We describe the opportunities available at Oak Ridge National Laboratory using the highest voltage electrostatic accelerator in the world, the 25-MV tandem of the Holifield Radioactive Ion Beam Facility (HRIBF), to measure two important AMS isotopes 14C and 36Cl. For the case of 36Cl, we measured the isotopic ratios 36Cl/Cl of various seawater samples taken from around the world finding significant variation in the content of this isotope. We have pushed the AMS technique to its limits in measuring the lowest isotopic ratio of 36Cl/Cl ever.
For radiocarbon measurements, the unique characteristics of the HRIBF tandem will allow unprecedented cleanliness. The vertical accelerator has a folded configuration in which both low- and high-energy acceleration tubes are contained in a single column. In the terminal, the ion beam first passes through the stripper becoming positively charged and allowing an early molecular destruction of the much more abundant molecules 13CH- and 12CH2-. After the stripper one charge state component is bent through an angle of 180° and injected into the high energy acceleration tube. The tandem operation at a high terminal voltage, allows to fully strip nearly 50% of 14C ions “on the way up”. The 180° magnet required to reverse the beam direction serves as an excellent filter for the selection of 14C6+. Further acceleration back to ground potential follows. This way all potential sources of instrumental background are virtually eliminated.
Finally we will discuss how two fields of research with apparently little in common — Radioactive Ion Beam Physics and AMS — complement each other in techniques to isolate the isobars of interest. Applications that require the highest sensitivity levels for AMS will be discussed.
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