Calls like this one to make international databases of nuclear "signatures" available publicly make sense to me as a way to help deter theft and transfer of nuclear materiel: http://www.nature.com/nature/journal/v443/n7114/full/443907a.html The rationale is that a state that manufactured material used in a detonation or dirty event could be identified and held accountable after the fact. I wonder how foolproof this might be, however. Wouldn't a state that is determined to transfer nuclear material to, e.g., a terrorist group, try to spoof their material? With the aid of the international database, in fact, they'd attempt to dope their material with just the right trace additives to make it look like it came from someone they consider to be an enemy. My question is, how difficult is it to do this, given that a nuclear country will have advanced capabilities in separation, metallurgy, etc.?