Highly Accurate Measurements Show Neutron Star “Skin” Is Less Than a Millionth of a Nanometer Thick

X-ray Burst Magnetar

Illustration of a powerful X-ray burst erupts from a magnetar — a supermagnetized version of a stellar remnant known as a neutron star. Credit: NASA’s Goddard Space Flight Center/Chris Smith (USRA)

Nuclear physicists make new, high-precision measurement of the layer of neutrons that encompass the lead nucleus, revealing new information about neutron stars.

Nuclear physicists have made a new, highly accurate measurement of the thickness of the neutron “skin” that encompasses the lead nucleus in experiments conducted at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility and just published in Physical Review Letters. The result, which revealed a neutron skin thickness of .28 millionths of a nanometer, has important implications for the structure and size of neutron stars.

The protons and neutrons that form the nucleus at the heart of every atom in the universe help determine each atom’s identity and properties. Nuclear physicists are studying different nuclei to learn more about how these protons and neutrons act inside the nucleus. The Lead Radius Experiment collaboration, called PREx (after the chemical symbol for lead, Pb), is studying the fine details of how protons and neutrons are distributed in lead nuclei.

“The question is about where the neutrons are in lead. Lead is a heavy nucleus – there’s extra neutrons, but as far as the nuclear force is concerned, an equal mix of protons and neutrons works better,” said Kent Paschke, a professor at the University of Virginia and experiment co-spokesperson.

Jefferson Lab’s Experimental Hall A is one of four nuclear physics research areas in the lab’s Continuous Electron Beam Accelerator Facility. Credit: DOE’s Jefferson Lab

Paschke explained that light nuclei, those with just a few protons, typically have equal numbers of protons and neutrons inside. As nuclei get heavier, they need more neutrons than protons to remain stable. All stable nuclei that have more than 20  protons have more neutrons than protons. For instance, lead has 82 protons and 126 neutrons. Measuring how these extra neutrons are distributed inside the nucleus is key input for understanding how heavy nuclei are put together.

“The protons in a lead nucleus are in a sphere, and we have found that the neutrons are in a larger sphere around them, and we call that the neutron skin,” said Paschke.

The PREx experiment result, published in Physical Review Letters in 2012, provided the first experimental observation of this neutron skin using electron scattering techniques. Following that result, the collaboration set out to make a more precise measurement of its thickness in PREx-II. The measurement was carried out in the summer of 2019 using the Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility. This experiment, like the first, measured the average size of the lead nucleus in terms of its neutrons.

Neutrons are difficult to measure, because many of the sensitive probes that physicists use to measure subatomic particles rely on measuring the particles’ electric charge through the electromagnetic interaction, one of the four interactions in nature. PREx makes use of a different fundamental force, the weak nuclear force, to study the distribution of neutrons.

“Protons have an electric charge and can be mapped using the electromagnetic force. Neutrons have no electric charge, but compared to protons they have a large weak charge, and so if you use the weak interaction, you can figure out where the neutrons are,” explained Paschke.

In the experiment, a precisely controlled beam of electrons was sent crashing into a thin sheet of cryogenically cooled lead. These electrons were spinning in their direction of motion, like a spiral on a football pass.

Electrons in the beam interacted with the lead target’s protons or neutrons either via the electromagnetic or the weak interaction. While the electromagnetic interaction is mirror-symmetric, the weak interaction is not. That means that the electrons that interacted via electromagnetism did so regardless of the electrons’ spin direction, while the electrons that interacted via the weak interaction preferentially did so more often when the spin was in one direction versus the other.

“Using this asymmetry in the scattering, we can identify the strength of the interaction, and that tells us the size of the volume occupied by neutrons. It tells us where the neutrons are compared to the protons.” said Krishna Kumar, an experiment co-spokesperson and professor at the University of Massachusetts Amherst.

The measurement required a high degree of precision to carry out successfully. Throughout the experimental run, the electron beam spin was flipped from one direction to its opposite 240 times per second, and then the electrons traveled nearly a mile through the CEBAF accelerator before being precisely placed on the target.

“On average over the entire run, we knew where the right- and left-hand beams were, relative to each other, within a width of 10 atoms,” said Kumar.

The electrons that had scattered off lead nuclei while leaving them intact were collected and analyzed. Then, the PREx-II collaboration combined it with the previous 2012 result and precision measurements of the lead nucleus’ proton radius, which is often referred to as its charge radius.

“The charge radius is about 5.5 femtometers. And the neutron distribution is a little larger than that – around 5.8 femtometers, so the neutron skin is .28 femtometers, or about .28 millionths of a nanometer,” Paschke said.

The researchers said that this figure is thicker than some theories had suggested, which has implications for the physical processes in neutron stars and their size.

“This is the most direct observation of the neutron skin. We are finding what we call a stiff equation of state – higher than expected pressure so that it’s difficult to squeeze these neutrons into the nucleus. And so, we’re finding that the density inside the nucleus is a little bit lower than was expected,” said Paschke.

“We need to know the content of the neutron star and the equation of state, and then we can predict the properties of these neutron stars,” Kumar said. “So, what we are contributing to the field with this measurement of the lead nucleus allows you to better extrapolate to the properties of neutron stars.”

