For 2 weeks in the fall of 2017, traces of the isotope ruthenium-106 wafted across Europe. The radioactive cloud was too thin to be dangerous, but it posed a mystery to scientists. Now, researchers at the French Institute of Radioprotection and Nuclear Security say the isotope may have been released from the Mayak nuclear facility in southern Russia. They argue the leak may have happened when technicians botched the fabrication of a cerium-144 source needed in the search for sterile neutrinos at the Gran Sasso National Laboratory in L’Aquila, Italy. The Russian government has vehemently denied that an accident took place, however.
There exist many isotopes of ruthenium, with atomic weights from 87 to 120; only seven are stable. Ruthenium-106 does not exist in nature, but has been synthesized from other nuclei; its half-life is just over a year. (Some of the others won’t last more than a minute or two.)