Search

The Gamow–Teller strength distributions for the β decay of the ground state as well as the lowest excited states of the rp-process waiting point nuclei 68Se and 72Kr are obtained within the complex Excited Vampir variational approach using realistic effective interactions and a rather large model space. The shape mixing is consistently described for both the states in the even–even parent and the states in the odd–odd daugther nucleus. The influence of the shape mixing accounted by the different effective interactions used and comparison with the available data are presented. The possible influence of the decay of the lowest excited states of the parent nuclei in the astrophysical environment of X-ray bursts is discussed. Gamow–Teller strength distributions, β -decay half-lives, and β -delayed neutron emission probabilities for neutron-rich Zr nuclei are investigated for the first time within the complex Excited Vampir approach using a large model space. Comparison with available data and predictions relevant for the astrophysical r process are presented.

We study the influence of shape mixing on the Gamow–Teller β decay of the nucleus 74Kr using the complex EXCITED VAMPIR variational approach for the description of the ground state of 74Kr and the 1 + states appearing within the β + -decay window in the daughter nucleus 74Br. The Gamow–Teller strength is obtained describing consistently the shape mixing in both parent and daughter nucleus. The theoretical results for the Gamow–Teller strength distribution as well as accumulated strength are compared with the available experimental data.

The complex Excited Vampir approach was applied to the description of the recently identified magnetic cascade of negative-parity states in the odd-odd 82Rb nucleus. Strong M1, ΔI = 1 transitions as well as rather weak crossover B(E2) strengths are obtained in agreement with the available data. The decrease of the B(M1) strengths with increasing spin is revealed by the calculations. A strong mixing of differently deformed states at the highest calculated spins is responsible for the trend of the B(M1;ΔI = 1) and B(E2;ΔI = 2) strengths.

We study the influence of Coulomb-induced isospin-mixing on the superallowed Fermi β decay using the complex EXCITED VAMPIR variational approach for the description of the lowest excited 0 + states in the two isovector triplets of nuclei 70 Se, 70 Br, 70 Kr and 74 Kr, 74 Rb, 74 Sr, respectively. As expected, the effects are small but nevertheless non-negligible. The calculated strength for the 74 Rb → 74 Kr Fermi β decay to the first excited 0 + state turns out to be very weak, in agreement with the experimentally given upper limit. However, particular non-analog Fermi branches to excited 0 + states with considerable strength are predicted to coexist with the superallowed decay in some of the presently investigated nuclei. This may give some hope for possible experimental detection.

Results are presented on shape coexistence at low and high spins in the mirror nuclei 70 Se and 70 Kr obtained within the complex version of the Excited Vampir variational approach. Effects of oblate–prolate coexistence and mixing at low, intermediate and high spins are discussed. Strong M1, ΔI=0 transitions are predicted. A detailed comparison with the available experimental information is performed.

Results are presented concerning shape coexistence and shape transition at low and intermediate spins in the N = Z nuclei 72 Kr, 76 Sr and 80 Zr obtained within the complex version of the Excited Vampir variational approach. The effects of possible contaminations due to the center-of-mass motion are discussed using a rough method to eliminate them at least approximately. The results indicate maximum influence on the observables depending on band mixing. Detailed illustration of the effects on the oblate–prolate mixing and the electromagnetic properties, as well as the alignments and particle occupations of the 0 g 9/2 spherical orbital is presented for the 72 Kr nucleus.

The pair structure and the average pairing gaps of realistic wave functions obtained within the complex Excited Vampir variational approach are investigated in order to evaluate the neutron-proton pairing correlations at low and high spins in medium mass N ∼- Z nuclei. The number of isovector Jπ = 0+ pairs is calculated for the lowest few 0+ states in two chains of nuclei in the A ∼- 70 mass region. The results indicate the dominant role played by the isovector neutron-proton pairing correlations in the structure of odd-odd N = Z nuclei and the reduction of their importance with increasing neutron excess in even-even nuclei. The evolution of particular isovector and isoscalar pairs with increasing angular momentum is analyzed for the odd-odd N = Z nucleus 74Rb and the even-even N = Z nucleus 72Kr. It turns out that in the nucleus 74Rb the neutron-proton correlations play an essential role for the alignment of the yrast positive-parity even-spin band.

