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Can a stable nucleus ever spontaneously become (temporarily) unstable?

  1. Jan 13, 2009 #1
    Hi folks,

    I have what might be a naive question about the decay of unstable nuclei. I've recently been reading about various decay processes (e.g. beta decay, electron capture etc.).

    Specifically, I'm wondering if it is ever possible for a stable nucleus (say, carbon-12 or any other stable nucleus) to spontaneously become unstable, and then to undergo further decay processes (e.g. a different sequence, resulting in, say, a lighter element)? I understand that the trigger for decay in an unstable nucleus involves quantum vacuum fluctuations, which provide the activation energy required, and that moves towards the ground state result in an increase in entropy. I guess I'm asking if there are vacuum fluctuations large enough to temporarily 'excite' a nucleus into an unstable state, such that an alternative sequence of decay processes can occur (resulting in a larger final entropy). Would the required initial decrease in entropy (to put the nucleus in an excited state) prevent this from occurring? I.e. I would think that this type of process cannot run backwards, but I'm not sure?

    Put differently: if we consider (hypothetically) an isolated, stable atom (with stable nucleus - again, e.g. carbon-12) floating alone in space, will it remain stable forever? (ignoring the possibility of proton decay etc...)

    Thanks all. Apologies if my question isn't very clear.

  2. jcsd
  3. Jan 13, 2009 #2
    Stable nuclei are called stable precisely because they can't decay. Uranium 235 can decay because you can find two or more nuclei that, combined, have the right number of neutrons and protons and weigh less than the original nucleus. There is a potential barrier that keeps it from decaying immediately and a quantum fluctuation is required for the nucleus to decay.

    Carbon-12 is stable because there's nothing it could decay to. Any combination of decay products would be heavier than the original nucleus. Law of conservation of energy prevents carbon-12 from decaying.
  4. Jan 14, 2009 #3

    Vanadium 50

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    And indeed, an unstable nucleus is stable until the moment it decays.
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