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Protons get zippier in neutron-rich nuclei

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Posted August 28, 2018

It’s a case of acting differently when outmanned — or rather out-neutroned. Protons appear to get extra pep in their step when they’re outnumbered by neutrons in the atom’s nucleus.

A new study carried out at the Department of Energy’s Thomas Jefferson National Accelerator Facility 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. The nuclear physics result, which has implications for the dynamics of neutron stars, has been published in the journal Nature.

Jefferson Lab’s CEBAF Large Acceptance Spectrometer collected the data in an experiment that found that increasing the number of neutrons as compared to protons in the atom’s nucleus also increases the average momentum of its protons. Image credit: Jefferson Lab

Jefferson Lab was one of 43 institutions involved in the experiment, which included William & Mary Department of Physics faculty among its collaborators. Physics Professor Keith Griffioen and Rob Fersch, a 2008 Ph.D. graduate, are among the co-authors on the Nature paper.

The study involves short-range correlations, a phenomenon in which protons and neutrons, or nucleons, may pair up briefly inside the nucleus. This brief pairing imparts the two partners with high momentum.

“The paper shows that neutron-proton pairs are much more likely than like-nucleon pairs, and that a universal model can explain all the observations,” Griffioen said.

Earlier research found that nucleons prefer to pair up with nucleons of a different type (for instance, protons prefer neutrons and vice-versa). The research, which only measured protons, had also hinted that a greater fraction of protons than neutrons may pair up in neutron-rich nuclei, thus giving the protons a higher-average momentum than neutrons.

“What differentiates this study from earlier ones is that this is the first time we detect the neutron,” says Meytal Duer, a graduate student at Tel Aviv University in Israel who is the lead author on the paper and who led the analysis effort. Including the neutron in the study, she says, also allowed the nuclear physics researchers to quantify the short-range correlations effect.

Duer and her colleagues re-analyzed data from an experiment conducted in 2004. In the experiment, the Jefferson Lab Continuous Electron Beam Accelerator Facility produced a 5.01 GeV beam of electrons to probe nuclei of carbon, aluminum, iron and lead.

“When the electron comes in and hits the nucleus, we have struck a proton or a neutron. We detected the scattered electron and the scattered proton or neutron,” Duer explains. “This is the first study to measure both protons and neutrons in short-range correlations and compare the high-momentum fraction carried by each.”

The researchers found that when they compared the lightest nucleus, carbon, to the progressively heavier and neutron-rich nuclei of aluminum, iron and lead, the fraction of high-momentum protons went up.

“When we’ve got 50 percent more neutrons in the nucleus, we’ve also got 50 percent more high-momentum protons than we had before,” says Or Hen, an assistant professor of physics at MIT and spokesperson for the data mining collaboration.

Hen explains that 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.

“A main result of this paper is that as the nucleus becomes more and more neutron rich, the pairing probability of neutrons saturates, while the pairing probability of protons grows,” he says.

Lawrence Weinstein, a lead coauthor and Professor & Eminent Scholar at Old Dominion University, says the result of that pairing preference has now been confirmed and quantified for the first time by this new finding.

“As you add neutrons to the nucleus, the fraction of neutrons that are high momentum stays the same, but the fraction of protons at high momentum increases by 50 percent,” he says. “Put another way, it looks like when you add neutrons to a nucleus, it makes the protons 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.

The researchers say the next step is to move forward with new experiments that may be able to get at least ten times as much data on a wider range of nuclei, as well as continuing their efforts to scrutinize the data that’s already been collected for more insights into these mysterious nucleon pairings and how they influence nuclear structure.

Source: NSF, William & Mary

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