A calculation of low-lying collective states in even-even nuclei

We present results of a calculation of the low-lying collective quadrupole states in even-even nuclei within the framework of the proton-neutron interacting boson model.     

Rotational properties of shell-model states infp-shell nuclei

The relation between the microscopic shell model and the collective rotor model is investigated. As the first step an extensive shell-model calculation is performed on about twentyfp-shell nuclei with massA=52–60. It turns out that, with the model space and the effective interaction chosen, the shell model is well able to reproduce the experimental data in this mass region. As the second step the shell-model wavefunctions are used to calculate energies, electromagnetic moments and transition rates of states with spin up toJ=16. As the third step the observables calculated with the shell model are used to investigate whether these microscopic results can be reproduced by a simple rotational model. About twenty pure axially symmetric rotor bands, generated by the shell model, could be localized. Their properties are presented and discussed.     

A shell-model treatment of (0+1) ħω states in A=4–16 nuclei (II)

The results of spurious-state free calculations in a complete (0+1)?ω model space on theA=4–16 nuclei are presented. The calculated spectra as well as electromagnetic properties of the normal- and nonnormal-parity states are compared with the experimental data. In general good agreement is obtained, while in particular the magnetic-dipole and electric-quadrupole moments are very well reproduced. The present results for the normal-parity states are similar to those of Cohen and Kurath. Several nonnormalparity states expected at rather low excitation energies and properties of very neutronrich nuclei have not been observed experimentally so far.     

Excited states of deformable even-even atomic nuclei

A method of calculating the collective excited states of even-even atomic nuclei is proposed. A formula depending on three parameters appropriate for calculating the energy ratios of the excited states of deformed and spherical nuclei as well as those in the intermediate region is obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 19–24, November, 1972.     

Excited collective states of heavy even-even nuclei

Quadrupole-type collective excitations of even-even nuclei are analyzed. In this analysis, transverse γ vibrations of the nuclear surface are taken into account effectively, while longitudinal beta vibrations remain free. A potential energy of the exponential form is used for free surface longitudinal beta vibrations. The behavior of the energy levels of excited states in the ground-state, β, and γ bands of heavy nuclei is studied, and the predictive potential of the model used is demonstrated for transfermium nuclei.     

Collective description of magnetic properties of even-even nuclei

A general treatment of magnetic properties of even-even nuclei in the framework of the collective model is proposed in a perturbation-theoretic approximation.     

A method of angular momentum projection for excited states in even-even nuclei

A method of angular momentum projection for the excited states in even-even nuclei is formulated within the framework of the Hartree-Bogoliubov theory. A preliminary application of this method is performed for the K=0^? band in ¹⁶²Er.     

Theory of the excited states of deformable even-even nuclei

The excited states of nonspherical even-even nuclei are discussed taking the change in nuclear shape during excitation into account.     

A translationally invariant shell-model treatment of (0+1) ħω states in a=4–16 nuclei (I)

Shell-model calculations on positive- and negative-parity states have been performed for thep-shell nuclei. The model space includes all 0?ω and 1?ω harmonic-oscillator basis states without the use of a core. The matrix elements of the mass-independent two-body interaction and the harmonic-oscillator size parameter are determined empirically from a fit to experimental energy levels. The interaction is required to satisfy translational invariance. The single-particle energies are compatible with this requirement. The spurious states are isolated exactly from the nonspurious states.     

Transition probabilities and static moments of excited states in even-even nuclei

A new approach is made to calculate the generalized single-particle density matrixp ^μv in even-even nuclei. It is shown that this quantity is included in the three particle response function, for which we derived a renormalized equation. Taking into account in a consequent way the effective particle-hole interaction we received a formula for the static moment of excited states and the transition probability between such states which is essentially different from the usual RPA theory.     

On the description of collective negative parity bands in even-even deformed nuclei

The collective bands of negative parity are studied via a microscopic approach where Coriolis forces and octupole correlations are treated on an equal footing. The apparent moment of inertia of these bands are nicely reproduced.     

10+ isomers observed in even-even 78-neutron nuclei

Isomeric states with spinI ^π=10⁺ were found in the nuclei¹³⁸Nd and¹⁴⁰Sm. These states are interpreted as having a [vh _11/2]^?2 configuration. The systematical behaviour of these isomers in the isotones¹³⁶Ce,¹³⁸Nd and¹⁴⁰Sm is discussed.     

Subthreshold photofission of even-even nuclei

Within quantum-mechanical fission theory, the angular distributions of fragments originating from the subthreshold photofission of the even-even nuclei ²³²Th, ²³⁴U, ²³⁶U, ²³⁸U, ²³⁸Pu, ²⁴⁰Pu, and ²⁴²Pu are analyzed for photon energies below 7 MeV. Special features of various fission channels are assessed under the assumption that the fission barrier has a two-humped shape. It is shown that the maximum value of the relative orbital angular momentum L _m of fission fragments can be found upon taking into account deviations from the predictions of A. Bohr’s formula for the angular distributions of fission fragments. The result is L _m ≈ 30. The existence of an “isomeric shelf” for the angular distributions of fragments from ²³⁶U and ²³⁸U photofission in the low-energy region is confirmed.     

High-spin states in89Nb,88Zr and88Nb with a shell-model description of theN=48 andN=47 nuclei

The reactions⁸⁹Y(α,4nγ),⁸⁹Y(α,p4nγ) and⁸⁹Y(α,5nγ) were used to populate high-spin states in, respectively,⁸⁹Nb,⁸⁸Zr and the previously unstudied nucleus⁸⁸Nb. These states were deduced via in-beam gamma-ray spectroscopy. The results of a shell-model study ofN=48⁸⁸Zr,⁹⁰Mo,⁸⁷Y,⁸⁹Nb andN=47⁸⁷Zr,⁸⁸Nb nuclei are compared to experiment.     

Microscopic description of mixed-symmetry states in nearly spherical nuclei

We adopt the quasiparticle-phonon model to investigate the phonon content and the proton-neutron symmetry of low-lying states recently discovered in nuclei around shell closure. The results are in overall agreement with experiments and consistent with the interacting boson model.