In order to obtain the necessary sensitivity for 10Be with a low energy AMS system not only the molecular background but also the very high intensity of the nuclear isobar 10B has to be suppressed. By using BeF2 instead of BeO as sample material the ratio of boron to beryllium in the beam can be reduced by 4 to 5 orders of magnitude. We have shown that it is possible to suppress the remaining 10B background by another 5 orders of magnitude in a high resolution ΔE-Er gas ionisation chamber even at a final beam energy of only 0.8 MeV. The necessary resolution has been obtained by a special choice of the detector entrance window and the preamplifier design. This allows now to detect 10Be down to a 10Be/9Be level of 10-14 with the 0.6 MV Tandem accelerator at PSI/ETH Zurich. In charge state 1+ a total transmission of 50 % is obtained which makes such a facility competitive with much larger systems.
Boron suppression by a degrader foil has also been investigated at the same terminal voltage and charge state. With an overall transmission of about 5 % a 10Be/9Be level of 10-12 was reached. For both techniques the limiting background is most probably caused by scattered 9Be from molecular components in the injected beam. It is expected that the detection limits can still be reduced significantly by introduction of an additional mass filter.
In proof of the successful development, the compact AMS system has been used to measure 10Be rainwater samples in form of BeF2. Comparison of the results to those obtained by a large 6 MV facility definitely confirms the competitiveness of this type of small scale equipment, now also for 10Be AMS.
See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)