New concepts of 10Be AMS at low energies

Isotopic ratios of 10Be/9Be with a background sensitivity of 2 × 10−14 can now be measured with the 0.6 MV Tandem accelerator of PSI/ETH Zurich using BeF2 instead of BeO as sample material and injecting BeF− to suppress the boron detector count rate. A new, high-resolution gas ionization detector with a thin silicon nitride entrance window and special preamplifier design are essential in order to achieve this sensitivity. A particle transmission of 50% is possible for charge state 1+. Due to the maximum count rate that can be accepted by the detector the BeF− currents are limited to a few hundred nA.Measurements of 10Be in rainwater samples performed under these conditions at the low terminal voltage of 0.6 MV with the new compact AMS system show good agreement with results using the conventional 10Be measurement setup on the 6 MV facility. Thus, this type of small scale equipment can now also be applied for 10Be AMS as long as 10Be/9Be ratios are sufficiently high and measurement time is not a limiting factor.Instead, using BeO and the degrader foil technique with a silicon nitride membrane the boron count rate can also be suppressed sufficiently but the 10Be/9Be background level is limited to 10−13 by scattered 9Be and the overall transmission is less than 5%.