nonyrast

Nonyrast states are nuclear excited states that do not lie on the yrast line for a given angular momentum. The yrast line comprises the lowest-energy state for each spin value, so nonyrast states are higher-energy states with the same spin that are not the energetically favored ones. The distinction helps organize the complex spectrum of a nucleus into a minimal-energy benchmark (yrast) and additional, higher-energy excitations (nonyrast).

The term yrast originated in nuclear structure research, with the yrast line concept attributed to early work by Bohr and Mottelson in the mid-20th century. Nonyrast states are simply those that lie above this line for their spin and can arise from various degrees of freedom, including vibrational excitations, multi-phonon modes, and configurations involving broken pairs or additional quasiparticles.

Nonyrast excitations include vibrational bands such as beta- and gamma-vibrations, as well as more complex two-quasiparticle or multiparticle-multihole configurations that appear at higher energy. In deformed nuclei, rotational bands built on nonyrast configurations coexist with the yrast rotational band, and transitions among these states enrich the observed gamma-decay schemes. The presence and properties of nonyrast states provide insight into deformation, pairing correlations, and the interplay between collective and single-particle motion.

Experimentally, nonyrast states are studied through in-beam gamma-ray spectroscopy, fusion-evaporation reactions, and Coulomb excitation. Assigning spin, parity, and configuration relies on observed gamma cascades, angular correlations, and multipole mixing, enabling mapping of nonyrast structures alongside the yrast line.