: The excitation energies of the single-particle normal and intruder levels in both 183 Tl and 187 Bi were measured for the first time via the α decay of 187 Bi produced in the 97 Mo( 92 Mo,pn) 187 Bi reaction. The previously unobserved 187 Bi ground state (h 9/2 ) to 183 Tl ground state (s 1/2 ) α transition was identified, establishing the 187 Bi intruder state excitation energy to be 112(21) keV, 70 keV less than that of the same level in 189 Bi.

We study a fragmentation model in order to describe the distribution of a collective, scissors-like 1+ state into a background of deformed two- and four-quasiparticle excitations. The coupling matrix elements for coupling the collective state to the underlying microscopic structures are studied. The fragmentation results are compared to the available data. One-nucleon transfer strength distributions are discussed with reference to the fragmentation of two-quasiparticle configurations. We also point out possible origins for intermediate structure in the fragmentation mechanism.

A version of the interacting boson model (IBM) is introduced that includes particle-like and hole-like bosons and allows the description of excitations across closed shells. The formal, algebraic aspects of the model are worked out in detail. Reduction of the dynamical algebra Up(6) ⊗ Uh(6) leads to the definition of intruder or I spin which labels the character of the bosons (particle-like or hole-like). The I-spin properties of the Hamiltonian and electromagnetic transition operators are discussed. Embedding of Up(6) ⊗ Uh(6) into a larger dynamical algebra gives rise to multiplets that connect either states in different nuclei with the same I spin [U(12)], or states that differ by two particles and two holes [U(6,6)], or states that differ by four particles or four holes [Sp(12)].

Particle-hole excitations near closed shells are incorporated in an extended interacting boson model (EIBM) thereby enlarging the region of applicability of this algebraic approach. We study the consequences of classifying various many-particle-hole (mp-nh) excitations into multiplets, according to dynamical symmetries of the EIBM. Applications to the Z = 50 region are presented for the RuCdTeBa nuclei. Here, we show (i) that the “global” smooth change in the intruder multiplet band structure is described by an O(6) → SU(3) symmetry transition and (ii) how the “local” mixing between the intruder structure and the regular anharmonic vibrational states (U(5)) radically changes in the above O(6) → SU(3) symmetry transition.

The even-even Cd nuclei near neutron number N = 66 are indicative of the presence of two rather distinct families of excitations: anharmonic quadrupole vibrations and more deformed intruder particle-hole excitations. We discuss these excitations as mainly coexisting families of states forming the global structure of these nuclei. Then, we investigate how local large perturbations can cause strong mixing between the two families. The coupling is studied in detail and a particular set of selection rules governing the mixing leads to the introduction of a new basis in order to discuss the interaction between vibrational and intruder excitations. Numerical applications are carried out for 112,114Cd.

We discuss the experimental evidence for many-particle many-hole excitations (mp-nh) at and near to closed shells. Besides a shell-model approach and in order to understand the salient features of the general behaviour of the 0+ intruder band heads, we mainly concentrate on a group theoretical classification. Using the boson model, various possibilities to classify intruder mp-nh excitations can be shown to exist. Ample evidence for the realization of these new symmetries is given in (i) the Z = 50 mass region and, in (ii) the Z = 82 Pb mass region. Finally, a possible extension to encompass also mp-nh excitations in light nuclei is presented.

We use linear energy-weighted sum rules within the proton-neutron interacting boson model to deduce a relationship between magnetic dipole and magnetic octupole transition probabilities. We then obtain a first estimate of the summed magnetic octupole strength to be expected in rare-earth nuclei.

We discuss the systematics of E2 and M1 transition strengths as obtained from experimental data in the rare-earth region. The similarity is studied in the light of the observed dependence of E2 and M1 transition strength on quadrupole deformation. Especially the "saturation" effect for these electromagnetic observables is discussed, starting from the Nilsson deformed shell-model. The data are compared to the present calculations, using a QTDA approach. The particular E2-M1 correlation is investigated within the IBM-2 using a sum-rule approach.

We point out that the centrifugal term originating from the rotational energy concentrates magnetic dipole 1+ strength in a dramatic way into a scissor-like state near Ex∼−3MeV. The importance of this term and its neglect in earlier studies is discussed. Applications to the realistic situation of 164Dy are carried out where residual quadrupole and spin interactions are not able to redistribute the strong concentration of 1+ strength in an appreciable way.

