Abstract
Many properties of the atomic nucleus, such as vibra- 21 tions, rotations and incompressibility can be interpreted 22 as due to a two-component quantum liquid of protons and 23 neutrons. Electron scattering measurements on stable nu- 24 clei demonstrate that their central densities are saturated, 25 as for liquid drops. In exotic nuclei near the limits of mass 26 and charge, with large imbalances in their proton and neu- 27 tron numbers, the possibility of a depleted central density, 28 or a “bubble” structure, was discussed in a recurrent man- 29 ner since the seventies. Here we report first experimental 30 evidence that points to a depletion of the central density of protons in the short-lived nucleus 34 31 Si. The proton-toneutron density asymmetry in 34 32 Si offers the possibility to 33 place constraints on the density and isospin dependence 34 of the spin-orbit force - on which nuclear models have dis- 35 agreed for decades- and on its stabilizing effect towards 36 limits of nuclear existence