What stabilizes a Neutron in a Nucleus from Neutron Decay?

Submitted by UnifiedQuantumField t3_z7o4ct in askscience

The question is simple, but the answer is difficult.

If you have a neutron in, say, a helium atom... that neutron is stable for billions of years.

Contrast this with a free neutron, which has a half life estimated to be somewhere between 10 and 15 minutes. Free neutrons decay into a proton, an electron and a gamma ray (and an antineutrino or something like that?)

So what is it that makes such a huge difference in the stability of neutrons (in a nucleus vs free)?

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mfb- t1_iy7vjh8 wrote

Nucleons in (stable) nuclei are bound, they have less energy than free particles. A helium-4 nucleus has less energy than a helium-3 nucleus plus a proton plus an electron (and also less energy than lithium-4 plus electron). The neutrons cannot decay because there is not enough energy.

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clocks212 t1_iy8yb5f wrote

Why/how is that? How can more particles = less energy?

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mfb- t1_iy8zc6x wrote

Attractive forces lower the energy of bound systems. It's the same concept as for e.g. a hydrogen atom, which has 13.6 eV less energy than the sum of isolated proton and isolated electron.

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bkinstle t1_iybztbf wrote

Is this what causes some atoms to be radioactive? Some neutron in the cluster of the nucleus isn't quite completely bound as well as the others so everyone it decays, and then the chunk of the nucleus it was holding together breaks off?

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mfb- t1_iyc0k3i wrote

If there is enough energy to do that decay then it will happen eventually, yes (unless something else happens before). That's how beta decay works.

> and then the chunk of the nucleus it was holding together breaks off?

There is nothing breaking off. The electron and an antineutrino escape.

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bkinstle t1_iyc10g9 wrote

Cool thanks for the reply.

When I said breaking off a chunk I was thinking about when an atom fissions. Does that happen as a process of natural decay or only when triggered for example by a neutron released from a nearby atom?

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mfb- t1_iyc1uyk wrote

Spontaneous fission is a decay mode of some nuclei, but that's different from beta decay.

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