A large atomic nucleus is like a chocolate truffle with a gooey interior and a harder shell. Inside, the nucleus contains a mixture of protons and neutrons. Outside, it's covered with a nearly pure layer of neutrons—the "neutron skin." Now, for the first time, nuclear physicists have measured the thickness of that skin in a fairly direct way. More-precise future measurements could transform the study of all nuclei and even of neutron stars.
"This single piece of data would provide an extremely useful and extremely important constraint on theoretical models," says Witold Nazarewicz, a nuclear theorist at the University of Tennessee, Knoxville, who was not involved in the new study.
In the past, experimenters have tried to measure the distribution of neutrons in nuclei by pelting the nuclei with protons, antiprotons, or particles called pions. These particles all interact with the nucleus through the strong nuclear force, which is so complicated that to interpret the results, researchers have to resort to approximate theoretical models. As a result, the answer you get depends on which model you use.
Now, one team has measured the neutron distribution in a way that relies a lot less on theory. At the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia, physicists with the Lead Radius Experiment (PREX) fired electrons at a thin sheet of lead-208. Each lead-208 nucleus has 82 protons and a whopping 126 neutrons, making it especially rich in the latter. Electrons do not feel the strong force but instead interact with the nucleus primarily through the electromagnetic force. So the tug of the protons' electric field deflects the trajectories of the electrons. By studying that deflection, researchers can measure the distribution of protons to determine a nucleus's "proton radius."
PREX researchers went a big step further to measure the neutron distribution and the "neutron radius" of lead-208  (from which they would subtract the proton radius to get the thickness of the neutron skin). To do that the team had to tease out the effects of a third, much fainter force, the weak nuclear force. Through that force, the electrons interact primarily with the neutrons in the nucleus. Unlike the electromagnetic force, the weak force affects electrons differently depending on which way they spin—whether spiraling to the right like a football thrown by a right-handed quarterback or to the left like a pigskin thrown by a southpaw. Thus, the weak force should produce a slight asymmetry in the deflection of right- and left-spinning electrons that can reveal the distribution of the neutrons.
So the PREX team bombarded lead-208 nuclei with pulses of electrons all spinning the same way—either to the right or to the left—and compared the results. "The thing that makes the experiment hard is that when you flip the spin [from one bunch to the next], you don't want to change anything else," says JLab's Robert Michaels, co-spokesperson for the PREX team. "If you change the energy or the trajectory of the beam, you introduce a systematic error" that could fake a signal.
PREX researchers measured a 0.656-parts-per-million asymmetry in the probability that right- or left-spinning electrons would be deflected by a certain angle. From that asymmetry, they deduced a neutron radius of 5.78 millionth of a nanometer, as they report in a paper in press at Physical Review Letters. Subtracting the known proton radius revealed a neutron skin 0.33 millionths of a nanometer thick, give or take about 50%.
So what's a measure of lead-208's neutron skin good for? Quite a lot. The incredibly complex mathematical theory of the nucleus contains terms that depend on the difference of the proton distribution and the neutron distribution. Measuring the neutron skin of lead-208 could bring key parameters in those terms into much sharper focus, Nazarewicz says. That, in turn, could lead to much better estimates of how many neutrons can be crammed into heavy nuclei or of which nuclei are involved in the so-called r process, a cascade of nuclear reactions inside exploding stars that forge half the elements heavier than iron throughout the universe, Nazarewicz says.
Nailing down such parameters would have equally big implications for the theory of neutron stars, says James Lattimer, a theoretical astrophysicist at Stony Brook University in New York state. "That directly tells you the radius of a neutron star [of a given mass] and a lot of other things like the thickness of its crust, the response of its surface to explosions, et cetera," Lattimer says.
Alas, the uncertainty on the PREX measurement is still too large to pin down the parameters, Lattimer says. "It's a very important experiment and has the potential to constrain theory very nicely, but it's not there yet," he says. JLab's Michaels says the PREX team will run the experiment next year and aims to reduce the uncertainty to one-third its current value. "Then it becomes a very interesting result," he says.
Something else physicists will be watching for: The PREX measurement suggests that the neutron skin of lead-208 is twice as thick as more-precise but model-dependent methods indicate. Right now, the PREX result has too much uncertainty to pose a direct challenge to earlier estimates. But if the new value holds up as the uncertainty shrinks, things could get really interesting, Nazarewicz says: "Then, there is something wrong with all theoretical models." There's a possibility to set your skin a-tingling.