Synopsis

Fixing a Million-Year Clock

Physics 8, s11
A better measure of an iron isotope’s half-life may lead to new ways of dating astrophysical events that unfold over millions of years.
A. Wallner et al., Phys. Rev. Lett. (2015)

Radioactive iron-60 ( 60Fe) is produced at the core of large stars and in supernovae, and it has a half-life of roughly a million years, so its abundance can be used to date astrophysical events on a similar time scale. Scientists have, for example, used the small amount of 60Fe deposited in deep-sea crust to trace the history of supernovae near our Solar System, which may have affected Earth’s climate in the past. But the best measures of 60Fe’s half-life—one performed in 1984, the other in 2009—disagree by nearly a factor of 2. Now, a new experiment settles the discrepancy, enabling more astrophysical studies based on the isotope, such as the monitoring of nucleosynthesis in stars.

To derive the half-life of a long-lived isotope, scientists use samples containing a known number of the nuclei and detect how many of them decay per second. In the case of 60Fe, its decays are monitored by detecting the gamma rays emitted by its daughter nucleus, cobalt-60. But the main uncertainty in earlier experiments has been the initial number of decaying 60Fe nuclei. Working with an iron sample extracted from irradiated copper, Anton Wallner, at the Australian National University, and his colleagues used accelerator mass spectrometry to determine the small concentration of 60Fe isotopes. By comparing this number to the concentration of 55Fe, another rare isotope, they were able to “cancel out” some of the systematic errors that plagued earlier experiments and accurately gauge the 60Fe amount. The half-life they find agrees well with the 2009 value; averaging the two together, Wallner et al. report a value of 2.60 million years and a 2% uncertainty.

This research is published in Physical Review Letters.

–Jessica Thomas


Subject Areas

AstrophysicsNuclear Physics

Related Articles

Do Merging Dwarf Galaxies Explain a Peculiar Gravitational-Wave Detection?   
Astrophysics

Do Merging Dwarf Galaxies Explain a Peculiar Gravitational-Wave Detection?   

The hard-to-explain masses of two coalescing black holes could be accounted for if they were the central black holes in two distant, tiny galaxies that merged. Read More »

Compiling Messages from Neutron Stars
Astrophysics

Compiling Messages from Neutron Stars

The combination of gravitational-wave and x-ray observations of neutron stars provides new insight into the structure of these stars, as well as new confirmation of Einstein’s theory of gravity. Read More »

Probing the Skin of a Lead Nucleus
Astrophysics

Probing the Skin of a Lead Nucleus

Researchers make the most precise measurement yet of the neutron distribution in a heavy nucleus, with implications for the structure of neutron stars. Read More »

More Articles