Ultra-sensitive measurements of 36Cl and 236U at the Australian National University

Some potential applications of the isotopes 36Cl and 236U require higher sensitivity than is normally required or achieved. For 36Cl, one such application is the measurement of the 36Cl/Cl ratio of seawater. We show that, with appropriate precautions to reduce the effect of ion-source memory, it is possible to reach blank levels below 10−16 in the 36Cl/Cl ratio. Measurements of a series of seawater and sea salt samples yield a blank-corrected 36Cl/Cl ratio of present-day sea water of 0.71 ± 0.08 × 10−15. For 236U, one potential application is as a prospecting tool, where sensitivities in the 236U/238U ratio at the 10−13 level are desirable. We demonstrate that this sensitivity can be achieved for 24 MeV uranium ions with time-of-flight detection over a 6.0 m flight path.