Activity measurement of 60Fe through the decay of 60mCo and confirmation of its half-life

The half-life of the neutron-rich nuclide, $^{60}\text{Fe}$ has been in dispute in recent years. A measurement in 2009 published a value of $(2.62 \pm 0.04)\times10^{6}$ years, almost twice that of the previously accepted value from 1984 of $(1.49 \pm 0.27)\times10^{6}$ years. This longer half-life was confirmed in 2015 by a new measurement, resulting in a value of $(2.50 \pm 0.12)\times10^{6}$ years. All three half-life measurements used the grow-in of the $\gamma$-ray lines in $^{60}\text{Ni}$ from the decay of the ground state of $^{60\text{g}}\text{Co}$ (t$_{1/2}$=5.27 years) to determine the activity of a sample with a known number of $^{60}\text{Fe}$ atoms. In contrast, the work presented here measured the $^{60}\text{Fe}$ activity directly with the 58.6 keV $\gamma$-ray line from the short-lived isomeric state of $^{60\text{m}}\text{Co}$ (t$_{1/2}$=10.5 minutes), thus being independent of any possible contamination from long-lived $^{60\text{g}}\text{Co}$. A fraction of the material from the 2015 experiment with a known number of $^{60}\text{Fe}$ atoms was used for the activity measurement, resulting in a half-life value of $(2.58 \pm 0.15)\times10^{6}$ years, confirming again the longer half-life. In addition, $^{60}\text{Fe}$/$^{56}\text{Fe}$ isotopic ratios of samples with two different dilutions of this material were measured with Accelerator Mass Spectrometry (AMS). Combining these with our activity measurement resulted in a half-life value of $(2.55 \pm 0.44)\times 10^{6}$ years, again agreeing with the longer half-life.