I. Ragnarsson

Rotational bands and particle-hole excitations at very high spin

Abstract A method of calculating high-spin states of nuclei within the cranked Nilsson-Strutinsky framework is presented and discussed in some detail. With this method, various high-spin features of nuclei are studied, such as shape coexistence, shape changes, band crossings and band terminations. Nuclei with different mass numbers such as 106 Pd, 118 Te, 158 Yb and 187 Au are used to exemplify the formalism. Comparisons are made with observed discrete high-spin states in 160, 165, 168 Yb, 168 Hf, 171 Ta and 130 Ce.

Microscopic study of the high-spin behaviour in selected A ~= 80 nuclei

Abstract The collective and non-collective high-spin configurations in selected A ⋍ 80 nuclei are analysed in detail using a shell-correction approach taking care of individual configurations and the pairing self-consistent cranking method with a non-axially-deformed Woods-Saxon potential. Shape transitions, shape coexistence, band-termination effects and alignment processes are discussed.

Nuclear shell structure at very high angular momentum

Abstract A cranked modified-oscillator model (with triaxial shape coordinates ϵ and γ) is used to study the nuclear potential-energy surface (based on a Strutinsky type of shell correction method) for very high angular momenta (30 ≦ I ≦ 100). For this region of spin, pair correlation is assumed to have collapsed. The influence of rapid rotation on the shell structure has been studied in the light and heavy rare-earth region as well as the Te-Ba region. Preliminary studies have also been made in the regions of superheavy and light nuclei. The possible occurrence of yrast traps is discussed.

The breaking of intrinsic reflection symmetry in nuclear ground states

Abstract Negative-parity excited states of doubly even nuclei have earlier been attributed to vibrational excitations. This paper shows that an interpretation starting from a reflection asymmetric intrinsic state is more appropriate for certain nuclei in the radium region. Theoretical evidence for stable octupole deformation comes from a deformed shell-model calculation in which we use a single-particle potential with a realistic radial shape and a finite-range interaction for the surface energy. The octupole effect systematically improves the agreement between theoretical and experimental masses. The low-lying O + excitations observed in experiment are compatible with the calculated collective octupole potentials. The possibility of obtaining further evidence from the spectroscopy of odd-mass nuclei is considered in an exactly solvable model, which shows that the smaller energy splitting observed in odd- A parity doublets mainly reflects single-particle fragmentation of the collective mode. The systematics of theoretical shell structure and experimental spectroscopy suggests the presence of other regions of octupole collectivity near the limits of stability.

Nuclear core-quasiparticle coupling

Abstract Relations between different core-particle coupling models based on the Bohr Hamiltonian are discussed, and in particular the γ-unstable and rigid 30° triaxial cores are compared quantitatively. A new formulation is developed employing a rigid triaxial rotor core and the strong-coupling basis. Within the limitations imposed by this model core, it is equally applicable in the limits of small and extremely large deformations. Sample calculations are made for the nuclei 185Re and 75, 77Se. The results for the latter support earlier predictions of an oblate-prolate shape transition in this region of nuclei, but also indicate difficulties in the treatment of pairing. As another application, the wave functions and excitation energies of decoupled states are illuminated from the point of view of the strong-coupling basis, and the effects of hexadecapole deformation and j-mixing are examined.

The role of high-N orbits in superdeformed states

Abstract It is investigated how particles in high- N shells affect the moment of inertia, (2) , and the quadrupole moment, Q 0 , of superdeformed high-spin states. The different behaviour of (2) versus the rotational frequency observed for superdeformed states in 152 Dy and 149 Gd is explained in terms of different occupations of a few high- N orbits. Predictions for (2) and Q 0 for the neighbouring nucleus 148 Eu are presented.

TERMINATION OF ROTATIONAL BANDS : DISAPPEARANCE OF QUANTUM MANY-BODY COLLECTIVITY

Abstract One of the most interesting features of nuclei is the process by which specific configurations, manifest as collective rotational bands at intermediate spin values, gradually lose their collectivity and terminate in a non-collective state at the maximum spin which can be built within the configuration. Recent advances in both experiment and theory allow the study of this nuclear structure feature in detail. The bands, which show such a continuous transition from high collectivity to a pure particle–hole (terminating) state, are generally called terminating bands or to underline their continuous character, smooth terminating bands . The best examples of such bands known at present are in the neutron-deficient A ∼110 mass region, where terminating configurations involving proton particle–hole excitations across the Z =50 gap can be observed over their entire spin range. The main features of band termination as a specific high-spin phenomenon inherent to finite many-fermion strongly interacting systems are overviewed, based mainly on the configuration-dependent cranked Nilsson–Strutinsky approach. The extensive experimental results on smooth terminating bands in the A ∼110 mass region, which have been addressed by these theoretical calculations, are presented along with the theoretical comparisons in a systematic way. In addition, specific features of band termination in other parts of the periodic chart and other possible theoretical approaches are briefly reviewed.

Shell structure in nuclei

Abstract A review of shell structure for spherical and a variety of deformed nuclei is presented. The microscopic-macroscopic method of Strutinsky is used to calculate potential energy surfaces with the pure harmonic oscillator and the modified harmonic oscillator. New sets of “magic numbers” for a variety of different prolate, oblate and axially asymmetric shapes are generated. Experimental evidence for the special stability caused by these shell effects is presented with special emphasis on the lightest and heaviest nuclei where the effects are most pronounced. The radial diffuseness parameter is treated as a Strutinsky variable and its significance in extrapolating into the superheavy region considered. The calculation of shell effects for high spin states is also reviewed.

Shell structure for deformed nuclear shapes

Abstract Eigenvalues for the harmonic oscillator without l · s or l 2 terms suggest a deformed shell structure for nuclei with axes ratios 2 : 1 and deformation ϵ = 0.6 with corresponding nucleon “magic numbers” 2, 4, 10, 16, 28, 40, 60, 80 110 and 140, subject to small modifications due to spin-orbit and other correction terms. Experimental evidence of reasonably stable highly deformed structures corresponding to nucleon numbers 16, 20, 28, 40 and 60 (64) is presented. Attempts to calculate the corresponding potential energy surfaces using the Strutinsky shell correction method are described.

Islands of high-spin yrast isomers

Abstract The variation in nuclear deformation with angular momentum is considered in a wide region of nucleides, 34 ≦ Z ≦ 94. Cases of spin trajectories involving rotation around a symmetry axis, oblate or prolate, are surveyed as likely candidates for yrast traps. A few representative yrast spectra are also given and trap mechanisms discussed.