Session NB: Invited Session: New Developments in the Study of Exotic Neutron-rich Nuclei

Ab initio valence-space theory for exotic nuclei

Recent advances in ab initio nuclear structure theory have led to groundbreaking predictions in the exotic medium-mass region, from the location of the neutron dripline to the emergence of new magic numbers far from stability. Playing a key role in this progress has been the development of sophisticated many-body techniques and chiral effective field theory, which provides a systematic basis for consistent many-nucleon forces and electroweak currents. Within the context of valence-space Hamiltonians derived from the nonperturbative in-medium similarity renormalization group (IM-SRG) approach, I will discuss the importance of 3N forces in understanding and making new discoveries in the exotic $sd$-shell region. Beginning in oxygen, we find that the effects of 3N forces are decisive in explaining why $^{24}$O is the last bound oxygen isotope, validating first predictions of this phenomenon from several years ago. Furthermore, 3N forces play a key role in reproducing spectroscopy, including signatures of doubly magic $^{22,24}$O, and physics beyond the dripline. Similar improvements are obtained in new spectroscopic predictions for exotic fluorine and neon isotopes, where agreement with recent experimental data is competitive with state-of-the-art phenomenology. Finally, I will discuss first applications of the IM-SRG to effective valence-space operators, such as radii and $E0$ transitions, as well as extensions to general operators crucial for our future understanding of electroweak processes, such as neutrinoless double-beta decay.   

Studies of Neutron-Rich Nuclei with 3-MeV/u Beams

The study of shapes and collectivity in atomic nuclei has been a major focus of nuclear structure ever since the observation of large electric quadrupole moments in the first half of the 20th century. A leading challenge has been to experimentally establish regions of spherical shape and regions of prolate, triaxial, and oblate deformed shapes, with the latter being very limited. Another challenge has been to understand the evolution of shell structure, the emergence of collectivity, and their connection to shapes. Radioactive beams have and will continue to expand these inquiries and our understanding of nuclear structure. A survey of equipment, techniques, and results from recent experiments in the Sn-132 and Mo-Ru neutron-rich regions will be presented. These experiments were conducted at the HRIBF-ORNL and CARIBU-ANL facilities using CLARION-BAREBALL and GRETINA-CHICO2, respectively. Furthermore, an outlook towards ReA3-NSCL will be given. An emphasis will be placed on unique opportunities with 3-MeV/u beams.   

Shape coexistence in and near $^{68}$Ni

The nuclei in the vicinity of $^{68}$Ni have been the subject of considerable experimental and theoretical work focused on studying the evolution of nuclear structure. Situated at the $Z = $ 28 proton shell closure and the fragile $N = $ 40 subshell closure, $^{68}$Ni is an important nucleus to understand as a progression is made from stable to increasingly exotic nuclei. The nature and decay of the first excited state in $^{68}$Ni has been thoroughly investigated in recent years. The first excited state has a spin and parity of 0$^{+}$, can be described by the excitation of neutrons across the $N = $ 40 gap, and has been interpreted as a moderately oblate-deformed state that coexists with the spherical ground state. A second low-energy excited 0$^{+}$ state is also known to exist in $^{68}$Ni. Based on comparisons with theoretical calculations, the second excited 0$^{+}$ state has been proposed to be strongly prolate deformed and based primarily on the excitation of protons across the $Z = $ 28 gap, leading to the inference that three different 0$^{+}$ states with three distinct shapes coexist below 3 MeV in $^{68}$Ni. Additional studies suggest that shape coexistence is not unique to $^{68}$Ni in this neutron-rich region near $Z = $ 28. For instance, in the neighboring even-even isotope $^{70}$Ni, theory predicts that a prolate-deformed minimum in the potential energy surface occurs at even lower energy than in $^{68}$Ni, and experimental evidence is consistent with the theoretical prediction. The results of recent experiments studying shape coexistence in the region, particularly investigations of $^{68,70}$Ni, will be presented and theoretical interpretations will be discussed.