Nucleus emitting a Photon

[AKG]

What does it mean for a nucleus to emit a photon, and what does it mean to talk about the "recoil energy" of the nucleus. If I'm given a mass for a nucleus and an energy for an emitted photon, how can I calculate the recoil energy of the nucleus?

[vanesch]

If I'm given a mass for a nucleus and an energy for an emitted photon, how can I calculate the recoil energy of the nucleus?

Conservation of momentum !

cheers,
Patrick.