For three months in 2019, physicists blasted a beam of electrons at lead atoms that were held in place by foil made of diamond. The team was trying to determine the thickness of the neutron skin, the sheath of neutrally charged neutrons that surrounds the positively charged protons in a lead atom’s nucleus. They succeeded.
The neutron skin of lead-208 is 0.28 femtometers — 0.28 trillionths of a millimetre — the team determined, increasing the estimated thickness of the skin by a tenth of a femtometer compared to previous calculations. That’s a big shift, on the atomic scale.
Teasing out the measurement was a lot like “knowing you got this tiger by the tail,” Kent Paschke, a physicist at the University of Virginia and co-author of the new study, said in a phone call. It took three months of intense runs of the laser, enduring power outages and round-the-clock monitoring of the system. The team wasn’t sure if they’d be able to complete the work in the three months they were given. But ultimately, the atomic-scale marathon turned up an exact measurement, one that redefines our understanding of the size of the neutron skin.
Previous calculations for the skin relied on more roughly drawn estimates and assumptions; the researchers hope this new measurement becomes a fundamental component for future observations on both the nuclear and stellar scales. They did their work at the Continuous Electron Beam Accelerator Facility at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. The measurement is the culmination of the second rendition of the Pb Radius Experiment, or PREX-II, and the team’s results are published today in the Physical Review Letters.
“This measurement is exciting to the scientist because it makes a measurement of this neutron radius with the least assumptions ever done,” said study co-author Krishna Kumar, an experimental nuclear physicist at the University of Massachusetts Amherst, in a video call. “That’s what experimental scientists live for.”
By measuring how electrons of different spins scattered off the lead nuclei, the team was able to figure out the thickness of the neutron skin, a measurement that’s previously been difficult to determine because neutrons have no electric charge. To draw a bead on the thickness of the neutron skin, the team took measurements using the weak nuclear force, rather than the electromagnetic force that electrons and protons so readily display.
That particular lead isotope, lead-208, was chosen because of its size and structure; it’s the biggest superstable nucleus physicists know of, and, perhaps more importantly, it’s doubly “magic,” meaning that both its protons and neutrons completely fill up their orbital shells.
“Lead-208 is particularly useful because it approximates uniform nuclear matter,” Paschke said. “You need those theoretical techniques to describe big, heavy things.”
Ah, physics, a field of extremes. In this case, the examination of the skin of the neutrons surrounding an atom’s nucleus has implications for understanding of neutron stars, the densest objects in our universe besides black holes. Neutron stars are the collapsed cores of dead stars; they are so dense that experts aren’t quite sure what lies at their core. It’s been suggested that they may be the source of axions, a candidate to explain dark matter.
“The pressure of neutron matter holds a neutron star up against gravity and keeps it from collapsing to a black hole,” said study co-author Chuck Horowitz, an astrophysicist at the University of Illinois, in an email. “We find a relatively thick neutron skin in Pb [lead], and this implies a high pressure and suggests that neutron stars are relatively big.”
The hope riding on the lead neutron skin thickness is that astrophysicists will better understand the properties of neutron stars. The collisions of neutron stars have been observed by the gravitational waves their mergers generate; since neutron stars are densely packed nuclear matter, their cores remain enigmatic — they could host new physics, in the form of new states of matter. Sometimes, examining the itsy-bitsy behaviours of subatomic particles can tell you more about a star than peering at it through a telescope.