The decay of a proton to neutron is :

A free proton cannot decay into a neutron because the neutron is heavier than the proton. The mass of a neutron is approximately $$939.565$$ MeV/c$$^2$$, while the mass of a proton is approximately $$938.272$$ MeV/c$$^2$$. For a free proton, the decay $$p \to n + e^+ + \nu_e$$ would violate energy conservation since the products are heavier than the parent.

However, inside a nucleus, a proton can convert to a neutron through $$\beta^+$$ decay (positron emission): $$p \to n + e^+ + \nu_e$$. This becomes possible because the difference in nuclear binding energies between the parent and daughter nuclei can supply the required mass-energy deficit.

Therefore, the decay of a proton to a neutron is possible only inside the nucleus.