The giant monopole resonance in the Sn isotopes: Why is Tin so fluffy?

The isoscalar giant monopole resonance (GMR) has been investigated in a series of Sn isotopes (A=112-124) using inelastic scattering of 400-MeV a particles at extremely forward angles (including 0 degrees). The primary aim of the investigation has been to explore the role of the symmetry-energy term in the expression for nuclear incompressibility. It is found that the excitation energies of the GMR in the Sn isotopes are significantly lower than those expected from the nuclear incompressibility previously extracted from the available data on the compressional-mode giant resonances.