What is isomeric transition?

Isomeric transition is a type of radioactive decay in which an excited nucleus releases excess energy without changing its number of protons or neutrons. The nucleus remains the same element and isotope but moves from a higher-energy (metastable) state to a lower-energy state, usually by emitting a gamma photon.

Isomeric transition is the emission of gamma radiation from an excited nucleus without changing its atomic or mass number.

This process occurs when a nucleus has already undergone a transformation (such as beta decay) and is left in an excited configuration. Instead of changing its composition further, it simply releases energy and becomes more stable energetically.

Isomeric transition is particularly important in nuclear medicine because it produces gamma radiation without altering the atomic number, making it ideal for diagnostic imaging.

Understanding the physics

After certain types of radioactive decay, the daughter nucleus may not immediately settle into its lowest possible energy state. Instead, it may exist in an excited configuration. This excited nucleus has excess internal energy but an otherwise stable neutron-to-proton ratio.

In isomeric transition, the nucleus releases this excess energy as a gamma photon. Because no nucleons are gained or lost, both the atomic number and mass number remain unchanged.

Some nuclei exist in a relatively long-lived excited state known as a metastable state, denoted by an “m” after the mass number (for example, Tc-99m). These metastable states persist long enough to be useful clinically before decaying to the ground state via gamma emission.

In some cases, instead of emitting a gamma photon, the nucleus transfers its energy directly to an orbital electron, which is then ejected. This process is known as internal conversion. Although no gamma photon is emitted in internal conversion, the nucleus still transitions to a lower energy state.

Isomeric transition does not alter the identity of the element. It is therefore distinct from beta or alpha decay, which change the atomic number.

Where this matters clinically

Isomeric transition is the basis of many diagnostic radionuclides, most notably technetium-99m (Tc-99m). Because it emits a gamma photon without emitting high-energy charged particles, it allows external detection with relatively low tissue damage. This makes it ideal for SPECT imaging.