Islands of insight in the nuclear chart

Abstract

The ''island of inversion'' nucleus 32 Mg has been studied by a (t, p) two neutron transfer reaction in inverse kinematics at REX-ISOLDE. The shape coexistent excited 0 þ state in 32 Mg has been identified by the characteristic angular distribution of the protons of the ÁL ¼ 0 transfer. The excitation energy of 1058 keV is much lower than predicted by any theoretical model. The low -ray intensity observed for the decay of this 0 þ state indicates a lifetime of more than 10 ns. Deduced spectroscopic amplitudes are compared with occupation numbers from shell-model calculations. The evolution of shell structure in exotic nuclei as a function of the proton (Z) and neutron (N) number is currently at the center of many theoretical and experimental investigations [1,2]. It has been realized that the interaction of the last valence protons and neutrons, in particular, the monopole component of the residual interaction between those nucleons, can lead to significant shifts in the single-particle energies, leading to the disappearance of classic shell closures and the appearance of new shell gaps [3]. A prominent example is the collapse of the N ¼ 20 shell gap in the neutron-rich oxygen isotopes where instead a new magic shell gap appears for 24 O at N ¼ 16 [4,5]. Recent work showed that the disappearance of the N ¼ 20 shell can be attributed to the monopole effect of the tensor force [3,6,7]. The reduced strength of the attractive interaction between the proton d 5=2 and the neutron d 3=2 orbitals causes the d 3=2 orbital to rise in energy and come closer to the f 7=2 orbital. In regions without pronounced shell closures correlations between the valence nucleons may become as large as the spacing of the single-particle energies. This can thus lead to particle-hole excitations to higher-lying single-particle states enabling deformed configurations to be lowered in energy. This may result in low-lying collective excitations, the coexistence of different shapes at low energies or even the deformation of the ground state for nuclei with the conventional magic number N ¼ 20.Such an effect occurs in the ''island of inversion'', one of most studied regions of exotic nuclei in the nuclear chart. In this region of neutron-rich nuclei around the magic number N ¼ 20 strongly deformed ground states in Ne, Na, and Mg isotopes have been observed [8][9][10][11]. Because of the reduction of the N ¼ 20 shell gap, quadrupole correlations can enable low-lying deformed 2p-2h intruder states from the fp shell to compete with spherical normal neutron 0p-0h states of the sd shell. In this situation the promotion of a neutron pair across the N ¼ 20 gap can result in deformed intruder ground states. Consequentially, the competition of two configurations can lead to the coexistence of spherical and deformed 0 þ states in the neutron-rich

The In-Medium Similarity Renormalization Group: A novel ab initio method for nuclei

“…The complex interplay of nuclear interactions, many-body correlations, and, in the dripline region, continuum effects, can cause strong competition between configurations with different intrinsic structures. This manifests in phenomena like the erosion and emergence of shell closures [24,26,29,139], or the appearance of the so-called islands of inversion (see, e.g., [140]). Their description requires a true multi-reference treatment.…”

In-beam γ -ray spectroscopy of Cr62,64

“…The islands of inversion around 12 Be (N = 8) [1], 32 Mg (N = 20) [2][3][4], and 42 Si [5,6] (N = 28) have revealed the existence of structural changes for neutron-rich systems, such as the breakdown of magic numbers and rapid shape changes among neighboring nuclei. These regions have been invaluable in unraveling the driving forces of shell evolution including, most broadly, a variety of correlation effects and, specifically, the role of monopole interactions due to the various constituents of the NN interaction such as the tensor and central forces [4,[6][7][8]. The newest addition to the group of islands of inversion is centered on 64 Cr (N = 40) [9], where Cr and Fe isotopes are among the most deformed in the region [10][11][12][13] in spite of an originally proposed N = 40 harmonic oscillator shell gap [14].…”

Reaching into the N = 40 Island of Inversion with Nucleon Removal Reactions

“…In nuclear models, this is theorized to be caused by the strong quadrupole-quadrupole interaction producing a nuclear shape transition in which highly-correlated many-particle-many-hole configurations become energetically more favored than the normal-order (spherical) ones [4]. Such islands of inversion are characterized by rapid structural changes and shape coexistence [5,6], providing insight into nuclear structure physics far from stability [7]. Large-scale shell-model calculations with the LNPS (Lenzi-Nowacki-Poves-Sieja) effective interaction [4] in the full f p shell for protons and the f 5/2 , p 3/2 , p 1/2 , g 9/2 , and d 5/2 orbitals for neutrons have confirmed the picture described above, with many successful predictions that preceded experimental results [3].…”