We present the results of nuclear-resonance fluorescence measurements at the S-DALINAC for a chain of evenvl isotopes 148–154Sm, covering the excitation-energy region of 2–4 MeV. The orbital M1 strength distributions are compared with recent microscopic calculations. The summed E1 strengths are discussed in terms of macroscopic clustering and octupole deformation prescriptions of Iachello.

We point out that a recently derived M1 sum rule by Ginocchio expressing a relation with the ground-state d-boson number expectation value holds under more general conditions that for good F-spin states. We also point out a new relation connecting the variation of the summed M1 strength to the nuclear isotopic shifts.

The M1 strength distribution is studied in rare‐earth nuclei, starting from the Nilsson model and using the Quasi‐Particle Random Phase Approximation (QRPA), including monopole as well as quadrupole pairing, Coriolis mixing and residual quadrupole and spin‐isospin interactions. For the nucleus 164Dy the importance and effect of the different terms in the Hamiltonian are studied. Results are compared with experimental observations. Special attention is given to the higher‐lying spin strength in view of a recent (p,p’) experiment performed at TRIUMF on 154Sm.

We show that, starting from density‐dependent nucleon‐nucleon interactions (extended Skyrme forces), one is able to construct an interaction that leads to correct saturation properties in the nucleus and also behaves as a good effective interaction in describing low‐lying excitations. This is illustrated in various mass regions. In those regions where many protons and neutrons determine the nuclear structure, one has to resort on collective model descriptions. We shortly point out the importance of symmetry considerations (the Interacting‐Boson model). More in particular, we concentrate on those modes of motion where the interplay of protons and neutrons is dominant: the M1 scissor mode, intruder 0+ excitations and shape coexistence. Results for the Cd region, the Pb region and the N=20 nuclei are presented.

We study low-lying 1+ excitations in several rare-earth nuclei using the quasiparticle random-phase approximation (QRPA). Besides separable quadrupole and spin forces, we include quadrupole pairing and Coriolis mixing and illustrate their effect on the results (Ex(1+), B(M1; 0+ → 1+)). The validity of the proposed macroscopic scissors picture is evaluated. A comparison of the results obtained in the present calculations with the complete set of available data is given.

A systematic study of magnetic dipole strength in the even-even Gd-148-160 nuclei and over the much larger region of heavy rare-earth nuclei is carried out. Within a quasiparticle description for the Nilsson model, all possible two-quasiparticle K-pi = 1+ states are constructed and thus, calculating the corresponding B(M1) reduced transition probabilities, information on the total summed magnetic dipole strength and on its fragmentation is obtained. We get as our main results that most of the M1 excitation strength comes from two-quasiparticle (2qp) excitations, with states up to E(x) less-than-or-equal-to 4 MeV being mainly orbital in nature. The higher energy region 4 MeV < E(x) less-than-or-equal-to 9 MeV is characterized mainly by spin-flip M1 strength. The results are compared with existing experimental (e, e'), (gamma, gamma') and (p, p') data and one nucleon transfer results for Dy-164. Some general features are discussed.

The hyperfine structure and isotope shifts of short-lived gold isotopes with 185 ⩽ A ⩽ 190 and the 112− isomer of 189Au have been investigated by application of on-line resonance ionization mass spectroscopy. A detection efficiency of ε = 10−8 for gold atoms was observed at a background of about one event per 1000 laser shots. The deduced charge radii show a drastic change between A = 187 and A = 186 which is interpreted as an onset of strong deformation (β2 ≈ 0.25) in 186Au and 185Au due to the influence of the π 1h92 intruder orbital.

Starting from the U(5) and SU(3) dynamic symmetries, perturbations on an F-spin scalar IBM-2 Hamiltonian are studied. The parameters describing the perturbations are chosen such as to introduce only the F-spin vector and the F-spin tensor terms of rank two. Effects on the F-spin structure of the wave functions, excitation energies and electromagnetic transition probabilities are studied for the lowest symmetric and mixed-symmetry states. If possible, numerical results are compared to analytic expressions derived via the first-order perturbation calculations.

We point out that the description of intruder states, incorporating particle-hole (ph) excitation across a closed shell in the spherical shell model or a description starting from the Nilsson model are equivalent. We furthermore indicate that the major part of the nucleon-nucleon interaction, responsible for the low excitation energy of intruder states comes as a two-body proton-neutron quadrupole interaction in the spherical shell model. In the deformed shell model, quadrupole binding energy is gained mainly through the one-body part of the potential.