The `beta - decay` process , discovered around `1900` , is basically the decay of a neutron `n`. In the laboratory , a proton `p` and an electron `e^(bar)` are observed as the decay product of neutron. Therefore considering the decay of 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 rarr p + e^(bar) + bar nu _(e) , ` around `1930` , Pauli explained the observed electron energy spectrum. Assuming the anti-neutrino `(bar nu_(e))` to be massaless and possessing negligible energy , and the neutrino to be at rest , momentum and energy conservation principle are applied. From this calculation , the maximum kinetic energy of the electron is `0.8 xx 10^(6) eV` The kinetic energy carried by the proton is only the recoil energy. If the - neutrono had a mass of `3 eV// c^(2)` (where c is the speed of light ) insend of zero mass , what should be the range of the kinectic energy `K.` of the electron ?