We review the theoretical interpretation of experimental results on low and high spin states in some even mass A=60−80 nuclei within the complex versions of the Excited Vampir and Fed Vampir variational approaches. The coexistence of oblate and prolate deformed configurations and their mixing at low spin are found to be responsible for the irregular sequences of states and the isomeric decays identified at low excitation energy in even-even nuclei. This behaviour is determined essentially by the isoscalar neutron-proton interaction. At higher spins the coexistence of states with different deformations causes complicated multiple band structures and strong fragmentation of the B(E2) strengths. It also influences the occurence of almost identical bands at normal deformation in some even mass nuclei. In some odd-odd nuclei the shape coexistence furthermore suggests a possible mechanism to populate high spin isomeric states. For the ground state properties of N=Z nuclei the competition between neutron-proton and like-nucleon isovector pairing plays an important role. With increasing angular momentum the neutron-proton alignment dominates the like-nucleon one in the ground bands of odd-odd N=Z nuclei while in the doubly even N=Z nuclei both types of alignment are of comparable importance.

Lifetimes of a superdeformed band in Sr-82 were measured with the centroid shift method. The measured average quadrupole moment of this band corresponds to a quadrupole deformation of beta(2) approximate to 0.49, which is slightly smaller than both the theoretical prediction, and the measured deformation of the SD band in the neighboring isotone Zr-84. Lifetimes of high spin states of three normally deformed rotational bands in Sr-82 were also measured with the Doppler shift attenuation method technique. The quadrupole moments of these normally deformed bands show a decrease at the highest spins, supporting the predicted band terminations.

Recent investigations of the shape transition and shape coexistence phenomena dominating the structure of the even-even N ≃ Z nuclei in the A ≃ 80 mass region are extended to the odd-odd nucleus 78Rb. Special attention is paid to the structure of some “identical” bands which have been recently identified in 78 Sr and 78Rb. The ground band of78 Sr and the yrast as well as excited negative parity bands in 78Rb are studied within the EXCITED VAMPIR approximation using complex Hartree-Fock-Bogoliubov transformations in a relatively large model space. The results are compared with the available experimental data. The emerging picture reveals the role played by the strong quadrupole deformation on the appearance of identical bands as well as the influence of the shape coexistence on their evolution. Predictions for the electromagnetic and alignment properties of the bands are presented.

The $\ensuremath{\beta}$-feeding probabilities for three important contributors to the decay heat in nuclear reactors, namely ${}^{102,104,105}$Tc, have been measured using the total absorption spectroscopy technique. For the measurements, sources of very high isobaric purity have been obtained using a Penning trap (JYFLTRAP). A detailed description of the data analysis is given and the results are compared with high-resolution measurements and theoretical calculations.

High spin negative parity states have been studied in 68 Ge via in-beam γ-ray spectroscopy at HHIRF. Several close-lying 7−, 9−, and 11− states are observed with a 13− to tentatively 17− cascade feeding them. Numerous E1 crossing transitions between the negative and positive parity levels are observed, along with possible M1 connecting transitions between the low lying negative parity bands. Previous microscopic calculations of the shape coexisting positive parity levels have been extended to the negative parity levels. The calculations used the EXCITED VAMPIR model based on complex HFB transformations with neutron-proton correlations and parity mixing in the mean field. The complex decay pattern and multiple high spin negative parity bands can be explained by a variable strength of mixing of the different deformed configurations which are bunched in small excitation intervals.

