Einstein's Theory of Relativity is far from my area of specialty. I had a course in it, but at that time I was still a chemist, and didn't understand much of it.
I don't know that radioactive decay had anything to do with relativity. So far as I know it's the simple first order decay found in many chemical and physical processes. The basic statement leading to the equation below is that the rate of decay is proportional to the amount you have.
A = A0e-t/tau
where (Greek) tau is just a constant times the half-life. (half-life is really an awkward way to specify the decay parameter, and I think it's done that way so a layman can get a sort of feel for it) A is the amount that was radioactive at time = t and A0 was the amount that was radioactive at time t=0, and when you start is arbitrary as long as you measure t from the time you start (A being measured in number of atoms (or units of mass) in your fixed sized sample, no matter what the actual size is as long as it's constant (previously decayed + undecayed).) I think you can probably find tables (the most up-to-date ones) of half-lifes on the NIST website, if not, then almost any handbook of physics will have such a table.