Lead that has just been dug out of the earth was exposed to radiation from other minerals and cosmic radiation until very recently. On the other hand this batch

Lead that has just been dug out of the earth was exposed to radiation from other minerals and cosmic radiation until very recently. On the other hand this batch of ancient lead was at the bottom of a lake for 2000 years, so it had time to lose some of its radioactivity while being shielded by water.