The recent investigations of the shape coexistence phenomena dominating the structure of even-even and odd-odd nuclei in the A ∼ 70 mass region are extended to N ⋍ Z nuclei ranging from krypton to molybdenum. Positive-parity states with even angular momenta in 72 Kr, 76 Sr, 78 Sr, 80 Zr and 84 Mo are investigated within the EXCITED VAMPIR approximation using essentially complex Hartree-Fock-Bogoliubov transformations and a relatively large model space. The results are compared with the available experimental data. We obtain an oblate ground band and strong oblate-prolate mixing for N = Z = 36, almost pure prolate and oblate bands, with the yrast one being prolate in 76 Sr, maximum prolate deformation in 76,78 Sr and 80 Zr with very high-lying non-yrast bands and decreased prolate quadrupole deformation for the N = Z = 42 system. Predictions for the electromagnetic properties of the investigated states are presented, too.

The levels in have been studied employing the pn) reaction with the Bucharest tandem and the p) reaction at 95 MeV with the HHIRF tandem to enhance high spin states. New high spin states have been observed beginning at 1179.5 keV and they include a cascade up to 6092.2 keV, tentatively assigned . These results are compared with the theoretical calculations based on the VAMPIR models which also include neutron - proton mixing.

High spin states in $^{68}\mathrm{As}$ have been studied using the $^{56}\mathrm{Fe}$${(}^{14}$N,2n${)}^{68}$As, $^{58}\mathrm{Ni}$ ${(}^{12}$C,pn${)}^{68}$As, and $^{46}\mathrm{Ti}$${(}^{25}$ Mg,p2n${)}^{68}$As reactions. New high spin states have been observed including a band structure above the known 2160 keV isomeric state. A microscopic description based on the EXCITED VAMPIR model for negative and positive parity states in $^{68}\mathrm{As}$ is presented. A possible mechanism to populate the observed high spin isomeric state is proposed. The complex structure of the $^{68}\mathrm{As}$ nucleus is caused essentially by specific shape coexistence effects. \textcopyright{} 1996 The American Physical Society.

A microscopic description of the yrast states in the odd-odd nucleus 72 As is presented. The investigation is based on the FED VAMPIR approach, which for each yrast state under consideration solves the general Hartree-Fock-Bogoliubov problem with symmetry-projection before variation first for the dominant and then successively for the most important correlating configurations. Energies as well as electromagnetic properties of the resulting bands are analysed and, where possible, compared to the experimental data. Unfortunately, the overall agreement with the latter is not very satisfying. Reasons for this deficiency are discussed.

The recent investigations of the shape-coexistence phenomena dominating the structure of the even-even nuclei in the A ∼ 70 mass region are extended by introducing neutron-proton and unnatural-parity correlations in the mean field, while keeping the time-reversal invariance and the axial-symmetry restrictions. Selected low-lying states in 68Ge, 72Ge and 72Kr nuclei are investigated within the FED VAMPIR or EXCITED FED VAMPIR approaches. The results obtained using real and complex mean fields in a relatively large model space are compared with the experimental data. Similar qualitative features are obtained with both real and complex calculations concerning the evolution of the shape transition and shape coexistence with increasing spin and excitation energy. The results obtained indicate the importance of the new type of correlations introduced by the complex mean fields.

The Gamow-Teller strength distributions for the β decay of the ground state and the predicted lowest two excited 0 + states of 68 Se to 68 As are obtained with the complex excited VAMPIR variational approach using two effective interactions and a rather large model space. The shape mixing is described consistently for both the 0 + states in the even-even 68 Se and the 1 + states in the odd-odd 68 As. The influence of the shape mixing accounted by the different effective interactions used and a comparison with the available data are presented. The possible influence of the decay of the excited 0 + states in the astrophysical environment of x-ray bursts is discussed.

