Study of thermally excited nuclei through E1 and E2 decays from collective modes.

The nuclear system at the limit of excitation energy and angular momentum is here studied with γ-spectroscopy on the superdeformed nucleus [sup 143]Eu based on a EUROBALL experiment. The influence of thermal energy is investigated through the analysis of the quasi-continuum spectra formed by E2 transitions among states of excited rotational bands with energy extending up to 4–5 MeV above the yrast line. In particular, the effective lifetimes of the discrete rotational bands forming ridge structures in γ-γ coincidence matrices is measured by a Doppler Shift Attenuation Method. The obtained results, consistent with a quadrupole deformation of Q[sub t]approx. 10 eb, demonstrate that the nucleus maintains its collectivity with increasing excitation energy, supporting the rotational character of the excited nuclear rotation. The number of superdeformed discrete bands is also measured by a statistical analysis of the fluctuations in the number of counts of the ridge structures. It is found that the data are well reproduced by microscopic crancked shell model calculations including a two-body residual interaction and tacking also into account the decay-out process into the lower deformation minimum. In addition, the nuclear properties at higher excitation energies are investigated through the E1 γ-decay of the giant dipole resonance (GDR). It is found that the intensity of the superdeformed yrast and excited bands increases by a factor of approximately 2 when a coincidence with an high-energy γ-ray is required, showing the importance of the E1 cooling in the feeding mechanism of the superdeformed states. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]