Limits on the variability of coupling constants from the Oklo natural reactor

Abstract
Thermal neutron capture cross sections in fission fragments need to be known to great precision in the nuclear power industry. These cross sections are frequently dominated by extremely narrow neutron capture resonances. An analysis of isotopic abundances at the site of a prehistoric natural reactor at Oklo in West Africa suggests that such a resonance in samarium-149 has moved by less than 0.01 eV in the past 2 x 10 9 years and this is used to place limits on the variability of coupling constants over this period.