Physicists measure the neutron skin of a lead atom

For three months in 2019, physicists detonated a beam of electrons on atoms of lead held in place by a diamond foil. The team was trying to determine the thickness of the neutron skinot, the neutron sheath of neutral charge which surrounds positively charged protons in the nucleus of a lead atom. They succeeded.

The neutron skin of lead-208 is 0.28 femtometers – 0.28 trillionths of a millimeter – the team determined, increasing the estimated skin thickness by a tenth of a femtometer from previous calculations. It’s a big change, on an atomic scale.

TeasThe measurement was sort of like “knowing you had that tiger by the tail,” Kent Paschke, a University of Virginia physicist and co-author of the new study, said in a phone call. It took three months of intense laser operations, persistent power outages, and continuous system monitoring. The team were not sure they could complete the job within the three months they were given. But in the end, the atomic-scale marathon gave an exact measure, which redefines our understanding neutron size skin.

Previous calculations for the skin relies on coarser estimates and assumptions; the researchers hope that this new measurement will become a fundamental element for future observations at the nuclear and stellar scale. They did their job at Continuous electron beam accelerator installation at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. The measurement is the culmination of the second interpretation of the Pb Radius experiment, or PREX-II, and of the team’s results. are published today in Physical Review Letters.

“This measurement is exciting for the scientist because it makes a measurement of this neutron radius with the fewest assumptions ever made,” saidof them co-author Krishna Kumar, experimental nuclear physicist at the University of Massachusetts at Amherst, during a video call. “This is what experimental sciencentists live for. “

By measuring how electrons of different spins scatter over lead nuclei, the team was able to determine the thickness of the neutron skin, a measurement that was previously difficult to determine because neutrons have no electrical charge. . To draw a bead on the thickness of the neutron skin, the team took measurements using weak nuclear force, rather than the electromagnetic force that electrons and protons so easily display.

This particular isotope of lead, lead-208, was chosen because of its size and structure; it’s the largest superstable nucleus that physicists know of, and, perhaps more importantly, it’s doubly “magical,” meaning its protons and neutrons completely fill their orbital shells.

“Lead-208 is particularly useful because it approximates uniform nuclear material,” Paschke said. “You need these theoretical techniques to describe big and heavy things.”

Ah, physics, an area of ​​extremes. In this case, examining the skin of neutrons surrounding an atom’s nucleus has implications for understanding neutron stars, the densest objects in our universe in addition to black holes. Neutron stars are the collapsed nuclei of dead stars; they are so dense that experts are not quite sure what lies at their base. This is been suggested that they can be the source of axions, a candidate to explain dark matter.

“The pressure of neutron matter holds a neutron star against gravity and prevents it from collapsing into a black hole,” said study co-author Chuck Horowitz, an astrophysicist at the University of the Illinois, in an email. “We find a relatively thick neutron skin in the Pb [lead], and this implies high pressure and suggests that neutron stars are relatively large.

The hope that lies in the thickness of the skin of lead neutrons is that astrophysicists will better understand the properties of neutron stars. Collisions of neutron stars have been observed by gravitational waves generated by their fusions; since neutron stars are densely packed nuclear matter, their nuclei remain enigmatic – they could accommodate new physics, in the form of new states of matter. Sometimes examining the roving behaviors of subatomic particles can tell you more about a star than looking at it through a telescope.

After: Typical neutron star is just 13.6 miles wide, new ultra-precise measurement shows