Developments towards detection of Cs-135 at VERA

作者：Lachner Johannes^*; Ka**erger Magdalena; Martschini Martin; Priller Alfred; Steier Peter; Golser Robin

来源：Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms , 2015, 361: 440-444.

DOI：10.1016/j.nimb.2015.01.032

• Rubidium sputtering
• VERA
• Isobar separation
• Cs-135
• Negative ion formation

Radioisotopes produced in natural or anthropogenic fission are widely used for tracer studies of environmental processes, in nuclear forensics, and are important for nuclear waste disposal. Besides the well-known Cs-137, the longer-lived sister isotope Cs-135 (T-1/2 = 2.3 Myr) is also produced, and the combined measurement of the two isotopes would allow for assessment of contaminating sources. The insufficient suppression of the stable isobar Ba-135 presently prevents AMS measurements down to expected natural levels of Cs-135/Cs-133 approximate to 10(-11). Via the difference in electron affinities between Cs and Ba further isobar suppression should be achievable after the installation of the Ion-Laser-Interaction System (ILIAS) at VERA. We present a preparatory study on the performance of the 3 MV VERA AMS facility for Cs-135 concerning ion formation, transmission and detection. Since the usual Cs sputtering would obscure the Cs-135/Cs-133 ratio of a sample, Rb sputtering was successfully applied and tested also for various other typical AMS elements. Partial suppression of Ba-135 is possible with the extraction of Cs- and negative Cs-fluorides. Cs- currents of several 10 nA were extracted over hours from mg amounts of Cs2SO4 material. The transmission to various charge states was tested with gas (Ar, He) and foil stripping. Experiments showed that no suppression in the detection system is possible at high beam energies with the VERA facility. For this reason, gas stripping to low charge states (2(+), 3(+)) with transmissions up to 30% is favorable to guarantee optimal beam transport to the detector. In the present setup, utilizing a simple Bragg-type detector, the blank Cs-135/Cs-133 ratios from chemically pure samples are determined by the Ba-135 background to a value of (4.0 +/- 1.3).10(-9).

• 出版日期2015-10-15