Sm-146 – Feasibility studies to re-date the chronology of the Early Solar System
Dr Stefan Pavetich1, Keith Fifield1, Michaela B Froehlich1, Dominik Koll1, Zuzana Slavkovská1, Attila Stopic2, Steve G. Tims1, Anton Wallner1,3
1The Australian National University, Canberra, Australia, 2ANSTO, Sydney, Australia, 3Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany
AMS measurements of long-lived radionuclides can make significant contributions to the understanding of the temporal evolution of our early solar system. Samarium-146 has a half-life in the order of 100 Myr and decays via emission of α-particles into stable ¹⁴²Nd. Due to different geochemical behaviour and the radioactive decay of ¹⁴⁶Sm, the Sm-Nd isotopic system can serve as a chronometer for the early solar system and planetary formation processes. The half-life of ¹⁴⁶Sm, which provides the time scale for this clock, is in dispute. The most recent and notably precise measurements for the half-life are (103±5) Myr (adopted from [1,2]) and (68±7) Myr  and differ by more than 5 standard deviations. In addition to potentially resolving this discrepancy, developing AMS for ¹⁴⁶Sm might provide the means to study stellar nucleosynthesis on the proton rich side of the chart of nuclei and serve as radiometric tracer for geosciences.
Due to the extremely challenging task of separating ¹⁴⁶Sm from its stable isobar ¹⁴⁶Nd, to date the only AMS measurement of ¹⁴⁶Sm was performed at Argonne National Laboratory with energies in the order of ~880 MeV. At the Heavy Ion Accelerator Facility at ANU, the possibility to measure ¹⁴⁶Sm at energies of 200-250 MeV is being explored. Different sample materials, molecular negative ion beams and detector setups are investigated. So far, the lowest Nd backgrounds, from commercially available sample material without additional Nd separation were achieved using SmO₂- beams extracted from Sm₂O₃ samples.
In order to explore the limits of the Sm detection capabilities, Sm₂O₃ samples were irradiated with thermal neutrons in the reactor at ANSTO to produce the shorter lived ¹⁴⁵Sm (t1/2 = (340±3) d ) via ¹⁴⁴Sm(n,γ)¹⁴⁵Sm. The production of ¹⁴⁵Sm is easier and faster and the challenges in measuring ¹⁴⁵Sm via AMS are very similar to those measuring ¹⁴⁶Sm. In addition, ¹⁴⁵Sm has the potential to serve as a tracer for future reference materials for AMS measurements of Sm.
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Stefan Pavetich studied Physics at the University of Vienna. He received his PhD in Physics from the Technical University of Dresden in 2015. His PhD work focused on ion source development for AMS and was conducted at the Helmholtz-Zentrum Dresden-Rossendorf. Currently, he is a Postdoctoral Fellow in the Department of Nuclear Physics at the ANU investigating neutron -, and alpha capture reactions relevant for nucleosynthesis in stellar environments and developing AMS for non-routine radionuclides (Zr-93, Fe-60). He participated in interdisciplinary studies using AMS, including reconstruction of irradiation histories of meteorites and groundwater modelling in arid regions in Israel and Oman.