The Low-Energy Isobar Separator for Anions: Progress Report

Abstract

ABSTRACT. The suppression of interferences from atomic and molecular isobars is a key requirement for the extension of accelerator mass spectrometry (AMS) to the analysis of new cosmogenic isotopes and for increasing the range of applications for small AMS systems. In earlier work, it was shown that unwanted isobars can be eliminated by anion-gas reactions ). Recently, a prototype system in which such reactions could be applied to ions from an AMS ion source, the Isobar Separator for Anions (ISA), was described ). This system decelerates the beam of rare anions from keV to eV energies, guides them through a single radiofrequency quadrupole (RFQ) gas cell, and re-accelerates them for further analysis in a 2.5MV AMS system. Tests of this system with Cl and S anions and NO 2 gas showed a suppression of S with respect to Cl of over 6 orders of magnitude, with a transmission of ~30% for the Cl beam. In this work, results of the analysis of a range of standard reference materials are reported; these show the linearity of the system for measuring the 36 Cl/ 35 Cl ratio over a span of 2 orders of magnitude. Further tests, using the AMS system as a diagnostic tool, have provided clues about the loss of Cl at higher cell pressure and the nature of the residual low level of S transmission. These lead to the assessment of various gases for cooling the Cl -beam. Suppression measurements for 41 K in the analysis of 41 Ca, using NO 2 as a reaction gas, are also discussed. These preliminary measurements have provided data for the development of a more advanced system with separate cooling and reaction cells.

Fluoride sample matrices and reaction cells — new capabilities for isotope measurements in accelerator mass spectrometry

“…at a lower pressure) of the interfering anion, are needed. Similar searches for an inert cooling gas will be needed, but initial tests indicate that a light, inert gas will be best suited for this purpose [4].…”

“…[3]. Tests in which a real mass 36 beam, from a suite of 4 calibration samples 2 , was transmitted through the ISA demonstrated the linearity of the ISA analysis system over a range in 36 Cl/Cl ratios from 4 × 10 −11 to 4 × 10 −13 [4]. They also showed the ability of this system to remove molecular in addition to the atomic isobar interferences with the destruction of the 12 C trimer anion.…”

“…This has recently been demonstrated for the reduction of 36 S − in beams of 36 Cl − by over six orders of magnitude [3] and for the reduction of 41 KF − 3 in beams of 41 CaF − 3 by three orders of magnitude [4]. The latter example benefits greatly from the use of both these techniques.…”

“…In this case, the fluoride molecular anions were used [4]. For Ca, the predominant anion is CaF − 3 whereas for K it is KF − 2 (see fig.…”

RFQ Reaction Cells for AMS: Developments and Applications

“…The earliest AMS work progressed rapidly for cases where the most prolific atomic isobars did not form negative ions. These are to date limited to 14 N in the analysis of 14 C [4], 26 Mg for 26 Al and 129 Xe for 129 I analysis [5]. Beyond these, aside from very careful sample preparation techniques to exclude the element containing the isobar, the energy available in the accelerated AMS beams permits some additional strategies.…”