Unstable 60Fe could have been one of the most powerful heat sources for inducing planetary melting and differentiation in the early solar system. The appearance of 60Ni isotopic anomalies in presolar grains produced by the decay of short-lived 60Fe is a possible proof that 60Fe existed in the early solar system. This has important implications on the history of nucleosynthesis. Measurements with conventional SIMS have already been done, but the desired precision has not been reached due to molecular interferences.
Accelerator SIMS has proven to be capable of measuring concentrations of trace elements at the ppb- to ppt-level. It is a new challenge to measure precise isotope ratios at this concentration level. Since all isotopes of interest have low count rates none of them can be measured in a Faraday cup as in conventional AMS. Thus, the detection system has to be modified to acquire two or more isotopes of very similar masses at the same time in a particle detector.
At the PSI/ETH AMS facility it is possible to inject up to three different isotopes quasi simultaneously into the tandem accelerator with the fast beam bouncing system. In the focal plane of the high-energy mass spectrometer, two isotopes of a mass close to 60 amu will be separated by 37 mm per amu. With this narrow space it is not possible to place two or more different detectors side by side. Our approach is therefore to measure both isotopes in the same detector. For this purpose a position sensitive gas ionisation detector with a 76 mm wide entrance window was built, allowing to simultaneously accept up to three neighbouring masses around 60 amu. In order to measure the position, the first anode of the detector is divided into two saw tooth patterns. As a side effect of this single detector method, systematic differences between several counting systems do not have to be taken into account, which improves the intrinsic precision of the measurement.
First tests show that it is possible to measure the natural isotopic ratio of 61Ni/62Ni and 121Sb/123Sb to a precision well below 1%. We conclude that measurements in presolar grains should be possible to the desired accuracy with our set-up.
See more of Poster Session I
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)