Thermal neutron capture cross section of the radioactive isotope ${}^{60}\mathrm{Fe}$

Background: Fifty percent of the heavy element abundances are produced via slow neutron capture reactions in different stellar scenarios. The underlying nucleosynthesis models need the input of neutron capture cross sections.

Purpose: One of the fundamental signatures for active nucleosynthesis in our galaxy is the observation of long-lived radioactive isotopes, such as ${}^{60}\mathrm{Fe}$ with a half-life of $2.60\times {10}^6$ yr. To reproduce this $\gamma$ activity in the universe, the nucleosynthesis of ${}^{60}\mathrm{Fe}$ has to be understood reliably.

Methods: An ${}^{60}\mathrm{Fe}$ sample produced at the Paul Scherrer Institut (Villigen, Switzerland) was activated with thermal and epithermal neutrons at the research reactor at the Johannes Gutenberg-Universität Mainz (Mainz, Germany).

Results: The thermal neutron capture cross section has been measured for the first time to ${\sigma }_{\text{th}}=0.226{(}_{-0.049}^{+0.044})\text{b}$. An upper limit of ${\sigma }_{\text{RI}}<0.50\text{b}$ could be determined for the resonance integral.

Conclusions: An extrapolation towards the astrophysically interesting energy regime between $kT=10$ and 100 keV illustrates that the $s$-wave part of the direct capture component can be neglected.

Figure 1

The $s$-process reaction path between Fe and Ni. The isotope ${}^{60}\mathrm{Fe}$ is produced via a sequence of $(n,\gamma )$ reactions starting at the stable iron isotopes. Because of the short half-life of ${}^{59}\mathrm{Fe} (t_{1/2}\approx 45$ d), the production of ${}^{60}\mathrm{Fe}$ depends critically on the stellar neutron density.

Figure 2

The detection efficiency of the HPGe detector used for the activity measurements in Mainz. The efficiency used for determination of the number of ${}^{60}\mathrm{Fe}$ particles is given in Table 1. The solid line shows the least-squares fit to interpolate between the data points of the calibrated solution (red circles). For ${}^{88}\mathrm{Y}$ and ${}^{60}\mathrm{Co}$ the data points were corrected for cascade effects using the geant-3.21 package [15, 16]. The grey band represents the uncertainty of the fit.

Figure 3

The activity of the ${}^{60}\mathrm{Fe}$ sample was determined by the $\gamma$-ray cascade with energies of 1173 and 1332 keV emitted after the $\beta$ decay of the daughter nucleus ${}^{60}\mathrm{Co}$. Data are from Refs. [17, 21].

Figure 4

$\gamma$-ray spectrum for the determination of the ${}^{60}\mathrm{Co}$ activity taken 34 months after purification of the sample. The measurement time was $t_m=23$ h.

Figure 5

The $\gamma$-ray spectra of the Zr monitor foils used for the neutron fluence analysis normalized for different measuring times. Due to the smaller capture cross section of ${}^{96}\mathrm{Zr}$ in the thermal energy regime, the cadmium shielding affects the ${}^{97}\mathrm{Zr}$ signature to a much lower degree than that of ${}^{95}\mathrm{Zr}$.

Figure 6

The $\gamma$-ray spectrum of the activation without cadmium shielding measured for one, two, and three half-lives of ${}^{61}\mathrm{Fe}$, respectively. The upper panel provides an overview, and the lower panels show a zoom into the regions of the ${}^{61}\mathrm{Fe}$ lines at 1027 and 1205 keV. The background stems from the activation of the contaminants of the ${}^{60}\mathrm{Fe}$ sample.

Figure 7

A detailed view into the region around 1027 keV of the $\gamma$-ray spectrum after activation with the cadmium shielding, illustrating the absence of ${}^{61}\mathrm{Fe}$ lines. Therefore, only an upper limit can be deduced from the data.

Figure 8

The Maxwellian averaged cross section for ${}^{60}\mathrm{Fe}(n,\gamma )$. The present measurement of the cross section at $kT=25.3$ meV (red triangle) and an $0.00115/\sqrt{E}$ extrapolation to the astrophysical energy regime are indicated by the solid red triangle and the dashed red line, respectively. This extrapolation can be used to estimate the DC of the MACS at $kT=25$ keV (black dot) [9]. The astrophysical energy regime from $kT=10$ keV–100 keV (grey box) is clearly dominated by the resonant capture contribution. Below about 1 keV, the MACS based on the most recent version of the TENDL library (TENDL-2014 [24], blue line) are a factor of 3 above the current measurement. This indicates that the DC componenent is clearly overestimated in the library.