Nuclear isomer

A nuclear isomer is an excited state of an atomic nucleus with the same proton and neutron numbers as a lower-energy state, but a sufficiently long lifetime to be distinguished from it.

Nuclear isomeric transition: A transition from a metastable nuclear state to a lower-energy state, often accompanied by gamma-ray emission. This is the decay process that ends an isomer’s metastable lifetime.
Technetium-99m: A metastable state of technetium-99 that decays by gamma emission to the ground state. Its short-lived metastability underpins a widely used medical imaging isotope.
Technetium-99m scintigraphy: Medical imaging that detects gamma rays emitted as technetium-99m decays to technetium-99. This diagnostic technique exploits the isomer’s useful radiation and short half-life.
Nuclear ground state: The lowest-energy state of a particular atomic nucleus. An isomer is an excited state of the same nuclide, not its ground state.
Gamma decay: Radioactive decay in which an excited nucleus emits a gamma-ray photon without changing its proton or neutron numbers. Many nuclear isomers lose energy through this broader class of transitions.
Tantalum-180m: A metastable state of tantalum-180 whose decay has not been observed and whose lifetime is extraordinarily long. It is a striking example of an isomer surviving on timescales far beyond ordinary nuclear decays.
Single-photon emission computed tomography: A medical imaging method that reconstructs three-dimensional images from detected gamma-ray emissions. Technetium-99m is a common radiotracer for this imaging method.
Nuclear resonance: A short-lived excited nuclear state produced when a nucleus absorbs energy at a resonant energy. Resonances are generally too short-lived to be distinguished as metastable isomers.
Selection rule: A constraint on whether a physical transition can occur, derived from conserved quantities and the states’ quantum numbers. Angular-momentum and parity constraints can hinder an isomer’s decay.
Hafnium-178m2: A high-energy metastable state of hafnium-178 with a lifetime of about 31 years. Its long lifetime makes it a prominent case in discussions of high-energy nuclear isomers.

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