Be-10 measurements at the TANDY

Contact:
Dr. A. Müller
(tech. developments)
Dr, M. Christl
(routine measurements)

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-E_res 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-E_res 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 [3]. 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 [4].

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.

References

[1] A.M. Müller et al, NIM B 268 (2010), 843-846

(http://dx.doi.org/10.1016/j.nimb.2009.10.045)

[2] A.M. Müller et al, NIM B 287 (2012), 94-102

(http://dx.doi.org/10.1016/j.nimb.2012.06.012)

[3] A.M. Müller et al, NIM B 266 10 (208), 2207-2212

http://dx.doi.org/10.1016/j.nimb.2008.02.067

[4] A.M. Müller et al, NIM B 268 17-18 (2010), 2801 – 2807

(http://dx.doi.org/10.1016/j.nimb.2010.05.104)