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Accueil du site > FRANCAIS > Communication > Séminaires > Année 2008 > Evolution of the nuclear structure far from stability : A study of shape coexistence in neutron-deficient even-even polonium isotopes


Evolution of the nuclear structure far from stability : A study of shape coexistence in neutron-deficient even-even polonium isotopes

Bastin Beyhan, IKS KULeuven on behalf of IKS-Miniball REX-ISOLDE Collaboration
Vendredi 5 Décembre 2008 à 10h30 - Salle des Séminaires

In the region around the Z=82 shell closure with neutron number around midshell between N=82 and N=126, shape coexistence occurs at low excitation energy. This phenomenon is well-established in the neutron-deficient polonium isotopes as evidenced by low-lying rotational like bands intruding in the low-energy structure. Proton-pair excitations across the magic Z=82 along with the strong proton-neutron interaction in the vicinity of the neutron midshell are considered as the driving mechanism for shape coexistence in this region. The strong perturbation of the energy-level systematics in the very light polonium isotopes is also interpreted as arising from the interaction between the regular and intruder structures. While the onset of the deformation in the light Po isotopes is reasonably well established experimentally, questions remain concerning the sign of deformation and the magnitude of the mixing between different configurations. Furthermore, controversy is present with respect to the transition from the vibrational-like character of the heavier Po isotopes to the structure driven by shape coexistence as observed in the lighter Po isotopes. In the presentation, a new project to study shape coexistence in the even-mass neutron-deficient 198,200,202Po isotopes by using post-accelerated (up to 2.8 MeV/u) radioactive polonium beams from REX-ISOLDE (CERN) followed by Coulomb excitation in a ‘safe’ energy domain will be discussed. The gamma rays will be detected by the germanium MINIBALL detector array. Measurements of the differential cross section will allow us to deduce both the transition and diagonal matrix elements for these nuclei and, eventually combined with lifetime measurements, the sign of the quadrupole deformation in the polonium isotopes. In the future, the obtained reduced transition matrix elements will be compared to beyond mean field models and will serve as important bench marks to test of the model and interactions used. Also, the complementary results of the recent charge radii measurements of 193-204Po isotopes obtained using laser spectroscopy at ISOLDE will be presented.

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