Question

Discuss the gamma decay process with an example.

Answer 1

In α and β decay, the daughter nucleus is in the excited state most of the time. The typical life time of excited state is approximately 10^-11 s. So this excited state nucleus immediately returns to the ground state or lower energy state by emitting highly energetic photons called γ rays. In fact, when the atom is in the excited state, it returns to the ground state by emitting photons of energy in the order of a few eV. But when the excited state nucleus returns to its ground state, it emits a highly energetic photon (γ rays) of energy in the order of MeV. The gamma decay is given by

\[\ce{^A_ZX^* -> ^A_ZX + gamma (γ) rays}\]

Gamma emission

Here the asterisk (*) means excited state nucleus. In gamma decay, there is no change in the mass number or an atomic number of the nucleus.

Boron \[\ce{^12_5B}\] has two beta decay modes:

1. It undergoes beta decay directly into ground state carbon by emitting an electron of a maximum of energy 13.4 MeV.

2. it undergoes beta decay to an excited state of carbon \[\ce{^12_6C^*}\] by emitting an electron of maximum energy 9.0 MeV followed by gamma decay to ground state by emitting a photon of energy 4.4 MeV.
   It is represented by
   \[\ce{^12_5B -> ^12_6C + e- + \bar{v}}\]
   \[\ce{^12_6C^* -> ^12_6C + \gamma}\]