Brian R. Martin

“The discovery of the neutron was a crucial step in understanding nuclei, including radioactive ones. For example, beta decays are the transformation within a nucleus of a nucleon of one type, either a proton or a neutron, to the other. You may wonder how a proton can decay to a neutron if the neutron is heavier than the proton; conservation of energy would seem to make this impossible. However, while a proton not bound in a nucleus cannot transform to a neutron, it is possible in some circumstances for a proton within a nucleus to do so. This is because the proton can use the additional energy from the force that binds nucleons in the nucleus. Beta decay occurs if it results in the total energy of the final atom, taking into account the energy due to binding, being lower that that of the initial atom. The same applies to a neutron bound in a nucleus, whereas a free neutron can always decay to a proton.”
― Brian R. Martin, Particle Physics: A Beginner's Guide