[en] The IBA treatment of nuclear symmetries and transition regions is discussed. A new, extensive region of O(6) nuclei near A = 130 is presented and compared with the Pt isotopes. Nearly identical level schemes characterize both regions, including similar relations between the O(6) and O(5) steps in the dynamical symmetry chain decomposition, and similar discrepancies with the O(6) limit. Simple calculations that incorporate a triaxial component as an O(6) symmetry breaking mechanism remove these discrepancies in both regions. The evidence for a new, and perhaps the only, good region of SU(3) symmetry is given and discussed in a context of mixing with non-collective degrees of freedom. Finally, a systematic approach to transition regions is presented in which the key determinant of structure is the product N/sub π/.N/sub ν/ of the number of valence proton and neutron bosons. Recognition of this allows an extremely simple treatment of complex transition regions. As an example, calculations for the U(5)→O(6)→SU(3) sequence from A = 120-140, which involve only six constants for the entire region of approx. = 30 nuclei, are discussed. The use of N/sub π/.N/sub ν/ plots allows the study of the dissipation of shell and subshell gaps. Results for the A approx. = 100 and A approx. = 150 regions are presented and effective proton-boson numbers are extracted and compared with earlier results from g factor measurements and from microscopic calculations. 43 references

[en] Deformed nuclei represent perhaps the largest and best studied class of nuclear level schemes. The Interacting Boson Approximation (IBA) model is devised so as to provide a general framework for the description of low lying collective states in nuclei spanning vibrational, rotational (i.e., deformed) and axially asymmetric types as well as the transitional species intermediate between these limiting cases. The juxtaposition of these two statements makes it all the more surprising that until recently there had been no thorough test of the model in such nuclei. Partly, the explanation for this lies in the type of data required for an adequate test. Since the IBA predicts a broad range of collective states it requires a correspondingly thorough empirical test. Moreover, in deformed nuclei, though the characteristic predictions that distinguish the IBA from the traditional, familiar collective model of harmonic β and γ vibrations are important, their clearest manifestation occurs in very weak, hard-to-detect low energy transitions between excited vibrational bands (in particular between β and γ bands), that had not heretofore been systematically observed. The present brief summary will begin with a review of the properties of the (n,γ) reaction that render it a useful empirical tool for such studies, and follow this with a description of the results of the ¹⁶⁸Er study and the application of the IBA model to the resultant level scheme. The discussion will then be generalized to other deformed nuclei and to the inherent systematic predictions that must characterize the IBA for such nuclei. Many of these ideas will be related to the role of finite boson number in the IBA

[en] Although the IBA-1 contains no solutions corresponding to a rigid triaxial shape, it does contain an effective asymmetry arising from zero point motion in a γ-soft potential leading to a non-zero mean or rms γ. In the consistent Q Formalism (CQF) of the IBA, most results of calculation depend only on one parameter chi. A relation will be established between chi and the effective asymmetry parameter γ. The relation between the asymmetry occurring naturally in IBA-1 and the triaxiality arising from the introduction of cubic terms into the IBA Hamiltonian will be discussed. It will be shown that γ-band energy staggering is a particularly sensitive indicator of the degree of γ rigidity. Finally, an extensive new region of 0(6) like Xe and Ba nuclei near A=130 will be discussed. Their remarkable similarity to Pt will be explored. Deviations from the strict 0(6) limit can be described in terms of the interplay of soft and rigid axial asymmetry and calculations will be presented that interpret the Xe, Ba and Pt isotopes in this way. 25 references

[en] The widespread effects of the residual T=0 proton-neutron (p-n) interaction in the evolution of nuclear structure are discussed. Although these effects in inducing single nucleon configuration mixing, and hence in the development of non-spherical nuclear shapes, collectivity, and the associated shape and phase transitions have been known for four decades, it is only in recent years that their deep ramifications have become more fully appreciated. This had led to a unified phenomenological understanding of the role of the p-n interaction in nuclear collectivity and to, for example, the proposal of the N_pN_n scheme and the associated concept of the P factor, which is a normalized value of N_pN_n reflecting the average number of p-n interactions per valence nucleon. Simultaneously, experimentally-determined p-n matrix elements for many nuclei have been extracted: they disclose striking anomalies for N=Z nuclei, and intriguing microstructure. These developments and empirical results will be discussed along with microscopic calculations that can be used to interpret them. 18 refs., 13 figs

[en] The intimate relation between the IBM and axial asymmetry, appearing in its dynamical symmetries, in numerical calculations, in bandmixing effects, and in extensions to the model, is discussed

[en] The Interacting Boson Approximation (IBA) nuclear structure model can be expressed in terms of the U(6) group, and thereby leads to three dynamical symmetries (or group chains) corresponding to different nuclear coupling schemes and geometrical shapes. The status of the empirical evidence for these three symmetries is reviewed, along with brief comments on the possible existence of supersymmetries in nuclei. The relationships between these symmetries, the nuclear phase transitional regions linking them, and the residual proton-neutron interaction are discussed in terms of a particularly simple scheme for parameterizing the effects of that interaction. 34 refs., 15 figs

[en] The pervasive role of the proton-neutron interaction in nuclear structure is discussed. Particular emphasis is given to its influence on the onset of collectivity and deformation, on intruder states, and on the evolution of subshell structure. The N/sub p/N/sub n/ scheme is outlined and some applications of it to collective model calculations and to nuclei far off stability are described. The concept of N/sub p/N/sub n/ multiplets is introduced. 32 refs., 20 figs

[en] Calculations were performed for anti-aligned levels in ¹⁰⁹Pd. For a Nilsson model with pairing, variable moment of inertia, and Coriolis coupling, the favored levels were well reproduced, but the low-spin unfavored states exhibited serious disagreement with experiment results. Calculations with the IBFA were a significant improvement, in particular a regards the splitting of states of common spin. Comments on the source of this improvement are offered. 2 figures, 1 table

[en] In recent years the concept of dynamical symmetries in nuclei has witnessed a renaissance of interest and activity. Much of this work has been developed in the context of the Interacting Boson Approximation (or IBA) model. The appearance and properties of dynamical symmetries in nuclei will be reviewed, with emphasis on their characteristic signatures and on the role of the proton-neutron interaction in their formation, systematics and evolution. 36 refs., 20 figs

[en] The critical role of the residual valence p-n interaction in the development and evolution of collectivity, and the onset of shape/phase transitions, is discussed from the standpoint of phenomenological approaches, and empirical extraction of individual p-n interaction strengths, and simple model calculation of them. 22 refs., 18 figs