Size Isomers at Threshold States

n overview is provided for a new class of states, which we have named as the size isomers. Such states are weakly bound and have an exotic structure (cluster states, halo/‘‘skin’’). The development of methods for measuring the radii of nuclei in their short-lived excited states led to the discovery of new classes of states. The size of a nucleus defined by the radius of its nucleon (proton and neutron) density distribution and the proton charge distribution is one of the most fundamental and important nuclear characteristics. Nuclear radius determines the basic properties of nuclei and is a consequence of the fundamental features of the strong interaction. Up to now two groups of the size isomers were identified: the excited states with halos ( ^9 Be, ^11 Be, ^13 C, ^13 N) and some specific alpha-cluster states ( ^11 B, ^12 C, ^13 C). All the observed states are dilute, however, some indication to possible existence of more compact than the ground states was obtained as well (in ^13 C). The phenomenon of size isomerism occurred to be not a rare one especially if we take into account rotational bands based on some of such states. The structure of size isomers is related with some new features, e.g., rotating halos, halos in continuum, different types of quasimolecular configurations. Some rudimentary signs of alpha-particle condensation (a ‘‘ghost’’ of condensate) were observed in the Hoyle state of ^12 C, however, we cannot speak about confirmation of this ambitious theory. Analogs to the Hoyle state, expected in ^11 B, ^13 C, ^16 O, ^20 Ne near the α -emission thresholds, are considered.