The Isobar Separator for Anions at the A. E. Lalonde AMS Laboratory
Erin L. Flannigan1, Dr. Jean-François Alary2, Dr. Lisa M. Cousins2, Dr. Gholamreza Javahery2, Dr. William E. Kieser1,2
1University Of Ottawa, Ottawa, Canada, 2Isobarex Corp., Vaughan, Canada
Accelerator Mass Spectrometry (AMS) of isotopes with abundant negative ion-forming isobars often requires the use of large accelerators to achieve high sensitivity measurements. The Isobar Separator for Anions (ISA) is a radiofrequency quadrupole (RFQ) reaction cell system that provides selective isobar suppression of many of these isotopes in the low energy system, prior to injection into an accelerator. The ISA can then facilitate the measurement of these ions using smaller accelerators. A commercial version from Isobarex Corp. (Vaughan, Ontario, Canada) has been installed into a separate Research Line of the 3 MV tandem accelerator system at the A. E. Lalonde AMS Laboratory, University of Ottawa.
The Research Line consists of a Cs+ Sputter Ion Source that produces a 20-35 keV anion beam, which is energy and mass analyzed before injection into the ISA. The ISA includes a DC deceleration region, a combined cooling and reaction cell, and a DC re-acceleration region. The deceleration region reduces the beam energy to a level at which the RFQ cell can capture and contain the ions. The cell is filled with an inert cooling gas that has been experimentally selected to further reduce the ion energy and therefore facilitate charge exchange or other chemical reactions between the interfering isobar and the reaction gas, as well as enabling the highest transmission of the isotope of interest. A reaction gas, chosen to preferentially react with the interfering isobar, is also contained within the cell. RFQ segments along the length of the cell create a potential well, which limits the divergence of the traversing ions. DC offset voltages on these RFQ segments maintain a controlled ion velocity through the cell. After exiting the cell, the wanted ions are re-accelerated prior to injection into the tandem accelerator for conventional AMS analysis.
Here we present ISA characterization for best achieved sulfur suppression and chlorine transmission. These tests use NO₂ as the reaction gas due to its well-known exothermic reaction with sulfur anions but endothermic reaction with chlorine anions . Helium was selected as the cooling buffer gas, as it provided the best chlorine transmission of ~50% through the ISA column. Over six orders of magnitude reduction of sulfur-36 to chlorine-37 has been observed. Using the sulfur suppression from the ISA and the degree of dE/dx separation for 12 MeV ions in the detector offered by the 3MV-AMS system, a chlorine-36/chlorine-37 abundance sensitivity of ~7×10-¹⁵ was achieved.
The ISA will be used to measure chlorine-36 samples and its applications will soon be extended to additional isotopes involving, for example, the suppression of ⁹⁰ZrF₃- and ¹³⁵BaF₂-/¹³⁷BaF₂- in the measurements of ⁹⁰SrF₃- and ¹³⁵CsF₂-/¹³⁷CsF₂-, respectively.
 D.B. Dunkin et al. Chemical Physics Letters 15 (1972) 257.
Erin is a PhD Candidate in Physics at the A. E. Lalonde AMS Laboratory, University of Ottawa. She completed her MSc in Physics at the University of Guelph on emulating the Mars Curiosity rover Alpha-Particle X-ray Spectrometer using accelerator particle-induced X-ray emission.