Constraining the origin of stellar Iron-60

The radioactive nuclide Iron-60 with a half-life of 2.26 million years has attracted interest because of its importance in astrophysical scenarios. It is believed to be synthesized in massive stars, and the gamma rays associated with its decay have been detected by gamma-ray space observatories. By comparing the gamma-ray flux to that of from Aluminum-26, important constraints on stellar models can be obtained. However, the observed and predicted ratios of fluxes for Iron-60 and Aluminum-26 do not match, which is caused by uncertainties in the modelling of the stellar environments and in the nuclear-data inputs. If the nuclear-data uncertainties can be reduced, stellar processes can be better constrained.

One of the most important uncertainties in the nuclear data inputs is the beta-decay rate of Iron-59 in the stellar environments. In massive stars, the stable nucleus Iron-58 captures a neutron to form Iron-59, which then either captures another neutron to produce Iron-60 or beta-decays to Cobalt-59. Therefore, the beta-decay rate of Iron-59 determines the yield of Iron-60. Although the decay rate of Iron-59 has been accurately measured in laboratory experiments on earth, its decay rate could be significantly enhanced in stars. This is because a fraction of the Iron-59 nuclei will be in excited states due to the high temperature of the stellar environment. This strongly increases the decay rate compared to the situation in which all Iron-59 nuclei are in the ground state. Therefore, beta-decay rates from the excited levels in Iron-59 have to be measured to obtain accurate stellar decay rates.

To directly measure the beta-decay rates from the excited states of Iron-59, one would have to find a way to keep the Iron-59 nucleus in the excited states for an extended period, which is presently not feasible in the laboratory. However, it is possible to determine the relevant rates indirectly by using nuclear charge-exchange reactions. These reactions provide the information necessary to determine the relevant beta-decay rates in the astrophysical environment. A recent charge-exchange experiment performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University was successful in constraining the relevant Iron-59 beta-decay rates.

In the experiment, a Cobalt-59 target was bombarded by a beam of Hydrogen-3 particles. Helium-3 particles from the reaction were detected and analyzed in the S800 spectrograph and gamma-rays from the residual Iron-59 nucleus were detected by the GRETINA gamma-ray detector array. By counting the numbers of detected Helium-3 particles and associated gamma rays, the relevant beta-decay rates could be determined. As a consequence, it was determined that the yield of Iron-60 was 40% less in massive stars than previously thought. This result strongly reduces the discrepancy between the astronomical observations and the theoretical prediction for the ratio of Iron-60 and Aluminum-26 fluxes.

The results of the experiment were recently published in B. Gao et sl., Phys. Rev. Lett. 126