The β-decay process, discovered around 1900, is basically the decay of a neutron ($n$). In the laboratory, a proton ($p$) and an electron ($e^{-}$) are observed as the decay products of the neutron. Therefore, considering the decay of a neutron as a two-body decay process, it was predicted theoretically that the kinetic energy of the electron should be a constant. But experimentally, it was observed that the electron kinetic energy has continuous spectrum. Considering a three-body decay process, i.e. $n\to p+e^{-}+\overline{v_e}$,  around 1930, Pauli explained the observed electron energy spectrum. Assuming the anti-neutrino $\left(\overline{v_e}\right)$ to be massless and possessing negligible energy, and the neutron to be at rest, momentum and energy conservation principles are applied. From this calculation, the maximum kinetic energy of the electron is $0.8\times {10}^{6}eV.$ The kinetic energy carried by the proton is only the recoil energy