[<1/1,000,000th%]

How about cobalt thorium G?

I don't remember the decay schemes exactly, but if you start with a large nuclei like uranium, it'll run through a lot of the smaller, more radioactive species like cobalt as it decays into other elements. A lot of the really radioactive elements are so short-lived that they only occur when they decay from something else.

[AndrewC]

It was an attempt at a joke. Cobalt Thorium G is from Dr. Strangelove.