A photon produced in a transition between nuclear levels is known as a
gamma ray. In a transition from an initial energy 
to a
final energy 
, the energy of the emitted gamma is determined
by energy and momentum conservation. The excitation energy is shared
between the gamma and the kinetic energy of the recoiling nucleus:
where
(e.g for 
MeV, 
eV.)
In general, the transition energy and gamma energy may be taken as equal. The recoil is important in, e.g, the Mössbauer effect.