The unexpectedly stiff equation of state implied by the PREx result has deep connections to recent observations of colliding neutron stars made by the Nobel Prize-winning Laser Interferometer Gravitational-Wave Observatory, or LIGO, experiment. LIGO is a large-scale physics observatory that was designed to detect gravitational waves.

“As neutron stars start to spiral around each other, they emit gravitational waves that are detected by LIGO. And as they get close in the last fraction of a second, the gravitational pull of one neutron star makes the other neutron star into a teardrop – it actually becomes oblong like an American football. If the neutron skin is larger, then it means a certain shape for the football, and if the neutron skin were smaller, it means a different shape for the football. And the shape of the football is measured by LIGO,” said Kumar. “The LIGO experiment and the PREx experiment did very different things, but they are connected by this fundamental equation – the equation of state of nuclear matter.“

Reference: “Accurate Determination of the Neutron Skin Thickness of 208Pb through Parity-Violation in Electron Scattering” by D. Adhikari et al. (PREX Collaboration), 27 April 2021, Physical Review Letters.
DOI: 10.1103/PhysRevLett.126.172502

The PREx-II experimental collaboration includes 13 Ph.D. students and seven postdoctoral research associates, as well as more than 70 other scientists from about 30 institutions.

This work was supported by DOE’s Office of Science, the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Italian Istituto Nazionale di Fisica Nucleare (INFN).


View Comments

  • The method used for determination of thickness of neutron star's skiñ has good experimental arrangement.Result with value .28×10 to the power_15 meter has good confirmation with some mathematical aspects.So the result has utility for formation of Neutron Star at different stages.Thanks to the Auther.

  • Certainly,the result of the experiment with method appĺied sketches how atomic neuclei aŕe stable.

  • The tesult of the experiment to measure neitron star's skìn,with typical methòd adopted speaks how atomic neuçlei are stable.

  • The Thomas Jefferson National Accelerator? Clearly this is racist science and the result of white supremacy. 2+2 does not equal 4!

  • Highly accurate measurements? Lmbo...Folks just beleive anything they are fed by these dam liars that control the world system. Wake up Sheep. There is no outer space.

  • There are those who feel the meutron star is just that, a ball of free neutrons in a semi fluid soup.
    With only the single atomic nucleus for the entire star the change would be miniscule with regard to size.

    • No, the Equation Of State for the star core and this result potentially bearing on the crust thickness has ramifications for size and merger observations, see my comment with references.

      I also quotes an old model of neutron stars that agree with what you describe. But it is likely not as simple as that core-crust model, IIRC I think signals from magnetic reconnections have shown that neutron stars have inner structure (wandering magnetic poles, perhaps from convection despite the high pressures).

  • Duh. The neutron layer in a lead ATOM has a skin less than a millionth of a nanometer thick, not a neutron star. Read the article.

  • Jefferson Lab has an old but useful meeting presentation: .

    Page 4 show the 208Pb nucleus measurement, with neutrons dominating as much over the proton core as when that starts to taper off - the "skin" looks like a pressure effect as they say and not some sort of confinement.

    Page 14 show their 2009 model of a neutron star [NS] which has "solid crust (yellow) over liquid core (blue)" where "[Pb] neutron skin and NS crust are made out of neutron rich matter at similar densities". The thick Pb skin translates to a thinner (I think) but stiffer NS crust.

    Page 15 discuss LIGO measurements. "A solid crust can support an off axis mass quadrupole moment." "We find: neutron star crust is the strongest material known. It is 10 billion times stronger than steel. Very promising for GW searches."

    Re the crust I think signals from magnetic reconnections have show that neutron stars have inner variation (wandering magnetic poles). Science says on them ["Neutron stars may be bigger than expected, measurement of lead nucleus suggests"]: " In particular, a thicker neutron skin implies that neutron stars are less compressible than many theories predict, he says, which would make them bigger. In fact, in another paper published today in Physical Review Letters, Piekarewicz and colleagues calculate that the PREX result implies a radius between 13.25 and 14.25 kilometers for a run-of-the-mill neutron star 1.4 times as massive as the Sun. Most theories yield estimates closer to 10 kilometers."

    "But Miller notes that data from gravitational wave detectors may favor smaller, softer neutron stars. In 2017, physicists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and the Virgo detector in Italy spotted two neutron stars whirling into each other and merging, presumably to form a black hole. If the neutron stars were relatively large and stiff, then before the merger they should have started to deform each other through their gravity, Miller says. But LIGO and Virgo researchers saw no evidence of such tidal deformation in their signal, he says.

    However, Witold Nazarewicz, a nuclear theorist at Michigan State University, says it’s premature to worry about the astrophysical implications of the PREX result. He notes that the team measures only electron scattering asymmetry, and the theories the researchers use to convert it into the thickness of the neutron skin have their own uncertainties. And the value the team gets for the asymmetry may already conflict with measurements of other properties of the lead nucleus, Nazarewicz says. “I would like to know if everything is consistent with lead-208.”"

  • Why do these right-wing fundy SJW’s bother reading science articles? Don’t they have crosses to burn?

    Anyway I’m picturing a neutron skin on a lead nucleus that’s one neutron thick. Would it be the same on a neutron star?

  • I tryed to tell ya but there it is i told ya its not a bad thing at all lets enjoy this before its gone

Thomas Jefferson National Accelerator Facility

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