High-spin states in {sup 68}Ge and {sup 72}Se have been studied by in-beam, gamma-ray spectroscopy. In {sup 68}Ge the three known bands beginning at 8{sup +} are found to exhibit crossing transitions and other new positive-parity bands are discovered to feed into them. In {sup 72}Se only a single positive-parity yrast band is seen to high spin. Detailed, more realistic microscopic model calculations have been carried out for {sup 68}Ge and {sup 72}Se. There is striking agreement between the calculations and the new data for {sup 68}Ge, including a predicted new band with superdeformation, {beta}{similar to}0.42. The calculations for {sup 72}Se predict similar multiple band structures that are not seen experimentally.

Microscopic nuclear structure wave functions obtained within the EXCITED FED VAMPIR approach are used to calculate the charge densities for the ground states as well as the transition charge densities for the transitions from the ground to the first and second 2+ states in some doubly even Ge isotopes. The results are compared with the data extracted from elastic and inelastic electron scattering experiments. Except for the heavier isotopes, where the calculations fail to yield enough collectivity, rather nice agreement is obtained. This result strongly supports the theoretically predicted shape coexisting structures in the low spin states of the nuclei in this mass region.

We study the effects of the isospin-symmetry breaking on the superallowed Fermi {beta} decay of the ground state of {sup 82}Nb to {sup 82}Zr. Results on the analog as well as nonanalog {beta}-decay branches are self-consistently obtained within the complex excited VAMPIR approach. The {sup 82}Nb {yields} {sup 82}Zr {beta} decay to the first two excited 0{sup +} states with significant strength is predicted to coexist with the superallowed decay. The structure and electromagnetic properties of the analog and nonanalog low- and high-spin states in {sup 82}Nb and {sup 82}Zr are compared with the available experimental data.

The Gamow-Teller strength distributions for the {beta} decay of the ground state and the lowest-excited states in the nucleus {sup 72}Kr to the nucleus {sup 72}Br are obtained within the complex excited VAMPIR variational approach using a rather large model space. The shape mixing is consistently described for both the 0{sup +} and 2{sup +} states in the even-even {sup 72}Kr and the 1{sup +},2{sup +}, and 3{sup +} states in the odd-odd {sup 72}Br. The influence of shape mixing, comparison with the available data, and predictions concerning the half-life of the first excited 0{sup +} and yrast 2{sup +} states of {sup 72}Kr are presented.

The shape coexistence phenomena dominating the structure of the nucleus 68 Ge are reinvestigated by taking the dominant correlations on top of the symmetry-projected quasiparticle mean-field solutions of the earlier EXCITED VAMPIR description into account. It turns out that in spite of some quantitative changes the main qualitative features of the older description like, e.g., the occurrence of multiple band structures, the coexistence of many differently deformed configurations, the high level density for medium and high spin values, and the corresponding complex feeding pattern do persist. Qualitatively these features are confirmed by recent experimental data. In addition an EXCITED VAMPIR description of the positive-parity states with even angular momenta in the nucleus 70 Se is reported. The results for this nucleus are besides a few peculiarities qualitatively rather similar to those of our earlier calculations for the nucleus 72 Se.

Spherical Hartree--Fock calculations with projection onto zero total linear momentum before the variation are performed for the nuclei 4He, 12C, 16O, 28Si, 32S and 40Ca using a density--independent effective nucleon--nucleon interaction. The results are compared to those of usual spherical Hartree--Fock calculations subtracting the kinetic energy of the center of mass motion either before or after the variation and to the results obtained analytically with oscillator occupations. Total energies, hole--energies, elastic charge form factors and charge densities and the mathematical Coulomb sum rules are discussed., Comment: 16 pages, 13 postscript figures

Nuclei at or near the N=Z line are of particular interest as micro-laboratory for fundamental effects. The simulation of many astrophysical objects requires the knowledge of properties and decay rates of nuclei near the drip lines. The structure and dynamics of proton-rich A70 nuclei and neutron-rich A100 nuclei are influenced by shape coexistence and mixing. A realistic description of shape coexistence phenomena requires beyond-mean-field approaches. Results concerning the self-consistent description of exotic phenomena including anomalies in mirror energy differences of proton-rich nuclei, triple shape coexistence in neutron-rich N=58 Sr and Zr isotopes as well as Gamow-Teller β-decay strength distributions, half-lives, and β- delayed neutron emission probabilities of neutron-rich nuclei in the A100 mass region obtained in the frame of the complex Excited Vampir model using realistic effective interactions in large model spaces are presented.

