Quantification of 60Fe atoms by MC-ICP-MS for the redetermination of the half-life

Production and characterization of 60Fe standards for accelerator mass spectrometry

Accelerator Mass Spectrometry (AMS) is one of the most sensitive analysis techniques to measure long-lived radionuclides, reaching detection limits for isotopic ratios down to 10-15-10-16 in special cases. Its application portfolio covers nearly every field of environmental research, considering processes in the atmosphere, biosphere, hydrosphere, cryosphere, lithosphere and the cosmosphere. Normally, AMS measures the content of isotopes in comparison to a validated standard. However, in some cases like for example 60Fe, well characterized standard materials are difficult to produce due to the extreme rareness of the isotope. We report here on the manufacturing of a set of 60Fe standards, obtained by processing irradiated copper from a beam dump of the high-power proton accelerator (HIPA) at the Paul Scherrer Institute (PSI). The isotopic ratios of the standards have been adjusted via a dilution series of a master solution, isotopic content of which has been characterized by Multi Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS). In total, we produced three samples with isotopic ratios of 1.037(6)·10-8, 1.125(7)·10-10 and 1.234 (7)·10-12, respectively. The latter had already been applied in three pioneering AMS studies investigating the remaining signal of injected matter of nearby super novae explosions in sediment archives.

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.