Abstract
Isomers are metastable nuclear excitations with long half-lives, ranging from nanoseconds to years. In general, an isomer’s decay is inhibited by at least one of three physical constraints: spin isomers involve a large change in the magnitude of the angular momentum, often combined with low transition energy; K isomers require a large change in the direction of the angular momentum; and shape isomers arise due to a significant change in the shape of the nucleus. The long half-lives of isomers open up a variety of experimental techniques for studying their properties, which themselves give key information about the nuclear structure. Isomers can also be valuable in providing increased sensitivity for the investigation of exotic nuclei, far from the valley of β-stability. Furthermore, by virtue of their electromagnetic decay, isomers have applications that may differ from those of nuclear ground states, including critical roles in nuclear astrophysics and, more generally, physics at the atomic/nuclear interface. All these topics are discussed in this chapter.