  Neutrino masses and mixing can be investigated by studying the behavior of a
radioactive bare nucleus which decays by emitting an electron into the open
atomic K shell BEFORE and DURING its weak decay by neutrino emission. The
initial nuclear state has a momentum spread required by Heisenberg. The wave
packet contains pairs of components with different momenta which can produce
neutrinos in two mass eigenstates with exactly the same energy and different
momenta. These neutrino amplitudes mix to produce a single electron-neutrino
state with the same energy. Since there is no information on which mass
eigenstates produced the neutrino this is a typical quantum mechanics
"two-slit" or "which path" experiment. A transition between the same initial
and a final states can go via two paths with a phase difference producing
interference and oscillations. Here the two paths are in momentum space
  A new oscillation phenomenon providing information about neutrino mixing is
obtained by following the nucleus before and during the decay. The analysis
starts with Stodolsky's proof that interference between states having different
energies cannot be observed in realistic experiments. Results then follow from
simple rigorous quantum mechanics without the hand waving and loopholes which
have confused many previous neutrino oscillation investigations.
