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How come neutrons in a nucleus don't decay?

It is known that free neutron decays in 15 minutes on average. Why is it much more stable when "placed" in nuclei?

Edit: I see that this question had been asked already (here) and can be closed.

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Murod Abdukhakimov
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Neutron decay produces a proton. Free neutron decays since this leads to a lower energy state. However, if this happens in a nucleus then due to the already existing positive charge of all the other protons, it will result in a higher energy state. In fact, the opposite may happen if a nucleus has an excess of protons: some of them may transform into neutrons. See beta decay for more on both transformations. On average the heavier the nucleus the higher the stable ratio of neutrons to protons.

Adam Zalcman
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  • Speaking classically, is this a dynamic equilibrium i.e. are neutrons decaying into protons but this is balanced by protons decaying into neutrons? Or I'd guess from a QM perspective, I'm asking are the neutrons and protons mixed in the nucleus? – John Rennie Feb 09 '12 at 10:05
  • Thanks, Adam. Does it mean that $\beta^{-}$ decay leads to nuclei with higher energy state? Perpetuum mobile? – Murod Abdukhakimov Feb 09 '12 at 10:48
  • @JohnRennie I doubt it.. Proton's don't decay: http://en.wikipedia.org/wiki/Proton_decay . More accurately, they haven't been observed to. Many experiments have been set up to find out if they decay, but they haven't found anything, pushing half-life estimates to $10^{(\text{I think 37)}} y$. Proton decay is predicted by many of the new theories, though. – Manishearth Feb 09 '12 at 12:57
  • @Manishearth Protons don't decay, but they may transform into neutrons in β+ decay. This happens in isotops with too many protons (compared to stable isotopes), e.g. C11, O15, K40. – Adam Zalcman Feb 10 '12 at 18:33
  • @MurodAbdukhakimov Both types of β decay occur when the final configuration has lower energy. β- occurs in the nuclei with too many neutrons (most famous one probably being C14 which transforms via β- into N14). β+ occurs in the nuclei with too many protons (see examples in the previous comment). – Adam Zalcman Feb 10 '12 at 18:45