A. Müller, M. Christl
The main problem of measuring 10Be is the interfering 10B and 9BeH, which can’t be separated from 10Be by the mass spectrometer. Especially at low beam energies (< 1MeV) background suppression becomes critically for 10Be.
For 10Be measurements on AMS systems with terminal voltages of 5 - 6 MV a sufficient suppression of the isobaric boron is achieved by applying a passive absorber cell in front of a ΔE-Eres gas ionization chamber. Because of the higher stopping power of boron (Z=5) compared with beryllium (Z=4) the boron ions stop in the absorber cell, while the beryllium ions reach the detector. With this technique a 10Be/9Be background ratio of <10-15 can be obtained.
With small accelerators the passive degrader foil method is used for boron suppression. Thereby beryllium and boron are separated after the passage of a thin degrader foil by an electrostatic deflector (ESA) due to their different stopping power in matter. A boron suppression of 3-4 orders of magnitude is achievable with the TANDY system operated at 525 kV by using a SiN degrader foil of 75 nm thickness.
The remaining 10B is then separated from 10Be by a low noise ΔE-Eres gas ionization chamber. Since 10B losses more energy in ΔE-anode than 10Be due to its higher stopping power boron produces more ionization charge (higher signal on the anode) than beryllium. For the second anode it is vice versa. This difference in production of ionization charge is used to separate 10Be from 10B. The used detector with low noise electronics (8-9 keV noise level proton equivalent) guarantees an efficient boron separation (see spectrum aside) [1,2].
First measurements of BeO samples with the TANDY machine resulted in a 10Be/9Be background level of about 10-13 which is 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. These 9Be ions can pass the electrostatic deflector in front of the detector and have therefore the same energy as the 10Be ions . Since ions with equal energy are deflected according to their masses in a magnetic field an additional 130° magnet was mounted after the ESA in order to remove the 9Be background. In consequence a machine 10Be/9Be background level of 1*10-15 can be 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 10-13% by realizing an achromatic arrangement of the ESA and the 130° magnet and by using Helium as stripper gas instead of Argon. A value which is also comparable to those obtained at typical 10Be AMS facilities. At the present date investigations are performed for further optimizations of the efficiency.
 A.M. Müller et al, NIM B 268 (2010), 843-846
 A.M. Müller et al, NIM B 287 (2012), 94-102
 A.M. Müller et al, NIM B 266 10 (208), 2207-2212
 A.M. Müller et al, NIM B 268 17-18 (2010), 2801 – 2807
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