Formation of α clusters in dilute neutron-rich matter

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