Developments at the Cologne 10 MV AMS system for ${}^{60}$Fe measurements

The focus of this thesis is the development of ${}^{60}$Fe isotopic ratio measurements with Accelerator Mass Spectrometry (AMS) at the Cologne 10 MV tandem accelerator system. Within this work an extensive range of investigations and developments of the 10 MV AMS system were conducted. These increased the reachable statistics and enables the necessary system stability to allow long-term measurements and especially low-level isotopic ratio measurements. The comparison between initial and latest conducted ${}^{60}$Fe AMS measurements revealed a significant decrease of required measurement duration by at least 95 per cent, a factor of 20, to 1.49 days on average for a theoretical ${}^{60}$Fe event of a 1 $\cdot$ 10${}^{-16}$${}^{60}$Fe/Fe ratio. Therefore, the system is now capable of similar statistics in reasonable measurement duration as established AMS laboratories. For the first time, full reference sample sequence measurements were conducted at the system which is a mandatory criterion for a functioning AMS system. Furthermore, the data acquisition was coupled to the 10 MV AMS control system. With that, automated AMS measurements of ${}^{60}$Fe and ${}^{14}$C were conducted for the first time at the 10 MV AMS system. The key component for isobar suppression of ${}^{60}$Ni in ${}^{60}$Fe measurements at the Cologne setup is the gas-filled magnet. For the optimization of the used gas parameters for ${}^{60}$Fe measurements and for a deeper understanding of the inner processes of particles passing the gas-filled magnet, a wide range of investigations were conducted. Thereby, the ion beam shape of ${}^{60}$Ni particles a from a ${}^{60}$Fe blank material was investigated after the gas-filled magnet and for the first time within the gas-filled magnet for different gas pressures and gas types. The measurements in the gas-filled magnet investigated the beam shape and trajectory in the dispersive axis. For the first time, it was possible to measure the ions deviation from the magnet optical axis which increases with increased gas pressure to outer trajectories. In nitrogen gas, the ion widths show a characteristic shape including a minimum in comparison to the gas pressure. For the first time, it was determined that the minimum of this shape shifts for different passed gas paths. The measurements with helium gas showed smaller positional deviations from the optical axis and significantly larger widths for the measured gas pressure range. The analysis of the two-dimensional profiles after the magnet was used to determine the optimal gas settings in terms of transmission for a subsequent symmetrical detector window. From the investigations, the optimum gas parameter from the measured options is nitrogen gas at 3 mbar gas pressure. The additional measurements of the separation factor between ${}^{60}$Fe and ${}^{60}$Ni in dependence of different nitrogen gas pressures revealed an increase in separation factor up to a pressure of 5 mbar with increased gas pressure. Finally, the two-dimensional profile of ${}^{60}$Fe was analyzed and compared to the available subsequent detector entrance windows. It revealed that the high transmission loss in the initially conducted ${}^{60}$Fe measurements were caused by the size of the respective entrance window. The experiments were simulated with an in-house code, which was further developed within this thesis, and an externally available simulation code. The approaches for the in-house code including a semi-empirical approach for the mean charge states, for inclusion of the gas density effect, and charge state distribution widths was revised. It enables sufficient qualitative and moderate quantitative agreement with the experimental values for nitrogen gas. The measured separation factors could be qualitatively reproduced by the in-house code. The investigations made clear that a general full established description of the inner processes within the gas-filled magnet is not sufficiently possible at the present state of research for a range of the required approaches. Finally, ${}^{60}$Fe AMS measurements were developed at the 10 MV AMS system. Therefore, a characterization of the system for iron measurements was conducted. The values for the present system are comparable to values given for larger AMS systems measuring ${}^{60}$Fe. Thereby, for the first time successful AMS measurements using the gas-filled magnet at the Cologne 10 MV setup were conducted. Furthermore, for the first time in Cologne, successful AMS measurements of ${}^{60}$Fe reference sample sequences were achieved. Thereby, two sample sequence measurements of standard and background samples were conducted using different-sized detector entrance windows. For the smaller window a transmission from the measurement position of the stable reference isotope to the particle detector of 28.41 percent was achieved with a corrected background level of ${}^{60}$Fe/Fe=4.53${}_{-1.46}^{+3.85}$$\cdot$ 10${}^{-15}$. With the larger window a transmission of 45.11 percent at a corrected background level of ${}^{60}$Fe/Fe=1.46${}_{-0.30}^{+0.47}$$\cdot$ 10${}^{-14}$ was determined.