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

The nuclear system at the limit of excitation energy and angular momentum is here studied in the case of the superdeformed nucleus 143 Eu using $\gamma$ -spectroscopy techniques. The data are based on a EUROBALL experiment using the reaction 37 Cl + 110 Pd $\to^{143}$ Eu + 4n. The influence of thermal energy on superdeformed configurations 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 $\gamma$ - $\gamma$ coincidence matrices is measured by a Doppler Shift Attenuation Method. The deduced quadrupole deformation of $Q_t \approx$ 10 eb indicates that the nucleus maintains its collectivity with increasing excitation energy, supporting the superdeformed character of the excited nuclear rotation. The obtained number of superdeformed discrete bands forming the ridge structures is found in good agreement with microscopic cranked shell model calculations including the decay-out process into the lower deformation minimum. In addition, the nuclear properties at higher excitation energies are investigated through the E1 $\gamma$ -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 1.6 when a coincidence with a high-energy $\gamma$ -ray is required, showing the importance of the E1 cooling in the feeding mechanism of the superdeformed states