Gamma decay

Last revised by Yvette Mellam on 27 Feb 2023

Gamma decay refers to the release of a gamma (γ) ray photon, a form of high energy electromagnetic radiation, due to radioactive decay of a nucleus. Typically, the energy spectra is in the ~100 keV to ~10 MeV range 1.

Gamma decay is a mode of radioactive decay. It differs from alpha and beta decay in that it does not involve a change to a different daughter nuclide. Typically, radiative decay proceeds the aforementioned particle decay as the resulting daughter nucleus resides in an energetic (excited) state 1,2. Analogous to the production of x-rays, a gamma photon is produced as the nucleus transitions from this excited state to a lower energy state. The energy difference between these states defines the frequency of the released photon. Gamma rays have energies far greater than that of similar atomic process and therefore have high penetration depths. The depth exceeding that of alpha and beta decay. The nuclear reaction describing gamma decay may be written as;

AmX --> AX + γ

The superscript above the parent nucleus indicates an excited nuclear state. Note that the chemical isotope remains unchanged while the overall energy (internal binding energy per nucleon) changes. Equivalent nuclei with differing energies are termed nuclear isomers 2

Due to the large energies involved in radioactive decay a daughter nucleus may undergo gamma decay many times before residing in its lowest (ground) energy state. During these process the nucleus may fall into a metastable state, a state whose half-life is longer than that of an ordinary excited state but shorter than that of the ground state. Excited nuclear states typically have a half-life of the order of picoseconds, while a metastable state, by definition, is at least 1,000 times greater (and maybe on the order of weeks to years) 3

A metastable state is possible as the transition to a lower nuclear energy state is, quantum mechanically, highly unlikely but not impossible. This is termed a 'forbidden transition' and is defined by conservation laws and the stochastic nature of nuclear radiation 3.

A popular clinical metastable isomer is Tc-99m. Having a convenient half-life of six hours and a lower gamma decay photon of 141 keV makes it a useful nuclear isomer for single photon emission computed tomography 4.

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