The “island of inversion" from a nuclear moments perspective and the g factor of 35Si

This paper reviews recent results from electromagnetic moment measurements on isotopes in the island of inversion around N=20. The obtained moments on neutron rich Na, Mg, Al and Si isotopes allow to draw conclusions on the amount of intruder components in their ground state wave function, demonstrating a gradual transition from the normal sd-shell region into the island of inversion, starting at N=18 for Na, N=19 for Mg and N=20 for Al isotopes. A measurement of the ground state g factor of ³⁵Si (N=21), using a polarized fragment beam at GANIL, is discussed in more detail. The magnetic moment μ(³⁵Si, I^π= 7/2^-) = (-)1.638(4) μ_N is consistent with a normal ground state structure, dominated by a νf_7/2 neutron.     

Exploring the low-Z shore of the island of inversion at n=19

The technique of invariant mass spectroscopy has been used to measure, for the first time, the ground state energy of neutron-unbound (28)F, determined to be a resonance in the (27)F+n continuum at 220(50) keV. States in (28)F were populated by the reactions of a 62 MeV/u (29)Ne beam impinging on a 288 mg/cm(2) beryllium target. The measured (28)F ground state energy is in good agreement with USDA/USDB shell model predictions, indicating that pf shell intruder configurations play only a small role in the ground state structure of (28)F and establishing a low-Z boundary of the island of inversion for N=19 isotones.     

Nuclear charge radii of neutron-deficient lead isotopes beyond N = 104 midshell investigated by in-source laser spectroscopy

The shape of exotic even-mass (182-190)Pb isotopes was probed by measurement of optical isotope shifts providing mean square charge radii (delta(r(2))). The experiment was carried out at the isolde (cern) on-line mass separator, using in-source laser spectroscopy. Small deviations from the spherical droplet model are observed, but when compared to model calculations, those are explained by high sensitivity of delta(r(2)) to beyond mean-field correlations and small admixtures of intruder configurations in the ground state. The data support the predominantly spherical shape of the ground state of the proton-magic Z=82 lead isotopes near neutron midshell (N=104).     

Mg,Si和S同位素链的壳反常

采用考虑轴向形变的相对论平均场理论,以双中子、双质子分离能隙δ_2n/δ_2p作为壳封闭的特征,分析了Mg,Si和S同位素链的壳封闭和壳反常,发现Mg同位素链在N=12,16处,Si同位素链在N=14,S同位素链在N?=16处出现明显的壳封闭结构,Si同位素链在N=28处壳层结构并没有消失.Mg,Si和S同位素链在N=20处壳效应并不太明显.     

Nuclear charge radii of (21-32)Mg

Charge radii of all magnesium isotopes in the sd shell have been measured, revealing evolution of the nuclear shape throughout two prominent regions of assumed deformation centered on (24)Mg and (32)Mg. A striking correspondence is found between the nuclear charge radius and the neutron shell structure. The importance of cluster configurations towards N=8 and collectivity near N=20 is discussed in the framework of the fermionic molecular dynamics model. These essential results have been made possible by the first application of laser-induced nuclear orientation for isotope shift measurements.     

Low-lying states and isospin excitation in the Ge isotopes

The level structure of 64-70Ge isotopes has been studied within the framework of the interacting boson model-3 (IBM-3). The symmetry character in the proton and neutron degrees of freedom of the energy levels has been investigated. The isospin excitation states (T>Tz) have been assigned for the 64Ge (N=Z) nucleus. Some intruder states in these nuclei have been suggested. The calculated energy levels and transition probabilities are in good agreement with recent experimental data. The study indicates that the Ge isotopes are in transition from γ-unstable to vibrational.     

Intruder configurations in the A=33 isobars: 33Mg and 33Al

The beta decay of 33Mg (N=21) presented in this Letter reveals intruder configurations in both the parent and the daughter nucleus. The lowest excited states in the N=20 daughter nucleus, 33Al, are found to have nearly 2p-2h intruder configuration, thus extending the "island of inversion" beyond Mg. The allowed direct beta-decay branch to the 5/2{+} ground state of the daughter nucleus 33Al implies positive parity for the ground state of the parent 33Mg, contrary to an earlier suggestion of negative parity from a g-factor measurement. An admixture of 1p-1h and 3p-3h configurations is proposed for the ground state of 33Mg to explain all of the experimental observables.     

