Protons Get Zippier in Neutron-Rich Nuclei
A new study has confirmed that increasing the number of neutrons as compared to protons in the atom’s nucleus also increases the average momentum of its protons.
Prior research measuring protons in nuclei found that protons and neutrons, collectively called nucleons, may pair up briefly inside the nucleus of the atom in short-range correlations. These paired particles are imparted with high momentum in comparison to non-paired nucleons. This new study measured both protons and neutrons and found that in nuclei with more neutrons than protons, a greater fraction of protons pair up, thus giving the protons a higher-average momentum than the neutrons.
Adding extra neutrons to a neutron-rich nucleus causes its protons to move faster. The result also has implications for the dynamics of neutron stars, which are made up of about five percent protons. Due to their high average momentum as compared to neutrons, the protons may have an out-sized effect on neutron star structure.
Researchers previously obtained direct evidence that protons and neutrons pair up briefly in the nucleus, a phenomenon called a short-range correlation. Further, nucleons involved in a short-range correlation carry higher momentum than unpaired ones in a range of nuclei, including carbon, aluminum, iron and lead. The new result includes data taken on neutrons. It confirms these previous results and quantifies the short-range correlations effect in nuclei, showing that in nuclei in which neutrons outnumber protons, the minority protons are more likely be found in a short-range correlation and thus carry higher average momentum than the majority neutrons. This may be a consequence of the pair preference phenomenon exhibited by short-range correlations. That is, protons and neutrons both prefer to pair with particles different from themselves by 20 to 1. In neutron-rich nuclei, the few protons will have more pairing opportunities than the plentiful neutrons. The result also has implications for the dynamics of neutron stars, which are made up of about five percent protons. Due to their high average momentum as compared to neutrons, the protons may have an out-sized effect on neutron star structure. This result comes from a data-mining initiative, in which data from an experiment conducted in 2004 were re-analyzed to look for this phenomenon. The data were taken in the CEBAF Large Acceptance Spectrometer, which almost completely surrounds an experimental target and can collect more than 2 TB of data a day. In the experiment, a 5 GeV beam of electrons probed the nuclei of carbon, aluminum, iron and lead.
Massachusetts Institute of Technology
Old Dominion University
The research was supported by the U.S. Department of Energy’s Office of Science. It was also supported by the National Science Foundation, the Israel Science Foundation, the Chilean Comisión Nacional de Investigación Científica y Tecnológica, the French Centre National de la Recherche Scientifique and Commissariat a l’Energie Atomique the French-American Cultural Exchange, the Italian Istituto Nazionale di Fisica Nucleare, the National Research Foundation of Korea, and the UK’s Science and Technology Facilities Council.
The CLAS Collaboration, "Probing high-momentum protons and neutrons in neutron-rich nuclei." Nature 560 617 (2018).