Beta decay is generally displayed as below with a fairly significant intensity of electrons emitted with KE ~ 0. In this case, all the energy of the decay is given to the neutrino and the momentum is conserved between the neutron and the recoiling nucleus. However, for free neutron decay, I am under the impression that the intensity for electron with KE ~ 0 drops to zero such that the curve shown below is effectively shifted to the right until the intensity at 0 electron energy is 0. If that is indeed correct what accounts for this difference between the two decays? Shouldn't the proton from free neutron decay provide the same effective function as the recoiling nucleus and allow for decay in which the neutrino absorbs virtually all the energy ~782 keV?