Microscopic calculations for upper-fp,g<Subscript>9/2</Subscript> shell nuclei: Ge &; Se

Shell-model calculations for isotopes of Ge and Se are reported where valence nucleons beyond the N = 28 = Z core occupy levels of the normal parity upper-fp shell (f_5/2,p_3/2,p_1/2) and the unique parity g_9/2 intruder configuration. Results are given for realistic interactions of the Kuo-Brown-3 type with various model space truncations that key in on the number of nucleon pairs allowed to occupy the intruder level. Electromagnetic (E2 & M1) rates as well as decay probabilities are calculated, some of which are key in determining the structure of “waiting point” nuclei that regulate certain nucleo-astrosynthesis processes. The role of the intruder level, which is treated on an equal footing with the normal parity levels, is shown to be important for reproducing structural details. The levels of the upper-fp shell are handled within the framework of a normal ls-coupled basis as well as its pseudo-SU(3) counterpart, and respectively, the g_9/2 as a single level and as a member for the complete gds shell. The second of these two approaches, namely, the SU(3) picture, allows one to better probe the effect of deformation.     

Shape coexistence in the light Po isotopes

The systematics of the even light Po isotopes (N ⩽ 126) are studied in the framework of the Particle-Core Model. The strong perturbation of the systematics in the very light isotopes is interpreted as arising from the interaction between regular and intruder structures. Results of Potential Energy Surface (PES) calculations and predictions of the Pairing Vibration Model support this interpretation. The mixing between the regular and intruder structures is studied within the IBM-2 and in a simple two-state mixing picture. Matrix elements of the interaction and their spin dependence are extracted. The ‘reconstructed systematics’ show the coexistence of a spherical structure, which varies little with the neutron number, with an intruder band, strongly lowered in energy as the neutron number approaches midshell. The crossing of the two configurations takes place over a few isotopes; the intruder band becomes the ground-state configuration in ¹⁹²Po.     

Structure analysis of 159Sm and properties of odd-mass neutron-rich nuclei in mass-160 region

Recent fission experiment data provide interesting structure information for neutron-rich nuclei in the mass A ∼ 160 region. We apply the projected shell model to study the strongly-deformed, neutron-rich Sm isotopes. We perform calculations for rotational bands up to spin I = 20 (29/2) for even-even (odd-neutron) Sm isotopes, and analyze the band structure of low-lying states with quasiparticle excitations. Emphasis is given to rotational bands based on one-quasiparticle (1-qp) configurations in the odd-mass ¹⁵⁹Sm. The ¹⁵⁹Sm result is discussed together with those of the even-even isotopes ^158,160Sm. New bands in ¹⁵⁹Sm based on neutron 1-qp 1/2⁻ and 5/2⁺ configurations are predicted. Electromagnetic transition probabilities are discussed.

     

Low-lying states and isospin excitation in the Ge isotopes

The level structure of ^64-70Ge isotopes has been studied within the framework of the interacting boson model-3（IBM-3） . The symmetry character in the proton and neutron degrees of freedom of the energy levels has been investigated. The isospin excitation states（T 〉 Tz） have been assigned for the ^64Ge（N = Z） nucleus. Some intruder states in these nuclei have been suggested. The calculated energy levels and transition probabilities are in good agreement with recent experimental data. The study indicates that the Ge isotopes are in transition from γ-unstable to vibrational.     

Shape coexistence and the N = 28 shell closure far from stability

The masses of 31 neutron-rich nuclei in the range A = 29-47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S, and P isotopes are seen to exhibit a change in shell structure around N = 28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. Evidence for shape coexistence is provided by the observation of an isomer in 43S.     

Measurement of the spin and magnetic moment of 31Mg: evidence for a strongly deformed intruder ground state

Unambiguous values of the spin and magnetic moment of 31Mg are obtained by combining the results of a hyperfine-structure measurement and a beta-NMR measurement, both performed with an optically polarized ion beam. With a measured nuclear g factor and spin I=1/2, the magnetic moment mu(31Mg)=-0.88355(15)mu(N) is deduced. A revised level scheme of 31Mg (Z=12, N=19) with ground state spin/parity I(pi)=1/2(+) is presented, revealing the coexistence of 1p-1h and 2p-2h intruder states below 500 keV. Advanced shell-model calculations and the Nilsson model suggest that the I(pi)=1/2(+) ground state is a strongly prolate deformed intruder state. This result plays a key role for the understanding of nuclear structure changes due to the disappearance of the N=20 shell gap in neutron-rich nuclei.     

