Nuclear excited state

A nuclear excited state lies above the same nucleus's ground state in internal energy. spin, parity and approximate isospin help classify it. Single-particle rearrangements, broken pairs and collective deformation/vibration give different excitation patterns and can mix. Below particle-emission thresholds, gamma decay and internal conversion are important de-excitation routes; an excitation energy is not necessarily the energy of every emitted gamma ray in a cascade.

A nuclear isomer is a relatively long-lived metastable nuclear state. Gamma de-excitation can be hindered by a large angular-momentum change, high electromagnetic multipolarity or small overlap with lower states. Its lifetime and decay channels depend on the actual level structure; metastability is not an absolute prohibition on decay.