57. The decay scheme is n p+e- +½. The electron kinetic energy is a maximum if no neutrino is emitted.
Then, Kmax =(mn - mp - me)c2, where mn is the mass of a neutron, mp is the mass of a proton, and
me is the mass of an electron. Since mp + me = mH, where mH is the mass of a hydrogen atom, this can
be written Kmax =(mn - mH)c2. Hence, Kmax = (840 × 10-6 u)c2 = (840 × 10-6 u)(931.5MeV/u) =
0.783 MeV.