129I AMS at 0.5 MV tandem accelerator

The 129I measurement program has been established at the 0.5 MV ‘Tandy’ accelerator of the PSI/ETH Zürich AMS facility. This development was made possible by using a SiN window instead of Mylar one in a gas ionization detector. The setting up of the 129I measurement at Tandy is simple, the acquired performance is stable and reliable, and the quality of results is equal to or better than at our larger EN-tandem. With this setup, high sample throughput, which is required in many 129I studies, can be easily achieved. The measurements are performed in the +3 charge state. At this charge state the major difficulty in the 129I+3 identification is caused by a highly abundant 43+1 (m = 43, q = +1) molecule interference. This is a positive molecular ion, because its intensity reduces exponentially with an increase in gas stripper pressure. We conclude that this molecule is 27Al16O+ (m/q = 43/1 = 129/3) and comes from the break-up of (Al2O3 + Al)− (m = 129) precursor at the terminal: (Al2O3 + Al)− → 27Al16O+. The expected isobaric interferences 43Ca+1 and 86Sr+2, which also originate from the break-up of molecules in the stripper, were found to be low and do not disturb the 129I+3 measurements. The best repeatable performance with our standard sample material was achieved at 0.14 μg/cm2 Ar gas stripper pressure with machine blanks showing ∼6 × 10−14 normalized 129I/I ratio and 9% transmission through the accelerator. However, high 27Al16O+ molecular rates were observed from the user samples, and in order to destroy these molecules we had to increase the stripper pressure to ∼0.22 μg/cm2. This increase in the stripper pressure degraded the machine blank values to ∼9 × 10−14 and reduced transmission to 8%. Nevertheless, the achieved measurement conditions are sufficient for measurement of nearly all 129I samples that have been submitted to PSI/ETH over the last few years.