Recent data on isoscalar giant monopole resonance (ISGMR) in the calcium isotopes 40,44,48 Ca have suggested that K τ , the asymmetry term in the nuclear incompressibility, has a positive value. A value of K τ > 0 is entirely incompatible with present theoretical frameworks and, if correct, would have far-reaching implications on our understanding of myriad nuclear and astrophysical phenomena. This paper presents results of an independent ISGMR measurement with the 40,42,44,48 Ca(α, α ) reaction at E α = 386 MeV. These results conclusively discount the possibility of a positive value for K τ , and are consistent with the previously-obtained values for this quantity.

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The surface of neutron-rich heavy nuclei, with a neutron skin created by excess neutrons, provides an important terrestrial model system to study dilute neutron-rich matter. By using quasi-free α cluster–knockout reactions, we obtained direct experimental evidence for the formation of α clusters at the surface of neutron-rich tin isotopes. The observed monotonous decrease of the reaction cross sections with increasing mass number, in excellent agreement with the theoretical prediction, implies a tight interplay between α-cluster formation and the neutron skin. This result, in turn, calls for a revision of the correlation between the neutron-skin thickness and the density dependence of the symmetry energy, which is essential for understanding neutron stars. Our result also provides a natural explanation for the origin of α particles in α decay.