129I AMS at 0.5 MV tandem accelerator

The I-129 measurement program has been established at the 0.5 MV 'Tandy' accelerator of the PSI/ETH Zurich 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 I-129 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 I-129 studies, can be easily achieved. The measurements are performed in the +3 charge state. At this charge state the major difficulty in the I-129(+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 (AlO+)-Al-27-O-16 (m/q = 43/1 = 129/3) and comes from the break-up of (Al2O3 + Al)(-) (m = 129) precursor at the terminal: (Al2O3 + Al)(-) -> (AlO+)-Al-27-O-16. The expected isobaric interferences Ca-43(+1) and Sr-86(+2), which also originate from the break-up of molecules in the stripper, were found to be low and do not disturb the I-129(+3) measurements. The best repeatable performance with our standard sample material was achieved at 0.14 mu g/cm(2) Ar gas stripper pressure with machine blanks showing similar to 6 x 10(-14) normalized I-129/I ratio and 9% transmission through the accelerator. However, high (AlO+)-Al-27-O-16 molecular rates were observed from the user samples, and in order to destroy these molecules we had to increase the stripper pressure to similar to 0.22 mu g/cm(2). This increase in the stripper pressure degraded the machine blank values to similar to 9 x 10(-14) and reduced transmission to 8%. Nevertheless, the achieved measurement conditions are sufficient for measurement of nearly all I-129 samples that have been submitted to PSI/ETH over the last few years. (C) 2009 Elsevier B.V. All rights reserved.