Method developments for accelerator mass spectrometry at CologneAMS, Mn-53/He-3 burial dating and ultra-small (CO2)-C-14 samples

Accelerator mass spectrometry (AMS) has a great potential for various geological applications. The precision and sensitivity is directly linked to technical developments. Therefore CologneAMS, the center for accelerator mass spectrometry of the University of Cologne (Dewald et al., 2013a, 2013b), does extensive work on method developments for isotopic ratio measurement, for new applications and to improve the precision. We present two important technical developments: (1) dating of sediments with Mn-53 and (2) ultra-small (CO2)-C-14 samples. (1) The age determination of discontinuously deposited sediments by measurement of cosmogenic Mn-53 and He-3 concentration in iron-titaniumoxides (hematite, magnetite, titanomagnetite, ilmenite), where the measurement of the medium mass isotope Mn-53 is difficult because of the interfering isobar Cr-53. It can be measured at only 2 laboratories worldwide. The new AMS system at the Cologne 10 MV FN tandem accelerator is therefore designed especially for the measurement of medium mass isotopes, e.g. Mn-53. It provides the opportunity to use several detector systems in different combinations: degrader foils with an electrostatic analyzer, a 4 m time-of-flight system, a new 135 degrees gas-filled magnet, different gas ionization detectors and a projectile x-ray detector. With the implementation of the new 135 degrees gas-filled magnet to the 10 MV FN-Tandem accelerator AMS beamline the first tests have shown an increase in transmission and reduction of blank value in comparrison to the measurement with the degrader foil technique (Schiffer et al. 2018). The background level was reduced by a factor of 3.3 and the error normalized sensitivity (Schiffer et al. 2018) was increased by a factor of 1.6. (2) For the measurement of extremely low carbon from cosmogenic in-situ produced C-14 or from C-14 content of bulk sediments, with 0.01% carbon, technical developments are required to measure low concentrations in terrestrial rocks of 100,000 atoms g(-1) in quartz and to trace organic compounds with 10-50 mu g of (CO2)-C-14 samples. It turned out, that the measurement of (CO2)-C-14 micro-samples with the gas-ion source of the Cologne 6 MV AMS system in combination with a gas-handling system is an alternative to the development of a micro-scale graphitization. Modern 1 mu g CO2 samples,-)(C-14/C-12)= 1.18 x 10(-12) can be measured with this system with a precision of 2.8%. The overall efficiency is 4%-6% and the reproducibility of standard samples is 0.9% with a blank level of 3-6 x 10(-15)