text: Similar investigations into the isotopic ratios of ruthenium at Oklo found a much higher  concentration than otherwise naturally occurring (27–30% vs. 12.7%). This anomaly could be explained by the decay of  to . In the bar chart the normal natural isotope signature of ruthenium is compared with that for fission product ruthenium which is the result of the fission of  with thermal neutrons. It is clear that the fission ruthenium has a different isotope signature. The level of  in the fission product mixture is low because fission produces neutron rich isotopes which subsequently beta decay and  would only be produced in appreciable quantities by double beta decay of the very long-lived (half life  years) molybdenum isotope . On the timescale of when the reactors were in operation, very little (about 0.17 ppb)  decay to  will have occurred. Other pathways of  production like neutron capture in  or  (quickly followed by beta decay) can only have occurred during high neutron flux and thus ceased when the fission chain reaction stopped.