Assessment of 129I measurement at low MV tandem voltages

Significantly higher efficiencies for 129I measurement by Accelerator Mass Spectrometry (AMS) would be possible if 129I3+ ions could be counted at terminal voltages as low as 1 MV. However, at M/Q = 3, the molecular interference was anticipated to be severe; this has prevented the use of charge 3+ from being adequately considered. Instead, charge 5+ has been used at higher terminal voltages because of the minimal interference. During a recent performance assessment with charge state 3+, a background of 129I/127I ≈ 10−14 was readily obtained and few 86Sr2+ and 43Ca1+ ions were encountered. This surprising result is possibly due to the very low binding energy and consequent poor stability of anions such as CaSr− and Ca3-. Some triply charged molecular ions were found in the energy spectrum at lower stripping gas pressure but their interference with the detection of 129I3+ was readily suppressed with the Ar gas stripper operated at normal thickness. It now appears that as long as the prepared AMS samples are of good chemical purity, the molecular fragments can be expected to remain quite low in intensity and readily resolved by the final detector. As a result, charge state 3+ can be used for 129I measurements at lower terminal voltages with higher overall efficiency.