Precision Pb and S isotopic ratio measurements by microbeam AMS

Measurements of Pb and S isotopes with precision of better than 1 permil ha ve been achieved using AUSTRALIS (A.M.S. for Ultra Sensitive TRAce element and Isotopic Studies), a microbeam AMS system based on a 3 MV Tandetron at the HIAF laboratory. The high precision is made possible by a fast bouncing system both at the low and the high energy ends, to counter the effect of instabilities in the ion source and beam transport system. The system is ai med at enabling in situ microanalysis of geological samples for radiogenic and stable isotope data free from molecular and isobaric interferences. The microbeam source, developed from a HICONEX source, produces a 30 mum diame ter Csi beam routinely, and includes a high magnification sample viewing sy stem in the reflected geometry, facilitating sample positioning and tuning of the primary beam. Measurements were conducted at 1.5-2 MV terminal volta ge. Lead isotopes are measured as pb(4+) ions, from PbS- injected ions. Sul fur isotopes are measured as S2+ or S3+ ions from injected S- ions. Fractio nation effects still vary similar to2% over longer periods, attributable to the slight variation in some manual controls of the accelerator. This is e xpected to be improved when all controls are brought under computer control . (C) 2000 Elsevier Science B.V. All rights reserved.