γ-ray fluxes in Oklo natural reactors

Abstract

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. have been undertaken by many groups investigating whether the fine structure constant α has changed over the 2 GY period since the reactors operated. As first pointed out by Shlyachter [2], the samarium isotopic ratios are sensitive to the value of α through the overlap of the 149 Sm E 0 =97.3-meV neutron resonance with the thermal and epithermal portions of the neutron flux in the reactor. While the majority of Oklo analyses [3][4][5][6][7], have been consistent with no shift in the resonance energy, and therefore no change in α, a change has been argued for from astronomical observations [8].All Oklo analyses make assumptions about the operating temperatures of the reactors. But there is as yet no agreement on what these temperatures actually were. Utilizing the 176 Lu/ 175 Lu isotope ratio method to determine temperatures was recently revisited by Gould and Sharapov [9] . The method is based on the temperature dependence of the large thermal neutron capture cross section of 176 Lu (natural abundance 2.599 % [10]), and on knowing with certainty the (small) ground state branching ratio for thermal neutron neutron capture on the more abundant lutetium isotope, 175 Lu (natural abundance 97.401 %). The dominant capture branch σ

Oklo reactors and implications for nuclear science

“…However, the γ-ray fluxes in Oklo reactors obtained in Ref. [45] prove to be many orders of magnitude too low for these (γ, γ ′ )-reactions to impact application of lutetium thermometry.…”

“…An alternate explanation for problems with lutetium thermometry, burn up of 176 Lu in the γ-ray flux of the reactors, was investigated in Ref. [45]. Again, improved nuclear data is required, this time for the integral cross sections [56,57] for photo-excitation of the specific excited states of 176 Lu which decay to metastable 176m Lu.…”

“…Lanthanum isotopic data can, in principle, also give estimates of the neutron fluence[25], but, in practice, such estimates are limited by the lack of isotopic data, and are also more susceptible to uncertainties due to the variability in natural elemental abundance.…”