My bad - it was indeed Polonium-210. A slip of the mind on my part. But perhaps you'd now like to confirm that of all the possible tools of assassination, this is among the most difficult to handle, the least likely to be used. There are countless easier ways of bumping off an enemy, given the difficult logistics of this operation.
4.2-scale earthquake felt in Dead Sea, Jerusalem area (Haaretz)
from the article: U.S. website: Mossad killed Iranian nuclear physicist