Excitation energies and electromagnetic properties of the low energy spectra of21Ne and25Mg have been calculated using the Multi-Configuration-Hartree-Fock (MCHF) model. BothT=0 andT=1 pairing correlations are found to be simultaneously important in those odd mass nuclei. Furthermore, though axial symmetry was requested, quite good agreement with the experimental data is reached.

This article is intended as a review of models describing high and very high spin states in nuclei. It further gives examples of the application of these models to data. (i) First, an outline of the cranked Hartree-Fock-Bogoliubov (CHFB) theory is given. It is applied to the first and the second anomaly of the moment of inertia normally called backbending (bb). (ii) The CHFB approach is extended to n quasi-particle (q.p.) states. The role of the odd protons for bb is investigated. (iii) The difficulties of the cranking model with the total angular momentum conserved only on the average are studied. (iv) An exact angular momentum conserving model in which valence nucleons in a vi 13 2 (or πh 11 2 ) shell are coupled to a core is introduced. It explains the oscillating behaviour of bb as a function of the neutron (or for second bb of the proton) number. (v) The decay of a highly excited and rapidly rotating compound nucleus after a heavy ion (HI) fusion reaction into particles and γ rays is described by the Monte Carlo method. Special attention is paid to the competition between collective and single particle γ ray decay. (vi) Different methods for calculating the deformation energy surface (DES) at high spins are discussed. The occurrence of yrast isomers is investigated. Furthermore, the nuclear stability against fission, particle emission and large scale shape changes is discussed.

Recent theoretical investigations revealed two different mechanisms for the backbending in 130Ce and 128Ba. While in 130Ce the well-known alignment of two nucleons at the beginning of the π h 11 2 shell is found, in 128Ba the effect is produced by the particles in the more than half-filled ν h 11 2 orbit and hence is preceded by the scattering of a nucleon pair in the nearby ν g 7 2 shell in order to increase the available alignment energy. The present paper is devoted to the possible experimental fingerprints of these different mechanisms. Using the wave functions already obtained, the total spin of each state is decomposed into the individual contributions from the different shells. Furthermore g-factors, static and dynamic quadrupole and also static hexadecapole moments are calculated. It turns out that the only decisive measurable quantities are the occupations of the relevant shells before and after the backbend, which are hardly accessible experimentally, at least at present.

Backbending due to the rotational alignment of two i13/2 neutrons oscillates as a function of the neutron number. This is explained in a particle number and angular momentum conserving model. The neutrons in the i13/2-shell are described by shell model wave functions and are weakly coupled to the rotation of the core. It is shown that the alignment is arranged by the nucleus by scattering two i13/2 neutrons mainly from the Ω = 1/2 and Ω = 3/2 i13/2 orbits into the core. The aligned band has therefore in average two neutrons less in the i13/2-shell. Interaction between the valence nucleon and the core is due to the pairing force. It is shown that this interaction has j -3/2 zeros. No interaction between the ground band and the aligned band means strong backbending while a strong interaction means no backbending. In the second part of this contribution we explain the behaviour of the second anomaly of the moment of inertia by the alignment of two h11/2 protons corresponding to the same oscillating behaviour as discussed before. In this way we can explain that for the N = 90 isotones 66156Dy shows no backbending, 68158Er shows upbending and 70160Yb shows a strong second anomaly of the moment of inertia.

