By Edwin CartlidgeFeb. 14, 2018 , 2:35 PM
For 2 weeks in September and October last year, traces of the humanmade isotope ruthenium-106 wafted across Europe, triggering detectors from Norway to Greece and Ukraine to Switzerland. The radioactive cloud was too thin to be dangerous, containing no more than a few grams of material, but its origin posed an outsize mystery.
Now, scientists at the French Institute of Radioprotection and Nuclear Security (IRSN) in Paris say the isotope may (...)