Some atomic nuclei have thin skin: they are surrounded by a thin shell of neutrons.
Now physicists know how thick that neutron skin is for a particular type of nucleus. The skin of lead-208, a variety of lead with 126 neutrons in addition to its 82 protons, is about 0.28 trillion millimeters thick, according to online researchers April 27 in Physical Review Letters.
The core of lead-208 is roughly spherical, a proton ball embedded inside a slightly larger neutron ball. Measuring the difference between the sizes of the spheres reveals the thickness of the elegant skin of lead neutrons.
Measuring the size of the proton sphere is relatively simple: physicists can shoot electrically charged particles against the nucleus and study how particles disperse from positively charged protons. But because neutrons have no electric charge, the volume they enclose is more difficult to measure.
Therefore, researchers in the Lead Radius Experiment, or PREX-II, at Jefferson Lab in Newport News, Virginia, employed a special technique. The team measured how electrons dispersed differently to the nucleus depending on their rotation or angular momentum. Because electrons interact with neutrons at different speeds depending on the direction of their rotation, the experiment revealed the width of the neutron sphere, allowing researchers to calculate the thickness of the neutrons' skin.
Neutron skin was a little thicker than physicists had predicted. That’s a result that “makes everyone sit back and pay attention and start questioning their assumptions,” says PREX-II physicist and co-spokesman Krishna Kumar of the University of Massachusetts Amherst.
A future measurement will test the skin of another nucleus, calcium-48. Measures together could help scientists better understand theories of the atomic nucleus and other realms where neutrons are crowded together like neutron stars, extremely dense dead stars and composed mainly of neutrons (SN: 20/04/21).