A new, highly accurate measurement of the thickness of the neutron “skin”

Physicists net neutron star gold from the measurement of lead.

Studying nuclei could help scientists determine how 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 the lead. Lead is a heavy nucleus – there are extra neutrons; however, all things considered, an equivalent blend of protons and neutrons works better.

Recently, scientists at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility have made a new, highly accurate measurement of the thickness of the neutron “skin” that encompasses the lead nucleus. The outcomes of their experiment revealed that the neutron skin has a thickness of .28 millionths of a nanometer.

Kent Paschke, a professor at the University of Virginia and experiment co-spokesperson, explained, “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 a 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.”

The study is the first experimental observation of this neutron skin using electron scattering techniques. It could help physicists make a more precise measurement of its thickness in PREx-II.

It is quite challenging to measure neutrons due to 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.

Paschke said, “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.”

During the experiment, physicists precisely sent a controlled beam of electrons 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 associated with the lead target’s protons or neutrons either through the electromagnetic or the weak interaction. While the electromagnetic interaction is mirror-symmetric, the weak interaction isn’t. That means that the electrons that interfaced using electromagnetism did so, paying little heed to the electrons’ spin direction. In contrast, the electrons associated with the frail connection especially did so more regularly when the spin was in one direction versus the other.

Krishna Kumar, an experiment co-spokesperson and professor at the University of Massachusetts Amherst, said, “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.”

“The measurement required a high degree of precision to carry out successfully. The electron beam spin was flipped from one direction to its opposite 240 times per second throughout the experimental run. 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.”

Physicists later collected and analyzed electrons that had scattered off lead nuclei while leaving them intact. Thye then combined it with the previous 2012 result, and precision measurements of the lead nucleus’s proton radius, often referred to as its charge radius.

Paschke said, “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.”

Scientists noted, “this figure is thicker than some theories had suggested, which has implications for the physical processes in neutron stars and their size.”

Paschke said, “This is the most direct observation of the neutron skin. We find 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.”

Kumar said“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. 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.”

“As neutron stars start to spiral around each other, they emit gravitational waves that LIGO detects. 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 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. 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.”

Journal Reference:
  1. D. Adhikari et al. Accurate Determination of the Neutron Skin Thickness of 208Pb through Parity-Violation in Electron Scattering. DOI: 10.1103/PhysRevLett.126.172502

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