Backgrounds and blanks in I-129 measurements at the Australian National University
Prof. Keith Fifield1, Dr Michaela Froehlich1, Mr Dominik Koll1, Dr Stefan Pavetich1, Dr Zuzana Slavkovska1, A/Prof Stephen Tims1, Prof Anton Wallner1,2
1The Australian National University, , Australia, 2Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany
Although the AMS measurement of 129I is straightforward in principle due to the absence of a stable isobar, an inter-laboratory comparison exercise conducted more than 20 years ago (Roberts et al. 2000) indicated that there could be issues with both the chemistry and AMS of this isotope. In particular, there were clearly unidentified sources of AMS background at some laboratories. We are not aware that these issues have subsequently been fully resolved.
Recently, the number of 129I measurements at the ANU has been increasing, and it was considered timely to explore in rather more detail the questions of backgrounds and blanks at our facility. Specifically:
a) To date, Woodward iodine, which has an 129I/127I ratio of ~2×10-14, has been the lowest ratio material available and hence is widely used as carrier. We have made repeated measurements of this material in order to define its ratio more accurately. A number of other iodides have also been studied with a view to identifying lower-ratio material. In the event, 8 different iodides of varying provenance proved to have remarkably similar ratios, all of which were an order of magnitude higher than the Woodward iodine.
b) Any backgrounds are likely to be due to 127I7+ and 128Sb7+ ions. Hence, we are studying the extent to which 127IH2-, 127ID- and 128SbH- ions are produced in the ion source and injected into the accelerator along with 129I-.
c) Since our processing blanks have been consistently higher than our historical Woodward iodine material, we are making a series of measurements of the 129I content of the various chemicals used in sample processing. This includes the Woodward iodine carrier that we are using, which is from a different batch than the historical material.
Measurements are performed at a terminal voltage of 11 MV with nitrogen gas stripping, the 7+ charge state is selected, and the 88 MeV 129I7+ ions are detected in a gas-ionisation detector with an energy resolution of 1.3%.
These questions on background and blanks are, however, essentially independent of the available particle energies, and thus are also relevant for smaller AMS systems.
M.L. Roberts and M.W. Caffee, Nucl. Instr. Meth B172 (2000) 388-394
Keith Fifield initiated the AMS program on the ANU’s large 15 MV accelerator in the late 1980s. He has made significant contributions to developments for a wide range of isotopes from Be-10 to actinides, and is closely involved with a broad spectrum of applications.