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

Abstract

The half-life of the neutron-rich nuclide, {\fesixty} has been in dispute in recent years. A measurement in 2009 published a value of $(2.62\pm 0.04)\times 10^6$ years, almost twice that of the previously accepted value from 1984 of $(1.49\pm 0.27)\times 10^6$ years. This longer half-life was confirmed in 2015 by a second measurement, resulting in a value of $(2.50\pm 0.12)\times 10^6$ years. All three half-life measurements used the grow-in of the ${\hat{I}}^3$-ray lines in {\nisixty} 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 ${\hat{I}}^3$-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 ${}^{60\text{g}}\text{Co}$. 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 $(2.72\pm 0.16)\times 10^6$ 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.

14 pages, 8 figures, version submitted to PRC March 2017