Orateur
Dr
Jose Javier Valiente Dobon
(Laboratori Nazionali di Legnaro (INFN))
Description
Electromagnetic transition rates, in particular for E2 transitions, are a sensitive and well-studied probe of nuclear structure: their dependence on the nuclear wave function offers the possibility of strict tests of theoretical models. Usually, B(E2) rates are calculated in a restricted shell-model space and they are then renormalized with constant effective charges. However, even large-scale shell-model calculations may fall short in reproducing the experimental data, as in the case of proton-rich tin isotopes [1,2]. In this regard, there is a common but bad practice of neglecting effective three-body forces and two-body transition operators when calculating the B(E2) values which could be the origin of the problems encountered [3]. We have performed an experiment to measure the transitions rates from the seniority-isomers of semi-magic neutron-rich lead isotopes and they show indeed discrepancies with shell-model estimates. This region of the nuclide chart has been so far scarcely explored due to its high mass and neutron excess, which oblige to exploit fragmentations reactions with relativistic uranium beams. Consequently, neutron-rich nuclei beyond 208Pb were populated by using a 1 GeV*A 238U beam at GSI. The resulting fragments were separated and analyzed with the FRS-RISING setup [4,5]. Many neutron-rich isotopes were identified for the first time and a significant number of new isomers were hence discovered, enabling to study the structure of these isotopes. The new exotic isotopes observed extend up to 216Pb along the Z=82 shell closure and up to N=134 and N=138 for the proton-hole and proton-particle Tl and Bi nuclei, respectively. New isomers were observed in 212-216Pb, in 217Bi, in 211,213Tl and in 210Hg. In the talk, the experimental results will be presented as well as state-of-the-art shell-model calculations pointing out how the measured isomeric B(E2)s in neutron-rich lead isotopes seem to require state-dependent effective charges to be correctly reproduced. It will be shown how this is related with the aforementioned neglect of effective three-body forces, whose introduction improves the agreement with the experimental data. The unexpected structure of the very exotic 210Hg isotope will also be discussed.
References
[1] A. Banu et al., Phys. Rev. C 72, 061305(R) (2005).
[2] C. Vaman et al., Phys. Rev. Lett. 99, 162501 (2007).
[3] A. Poves et al., Phys. Lett. B 82, 319 (1979).
[4] H. Geissel et al., Nucl. Instr. Meth. B 70, 286 (1992).
[5] S. Pietri et al., Nucl. Instr. Meth. B 261, 1079 (2007).
Auteur principal
Dr
Jose Javier Valiente Dobon
(Laboratori Nazionali di Legnaro (INFN))
Co-auteurs
Dr
Andrea Gottardo
(LNL-INFN)
Dr
Giovanna Benzoni
(Milano-INFN)
