10Be low-energy AMS with the passive absorber technique

The passive absorber technique is one of the most common ways to suppress the 10B interference during 10Be measurements at facilities working with beam energies above 7 MeV. At lower energies, the range straggling complicates the application of absorbers, so that other suppression techniques are normally preferred. Several experiments were conducted at the SARA (hosted at Centro Nacional de Aceleradores, Seville, Spain) and VERA (Faculty of Physics, University of Vienna, Austria) AMS facilities to demonstrate the potential of the passive absorber technique also at and below 2.4 MeV. Two different absorber setups were installed and tested. For the detection of the rare isotopes both facilities used a gas ionization chamber optimized for light ions detection based on the same design. The absorber installed at the SARA facility was a combination of SiN foils and an isobutane gas volume, whereas VERA was equipped with an absorber constituted of a stack of SiN foils. In both cases, 10Be could be clearly separated from 10B and the use of a passive absorber at the entrance of the detector gave higher transmission compared to the degrader method. Depending on the absorber design, different background contributions could be identified: Rutherford scattering of 10B on the protons contained in the SiN foils and isobutane gas was responsible of a severe background at SARA, and fragments from 9Be1H molecules surviving the stripping process resulted in events partially overlapping the 10Be gate at VERA. The measured transmissions and background levels will be presented for the tested setups, as well as the advantages and disadvantages of each absorber design.
Tests at SARA were funded by the project FIS2015-69673-P of the Spanish Ministry of Economy and Competitiveness.