P. von Brentano

Singular character of critical points in nuclei

The concept of critical points in nuclear phase transitional regions is discussed from the standpoints of Q-invariants, simple observables and wave function entropy. It is shown that these critical points very closely coincide with the turning points of the discussed quantities, establishing the singular character of these points in nuclear phase transition regions between vibrational and rotational nuclei, with a finite number of particles.

New evidence for the E1 core polarization in spherical nuclei

Abstract The confrontation of the systematized transition rates B(E1,1−1→0+1) and B(E1,1−1→2+1) in even–even nuclei around closed shells, measured in systematic photon scattering experiments, reveals some salient features. These attributes emerge as the first evidence based on the systematic behaviour of experimental data for the dipole core polarization and are supported by earlier QPM calculations.

On the line-shape and lifetime determination in recoil distance Doppler-shift measurements

Abstract A method for the calculation of the γ-ray line-shapes observed in recoil distance Doppler-shift measurements of nuclear lifetimes is presented emphasising the case where a gate is set on a transition which feeds directly the level of interest. A description of such coincidence spectra is proposed for the first time. It is shown that a successful reproduction of the data requires to take into account the emission of γ-rays during the slowing-down of the ions in the stopper if the investigated lifetime is shorter than or comparable to the slowing-down time. The corresponding formalism for lifetime determination in the framework of the differential decay-curve method is developed. The new approach is illustrated by an application to experimental data obtained in the 110 Pd ( 28 Si , 4 n ) 134 Nd reaction at a beam energy of 125 MeV.

Two-phonon J = 1 states in even-mass Te isotopes with A = 122–130☆

Abstract Excited states of the nuclei 122,126,130 Te were populated via the (γ, γ′) reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL CLUSTER detector and a single HPGe detector. In all investigated nuclei two or three prominent dipole transitions were identified at E γ ≈ 3 MeV. The corresponding low-lying J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict at about 3 MeV one 1 − state arising from the coupling of the first quadrupole and the first octupole phonon, and one 1 + state arising from the coupling of the first and the second quadrupole phonon, where the latter has isovector character. Such an excitation mode can be considered as an analogue of the scissors mode in vibrational nuclei. The calculated transition strengths are compatible with experimental ones within a factor of about 1.5.

Quadrupole shape invariants in the interacting boson model

In terms of the interacting boson model (IBM), shape invariants for the ground state, formed by quadrupole moments up to sixth order, are studied in the dynamical symmetry limits and over the whole structural range of the IBM-1. The results are related to the effective deformation parameters and their fluctuations in the geometrical model. New signatures that can distinguish vibrator and {gamma}-soft rotor structures, and one that is related to shape coexistence, are identified. (c) 2000 The American Physical Society.

Quasideuteron states with deformed core

Abstract The M1 transitions between low-lying T =1 and T =0 states in deformed odd–odd N = Z nuclei are analyzed in the frames of the rotor-plus-particle model. Using the representation of an explicit coupling of angular momenta we show that strong coupling of the quasideuteron configurations to the axially deformed core results in a distribution of the total 0 + →1 + strength among a few low-lying 1 + states. Simple analytical formulae for B(M1) values are derived. The realization of the M1 sum rule for the low-lying 1 + , T =0 states is indicated. The calculated B(M1) values are found to be in good agreement with experimental data and reveal specific features of collectivity in odd–odd N = Z nuclei.

Spreading Width for Decay Out of a Superdeformed Band

The attenuation factor F responsible for the decay out of a superdeformed (SD) band is calculated with the help of a statistical model. This factor is given by 1/F = (1 + Gamma(down) / Gamma(S)). Here, Gamma(S) is the width for the collective E2 transition within the superdeformed band, and Gamma(down) is the spreading width which describes the mixing between a state in the SD band and the normally deformed (ND) states of equal spin. The attenuation factor F is independent of the statistical E1 decay widths Gamma(N) of the ND states provided that the Gamma(N) are much larger than both Gamma(down) and Gamma(S). This condition is generically met. Previously measured values of F are used to determine Gamma(down).

Crossing and anticrossing of energies and widths for unbound levels

Abstract The crossing and anticrossing properties of the energies and widths of two unbound levels under the influence of a symmetrical complex interaction are investigated. It is found that a sufficiently large variation of the difference of the “unperturbed” energies or of the widths leads always to a crossing of either the energies or the widths of the “perturbed” system. A particularly interesting result is that for a real off diagonal interaction there is a joint crossing of the “unperturbed” energies and of either the “perturbed” energies or the “perturbed” widths.

Test of the critical point symmetry X(5) in the mass A = 180 region

Based on the energy spectra and relative transition probabilities 176,178,180 Os turned out to be very promising candidates for testing the features of the critical point symmetry X(5). In order to also test absolute transition probabilities predicted in the framework of the X(5) symmetry lifetime measurements were performed at the Cologne FN tandem accelerator. The lifetimes of the first 2 + states in 176,178,180 Os were measured using electronic timing and in the case of 178 Os with the recoil distance Doppler shift (RDDS) technique using an 16 O-induced fusion evaporation reaction. In addition, a coincidence RDDS measurement was performed at the Laboratori Nazionali di Legnaro with the GASP spectrometer using the 154 Sm( 29 Si,5n) 178 Os reaction. The deduced transition quadrupole moments agree well with the X(5) predictions, thus further supporting an X(5) structure for 178 Os. The experimental data for 178 Os are compared to calculations in the framework of the interaction boson model (IBM) and of the general collective model (GCM).

Alternative interpretation of sharply rising E0 strengths in transitional regions.

It is shown that strong 0(+)(2)-->0(+)(1) E0 transitions provide a clear signature of phase transitional behavior in finite nuclei. Calculations using the interacting-boson approximation (IBA) show that these transition strengths exhibit a dramatic and robust increase in spherical-deformed shape transition regions, that this rise matches well the existing data, that the predictions of these E0 transitions remain large in deformed nuclei, that they arise from the specific d-boson coherence in the wave functions, and do not necessarily require the explicit mixing of normal and intruder configurations from different IBA spaces.