Settling the half-life of 60Fe: fundamental for a versatile astrophysical chronometer.

In order to resolve a recent discrepancy in the half-life of 60Fe, we performed an independent measurement with a new method that determines the 60Fe content of a material relative to 55Fe (t1/2=2.744 yr) with accelerator mass spectrometry. Our result of (2.50±0.12)×10(6) yr clearly favors the recently reported value (2.62±0.04)×10(6) yr, and rules out the older result of (1.49±0.27)×10(6) yr. The present weighted mean half-life value of (2.60±0.05)×10(6) yr substantially improves the reliability as an important chronometer for astrophysical applications in the million-year time range. This includes its use as a sensitive probe for studying recent chemical evolution of our Galaxy, the formation of the early Solar System, nucleosynthesis processes in massive stars, and as an indicator of a recent nearby supernova.

Novel method to study neutron capture of 235U and 238U simultaneously at keV energies.

The neutron capture cross sections of the main uranium isotopes, (235)U and (238)U, were measured simultaneously for keV energies, for the first time by combining activation technique and atom counting of the reaction products using accelerator mass spectrometry. New data, with a precision of 3%-5%, were obtained from mg-sized natural uranium samples for neutron energies with an equivalent Maxwell-Boltzmann distribution of kT ∼ 25 keV and for a broad energy distribution peaking at 426 keV. The cross-section ratio of (235)U(n,γ)/(238)U(n,γ) can be deduced in accelerator mass spectrometry directly from the atom ratio of the reaction products (236)U/(239)U, independent of any fluence normalization. Our results confirm the values at the lower band of existing data. They serve as important anchor points to resolve present discrepancies in nuclear data libraries as well as for the normalization of cross-section data used in the nuclear astrophysics community for s-process studies.