Say what you want about lead, it’s got a surprisingly thick skin—of neutrons, that is. In fact, the layer of neutrons on the outside of a lead nucleus is twice as thick as physicists thought, according to a new study. The seemingly abstruse result could have out-of-this-world implications: Neutron stars, the ultradense spheres left behind when stars explode in supernova explosions, could be stiffer and bigger than theory generally predicts.
“It’s a fantastic experimental achievement,” says Anna Watts, an astrophysicist at the University of Amsterdam who studies neutron stars. “It’s been talked about for years and years and years, and it’s so cool to finally see it done.”
An atom’s nucleus consists of protons and neutrons stuck together by the so-called strong nuclear force. Neutrons generally outnumber protons. Not by too much, however, as a large imbalance in the number of protons and neutrons increases a nucleus’ internal energy and can make it unstable. Theory generally predicts a large nucleus consists of a nearly equal mixture of proton and neutrons surrounded by a skin of pure neutrons.
It's the thickness of that skin that nuclear physicists with the Lead (Pb) Radius Experiment (PREX) at the Thomas Jefferson National Accelerator Facility have now measured. To do that, they bounced copious electrons off nuclei of lead-208, the most common isotope of the element, which has 82 protons and 126 neutrons. The negatively charged electrons interact with the positively charged protons mainly through the electromagnetic force, which deflects the electrons. Through such electromagnetic scattering, other physicists had previously measured the distribution of protons in the lead-208 nucleus and found that it extends to a radius of 5.50 fermi—a fermi being one-millionth of 1 nanometer.
To probe the neutrons, PREX physicists exploited the fact the electrons can interact with both protons and neutrons through the weak nuclear force. Feeble compared with the electromagnetic force, its strength depends on whether the incoming electron is spinning to the right—like a football thrown by a right-handed quarterback—or to the left. That handedness allowed PREX researchers to detect the influence of the weak force.
Researchers fired a beam of electrons, almost all spinning the same way, at the lead nuclei and measured the probability that they were deflected at a particular angle. Then, they flipped electrons so they spun the opposite way and looked for a one-part-in-1-million difference in the current of deflected electrons. That tiny asymmetry would signal the effect of the weak force, and its size would reveal the spatial spread of the neutrons. The physicists flipped the spin of the electrons 240 times per second, taking great care to ensure they didn’t change the energy, intensity, or trajectory of the beam.
The observed asymmetry implies the lead nucleus has a neutron skin 0.28 fermi thick, give or take 0.07, PREX researchers report today in Physical Review Letters. That measurement jibes nicely with a previous measurement reported by the PREX team in 2012, but the new data reduce the uncertainty by half. The more precise finding suggests the neutron skin of lead-208 is about twice as thick as theorists had predicted and other less direct experiments had indicated. “It has forced everyone to start scrutinizing their assumptions, and that is a dream for experimentalists,” says Krishna Kumar, a physicist at the University of Massachusetts, Amherst, and co-spokesperson for the PREX team.
Some of those assumptions involve, ultimately, the nature of neutron stars. Even though an atomic nucleus is several times less dense than a neutron star, the former can be used to make inferences about the later, explains Jorge Piekarewicz, a nuclear theorist at Florida State University. 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.
The jumbo size is plausible to Cole Miller, an astronomer at the University of Maryland, College Park, who works with NASA’s Neutron star Interior Composition Explorer (NICER), an x-ray telescope on the International Space Station. NICER researchers use the spectrum of radiation from a rotating neutron star to deduce its size and even map irregularities on its surface. The instrument has measured the radiation from two neutron stars 1.4 and 2.1 times as massive as the Sun and has found both to be about 13 kilometers in radius.
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.”
Still, the surprising PREX result will likely spur nuclear physicists and astrophysicists to reexamine the theoretical links between atomic nuclei and neutron stars, Piekarewicz says. “It’s a psychological jolt to the community.”