Nuclear Isomer
A nucleus in a long-lived excited state that decays to its ground state by gamma emission.
Formula / equation
denoted with m, e.g. ⁹⁹ᵐTc
Unit
MeV
In depth
Isomers have the same proton and neutron counts as the ground state but hold extra energy in a configuration from which decay is hindered, so the excited state persists far longer than usual. Their pure gamma emission with no accompanying particles makes them ideal for medical imaging.
Examples in the real world
Technetium-99m, whose 6-hour half-life and 140 keV gamma make it the workhorse of nuclear medicine.