Question

About twenty radioisotopes with ultra-long half-lives, from 82 million years (244- Plutonium) to 6 million billion years (50-Vanadium) are found in the earth’s crust, yet no radioisotopes with half-lives less than 70 million years (146- Samarium) are found. Why not? Use this fact to estimate a lower limit for the age of the earth. Hint: How many half-lives does it take for a radioisotope to decay into background levels?

Answer 1

I can't find it anywhere on the web but I think the number of half-lives at which a radioactive isotope is no longer a threat (from the radioactivity) is 10 half-lives.