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

Abstract 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 36 Cl and 10 Be. However, the efficiency is reduced due to the charge state distribution generated by the degrader-foil. For measurements injecting atomic 26 Al, 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 26 Mg 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°-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 27 Al at 40 MeV ion energy. Isobar separation for 36 Cl and 36 S 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.