γ-ray fluxes in Oklo natural reactors.

Background: Uncertainty in the operating temperatures of Oklo reactor zones impacts the precision of bounds derived for time variation of the fine structure constant α. Improved 176Lu/175Lu thermometry has been discussed but its usefulness may be complicated by photoexcitation of the isomeric state l76mLu by 176Lu(γ,γ') fluorescence. Purpose: We calculate prompt, delayed, and equilibrium γ-ray fluxes due to fission of 235U in pulsed mode operation of Oklo zone RZ10. Methods: We use Monte Carlo modeling to calculate the prompt flux. We use improved data libraries to estimate delayed and equilibrium spectra and fluxes. Results: We find γ-ray fluxes as a function of energy and derive values for the coefficients λγγ' that describe burn-up of 176Lu through the isomeric l76mLu state. Conclusion: The contribution of the (γγ') channel to the 176Lu/l75Lu isotopic ratio is negligible in comparison to the neutron burn-up channels. Lutetium thermometry is fully applicable to analyses of Oklo reactor data. [ABSTRACT FROM AUTHOR]