Nuclear physicists explore different nuclei to learn how protons and neutrons behave. For instance, they have found that nuclei made of just a few protons and neutrons typically contain close to an equal number of each. But as nuclei get heavier, they need to pack in more neutrons than protons to remain intact. These extra neutrons tend to stick to the outer edges of heavy nuclei and form a kind of neutron-rich “skin” around a more evenly distributed core. A recent experiment made what the researchers call the most direct observation of this neutron-rich skin to date in lead nuclei and measured its thickness as 0.28 femtometers, hundreds of thousands of times smaller than the diameter of an atom.
The neutron skin is thicker than some theories have suggested. This finding indicates that the internal density of lead nuclei is lower than expected. However, the internal pressure that neutrons in lead experience is higher than scientists once thought. The result has implications for the physical processes in neutron stars. It also has deep connections to recent observations of colliding neutron stars made by the Nobel Prize-winning Laser Interferometer Gravitational-Wave Observatory or LIGO experiment. Each experiment measured different things, and they both contribute new information to our understanding of neutron stars.
Nuclear physicists explore different nuclei to learn how protons and neutrons act inside the nucleus in everyday matter. The Lead (Pb) Radius Experiment (PREx) Collaboration recently used the Continuous Electron Beam Accelerator Facility, a Department of Energy (DOE) user facility, to learn about the neutron-rich “skin” of lead nuclei. The researchers sent a precise beam of electrons into a thin sheet of cold lead. These electrons were spinning in their direction of motion, like a spiral on a football pass. The physicists flipped the electron beam spin from one direction to its opposite 240 times per second throughout the experiment. Electrons in the beam mostly interact with target protons via the electromagnetic force. However, there is a subtle change introduced by the weak force causing electrons that interact via the weak force to preferentially interact with neutrons. What makes the weak force unique among fundamental forces is that it is not mirror symmetric. In this experiment, this broken symmetry meant that electrons spinning in a preferred direction scattered more often. Exploiting this difference allowed physicists to measure the neutron skin thickness as 0.28 femtometers. The result also reveals new details about neutron stars, such as how they interact as two stars merge together.
This work was supported by the DOE Office of Science, Nuclear Physics program; the National Science Foundation; the Natural Sciences and Engineering Research Council of Canada, and the Italian Istituto Nazionale di Fisica Nucleare.
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