The ${}^{59}$Fe(n, γ)${}^{60}$Fe Cross Section from the Surrogate Ratio Method and Its Effect on the ${}^{60}$Fe Nucleosynthesis

Abstract

The long-lived ${}^{60}$Fe (with a half-life of 2.62 Myr) is a crucial diagnostic of active nucleosynthesis in the Milky Way galaxy and in supernovae near the solar system. The neutron-capture reaction ${}^{59}$Fe(n,${\hat{I}}^3$)${}^{60}$Fe on ${}^{59}$Fe (half-life = 44.5 days) is the key reaction for the production of ${}^{60}$Fe in massive stars. This reaction cross section has been previously constrained by the Coulomb dissociation experiment, which offered partial constraint on the $E$1 ${\hat{I}}^3$-ray strength function but a negligible constraint on the $M$1 and $E$2 components. In this work, for the first time, we use the surrogate ratio method to experimentally determine the ${}^{59}$Fe(n,${\hat{I}}^3$)${}^{60}$Fe cross sections in which all the components are included. We derived a Maxwellian-averaged cross section of 27.5 $\pm$ 3.5 mb at $kT$= 30 keV and 13.4 $\pm$ 1.7 mb at $kT$= 90 keV, roughly 10 - 20% higher than previous estimates. We analyzed the impact of our new reaction rates in nucleosynthesis models of massive stars and found that uncertainties in the production of ${}^{60}$Fe from the ${}^{59}$Fe(n,${\hat{I}}^3$)${}^{60}$Fe rate are at most of 25%. We conclude that stellar physics uncertainties now play a major role in the accurate evaluation of the stellar production of ${}^{60}$Fe.

9 pages with 6 figures