The Hartree-Fock-Bogoliubov approach with spin and number projection before the variation extended to the description of nonyrast states (EXCITED VAMPIR) is used to describe simultaneously many properties of the 68,70,72,74Ge and 72,74Se nuclei. Low-lying spectra, neutron and proton occupation numbers for the spherical-basis orbitals, E0 and E2 transitions, and quadrupole moments are calculated and a fairly good agreement is obtained with experiment. The analysis of the EXCITED VAMPIR mean fields reveals that a strong mixing of prolate and oblate intrinsic quasiparticle determinants is responsible for the complex behaviour of the nuclei in A=70 region.

Hartree-Fock-Bogoliubov calculations with spin and number projection before the variation (VAMPIR) are performed for the nuclei 128 Ba and 130 Ce using a slightly renormalized Brueckner G -matrix as effective interaction in a rather large single-particle basis. The results are compared to those of Hartree-Fock-Bogoliubov calculations with projection after the variation, those of multiconfiguration calculations (MONSTER) and to experiment. In both nuclei the VAMPIR and the MONSTER approaches turn out to be of about the same quality and agree rather well with the experimental data. Analysis of the VAMPIR mean fields reveals that two somewhat different mechanisms are responsible for the backbending observed in the yrast bands of the two nuclei. While in 130 Ce the well-known alignment of two high- j quasiparticles (proton h 11 2 ) is found, in 128 Ba first a neutron pair is scattered from the h 11 2 to the g 7 2 orbit, and then the larger alignment energy of the less occupied neutron h 11 2 states produces the backbend. This latter effect is in agreement with the predictions of a simple model presented by us some years ago.

Shell-model and multi-configuration Hartree-Fock (MCHF) calculations in an Od 5 2 -1s 1 2 oscillator basis are compared for 20 Ne, 22 Ne and 24 Mg. The similarity of the calculated results in the two models shows that the MCHF method is a very good approximation to the shell-model calculations.

A microscopic study of the yrast bands of various doubly even nuclei in the rare earth region is presented. The calculations are based on a variational principle using angular momentum and particle number projected deformed BCS states as trial wave functions and the pairing plus quadrupole force as residual interaction. The influence of particle number projection and force parameters on the spectra and wave functions are studied as well as the interplay between Coriolis antipairing and deformation effects. Successes and shortcomings of the model are discussed.

A fully microscopic study of the proton radiative capture on 19 F with excitation of the multipole resonances in 20 Ne is presented. The calculation makes use of a version of the Feshbach formalism of nuclear reactions. The bound states are described as linear combinations of angular momentum projected one particle — one hole configurations in a deformed Hartree-Fock basis. The agreement for the 90° yields with the experimental data is excellent. Gross features as well as intermediate structures are nicely reproduced.

Multi-Configuration-Hartree-Fock (MCHF) calculations with angular momentum projection before the variation of the internal degrees of freedom have been performed for the nuclei20Ne and22Ne. This procedure yields different correlated intrinsic states for the different members of a rotational band. Thus the angular momentum dependence of correlations has been studied. Experimentally, the ground state spectra of20Ne and22Ne show properties similar to the phase transitions observed in some rare earth nuclei which have been well reproduced through the present calculations. The calculated spectra andBE2-values show a significant improvement compared to the ones obtained by variation before the angular momentum projection is effected.

Competition of strong and decoupling is studied systematically for odd-odd mass nuclei as a function of the position of the Fermi surfaceseF(π) andeF(υ) and the deformationsβ andγ. As an example we use80Br. For a prolate deformation the odd proton likes to decouple while the odd neutron couples strongly. This favours a ground state angular momentum of 5 to 6. The strong coupling is described by the admixture of ad5/2 nucleon. Only for the neutrons the admixture is so appreciably that it affects the spectrum and improves the agreement between theory and experiment for80Br.

