Wikipedia:
https://en.wikipedia.org/wiki/Isotopes_of_europium
It says the half life of the natural isotope 151Eu is 5x10^18 years - 1 alpha decay per two minutes per kilogram - so it wasn't realized it was unstable originally and they must be detecting low levels indeed. In fact, since alpha rays are easily stopped by the flimsiest shielding, I wonder if they aren't detecting a different type of radiation from some other elemental impurities in the Europium.