Shape coexistence in nuclei appears to be unique in the realm of finite many-body quantum systems It differs from the various geometrical arrangements that sometimes occur in a molecule in that in a molecule the various arrangements are of the widely separated atomic nuclei In nuclei the various ''arrangements'' of nucleons involve (sets of) energy eigenstates with different electric quadrupole properties such as moments and transition rates, and different distributions of proton pairs and neutron pairs with respect to their Fermi energies Sometimes two such structures will ''invert'' as a function of the nucleon number, resulting in a sudden and dramatic change in ground-state properties in neighboring isotopes and isotones In the first part of this review the theoretical status of coexistence in nuclei is summarized Two approaches, namely, microscopic shell-model descriptions and mean-field descriptions, are emphasized The second part of this review presents systematic data, for both even- and odd-mass nuclei, selected to illustrate the various ways in which coexistence is observed in nuclei The last part of this review looks to future developments and the issue of the universality of coexistence in nuclei Surprises continue to be discovered With the major advances in reaching to extremes of proton-neutronmore » number, and the anticipated new ''rare isotope beam'' facilities, guidelines for search and discovery are discussed« less

Shape coexistence in atomic nuclei

The atomic nucleus is a quantum many-body system whose constituent nucleons (protons and neutrons) are subject to complex nucleon-nucleon interactions that include spin- and isospin-dependent components. For stable nuclei, several decades ago, emerging seemingly regular patterns in some observables could already be described successfully within a shell-model picture that results in particularly stable nuclei at certain magic fillings of the shells with protons and/or neutrons: N, Z=8, 20, 28, 50, 82, 126. However, in short-lived, so-called exotic nuclei or rare isotopes, characterized by a large N/Z asymmetry and located far from the valley of β stability on the nuclear chart, these magic numbers, viewed through observables, were shown to change. These changes in the regime of exotic nuclei offer an unprecedented view at the roles of the various components of the nuclear force when theoretical descriptions are confronted with experimental data on exotic nuclei where certain effects are enhanced. This article reviews the driving forces behind shell evolution from a theoretical point of view and connects this to experimental signatures.

/pdf/evolution-of-shell-structure-in-exotic-nuclei-3lvumr2ez4.pdf

Evolution of shell structure in exotic nuclei

The BigRIPS in-flight separator, which became operational in March 2007 at the RI Beam
Factory (RIBF) at RIKEN Nishina Center, has been used to produce a variety of rare-isotope
(RI) beams by using in-flight fission as well as projectile fragmentation. Its major features are
large ion-optical acceptances and two-stage structure. Excellent performance in particle identification
is also an important feature. Efficient RI-beam production based on the in-flight scheme
has been made possible by these features of the BigRIPS separator, allowing us to greatly expand
the accessible region of exotic nuclei. An RI-beam delivery line following the BigRIPS separator
is designed to work as a forward spectrometer, called ZeroDegree. As a major experimental
device at RIBF, the ZeroDegree spectrometer has been used for a variety of reaction studies with
RI beams. In this paper, we present an overview of the BigRIPS separator and the ZeroDegree
spectrometer, emphasizing the capability and potential of the new-generation RI beam facility,
RIBF.

/pdf/bigrips-separator-and-zerodegree-spectrometer-at-riken-ri-22mu0n1uak.pdf

BigRIPS separator and ZeroDegree spectrometer at RIKEN RI Beam Factory

A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out at the RI Beam Factory at the RIKEN Nishina Center. Fission fragments were analyzed and identi...

Identification of 45 new neutron-rich isotopes produced by in-flight fission of a 238 U beam at 345 MeV/nucleon

A deformed relativistic Hartree Bogoliubov (RHB) theory in continuum is developed aiming at a proper description of exotic nuclei, particularly those with a large spatial extension. In order to give an adequate consideration of both the contribution of the continuum and the large spatial distribution in exotic nuclei, the deformed RHB equations are solved in a Woods-Saxon (WS) basis in which the radial wave functions have a proper asymptotic behavior at large distance from the nuclear center. This is crucial for the proper description of a possible halo. The formalism of deformed RHB theory in continuum is presented. A stable nucleus, Mg-20 and a weakly bound nucleus Mg-42 are taken as examples to present numerical details and to carry out necessary numerical checks. In addition, the ground-state properties of even-even magnesium isotopes are investigated. The generic conditions of the formation of a halo in weakly bound deformed systems and the shape of the halo in deformed nuclei are discussed. We show that the existence and the deformation of a possible neutron halo depend essentially on the quantum numbers of the main components of the single particle orbitals in the vicinity of the Fermi surface.

Deformed relativistic Hartree-Bogoliubov theory in continuum

The cross sections for single-neutron removal from the very neutron-rich nucleus $^{31}\mathrm{Ne}$ on Pb and C targets have been measured at $230\text{ }\text{ }\mathrm{MeV}/\mathrm{\text{nucleon}}$ using the RIBF facility at RIKEN. The deduced large Coulomb breakup cross section of 540(70) mb is indicative of a soft $E1$ excitation. Comparison with direct-breakup model calculations suggests that the valence neutron of $^{31}\mathrm{Ne}$ occupies a low-$\ensuremath{\ell}$ orbital (most probably $2{p}_{3/2}$) with a small separation energy (${S}_{n}\ensuremath{\lesssim}0.8\text{ }\text{ }\mathrm{MeV}$), instead of being predominantly in the $1{f}_{7/2}$ orbital as expected from the conventional shell ordering. These findings suggest that $^{31}\mathrm{Ne}$ is the heaviest halo system known.

Halo structure of the island of inversion nucleus 31Ne.

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Halo structure of the island of inversion nucleus 31Ne.