why is there still radioactive material on earth? Why hasn’t it all decayed?
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Naturally-occurring radioactive elements (almost) all decay very slowly, with half-lives in the millions or billions of years, and the Earth is young enough (~4.5 billion years) that some amounts of these isotopes are still around.
Even for those naturally-occurring elements with short half-lives, they can be produced from the decay of heavier elements with longer half-lives. The fastest-decaying natural isotope I could find is Fr-223 (22 minutes), which is a decay product of Uranium-235. That means that at any given time, some trace amount Fr-223 exists in natural Uranium deposits, constantly being replenished by the decay of the Uranium.
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