Fig. 1. ⁶⁰Fe nucleosynthesis in massive stars. (Image by LI Yutian)

Researchers from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences and their collaborators have recently made great progress in the study of the stellar beta-decay rate of ⁵⁹Fe, which constitutes an important step towards understanding ⁶⁰Fe nucleosynthesis in massive stars. The results were published in Physical Review Letters on April 12. 

Radioactive nuclide ⁶⁰Fe plays an essential role in nuclear astrophysical studies. It is synthesized in massive stars by successive neutron captures on a stable nucleus of ⁵⁸Fe and, during the late stages of stellar evolution, ejected into space via a core-collapse supernova.  

The characteristic gamma lines associated with the decay of ⁶⁰Fe have been detected by space gamma-ray detectors. By comparing the ⁶⁰Fe gamma-ray flux to that from ²⁶Al, which shares a similar origin as ⁶⁰Fe, researchers should be able to obtain important information on nucleosynthesis and stellar models. However, the observed gamma-ray flux ratio ²⁶Al/⁶⁰Fe does not match theoretical predictions due to uncertainties in both stellar models and nuclear data inputs.  

The stellar beta-decay rate of ⁵⁹Fe is among the greatest uncertainties in nuclear data inputs. During the nucleosynthesis of ⁶⁰Fe in massive stars, ⁵⁹Fe can either capture a neutron to produce ⁶⁰Fe or beta decay to ⁵⁹Co. Therefore, the stellar beta-decay rate of ⁵⁹Fe is critical to the yield of ⁶⁰Fe.  

Although the decay rate of ⁵⁹Fe has been accurately measured in laboratories, its decay rate may be significantly enhanced in stellar environments due to contributions from its excited states. However, direct measurement of the beta-decay rate from excited states is very challenging since one has to create a high-temperature environment as in stars to keep the ⁵⁹Fe nuclei in their excited states.  

To address this problem, researchers at IMP proposed a new method for measuring the stellar beta-decay rate of ⁵⁹Fe. “The nuclear charge-exchange reaction is an indirect measurement alternative, which provides key nuclear structure information that can determine those decay rates.” said GAO Bingshui, a researcher at IMP. 

The researchers carried out their experiment at the Coupled Cyclotron Facility at Michigan State University. In the experiment, a secondary triton beam produced by the cyclotrons was used to bombard a ⁵⁹Co target. Then the reaction products, ³He particles and gamma rays, were detected by the S800 spectrometer and GRETINA gamma-ray detection array. Using this information, the beta-decay rates from the ⁵⁹Fe excited states were determined. This measurement thus eliminated one of the major nuclear uncertainties in predicting the yield of ⁶⁰Fe. 

By comparing stellar model calculations using the new decay rate data with previous calculations, the researchers found that, for an 18 solar mass star, the yield of ⁶⁰Fe is 40% less when using the new data. The result points to a reduced tension in the discrepancy in ²⁶Al/⁶⁰Fe ratios between theoretical predictions and observations. 

"It is an important step towards understanding ⁶⁰Fe nucleosynthesis in massive stars and it will provide a more solid basis for future astrophysical simulations,” said LI Kuoang, the collaborator of Gao. 

This work was supported by the National Key Research and Development program and the Strategic Priority Research Program of CAS.    

  Fig. 2. ⁶⁰Fe yield in 18 solar mass star. Blue lines (LMP) are calculations based on previous decay rate, red lines (present work) are those based on the new measurement.  (Image from Physical Review Letters)