American Physical Society | via EurekAlert | 15 July 2010
An ancient clockParticle accelerator research helps narrow down the age of our galaxy
Physicists will soon have a better measure of the age of our galaxy, thanks to experiments described in a trio of papers appearing in the journal Physical Review C.
The papers report on experiments at the CERN neutron time-of-flight (n_TOF) facility and the Karlsruhe Van de Graaff accelerator that clarify the processes that affect the abundance of the element osmium-187. The element is created when rhenium-187 decays.
Because rhenium-187 was produced in the first stellar explosions after the birth of the galaxy, measuring the amounts of rhenium-187 and osmium-187 we observe today can provide an estimate of the galaxy's age. In effect, the elements act as a cosmic clock that started ticking when the galaxy was born.
Unfortunately, there are various processes that can affect the amounts of osmium we measure. Uncertainties in our understanding of those processes have limited the accuracy of the cosmic clock to more than a billion years.
The CERN and Karlsruhe experiments involve firing pulses of neutrons into an osmium target to determine how frequently the element is likely to capture neutrons and convert to another material. The data the researchers collected has reduced uncertainties in the rhenium-osmium cosmic clock to less than a billion years, allowing a better estimate of our roughly 14 billion year old galaxy.
APS Physics | 15 July 2010
Neutron physics of the Re/Os clock. I. Measurement of the (n,γ) cross sections of 186,187,188Os at the CERN n_TOF facility.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. Complimentary 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 1 Gyr, allowing a more accurate estimate for the age of our galaxy.
- Physical Review C 82 015802 (15 July 2010) DOI: 10.1103/PhysRevC.82.015802
- Physical Review C 82 015803 (15 July 2010) DOI: 10.1103/PhysRevC.82.015803
- Physical Review C 82 015804 (15 July 2010) DOI: 10.1103/PhysRevC.82.015804