferences possible. Violent charge exchange processes are exploited to remove binding electrons of light molecular ions to reach charge states of 3+ or higher where no molecular bound states exist. This elegant procedure was taken for granted as the “golden rule”. Instability of molecular ions in charge states lower than 3+ had been realized early, but the potential to simplify AMS technology had not been realized. Measurement of the molecule dissociation cross sections made substantial improvements of AMS instrumentation possible. Groundbreaking progress was made by introducing vacuum insulated accelerations stages feed by solid state power supplies. To utilize the unique properties of helium as stripping gas some technical obstacles were solved and by introducing fixed field magnets for the momentum selecting filter stages, power consumption was minimized. With these improvements state-of-the-art AMS instruments are comparable in size and complexity to high end traditional mass spectrometers. Due to their compact layout and based on optimized ion optical designs they are capable to measure carbon isotopic ratios with sup permille precision. Thus, quality of radiocarbon analyses is more and more no longer limited by the instrumental capabilities but rather by the quality and reproducibility of the sample preparation procedures.