Probing the Skin of a Lead Nucleus April 27, 2021• Physics 14, 58 Researchers make the most precise measurement yet of the neutron distribution in a heavy nucleus, with implications for the structure of neutron stars. Figure 1: A cartoon image of a lead-208 nucleus, showing the mixed proton-neutron core and the neutron “skin” (left). Measuring the thickness of the neutron skin offers clues about how neutron stars are structured (right).× Popular cartoon visualizations depict the protons and neutrons in a nucleus as colored marbles packed randomly into a sphere. In reality, heavy nuclei—in which neutrons tend to outnumber protons—are more differentiated, with the neutrons nudged radially outward. At the outer limits of such nuclei, the neutrons form a thin “skin” enclosing a core of mixed neutrons and protons (Fig. 1). Now, the Lead Radius Experiment (PREX) Collaboration at the Thomas Jefferson National Accelerator Facility in Virginia has determined the thickness of this neutron-rich skin in lead-208, a stable isotope with 44 more neutrons than protons [1]. The measurement, which addresses questions relating to all four fundamental forces of nature, yields insight into the structure of neutron stars, and it will have far-reaching implications for multimessenger astronomy and particle physics.