Session FA: At and Beyond the Limits: Extremely Neutron-rich and Neutron-unbound Nuclei near Oxygen

Three-nucleon forces for exotic oxygen isotopes

The oxygen isotopes, with an experimentally well established dripline ($N=16$) anomalously close to the valley of stability, provide an ideal laboratory to study the structure of extreme neutron-rich nuclei at and beyond the limits of existence. The emergence of $N=14,16$ as new magic numbers and properties of the unbound $^{25,26}$O isotopes pose particularly challenging benchmarks for models of nuclear forces and many-body methods aiming at a description of exotic medium-mass nuclei. At the heart of these efforts is three-nucleon (3N) forces, whose impact represents a current frontier in nuclear structure theory. I will discuss the first microscopic framework, based on chiral effective field theory and renormalization group methods, in which neutron-rich oxygen isotopes were explored from a systematic treatment of NN and 3N forces. In this approach we found that the repulsive effects of 3N forces were decisive in explaining why $^{24}$O is the heaviest oxygen isotope. Furthermore, 3N forces play a key role in reproducing spectra, including signatures of doubly-magic $^{22,24}$O, and unbound properties without empirical adjustments. Finally I will discuss subsequent progress in ab-initio efforts with 3N forces such as coupled cluster theory, in-medium similarity renormilazation group, and Green's function theory, where a consistent picture of the oxygen isotopic chain emerges, which is highly encouraging for first-principles calculations of exotic nuclei well into the medium mass region.   

Measuring oxygen isotopes beyond the neutron dripline: Two-neutron emission and radioactivity

The availability of rare isotope beams has made it possible to extend nuclear structure measurements to nuclei far away from stability. Drastic changes in the structure, properties, and available decay-modes of these exotic isotopes have been observed in comparison to their stable counterparts. The oxygen isotopic chain has been particularly interesting with observations of new shell closures at N=14 and N=16. The MoNA-LISA/Sweeper setup at the National Superconducting Cyclotron Laboratory at Michigan State University has allowed for studies of the oxygen isotopes to be extended beyond the neutron dripline. Recently, the $^{26}$O ground state was observed for the first time and shown to be unbound by less than 200~keV. The low energy ground state of the two-neutron unbound $^{26}$O opened the possibility for the discovery of two-neutron radioactivity. A new technique was developed to measure the lifetimes of neutron unbound nuclei in the picosecond range. This technique was applied to the $^{26}$O decay and a half-life of 4.5$_{~-1.5}^{+1.1}$~(stat.)~$\pm$3~(sys.)~ps was extracted. This corresponds to $^{26}$O having a finite lifetime at an 82\% confidence level and, thus, suggests the possibility of two-neutron radioactivity.   

Nuclear structures near and beyond the neutron drip line studied by breakup reactions at SAMURAI at RIBF

Some of the first results from kinematically complete measurements of breakup reactions on neutron-rich boron to oxygen isotopes, along and beyond the neutron drip line, are presented and discussed. These experiments were performed at the recently-commissioned large-acceptance multi-purpose spectrometer SAMURAI ({\bf S}uperconducting {\bf A}nalyser for {\bf MU}lti-particles from {\bf Ra}dio-{\bf I}sotope Beam) at the new-generation RI beam facility, RIBF, at RIKEN. The experiments aimed at probing the two-neutron Borromean halo nuclei, $^{19}$B and $^{22}$C, and at exploring the heavy oxygen isotopes, $^{25,26}$O, which are beyond the neutron drip line. The study of $^{19}$B and $^{22}$C has been made primarily by the Coulomb breakup, which is sensitive to the halo states and associated two-neutron correlations~[1,2]. $^{22}$C has drawn much attention due to the possibility that it has the largest halo known~[3]. In addition, $^{22}$C may also exhibit features consistent with the new magic number $N$=16, as was recently suggested by our inclusive measurement of the momentum distribution of $^{20}$C following breakup on a C target~[4]. $^{25}$O and $^{26}$O have drawn much attention since these unbound nuclei may have keys to understand why the neutron drip line ends anomalously closer to the stability for oxygen isotopes. $^{25}$O and $^{26}$O have been produced by the proton removal reactions on $^{26}$F,$^{27}$Ne, and $^{27}$F,$^{28}$Ne, respectively, at 220-250 MeV/nucleon. Preliminary data are shown and discussed. Finally, some perspectives on future projects using the SAMURAI facility are presented. \bigskip \\ \noindent [1] T.~Nakamura, Y.~Kondo, Clusters in Nuclei Vol. 2, Lecture Notes in Physics Vol.848, Springer, ed. C.Beck (2012).\\ \noindent [2] T.~Aumann, T.~Nakamura, Phys.Scr. {\bf T152}, 014012 (2013).\\ \noindent [3] K.~Tanaka {\it et al.}, Phys. Rev. Lett. {\bf 104}, 062701 (2010).\\ \noindent [4] N.~Kobayashi, T.~Nakamura {\it et al.}, Phys. Rev. C {\bf 86}, 054604 (2012).