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

TL;DR

This study accurately measures the neutron-capture cross section of $^{59}$Fe to improve understanding of $^{60}$Fe production in stars, revealing higher reaction rates and emphasizing the importance of stellar physics uncertainties.

Contribution

First experimental determination of the $^{59}$Fe(n,$ extgamma$)$^{60}$Fe cross section using the surrogate ratio method, including all reaction components, refining nucleosynthesis models.

Findings

Derived Maxwellian-averaged cross sections at 30 keV and 90 keV

Found reaction rates 10-20% higher than previous estimates

Uncertainties in $^{60}$Fe production are mainly due to stellar physics

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,$\gamma$)$^{60}$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 $\gamma$-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,$\gamma$)$^{60}$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,$\gamma$)$^{60}$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.