Yb− and 236UF5−—Two case studies of E/q and EM/q2 interferences in AMS

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The Lalonde 3MV-AMS system uses one large injection magnet and one large high energy magnet, providing both sufficient bending power and resolution in both the low and high energy mass and charge systems for all heavy elements. This allows two demanding AMS subjects to be investigated conclusively: one is the search for the elusive atomic negative ion Yb−, and the other is the use of 236UF5− to achieve sub-10−13 abundance sensitivities with a single analyzing magnet in the high energy system. Both studies require the AMS system to be capable of sufficiently removing intense E/q and EM/q2 interferences to the rare signals. In the case of the Yb− search, the complication caused by the fragmenting ytterbium hydride beams must be well resolved; in the case of 236U detection, its relative mass difference from the abundant 235,238U is among the smallest of all cases in AMS. In carrying out these investigations, a generalized method of analyzing the potential E/q and EM/q2 interferences in AMS is illustrated. The conclusions are: (1) With 12 kV ion source extraction voltage and 0.5 MV accelerator terminal voltage, which correspond to a maximum electric field gradient of 2.8 kV/cm along the ion trajectory from the sputter target surface to the entrance of the accelerator terminal, Yb− has not yet been observed by AMS using a Cs+ sputter ion source. (2) A best-possible abundance sensitivity of 236U/U ∼ 4 × 10−14 has been indicated using UF5− with just one large magnet in the high energy system.