Researchers say they've found the "obvious candidate" for the source of a mysterious radiation cloud that hovered over much of Europe last month before dispersing, NPR reports. More than 40 countries detected a radioactive isotope called ruthenium-106 in the atmosphere. Ruthenium is not found in nature, and Danish researcher Sven Poul Nielsen says the detection of it last month was "strange" because it's the first time it has ever been found without being accompanied by other types of radiation. Typically if ruthenium is in the atmosphere, something very bad has happened. For example, its detection was one of the first signs of the infamous 1986 meltdown at Chernobyl. But this time no country has announced an accident involving ruthenium-106.
Regardless, scientists now believe they know the source: the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. French and German analysis of weather patterns and ruthenium detection point to the Dimitrovgrad region. Also, one process that produces ruthenium as a byproduct is the creation of a popular medical isotope. RIAR is one of the few places that makes it. Scientists admit there are other possible sources of the ruthenium cloud, but RIAR makes the most sense. A spokesperson at RIAR says the facility hasn't produced any ruthenium in the past six months. And Russia is continuing to deny responsibility. "None of the enterprise of the Russian nuclear industry has recorded radiation levels that exceed the norm," the New York Times quotes Russian officials as saying. (Read more radiation stories.)