P. Bosetti

More than thirty bands in 177Re

Abstract The level scheme of 177Re has been extended extensively, with fragments of more than thirty bands, characterised by a given signature, observed. Alignments at low frequency in one-quasiparticle bands are interpreted as deformation effects and compared with the predictions of cranking calculations. The previously assigned one-quasiparticle π i 13 2 band is shown to be a mixture of three-quasiparticle and vibrational character. The non-observation of the one-quasiparticle π i 13 2 band is explained as being due to crossings of its s-band by competing bands. A number of high-K bands are also reported, including five- and seven-quasiparticle bands. Configuration assignments for these bands are aided by comparing experimental gκ values with “effective” gκ values, calculated with a semi-classical expression taking Coriolis mixing into account. The population of the yrast band was observed to oscillate with every four units of spin.

THE DISTRIBUTION OF THE ROTATIONAL TRANSITION STRENGTH IN WARM NUCLEI STUDIED THROUGH GAMMA-RAY CORRELATIONS

Abstract The study of damping of rotational motion applying te rotational plane mapping (RPM) method is presented and discussed. The aim of this technique is to extract the distribution of the rotational transition strength from an analysis of the shape of the “central valley” of two- and three-dimensional γ-ray spectra. The method is applied to a triple γ-coincidence data set of 162,163Tm nuclei formed in 37Cl+130Te reactions. The rotational transition strength is obtained as a function of rotational frequency for selected regions of entry states, and the width is found to be rather constant and approximately equal to 80 keV. This value is significantly smaller than the value predicted theoretically for the rotational damping width Γrot. Also the ratio between the observed depth and width of both the 2D and 3D valleys does not agree with the simple model adopted in the RPM method. These discrepancies point to the presence of both a wide and a narrow component in the distribution of rotational strength as extracted by the RPM method. The analysis of simulated spectra obtained on the basis of realistic band-mixing calculations, including residual interactions, confirms this behaviour.

Interactions between a multitude of rotational bands in well deformed odd nuclei: A new method for spin and parity assignment

Abstract Using the ( 18 O,5n) 163 Er reaction a multitude of rotational bands have been established with firm spin and parity assignments in 163 Er. In 17 out of 24 band crossings E2 cross-band transitions have been observed. The interaction strength varies between ∼ 1 and ∼ 50 keV with many of the values in the range 10–15 keV. These interactions sample a variety of the lowest (multi) -quasiparticle configurations. For some bands, in particular those with high K -values, the configurations can be established rather well. Quite complicated changes in the wavefunctions must occur at these crossings, and to explain the observed interaction strengths coupling to various vibrational degrees of freedom, in addition to possible residual neutron-proton interactions, may be needed.

Lifetime measurements of the rotational quasi-continuum transitions in 163 Tm

Abstract The lifetime of the unresolved transitions forming the rotational quasi-continuum spectra has been measured in 163 Tm using the Doppler shift attenuation technique. Spectra associated to different values of the coincidence folds, angular anisotropies and the dynamical moment of inertia were also obtained from the present experiment. The Doppler shift has been measured for discrete transitions, for the ridge structures in E γ 1 × E γ 2 spectra and for the rotational E2 bump in one dimensional spectra. The data were compared to the theory using a simulation model which makes use of microscopically calculated levels and transition probabilities. Above 1300 keV the results indicate collectivity with lifetime faster than 10 −14 s, while at lower E γ the results are well described by predictions with B (E2) = 400 WU.

Study of band mixing in the rotational quasi-continuum

Abstract The rotational continuum for normally deformed nuclei of the rare earth region is investigated making use of statistical analysis methods. With these techniques it is possible to extract information on the rotational motion from the region of high level density. In this connection, two points will be discussed; i) the important role of the residual interaction in the description of the experimental data and ii) the dependence of the mixing among rotational bands on the symmetry quantum numbers of the nuclear states. This last point was investigated for the nucleus 163Er for which it has been found that the γ-cascades feeding into low-K bands are different from those feeding high-K bands. This result indicates that the K-selection rules are effective for excited rotational bands within the angular momentum 30ℏ ≤ I ≤ 40ℏ.

The Giant Dipole Resonance in the superdeformed143Eu nucleus

The γ-decay of the Giant Dipole Resonance (GDR) built on excited nuclear states has been measured for the nucleus143Eu. The reaction110Pd(37Cl, 4n)143Eu at a beam energy of 165 MeV has been employed. This experiment aimed at searching the γ-decay of the GDR built on the superdeformed143Eu states, populated at high spins. High-energy γ-rays were detected in 8 large BaF2 scintillators in coincidence with discrete transitions measured with the NORDBALL array (in the configuration consisting of 17 HPGe detectors and a 2π multiplicity filter). The spectrum of high-energy γ-rays gated by low-energy transitions between states fed by the superdeformed states shows some excess of yield in the 7–10 MeV region with respect to that gated by transitions between states not populated by the superdeformed states. This excess should be due to the γ-decay of the the giant dipole oscillation along the superdeformed axis of the nucleus that is expected to have a frequency corresponding to ≈9–10 MeV (low-energy component of the GDR strength function). The measured excess, in spite of the large error bars, is found to be of the same order as predicted statistical model values.