JEE Main · 2021 · 18 Mar 2021 · Shift 2 · Modern Physics

A neutron is heavier than a proton, so a proton at rest in free space has too little energy to become one and a free proton does not decay. Inside a nucleus the energy accounting runs over the whole system: if the parent nucleus is heavier than the daughter together with the emitted positron and neutrino, the binding energy released by the rearrangement pays for the proton-neutron mass difference, and the conversion goes through as ${\beta }^{+}$ decay or electron capture. The decay of a proton to a neutron is possible only inside the nucleus.