Nuclear decay studies of rare isotopes

Abstract.

This article overviews recent developments and achievements on decay spectroscopic studies at the Radioactive-Isotope Beam Factory (RIBF) in RIKEN Nishina Center, which has come online in 2007 as the first 3rd-generation in-flight separator facility. The research territory on the chart of nuclides has been dramatically expanded both towards the proton-rich and neutron-rich frontiers with the advent of RIBF. Taking advantage of the world’s currently strongest RI beams, in conjunction with new experimental techniques and state-of-the-art detector systems, more than 400 species of rare isotopes have been explored in experimental programs dedicated to radioactive-decay measurements performed in the last decade. Some selected topics are introduced in this review. Excited levels investigated by $\gamma$-ray measurements following $\beta$ decay have provided significant information on the evolution of shell structures and nuclear shapes, and thus serve as a stringent benchmark for testing microscopic and macroscopic nuclear model calculations. Systematic studies of $\beta$-decay half-lives of very neutron-rich nuclei have an enormous impact on a better understanding of explosive nucleosynthesis in the rapid neutron-capture (r) process. On the proton-rich side of the $\beta$-stability line, new proton emitters have been identified, allowing an exploration of atomic nuclei beyond the proton drip line. New results obtained in a series of decay experiments include various types of nuclear isomers, for example, seniority, spin-trap, shape, and K isomers. The identification and characterization of such exotic isomers have revealed the underlying single-particle orbits and nucleon-nucleon correlations, as well as the change of nuclear shape and spin orientation. These new results will offer further insight into exotic nuclei and contribute to the development of new theories of nuclear structure and nucleosynthesis. In this review article, some of the ongoing research projects and future prospects for decay studies at RIBF are also presented.