From APS Synopsis: Getting Under the Neutron Skin

Heavy nuclei are believed to have a neutron-rich skin on the surface, and the thickness of this skin may have important implications for the physics of neutron stars.

Now the Lead Radius Experiment (PREx) Collaboration reports, in Physical Review Letters, electron scattering experiments that yield the thickness of the neutron skin in the heavy nucleus lead-208. Their preliminary results show that the skin’s thickness is about 0.33 millionths of a nanometer.

More:
http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.108.112502