The tin nuclei, representing the longest isotopic chain between two experimentally accessible doubly-magic 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 ncertainties 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 100 Sn.
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 106,108 Sn 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 100 Sn. 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.