Nuclear Structure Studies with the AGATA Array at Legnaro

Abstract

Results of three selected AGATA Demonstrator experiments performed at Legnaro National Laboratory of INFN (Italy), aiming at studying collective modes of excitations, such as rotation and vibration, under extreme conditions, are reported. Firstly, the K-hindrance to the γ decay is investigated in the warm rotating 174 W nucleus, focusing on the weakening of the selection rules of the K quantum number with excitation energy. A strong hindrance to the E1-decay between low-K and high-K rotational bands is found, giving evidence that K-mixing due to temperature effects is the leading phenomenon, contributing, for all discrete excited bands, to the progressive erosion of the K-quantum number with excitation energy. The second experiment concerns the study of the pygmy dipole resonance in the neutron rich 208 Pb nucleus, by inelastic scattering of 17 O at 20 MeV/u. The γ decay is measured with the AGATA Demonstrator coupled to an array of large volume LaBr 3 :Ce scintillators. A preliminary comparison with (γ,γ') data indicate that states in the 5-8 MeV energy interval belong to two different groups, one with an isoscalar character and the other with an isovector nature. Finally, the breaking of isospin symmetry is studied in the hot compound N = Z nucleus 80 Zr, by comparing the γ decay of the Giant Dipole Resonance from the fusion reactions 40 Ca+ 40 Ca at E beam = 136 MeV and 37 Cl+ 44 Ca at E beam = 95 MeV. Preliminary results show that the yield associated with the Giant Dipole Resonance is different in the two reactions because in self-conjugate nuclei the E1 selection rules forbid the decay between states with isospin I = 0. The experiment aims at providing information on the degree of isospin mixing at temperature around ≈ 2 MeV and to extrapolate the zero-temperature value, which is then compared with the latest predictions.