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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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marked as duplicate by Qmechanic, dmckee Feb 9 '12 at 19:46

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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.

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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 9 '12 at 10:05
    
Thanks, Adam. Does it mean that $\beta^{-}$ decay leads to nuclei with higher energy state? Perpetuum mobile? –  Murod Abdukhakimov Feb 9 '12 at 10:48
    
@JohnRennie I doubt it.. Proton's don't decay: 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 9 '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
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