Probing the Order-to-Chaos Region in SuperdeformedTb151andPb196Nuclei with Continuum

S. Leoni

¹

,

N. Blasi

²

,

A. Bracco

³

et al.

Abstract

The γ−decay associated with the warm rotation of the superdeformed (SD) nuclei 151 Tb and 196 Pb has been measured with the EUROBALL IV array. Several independent quantities provide a stringent test of the population and decay dynamics in the SD well. A Monte Carlo simulation of the γ-decay based on microscopic calculations gives remarkable agreement with the data only assuming a large enhancement of the B(E1) strength at low excitation energy, which may be related to the evidence for octupole vibrations in both mass regions.PACS numbers: 21.10. Re;23.20.Lv;25.70.Gh;27.70.+q;27.80.+w The thermally excited, rapidly rotating atomic nucleus is an ideal laboratory for studying structure properties beyond mean field. In particular, the γ-decay which cools the hot compound nucleus at high angular momentum is a powerful tool for investigating the transition from the chaotic regime around the particle binding energy to the cold, ordered system close to the yrast line [1,2,3]. This topic, mostly investigated in normally deformed (ND) nuclei [1,2,4,5,6,7], becomes particularly interesting and challenging, both experimentally and theoretically, in the case of very elongated systems. The study of superdeformed (SD) nuclei requires, in fact, high selectivity and high statistics experiments, in order to focus on the small fraction of the γ-decay (≈ few %) which finally gets trapped into the SD well and populates discrete rotational bands. At present, there is only partial experimental information available for few SD nuclei [8,9,10,11,12], whose interpretation is based on models which are either rather schematic [9,11,12] or depend on several parameters [8,10].The aim of this letter is to make a substantial step forward in the understanding of the population and decay of warm SD nuclei in the mass regions A=150 and A=190, where superdeformation is a well established phenomenon, by a comparative and comprehensive study of the nuclei 151 Tb and 196 Pb. For the first time, several independent observables are extracted from quasicontinuum γ-coincidence spectra. This provides strong experimental constraints on the dynamics of the γ-decay flow and of the tunneling through the potential barriers between SD and ND excited states, over a wide spin range. A key point of this work is the development of a new Monte Carlo simulation of the γ-cascades in the SD well. This makes use of discrete levels calculated microscopically with the cranked shell model. In addition, the coupling between excited SD and ND configurations is included, based on penetration probabilities obtained from a microscopical model [13]. It is anticipated that such a detailed study of the order-to-chaos region reveals the influence of nuclear structure effects. In particular, an enhancement in the E1 strength around 1-2 MeV, which could be related to octupole vibrations, is found to play an important role.The experiments were performed at the Vivitron in Strasbourg (F) with the EUROBALL IV array [14]. The reaction used were 130 Te( 27 Al,6n) 151 Tb (at 155 MeV) ...

Global properties ofKhindrance probed by theγdecay of the warm rotating174W nucleus

“…The experimental results, in terms of number of discrete excited bands of low-K and high-K nature, are interpreted via a simulation of the collective γ decay, based on microscopic cranked shell model (CSM) calculations at finite temperature [25,26]. This is a powerful tool to quantitatively describe nuclear structure properties of warm rotating nuclei, probing key aspects of their γ decay, as previously done in different regions of mass and deformation [20,[27][28][29][30][31]. It is anticipated that agreement between data and theory is obtained only by strongly quenching the decay strength of E1 statistical transitions between low-K and high-K discrete excited bands, resulting in reduced hindrance factors which follow the same trend, with excitation energy, as observed in spectroscopic studies of K isomers in the A ≈ 160-180 mass region [9,16].…”

Rotation of warm nuclei and superdeformation

“…However, it is found that the rotational damping width is smaller than the average distance d 2 between states, which interact via the residual two-body interaction. The ridges in Hg while they have a rather complex structure [79]. The number of bands however, is found to be smaller than in Hg 194 , as well as the fraction of the E2 strength found in the ridges, but the spin domain and excitation energy probed is quite different due to competition with fission and gating conditions and so, one cannot exclude that precursor-to-ergodic bands also exist in Pb 196 .…”

“…In particular, extensive work was done in connection with low-K/high-K structures, in order to investigate the transition between order and chaos in the atomic nucleus [51,67,[74][75][76]. In more recent yearstaking advantage of high statistics experiments with large arrays-statistical techniques have also become a powerful tool for a quantitative investigation of superdeformed configurations [46,[77][78][79][80][81][82][83] (the topic of the next section) and to The experimental average number of paths obtained from the fluctuation analysis of ridge structures in double and triple coincidence histograms (see figure 7), as a function of path length. The lines show predictions from the cranked shell model including a two-body residual interaction with different values of the interaction strength V 0 , as given in the legend.…”

The Legnaro National Laboratories and the SPES facility: nuclear structure and reactions today and tomorrow

“…Making use of the accelerated heavy ions provided by the Legnaro linac and of modern γ-detector arrays designed to select high-multiplicity cascades of γ-rays, discrete nuclear states up to the fission limit have been identified in a large variety of nuclei. Beyond the region of discrete states, studies of the quasi continuum at high spin have brought information on the order-chaos transitions [14,15]. Particularly at LNL, in connection with superdeformation the problems of the feeding and decay-out of the SD bands in different mass regions have attracted much interest [16,17].…”