Preliminary AMS measurements of 10Be at the CENTA facility

Published in Nuclear Instruments and Methods in Physics Research B 361 (2015) 139–142

The cosmogenic radionuclide ¹⁰Be is one of the several radionuclides studied mostly by accelerator mass spectrometry(AMS). It is used for a wide range of applications from climate research, exposure studies in geology to nuclear astrophysics.
The background during the AMS measurement of ¹⁰Be is mainly caused by the much more abundant stable isobar ¹⁰B. The conventional magnetic/electric mass spectrometers of an AMS system cannot distinguish between these two isobars, therefore additional separation is needed.
Negative BeO ions from blank samples (¹⁰Be/⁹Be ratio of 10^-15) were produced in the MC-SNICS (Multi Cathode Source of Negative Ions by Caesium Sputtering) ion source. The ¹⁰Be standard with ¹⁰Be/⁹Be isotopic ratio of (8.71 ± 0.24) x 10^-11 was used during measurements. After the ions were produced, they were accelerated by 60.5 kV potential, and electrostatically selected for the injection magnet. Masses of 25 AMU (⁹Be¹⁶O) and of 26 AMU (¹⁰Be¹⁶O) were separated by the injection magnet from the sputtered ions for ⁹Be and ¹⁰Be measurements, respectively. Separated ions were injected into the accelerator operating at 3 MV terminal voltage. For the beam tuning, the ⁹Be¹⁶O- molecules were injected to the accelerator with typical current of about 1 μA. By using the nitrogen gas, 30% of the Be ions were stripped to the 2+ charge state in the terminal of the accelerator. After the tuning of the beam to the detector, ¹⁰Be¹⁶O^- ions were injected into the accelerator and several peaks were registered in the ionization chamber.
The AMS analysis of ¹⁰Be in a standard sample was carried out for the first time in the new CENTA facility using the ionization chamber as the end of the line detector with SiN foil stack for background suppression of ¹⁰B isobar. This method has proved to be successful and a detection limit of 10^-12 for the ¹⁰Be/⁹Be isotopic ratio was reached without additional energy separation. It should be stressed that this detection limit was obtained without ahigh-resolution analysing magnet and energy separation, since only a switching magnet at 45 degrees was used as an analysing magnet. The obtained limit was mainly determined by the scattered ⁹Be²⁺ions (with the energy of 7.06 MeV) in the degraded vacuum inside the switching magnet, which were registered by the ionization chamber in the same region as the ¹⁰Be ions. It is expected that with a better vacuum, even a better detection limit could be achieved with this method.