T. Døssing

The decay out of superdeformed rotational bands

Abstract A phenomenological analysis is made of the decay of superdeformed states, using a statistical model of the coupling between superdeformed and normal states. We extract a phenomenological transmission coefficient of the barrier between superdeformed and normal states for the nuclei around 152Dy. The fluctuations in the depopulation probability as a function of angular momentum are found to be consistent with the statistical assumptions.

Excitation energies and spins of the yrast superdeformed band inHg191

The excitation energies and spins of the levels in the yrast superdeformed band of {sup 191}Hg have been determined from two single-step {gamma} transitions and the quasicontinuum spectrum connecting the superdeformed and normal-deformed states. The results are compared with those from theoretical mean-field calculations with different interactions. A discussion of pairing in superdeformed states is also included.

Probing the tilting mode in nuclear reactions

Abstract An expression is derived for the cross section near the beam in direct nuclear reactions. It contains information on the degree of excitation of the tilting mode, and it reduces to the standard result when applied to fission. An instructive application is made to the damped reaction 710 MeV 86 Kr + 139 La . In quasi-fission reactions the relaxation of the tilting mode is expected to be incomplete.

Covariance analysis of selection rules governing the γ−decay cascades of the rotational nucleus 164Yb

The γ-cascades feeding specific intrinsic configurations of 164Yb are studied making use of a statistical analysis of counts fluctuations in γ–γ coincident spectra. In particular, variance and covariance spectra are obtained starting from the measured (0,+), (0,−) and (1,−) gate-selected matrices. The analysis of the first ridge of the spectra yields about 25 excited bands carrying rotational energy correlations for the total spectrum, and about 10 in coincidence with each of the (α,π) configurations. The covariance of count fluctuations between the different (α,π) gates is expressed by an experimental correlation coefficient, r, which measures the degree of sharing of decay paths through the excited bands, and thereby the extent to which selection rules are obeyed by the decay. The correlation coefficient is around r≈0.3 for the ridge, and also in general rather small for valley fluctuations. Only at the highest valley energies, a tendency to larger correlation coefficients is seen, indicating some weakening of the selection rules over longer decay cascades. The results of the experimental analysis are compared to rotational damping model predictions based on cranked shell model plus residual interactions, and rather good agreement is found.

A complex integration technique to remove spurious states associated with spontaneously broken symmetries in RPA calculations

Abstract The removal of the spurious states associated with the violation of rotational and gauge invariance by mean field approximation from the RPA solutions describing the low-energy spectrum of nuclei is a notoriously difficult task, in particular in a rotating frame. A novel, numerically accurate yet technically simple method to accomplish this aim has been developed based on linear response theory. The results of calculations carried out for the deformed, superfluid, rotating nucleus 168 Yb testify to the power of the technique, the associated solutions respecting the variety of sum rules required to be fulfilled by the RPA solutions because of conservation laws.

Theoretical calculations of γ-ray feeding patterns for Gd, Dy and Er nuclei

Abstract The side-feeding patterns and the spectra of unresolved γ-lines are calculated for Gd, Dy and Er nuclei with neutron numbers from 82 to 86. Energies and configurations for states from the yrast line and approximately 2 MeV higher in energy are calculated as many-quasiparticle configurations in a deformed oblate potential. The γ-cascade is simulated by a Monte Carlo calculation. With a common average entry angular momentum of 35 h to the non-collective oblate states considered, the observed variation of intensity of the strongest γ-line with spin is well reproduced. The stretched dipole part of the continuum spectra is also well reproduced, whereas the stretched quadrupole part cannot be accounted for. These findings are in accordance with a two-stage picture of the γ-decay. First comes a cascade of stretched quadrupole transitions, probably of collective nature. This cascade feeds into numerous weakly oblate states around spin ≈ 35 h , and the decay proceeds first by weak transitions, mainly dipoles, and later by strong γ-lines.

Coupling between thermal and rotational motion in nuclei

Two aspects of rotational motion of thermally excited nuclei are discussed. First, the schematic cases of ordered or chaotic motion of intrinsic and rotational motion are presented: discrete rotational bands, ergodic bands, damping of the rotational motion. Secondly, the strength functions of mixing of basis bands and of EP-transitions over one or two decay steps are discussed. The two-step E2-strength displays both a narrow and wide component, yielding different types of information about the basis bands and their interaction. Indications of the presence of a narrow component is found in energy correlations of consecutive transitions on displaced rotational bands of a three dimensional spectrum for the nucleus “‘Yb.

Experimental study of the nuclear rotational motion with statistical analysis methods

Abstract The γ-decay of excited rotating nuclei in the high level density region a few MeV above the yrast line has been studied through a statistical analysis of the fluctuation of counts in γ-γ coincident spectra. In particular, making use of the covariance technique between spectra gated by different intrinsic configurations, one can measure how the cascades feeding into different selected bands are similar. The aim is to learn about the transition between order to chaos in the nuclear many-body system, in terms of the validity of selection rules associated with the quantum numbers of the intrinsic structure. Experimental results on the nucleus 164Yb are presented and compared to model predictions based on cranked shell model calculations including a two-body residual interaction.

The rotational quadrupole moment of continuum-states at high-spin in 164Yb

Abstract The lifetime of the unresolved transitions forming the rotational quasi-continuum spectra has been measured in 164 Yb using the Doppler shift attenuation method. The valley structure in E γ 1 × E γ 2 spectra was analysed using a new technique, based on fluctuation patterns, which provided for the first time experimental lifetimes for rotational damped transitions. The present results show, as predicted by the rotational damping model, that the collectivity persists for highly excited bands. In particular, agreement is found between the experimental results, for discrete and damped transitions, and the predictions corresponding to Q t = 5.5 eb, for the entire interval of γ-ray energies studied.

Decay-Out Of Superdeformed Bands By Coupling To Ordered Or Chaotic Spectra

Different treatments of the decay-out of superdeformed bands are reviewed, focusing on the role of the normally deformed doorway states to which the superdeformed band primarily couples at decay-out. A schematic description based on sparse GOE matrices is formulated for the normally deformed states including one doorway state. This allows for a gradual change of the normally deformed spectrum from an ordered to a chaotic situation by means of the effective dimensionality parameter. It is found that the distribution of the small normally deformed admixtures into the superdeformed band, through which it decays, depends sensitively on the effective dimensionality parameter. Analyzing the measured distribution of transition strength of decay-out gamma-ray lines in 194Hg, it is concluded that the spectrum of normally deformed states is closer to the chaotic than the ordered situation at the energy and angular momentum of the superdeformed band. ©2005 American Institute of Physics