SEZIONE DI FIRENZE
The tin nuclei, representing the longest isotopic chain between two experimentally accessible doublymagic
nuclei, provide a unique opportunity for systematic studies of the evolution of basic nuclear
properties when going from very neutron-deficient to very neutron-rich species. A little over a decade
ago, they were considered a paradigm of pairing dominance: the excitation energies of the first 2+ and
4+ states are rather constant along the Sn isotopic chain, and the B(E2; 2+→0+) values for isotopes
with A>116 present a parabolic behavior expected for the seniority scheme. On the other hand, the
B(E2; 2+→0+) values measured for neutron-deficient Sn isotopes remain constant with N.
Unfortunately, the lack of information on B(E2; 4+→2+) strengths in light Sn nuclei, combined with
large experimental uncertainties on the B(E2; 2+→0+) values, prevent firm conclusions on the shell
evolution in the vicinity of the heaviest proton-bound N=Z doubly-magic nucleus 100Sn.
To remedy this, the first lifetime measurement in neutron-deficient tin isotopes was carried out using
the Recoil Distance Doppler-Shift method, providing a complementary solution to the previous
Coulomb-excitation studies. Thanks to the unusual application of a multi-nucleon transfer reaction,
together with unprecedented capabilities of the powerful AGATA and VAMOS++ spectrometers, the
lifetimes of the 2+ and 4+ states in 106,108Sn have been directly measured for the very first time.
Large-scale shell-model calculations were performed to account for the new experimental results. In
particular, the comparison of the B(E2; 4+→2+) values with the theoretical predictions shed light on
the interplay between quadrupole and pairing forces in the vicinity of 100Sn. An interpretation has
also been proposed for the anomalous B(E2; 4+→2+)/B(E2; 2+→0+) ratio observed not only for the Sn
isotopes, but also in other regions of the nuclear chart. (Marco Siciliano)