Note that there is a similar particle-physics demand for lead from Roman-Empire warships ("ancient lead"). But in that case, it doesn't have anything to do with anthropogenic radiation. Rather, lead that is in the ground is constantly kept slightly radioactivate by background decays. But lead that has been pulled out of the ground and allowed to sit unmolested for a thousand years will deactivate.
I saw a talk on one of these experiments while I was a grad student. Apparently their lead source had "historical significance" according to the government of France, and getting it out of the country required beurocratic maneuvers of questionanable legality.
The real shame is that ancient lead isn't the only alternative, just the cheapest. Lead could be spun in centrifuges and then securely cast to remove such contaminants. This has been done with silicon, in that case to remove unwanted isotopes. It is a wildly expensive process but at least it is a process that could produce an infinite amount of product, rather than ancient lead of finite supply.
But what compound? PbH4 isn't stable. PbF4 is barely stable. Maybe Pb(CH4)4 ??? Naively, I guess that you'd get even better mass resolution. Molectual mass of UF6 being ~352, vs ~267 for Pb(CH4)4. Good use for all those leftover centrifuges ;)
You're getting downvoted because you're nitpicking vocabulary. Most people understood "infinite amount of product" to mean "functionally infinite, given the low demand and large supply."
