Synopsis: An ancient clock

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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