Progress in the structural studies of proton emitters

Abstract

Studies of nuclei far from the β stability line have been at the forefront of nuclear structure research. Proton radioactivity is an important decay mode for nuclei near the proton drip-line. Studies of this decay mode can reveal valuable information on exotic nuclear structure and provide deep insights on the organization of nuclei in extreme conditions. For example, recent advances in modern experimental techniques have allowed the measurement of the detailed rotational structure of proton emitters. Yu et al. of the Oak Ridge National Laboratory identified states in the ground-state proton emitter 151Lu, which were populated by using the recoil-decay tagging technique. The systematic data suggested a possible isomeric level at high spin in 151Lu. In addition, by using the same technique, Seweryniak et al. of the Argonne National Laboratory observed rotational bands feeding the ground state and the isomeric state in the proton emitter 141Ho. These authors showed that, by using a state-of-the-art detection system, it is possible to study excited states in nuclei produced with sub 100 nb cross-sections. These new experimental data have given us valuable information for understanding the interactions in exotic systems, which motivate further theoretical development. For example, in the experimental work of Seweryniak et al., the authors challenged existing theoretical models to include non-axial degrees of freedom in the nuclear potential in calculations of proton decay rates. In this article, we provide a brief review on the recent progress in the studies of proton emitters, with an emphasis on introducing exciting theoretical models. A good model should not only yield the decay rates, but should also contain microscopic degrees of freedom that provide information on the detailed structure. This requires developing theoretical models that extend beyond the semi-classical methods, for example those based on the particle-rotor model. We mention several such attempts that are documented in the literature, including the relativistic Hartree-Bogoliubov model of Vretenar, Lalazissis, and Ring, and the most recent application of the projected shell model by the current authors.