Detection of (135) Cs by accelerator mass spectrometry

The ability to measure both (135) Cs and (137) Cs can provide an estimate of the age and source of Cs isotopes in an environmental sample. Accelerator mass spectrometry (AMS) consistently reports lower abundance sensitivities than other techniques and, with the addition of an on-line reaction cell, simpler isobaric suppression. Therefore, an AMS methodology was developed to measure Cs isotopes using CsF2- as the initial anion.

The ion beam is passed through the Isobar Separator for Anions (ISA) where it is captured by radiofrequency quadrupoles in a gas cell before injection into the tandem accelerator. In the ISA, the beam reacts with O2 gas, selectively removing the BaF2- and leaving the Cs analyte to be reaccelerated and sent through the remainder of the AMS system.

The BaF2- signal was attenuated by a factor of 10(5) in the ISA while 25% of the original CsF2- current was transmitted into the accelerator. (135) Cs was measured without any interference from (133) Cs to an abundance sensitivity of 1.3 × 10(-10) . The abundances of four stable Ba isotopes (masses 133, 134, 135 and 137) were measured and no isotope-dependent bias was detected using the ISA in vacuum.

The results demonstrate the feasibility of measuring long-lived Cs isotopes without Ba interference by AMS with on-line isobar separation and the ability to use shorter lived Cs isotopes for yield tracing.