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MS: Fachverband Massenspektrometrie

MS 4: Accelerator Mass Spectrometry (AMS) II

MS 4.5: Talk

Tuesday, March 11, 2008, 12:00–12:15, 3E

¹⁰Be AMS measurements below 1MeV — •Arnold Milenko Müller, Marcus Christl, Max Döbeli, Martin Suter, and Hans-Arno Synal — Ion Beam Physics, Paul Scherrer Institute and ETH Zurich, 8093 Zurich, Switzerland

For ¹⁰Be measurements on 5-6 MV AMS systems sufficient suppression of isobaric boron and a ¹⁰Be/⁹Be background ratio of <10⁻¹⁵ is achieved by a passive absorber in front of a gas ionization chamber. With small accelerators the passive degrader foil method is used. Thereby Be and B are separated after passage of a degrader foil by an electrostatic deflector due to their different stopping power. A boron suppression of 4 orders of magnitude was achieved with our 600 kV machine by using a 90nm thick Si₃N_3.1 foil as degrader. The remaining ¹⁰B is separated from ¹⁰Be by a low noise ΔE-E_res gas ionization chamber. First measurements of BeO samples with the 600kV machine resulted in a ¹⁰Be/⁹Be background level of around 10⁻¹³, about 2 orders of magnitude above the intended value. Further investigations indicated that an additional background originates from ⁹Be reaching the detector by scattering processes. Since these ions can pass the electrostatic deflector in front of the detector they must have the same energy as the ¹⁰Be ions. Therefore an additional magnet was mounted after the ESA in order to remove the scattered ⁹Be. In consequence a ¹⁰Be/⁹Be background level of <5·10⁻¹⁵ was achieved, which is now competitive with the performance of bigger machines. In addition the transmission from the low energy side to the detector was improved from 2.5% to about 7.5% due to the energy focusing of the magnet.