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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
59
Fe, which constitutes an important step towards understanding
60
Fe nucleosynthesis in massive stars. The results were published in
Physical Review Letters
on April 12.
Radioactive nuclide
60
Fe plays an essential role in nuclear astrophysical studies. It is synthesized in massive stars by successive neutron captures on a stable nucleus of
58
Fe 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
60
Fe have been detected by space gamma-ray detectors. By comparing the
60
Fe gamma-ray flux to that from
26
Al, which shares a similar origin as
60
Fe, researchers should be able to obtain important information on nucleosynthesis and stellar models. However, the observed gamma-ray flux ratio
26
Al/
60
Fe does not match theoretical predictions due to uncertainties in both stellar models and nuclear data inputs.
The stellar beta-decay rate of
59
Fe is among the greatest uncertainties in nuclear data inputs. During the nucleosynthesis of
60
Fe in massive stars,
59
Fe can either capture a neutron to produce
60
Fe or beta decay to
59
Co. Therefore, the stellar beta-decay rate of
59
Fe is critical to the yield of
60
Fe.
Although the decay rate of
59
Fe 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
59
Fe nuclei in their excited states.
To address this problem, researchers at IMP proposed a new method for measuring the stellar beta-decay rate of
59
Fe. “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
59
Co target. Then the reaction products,
3
He 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
59
Fe excited states were determined. This measurement thus eliminated one of the major nuclear uncertainties in predicting the yield of
60
Fe.
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
60
Fe is 40% less when using the new data. The result points to a reduced tension in the discrepancy in
26
Al/
60
Fe ratios between theoretical predictions and observations.
“It is an important step towards understanding
60
Fe 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.
This part of information is sourced from https://www.eurekalert.org/pub_releases/2021-04/caos-ssl041621.php