Laser spectroscopy of the bismuth isotopes

Isotope shifts and hyperfine structures of the bismuth isotope chain have been studied on the 306.7 nm line in off-line measurements using gas cell laser spectroscopy and atomic beam spectroscopy. The changes in nuclear mean square charge radii and the nuclear magnetic and spectroscopic quadrupole moments have been deduced. The neutron-rich isotopes are the first isotones of Pb to be measured immediately above the N=126 shell closure. A remarkable correspondence between the nuclear charge radii of the Bi and Pb isotope chains is demonstrated by a King Plot analysis. The relationship between nuclear shapes and the charge radii can be understood in the framework of the spherical shell model using few-nucleon configurations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Decay spectroscopy of neutron-rich nuclei with A ≃ 100

We review structure data obtained by decay spectroscopy of neutron-rich nuclei of mass close to 100. Emphasis is put on the contribution of experiments at IGISOL in the nineties. They confirmed the earlier postulated shape coexistence in the fast shape-transition region between N = 58 (spherical ground states and low collectivity) and N = 60 (strong axial deformation). A detailed spectroscopic study of the A = 99 chain established the upper-Z limit of the N = 56 shell closure region with ⁹⁹Nb, owing to striking similarities with ⁹⁷Y. A consequence of the N = 56 closure is that the s _1/2 odd-neutron becomes the ground state of the most neutron-rich N = 57 isotones, starting with ⁹⁹Mo, instead of the degenerated d _5/2 and g _7/2 subshells familiar in the tin region. Consequences on the change of spin on astrophysical r-process calculations are briefly discussed. Finally, we say a few words about neutron-rich rhodium and palladium isotopes near the neutron midshell where regular and intruder states coexist very close to each other.     

New beyond-mean-field theories: examination of the potential shell closures at N=32 or 34

A beyond-mean-field theory of new generation has been developed and applied for the first time to discuss the controversial N=32 and/or N=34 shell closures and the puzzling behavior of the transition probabilities from the ground to the first 2(+) state in the titanium isotopes. In the numerical applications, the finite range density dependent Gogny interaction has been used. As compared with the experimental data for several calcium, titanium, and chromium isotopes, we obtain a good agreement for the excitation energies and a reasonable one for the transition probabilities. Our calculations support a shell closure for N=32 but not for N=34.     

Laser Spectroscopy and \beta-NMR Measurements of Short-Lived Mg Isotopes

The feasibility of studying the neutron-rich ²⁹Mg, ³¹Mg, and ³³Mg isotopes has been demonstrated with the laser and β-NMR spectroscopy setup at ISOLDE/CERN. The values of the magnetic moment and the nuclear spin of ³¹Mg are reported and reveal an intruder ground state. This proves the weakening of N = 20 shell gap and places this nucleus inside the so-called ‘island of inversion.’ The experimental setup and technique, as well as the results, are presented.     

Nuclear clusters and nuclear molecules

Clustering has long been known to be influential in the structure of ground and excited states of N=Z$N=Z$ nuclei. States close to the decay thresholds are of particular interest, as clustering becomes dominant. Recent studies of loosely bound light neutron-rich nuclei have focused attention on structures based on clusters and additional valence neutrons, which give rise to covalent molecular binding effects. These nuclear molecules appear only at the extremes of deformation, in the deformed shell model they are referred to as super- and hyper-deformed. The beryllium isotopes provide the first examples of such states in nuclear physics. Further nuclear molecules consisting of unequal cores and also with three centres can be considered. These arise in the isotopes of neon and carbon, respectively. Molecular states in intrinsically asymmetric configurations give rise to parity (inversion) doublets. Examples of recent experiments demonstrating the molecular structure of the rotational bands in beryllium isotopes are presented. Further experimental evidence for bands as parity doublets in nuclei with valence neutrons in molecular orbits is also analysed. Work on chain states (nuclear polymers) in the carbon isotopes is discussed. These are the first examples of hyper-deformed structures in nuclei with an axis ratio of 3:1. Future perspectives are outlined based on a threshold diagram for covalent nuclear molecules with clusters bound via neutrons in covalent molecular configurations.     

Shell-model studies of the N=14 and 16 shell closures in neutron-rich nuclei

Shell-model studies on the N=14 and 16 shell closures in neutron-rich Be, C, O and Ne isotopes are presented. We calculate, with the WBT interaction, the excited states in these nuclei. The calculations agree with recent experiment data. Excited energies and B(E2) values are displayed to discuss the shell closures. Our results support the N=16 shell closure in these isotopes, while indicating a disappearance of N=14 shell closure in Be and C isotopes.