The intrashell quartet states proposed earlier on the basis of intrinsic multiconfiguration Hartree-Fock calculations are reexamined for /sup 20/Ne and /sup 24/Mg in angular momentum projection formalism and supports in favor of the existence of such states are obtained. Quartet correlation in excited states of /sup 22/Ne has also been investigated. (AIP)

Opposite to the strong-coupling scheme, in which the back bending effect is discussed as an effect of the Coriolis force in the rotating body fixed frame, we have studied the effect in lab coordinates, in which a Coriolis force does not exist. For this purpose exact solutions of the ( i 13 2 ) N ν problem have been weakly coupled to a rigid rotor core spectrum. In addition to the usual quadrupole coupling also a pairing type interaction was included, which allows for the exchange of neutron pairs between the core and the valence system. It was found that this pairing field has drastic effects on the results and should hence not be neglected.

Parameter-free, microscopic predictions are presented for the B (M1)↑ strength distributions in some deformed even-even nuclei of the mass A = 130 region. An extensive fragmentation of the M1 strength, being predominantly of isovector character, is obtained. The major strength is typically split into two regions around 4.0 and 5.0 MeV with the largest transitions having B ( M 1)↑ values of about 1.5–3.0 μ 0 2 . The total strenghts vary from 6.0 to 9.0 μ 0 2 , a value obtained usually for a single collective 1 + state of phenomenological models. The predicted B ( M 1)↓ branching from the 1 + states is sensitively state dependent, the branching to other than the 0 1 + and 2 1 + states being anywhere between 10% and 200% of the joined intensity to these two lowest levels.

We present a systematic study of even-even nuclei in the mass A = 130 region using a microscopic nuclear structure model, in which the nuclear wave functions are approximated by linear combinations of number- and spin-projected zero-quasiparticle and two-quasipartile determinants obtained from a self-consistent Hartree-Fock-Bogoliubov mean field. With a fixed hamiltonian, being a slightly renormalized Brueckner G-matrix based on the Bonn potential the model is able to reproduce the main trends of empirical energy and electromagnetic properties of the transitional, soft nuclei in the region. The empirical nature of the crossing ( πh 11 2 ) 2 and ( vh 11 2 ) 2 superbands in Ce and Ba isotopes, respectively, is confirmed. The calculations suggest a particular interpretation of the negative-parity bands observed for 128Ba and 124Xe. Furthermore a rich variety of empirically unknown excited 2 q.p. bands with positive parity is predicted to exist throughout the region. We suggest an empirical method to deduce the low-end termination of such bands based on comparisons of microscopic electric quadrupole moments to those of the rotation model. The model fails to account for the low-lying quasi-gamma bands in Ce and Ba isotopes but indicates a strong contribution of proton 2 q.p. excitations for this band in Xe isotopes.

The Generator-Coordinate-Method is applied to describe yrast-bands and excited vibrational bands of pairing-vibrational orβ-vibrational type in deformed rare earth nuclei. The model wave functions are supplied by the Pairing+Quadrupole-Model with angular momentum — and particle number projection. Results are shown for the nucleus170Yb. The question if backbending may be produced by the intersection of the ground and the neutron pairing vibrational band is discussed.

In addition to the quartet excitations between major oscillator shells (intershell quartet states), we propose quartet excitations within a shell (intrashell quartet states). A phenomenological quartet shell model is devised to predict the energies of these states. Evidence for such excitations is also found in a microscopic multiconfiguration Hartree-Fock calculation.

The multi-configuration Hartree-Fock (MCHF) theory, a variational method for the self-consistent solution of the nuclear many-body problem with the inclusion of correlations, is applied to 12 C. The force is the Yale potential in the form of the Yale-Shakin matrix elements. 40 different configurations are included. The gain in the total binding energy compared to HF in the same single-particle basis and with the same force is 1.20 MeV. The MCHF theory yields the first excited 0 + state and the first excited 2 + state in good agreement with the experimental data.

Band mixing calculations are performed for the nucleus 166 Yb using angular momentum and particle number projected Hartree-Fock-Bogoliubov wave functions. The calculated spectrum for the yrast band is compared with that obtained from a non-selfconsistent Nilsson + BCS prescription.