"Neutrons don't decay in nuclei because no available states" incorrect? Hello, If I understand correctly, the argument for a neutron (usually) not decaying when in a nucleus, is that the resulting proton would then have to occupy a high energy level, the lower levels already being occupied by the protons that are already there. But that argument presupposes that a particle has to be in an energy eigenstate (or at least immediately after decaying). Is there any argument for this? There are an infinite number of states with average energies lower than that "high energy state" it would be obliged--according to the traditional argument--to occupy.