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[en] The generalized harmonic oscillator brackets form the transformation matrix for the transition from the single-particle coordinates to the centre of mass and relative coordinates in a two-particle harmonic oscillator basis. Unequal masses of particles are considered. A recently published closed form for the oscillator bracket (Dobes, 1977) is used. Two subprograms are presented, one for the evaluation of a single bracket, the other for simultaneous calculation of a set of brackets with fixed angular momenta. (Auth.)

[en] The Hartree method with angular momentum projection performed before variation is used to investigate nuclear shapes in the interacting boson approximation. It is shown that triaxial shapes are obtained in this procedure. (orig.)

[en] Aspects of the F-spin mixing mechanism in the proton-neutron interacting boson model (IBM-2) are studied. Various F-spin breaking terms in the IBM-2 hamiltonian are considered and their effect on M1 transitions is investigated. Similar results, which are in agreement with the available experimental M1 data, are obtained with two different F-spin breaking terms. Microscopic predictions of these terms are discussed. (orig.)

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[en] The development of radioactive beam facilities makes it possible to access N=Z nuclei up to ¹⁰⁰Sn. In such nuclei, neutron-proton (np) pairing is important. The features of np pairing are discussed. In particular, the two-nucleon transfer strengths may be sensitive indicators for this effect. The standard theoretical pairing approximations, however, fail in the regime of np pairing. Analysis suggests that the interaction between pairs is an important contribution implying four-nucleon correlations. The ground state is presented as a condensate of alpha-like four-nucleon clusters. The possible experimental consequences of the four-body clustering are highlighted

[en] Starting with the mapping of the quadrupole collective states in the fermion space onto the boson space, the fermion nuclear problem is transformed into the boson one. The boson images of the bifermion operators and of the fermion Hamiltonian are found. Recurrence relations are used to obtain approximately the norm matrix which appears in the boson-fermion mapping. The resulting boson Hamiltonian contains terms which go beyond the ordinary SU(6) symmetry Hamiltonian of the interacting boson model. Calculations, however, suggest that on the phenomenological level the differences between the mapped Hamiltonian and the SU(6) Hamiltonian are not too important. 18 refs.; 2 figs

[en] Geyer's similarity transformation between the Dyson boson operators and the seniority boson operators is generalized to the case of several non-degenerate-j shells. With restriction to the lowest collective s- and d-bosons, simple expressions for the parameters of the boson quadrupole operator are obtained. Calculated parameters are compared with the microscopic predictions from the extended Otsuka-Arima-Iachello method, with results of simple formulas by Scholten and Dieperink, and also with the phenomenological interacting boson model parameters. (orig.)