Settling the Half-Life of ${}^{60}\mathrm{Fe}$: Fundamental for a Versatile Astrophysical Chronometer

In order to resolve a recent discrepancy in the half-life of ${}^{60}\mathrm{Fe}$, we performed an independent measurement with a new method that determines the ${}^{60}\mathrm{Fe}$ content of a material relative to ${}^{55}\mathrm{Fe}$ ($t_{1/2}=2.744\mathrm{yr}$) with accelerator mass spectrometry. Our result of $(2.50\pm 0.12)\times {10}^6\mathrm{yr}$ clearly favors the recently reported value $(2.62\pm 0.04)\times {10}^6\mathrm{yr}$, and rules out the older result of $(1.49\pm 0.27)\times {10}^6\mathrm{yr}$. The present weighted mean half-life value of $(2.60\pm 0.05)\times {10}^6\mathrm{yr}$ 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.

Figure 1

The decay scheme of ${}^{60}\mathrm{Fe}$ through ${}^{60}\mathrm{Co}$ to ${}^{60}\mathrm{Ni}$, and the ${}^{60}\mathrm{Co}$ ingrowth measurement (inset). All data are from Refs. [25, 26], except for the half-life of ${}^{60}\mathrm{Fe}$, which is the mean value of Ref. [28] and of the current work. Inset: ingrowth of the ${}^{60\mathrm{g}}\mathrm{Co}$ activity from the decay of ${}^{60}\mathrm{Fe}$ observed through the 1173.2 keV (squares) and the 1332.5 keV (circle) $\gamma$-transitions in ${}^{60}\mathrm{Ni}$. Measurements were performed for a period of 4 yr after separation of ${}^{60\mathrm{g}}\mathrm{Co}$ from ${}^{60}\mathrm{Fe}$. The lines are fits to the data points by Eqs. (1a) and (1b), respectively.

Figure 2

Identification spectra for ${}^{60}\mathrm{Fe}$ demonstrating a clear separation from its isobar ${}^{60}\mathrm{Ni}$. Displayed are third ($\mathrm{\Delta }\mathrm{E}3$) vs second energy-loss signal ($\mathrm{\Delta }\mathrm{E}2$) (lower) and second vs fourth energy-loss signals (upper plot). Note, this sample has an isotope ratio ${}^{60}\mathrm{Fe}/{}^{\text{nat}}\mathrm{Fe}$ of $\sim {10}^{-12}$, background levels are at $<{10}^{-16}$, and the samples used for the half-life measurements were $>{10}^{-9}$ (the small third peak originates from a pulser signal).