Activity measurement of ${}^{60}\mathrm{Fe}$ through the decay of ${}^{60m}\mathrm{Co}$ and confirmation of its half-life

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

Figure 1

Full decay scheme for ${}^{60}\mathrm{Fe}$. Thick white arrows indicate decays that happen more prevalently (100% or almost 100% for each) and gray dashed arrows indicate other possible decays that occur (data taken from [16]).

Figure 2

Lead castle for the low-background counting station. Two planar HPGe detector heads fit inside of this lead castle construction, one on the left side of this picture and the other exactly opposite (not visible here). The lead castle is two layers of lead bricks thick on all sides. The aluminum structure surrounding the lead is a winch system for removing a section of the top two layers, allowing easy access to the inside and eliminating line-of-sight issues. For this work, the detector heads were 12.5 $\pm$ 0.25 mm from the target (or 17.5 $\pm$ 0.25 mm for the last measurement) with a plastic target holder centered between them, which would hold samples and calibration sources at the same location.

Figure 3

Typical spectrum of a calibration measurement using the customized ${}^{241}\mathrm{Am}$ reference source, on detector 1, for 3600 s of live time. Counts per bin are plotted as a function of energy (keV). Several peaks of interest are labeled, in particular the ${}^{237}\mathrm{Np}$ L x-ray peaks, the $\gamma$ rays of ${}^{241}\mathrm{Am}$, and the true coincidence summing peaks of the predominant $\gamma$ ray with the ${}^{237}\mathrm{Np}$ L x rays.

Figure 4

Detection efficiencies [labeled as Eff% in Eq. (1)] for the two planar HPGe detectors used, as measured with a ${}^{241}\mathrm{Am}$ calibration source using the 59.54-keV $\gamma$ ray and corrected for true coincidence summing effects. The percent error is 2.3%, as detailed in Table 3. Detector 1 is shown as black squares and detector 2 as red circles. The substantial change in efficiency between sets 1–9 and set 10 comes from changing the distance between the detectors and the source. The detectors for sets 1–9 are 12.5 $\pm$ 0.25 mm from the target and for the final set, set 10, are (17.5 $\pm$ 0.25) mm from the target. For each set, the target holder was in the same location for the Fe-1 activity sample and the ${}^{241}\mathrm{Am}$ source. For the final calculations on the Fe-1 sample, the efficiencies measured before the sample were used to scale the activity.

Figure 5

24-h runs on the Fe-1 sample (in red, upper line) and the background (in blue, lower line). Counts per energy bin are on the $y$ axis and energy in keV is on the $x$ axis. Note the background peak at 63.3 keV. This peak is from the decay of ${}^{234}\mathrm{Th}$ in the ${}^{238}\mathrm{U}$ decay chain. This peak is well separated from our peak of interest at 58.6 keV but can act as a good test of the background subtraction technique. The shift in the background continuum when the Fe-1 sample is in place is due to the internal bremsstrahlung photons from the electron capture decay of ${}^{55}\mathrm{Fe}$ which is present in the original ${}^{60}\mathrm{Fe}$ material [12].

Figure 6

Results of the Fe-1 activity sample where each set occurs over a 6-day real-time period. Both detectors are given here, one in black squares and the other in red circles. In the gray band, the final average number of 1.202 Bq and a 3.9% uncertainty is shown, as calculated from Table 3.

Figure 7

Left column illustrates the crossover technique with Bragg curve spectroscopy with a mass 58 beam, ${}^{58}\mathrm{Ni}$ and ${}^{58}\mathrm{Fe}$. Mass 58 is a good approximation for the behavior observed at mass 60. The position of the beam particles as they exit the spectograph magnet is recorded by a PGAC detector and plotted on the $x$ axis in the left column plots. Following the PGAC is an ionization chamber (IC), split into four anodes. Each anode records the amount of energy deposited. (a) Energy deposited in the first anode is plotted on the $y$ axis. Here the isobar ${}^{58}\mathrm{Ni}$ loses energy at a higher rate than ${}^{58}\mathrm{Fe}$. (c) Energy deposited in the third anode of the IC is plotted. Here ${}^{58}\mathrm{Fe}$ is losing energy at a higher rate than ${}^{58}\mathrm{Ni}$. The right column shows the energy loss of anodes 1 and 2 plotted against the energy loss in anode 3: (b) data for the Fe-4 material and (d) data for the Fe-1 material. By using the crossover technique, good separation between the isobars of nickel and iron is observed.

Figure 8

All half-life measurements of ${}^{60}\mathrm{Fe}$ including this work. Note there is no $y$ axis. The individual measurements are separated out for ease of the reader. The most recent measurements of Rugel et al. [11], Wallner et al. [12], and this work agree on a longer half-life than the previously accepted value of Kutschera et al. [10].