Operating the 120 degrees Dipol-Magnet at the CologneAMS in a gas-filled mode

Routine measurements of the last years at the 6 MV AMS system showed a good isobar suppression for all light isotopes, measured with the Degrader-Foil-Method, especially Cl-36 and Be-10. However, the efficiency is reduced due to the charge state distribution generated by the degrader-foil. For measurements injecting atomic Al-26, the ionization efficiency becomes the bottleneck, which leads to a relatively low ion source output. In the case of AlO-, the output is expected about one order of magnitude higher. However, to use AlO- the isobar interference of Mg-26 will become an issue. It has to be suppressed in several steps, e.g. by different dE/dx methods like TOF, degrader-foil or gas-filled magnet. In order to minimize the efficiency losses generated by the degrader-foil method, the 120 degrees-magnet at the 6 MV AMS system was modified to enable a gas-filled operation, and therefore focus the beam to a mean charge state. A constraint for all modification of the detection beam line was that the operation of the degrader-foil method is not affected. In this paper, we report on the performed construction work, as well as test measurements for Al-27 at 40 MeV ion energy. Isobar separation for Cl-36 and S-36 was measured as a function of the gas pressure. We compare measured data with Monte Carlo simulations performed with a computer code developed by our group. In addition, we will present a new dedicated gas ionization chamber designed for particle detection directly after the gas-filled magnet.