Positron emission (Beta + decay)

[OJFord]

Why does β⁺ decay occur?

If protons are stable and all baryons eventually decay to protons, then how is it that protons can 'decay' to a neutron?

What causes this to happen?
(I assume it isn't as frequent as β^- decay, or up and down quarks would be coming and going constantly, and we'd be inundated by electron neutrinos and electrons)

 

[jtbell]

Free protons do not undergo β⁺ decay because a neutron has more mass than a proton.

However, protons in a nucleus can undergo β⁺ decay if the mass of the final nucleus is less than the mass of the initial nucleus, so that energy can be released. In effect, the difference in the binding energies of the two nuclei provides the energy needed to make the decay "go".