For a neutron to decay, the resulting proton requires an available state at lower energy than the initial neutron state, but in stable nuclei the possible lower energy states are all filled, meaning they are each occupied by two protons with spin up and spin down.
The resulting proton requires an available state at lower energy than the initial neutron state. This is for a neutron to decay. The possible lower energy states are all filled, meaning they are each occupied by two protons with spin up and spin down. This was in stable nuclei. This was for a neutron to decay.