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Homework Help: Atom decay to ground state

  1. Oct 8, 2012 #1
    1. The problem statement, all variables and given/known data
    An atom of mass M decays from an excited state to the ground state with a change in mass of ΔM<<M. In the decay process, the atom releases a photon. Use the laws of energy and momentum to determine the energy of the photon, assuming the atom decays from rest.

    2. Relevant equations
    The 4-momenta of the atom before the emission:


    After the emission:

    P=(M-ΔM)c2, -pxc,0,0)

    And the photon:


    p = hf

    My main concern here is that this seems too straightforward. The book labels this problem as a challenging one. I think I'm missing a subtlety in the problem.
  2. jcsd
  3. Oct 8, 2012 #2
    Found it. The energy of the photon isn't delta(m)c^2 as the atom is moving. So in the atom's frame, the photon is redshifted. The energy of the photon (which is hf in a rest frame) is now hf', where f' is the shifted frequency due to the transverse relativistic doppler effect.
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