Lutetium thermometry for Oklo natural reactors: a new look at old data

TL;DR

This paper reevaluates Lutetium thermometry for Oklo reactors using new neutron capture data, suggesting more accurate temperature estimates and highlighting the need for improved measurements to refine bounds on fundamental constants.

Contribution

It introduces updated temperature predictions for Oklo reactors based on new measurements of Lu-175 neutron capture branching ratios, improving thermometry reliability.

Findings

Estimated reactor zone temperature: 100 ± 30°C

Previous estimates: 350-500°C

Better B^g measurements needed for confirmation

Abstract

Lutetium thermometry has been used to analyze Oklo natural nuclear reactor zones but leads to widely varying and puzzling predictions for the temperatures $T_O$ which in turn impacts bounds on time variation of the fine structure constant $\alpha$. We revisit results for reactor zone RZ10 in light of new measurements of the isomer branching ratio $B^g$ in $^{175}$Lu neutron capture at 5 and 25 keV. We recalculate predictions for $T_O$ as a function of $B^g$ using realistic models of the Oklo neutron flux. We find $T_O = 100 \pm 30$ C using a new value of $B^g$, in contrast to $350 < T_O < 500 $ C using the evaluated value at thermal energy. Lutetium thermometry can be applicable to analyses of Oklo reactor data, but a better measurement of $B^g$ with thermal neutrons is needed to confirm the reliability of temperature predictions.