The atomic nucleus is the small dense region at the center of an atom, consisting of protons and neutrons held together by nuclear forces or strong residual forces.
The protons and neutrons, collectively called nucleons, share energy and momentum in tight quarters within the nuclei. Exactly how they share the energy that keeps them within the nucleus and how they are distributed is one of the key puzzles for nuclear physicists.
Researchers at Lawrence Livermore National Laboratory (LLNL) and Washington University in St. Louis attempted to answer these questions by leveraging data from nuclear scattering experiments. They made stringent constraints on how neutrons and protons arrange themselves in the nucleus. Their research is published in Physical Review C and Physical Review Letters.
Their analysis shows that a tiny fraction of neutrons and protons possess much of the overall energy that keeps them bound in the nuclei, roughly 50% more than expected from standard theoretical treatments.
The study also makes new predictions for “neutron skin,” the neutron-rich outer layer of heavy nuclei. However, it’s not easy to directly measure neutron skins. In 2010, the Lead Radius Experiment or PREX, provided the first model-independent neutron skin measurement for lead-208. But it was swamped by uncertainties. Results of PREX II, a more precise follow-up experiment, is expected to release soon.
Understanding how nuclear asymmetry energy changes with density is essential to determine the neutron star structure and the elements that are likely to be synthesized in neutron star mergers
According to Cole Pruitt, LLNL postdoc, and lead author of both papers, their results quantitatively indicate how asymmetry, charge, and shell effects contribute to neutron skin generation and drive a disproportionate share of the total binding energy to the deepest nucleons. A comprehensive model should not only reproduce integrated quantities (like the charge radius or total binding energy) but also specify how nucleons share momentum and energy, all while being realistic about the model uncertainty of its predictions.
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