Time variability of α from realistic models of Oklo reactors

We reanalyze Oklo Sm-149 data using realistic models of the natural nuclear reactors. Disagreements among recent Oklo determinations of the time evolution of alpha, the electromagnetic fine structure constant, are shown to be due to different reactor models, which led to different neutron spectra used in the calculations. We use known Oklo reactor epithermal spectral indices as criteria for selecting realistic reactor models. Two Oklo reactors, RZ2 and RZ10, were modeled with MCNP. The resulting neutron spectra were used to calculate the change in the Sm-149 effective neutron capture cross section as a function of a possible shift in the energy of the 97.3-meV resonance. We independently deduce ancient Sm-149 effective cross sections and use these values to set limits on the time variation of alpha. Our study resolves a contradictory situation with previous Oklo alpha results. Our suggested 2 sigma bound on a possible time variation of alpha over 2 billion years is stringent: -0.11 <=Delta alpha/alpha <= 0.24, in units of 10(-7), but model dependent in that it assumes only alpha has varied over time.