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

The half-life of the neutron-rich nuclide Fe-60 has been in dispute in recent years. A measurement in 2009 published a value of (2.62 +/- 0.04) x 10(6) years, almost twice that of the previously accepted value from 1984 of (1.49 +/- 0.27) x 10(6) yr. This longer half-life was confirmed in 2015 by a second measurement, resulting in a value of (2.50 +/- 0.12) x 10(6) yr. All three half-life measurements used the grow-in of the.-ray lines in Ni-60 from the decay of the ground state of Co-60 (t(1/2) = 5.27 yr) to determine the activity of a sample with a known number of Fe-60 atoms. In contrast, the work presented here measured the Fe-60 activity directly via the 58.6 keV gamma-ray line from the short-lived isomeric state of Co-60 (t(1/2) = 10.5 min), thus being independent of any possible contamination from long-lived Co-60g. A fraction of the material from the 2015 experiment with a known number of Fe-60 atoms was used for the activity measurement, resulting in a half-life value of (2.72 +/- 0.16) x 10(6) yr, confirming again the longer half-life. In addition, Fe-60/Fe-56 isotopic ratios of samples with two different dilutions of this material were measured with accelerator mass spectrometry to determine the number of Fe-60 atoms. Combining this with our activity measurement resulted in a half-life value of (2.69 +/- 0.28) x 10(6) yr, again agreeing with the longer half-life.