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

The half-life of the neutron-rich nuclide, {fesixty} 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 second 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 { isixty} from the decay of the ground state of $^{60}text{Co}$ (t$_{1/2}$=5.27 years) to determine the activity of a sample with a known number of {fesixty} atoms. In contrast, the work presented here measured the {fesixty} activity directly via the 58.6 keV $gamma$-ray line from the short-lived isomeric state of $^{60}text{Co}$ (t$_{1/2}$=10.5 minutes), thus being independent of any possible contamination from long-lived $^{60text{g}}text{Co}$. A fraction of the material from the 2015 experiment with a known number of {fesixty} atoms was used for the activity measurement, resulting in a half-life value of $(2.72 pm 0.16)times10^{6}$ years, confirming again the longer half-life. In addition, {fesixty}/{fe} isotopic ratios of samples with two different dilutions of this material were measured with Accelerator Mass Spectrometry (AMS) to determine the number of {fesixty} atoms. Combining this with our activity measurement resulted in a half-life value of $(2.69 pm 0.28)times 10^{6}$ years, again agreeing with the longer half-life.