Synopsis

# An ancient clock

Physics 3, s96
Refining a cosmic clock gives a more accurate reading of the age of our galaxy.

In three papers published in Physical Review C, the international Neutron Time-Of-Flight (n $_$TOF) collaboration and scientists working at the Karlsruhe Van de Graaff accelerator in Germany present data and calculations that better characterize a unique clock for measuring the age of our galaxy.

Rhenium $-187$ ( $187Re$), an element similar in mass to gold, was produced in the first stellar explosions after the birth of our galaxy. $187Re$ beta-decays into osmium $-187$ ( $187Os$) with a half-life of 41 billion years—slowly enough so that the relative abundance of $187Re$ to $187Os$ provides a good measure of the time that has elapsed since our galaxy first formed.

However, additional nuclear processes, other than the decay of $187Re$, change the abundance of $187Os$, which can cause error in the rhenium-osmium clock. The n $_$TOF work is therefore aimed at precisely determining the neutron-capture cross sections of $187Os$ and its adjacent osmium isotopes, $186Os$ and $188Os$, which allows one to make an accurate subtraction of this direct contribution. In tandem, the Karlsruhe experiments probe reactions from excited nuclear states that are expected at the high temperatures present in stellar cores.

The n $_$TOF facility bombards a target of osmium nuclei with a pulse of neutrons produced by the $20$- $GeV$ CERN proton synchrotron accelerator to make precise measurements of these neutron cross sections. The experiment is specially designed to simultaneously measure the reaction cross sections for many different neutron energies, yielding thermally averaged cross sections relevant to the hot stellar production process. Complementary measurements of the inelastic scattering cross section were performed at the Karlsruhe $3.7$- $MV$ Van de Graaff accelerator and together with the n $_$TOF results and a detailed analysis, provide an updated assessment of the $Re$/ $Os$ cosmochronometer.

The new data limit the nuclear physics uncertainties for the rhenium-osmium clock to less than $1Gyr$, allowing a more accurate estimate for the age of our galaxy. – Brad Filippone

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