γ-decay

1. Definition

The nucleus in an atom can be found in an excited 
state or ground state. The involved energy of excitation 
is of the order of 1 MeV, compared to a few eV for atomic 
energy level. It is due to the great strength of nuclear 
interactions. 
When a nucleus  is placed in an excited state (by 
bombardment with high energy particle or by radioactive 
transformation), it can decay to the ground state by 
emitting one or more photons called gamma rays photons 
, with typical enrgies of 10 keV to 5 MeV.
This process is called γ-decay.

2. Example:

The nuclide Cesium137 undergoes two β minus 
decays. The most probable is the one that leads to the 
excited (metastable) *Cesium137, which emits a photon 
(γ-decay) 0.662 MeV of energy before being transformed 
into a stable nuclide Ba137 at the ground state.

The Cesium-137 (137,55) has a half-life of about 30 years.
Its activity is about of 3.20 x 10 12 Bq per gram.

The nuclide Cesium137 is used to calibrate 
radiation-detection equipments. The emitted photons of 0.662 
MeV energy can be useful in food irradiation and in the 
radio-oncology (radiotherapy of cancer).