Neutron moderation in the Oklo natural reactor and the time variation of α

In previous analyses of the Oklo (Gabon) natural reactor to test for a possible time variation of the fine-structure constant α, a Maxwell-Boltzmann low energy neutron spectrum was assumed. We present here an analysis where a more realistic spectrum is employed and show that the most recent isotopic analysis of samples implies a decrease in α, over the last 2×10⁹ years since the reactor was operating, of (α_past-α_now)/α⩾4.5×10^-8 (6σ confidence). Issues regarding the interpretation of the shifts of the low energy neutron absorption resonances are discussed.