Where do neutrons go? The elusive answer to such a seemingly simple question provides fundamental new insights into the structure of both atomic nuclei and neutron stars. To place the question in the proper context, consider lead-208, the element’s most abundant isotope, which contains 82 protons and 126 neutrons. As the heaviest known doubly magic nucleus, 208Pb holds a special place in the nuclear-physics community. Just as noble gases with filled electronic shells exhibit low levels of chemical reactivity, doubly magic nuclei with filled proton and neutron shells display great stability. Because 208Pb is heavy, the Coulomb repulsion among its protons leads to a large neutron excess. The Lead Radius Experiment, or PREX, at the Thomas Jefferson National Accelerator Facility in Virginia was built to measure the location of 208Pb’s 44 excess neutrons. In turn, a detailed knowledge of the neutron distribution in 208Pb illuminates the structure of a neutron star.

Full article: https://physicstoday.scitation.org/doi/10.1063/PT.3.4247