How many neutrons can you cram into an atom? More than physicists thought

Physicists in Japan have blasted out the heaviest calcium nuclei ever seen—each containing the 20 protons needed to make the element, but with a huge number—40—of neutrons. That’s twice as many neutrons as in calcium’s most common form, and a couple more than the previous record. The finding suggests it may be possible to cram even more neutrons into nuclei than previously thought, and it could have implications for the theory of neutron stars.

“This is indeed an important and interesting finding,” says Daniel Phillips, a theoretical nuclear physicist at Ohio University in Athens. Physicists’ models of nuclear structure are tuned to more common nuclei with roughly equal numbers of protons and neutrons, he says, and scientists need to know how much those theories err as they extrapolate them to nuclei with more lopsided ratios of protons and neutrons.

The atomic nucleus consists of protons and neutrons held together by the nuclear strong force. The number of protons determines an atom’s identity as a chemical element; the number of neutrons determines the isotope of that element. Textbooks often depict a nucleus as so many protons and neutrons stuck together like gumdrops, but real nuclei are far more complicated. Although it’s made of discrete particles, the average nucleus acts more like a droplet of fluid with surface tension. At the same time, however, nuclei have abstract quantum energy shells and can be more tightly bound when they have magic numbers of protons or neutrons that fill those shells—just as, on a larger scale, atoms are more inert when they have filled shells of electrons. Furthermore, protons and neutrons can form fleeting pairs and trios that also change a nucleus’s properties and stability.

Theorists use different models to account for these competing behaviors. For relatively light nuclei, ab initio models tackle the interactions of individual protons and neutrons head on. But such models bog down for heavier nuclei, so theorists employ more approximate models based on “density functionals” that treat the distribution of protons and neutrons as continuous variables. The dozens of such models can disagree on things as basic as how many neutrons will stick to a nucleus, a limit that physicists often visualize on a gridlike chart. On the chart, which shows the number of protons on the vertical axis and the number of neutrons on the horizontal axis, known and predicted nuclei form a pickle-shaped swath whose lower boundary marks the “neutron drip line”: the maximum number of neutrons a nucleus can hold. Physicists don’t know exactly where the drip line is.

Now, a 30-member team from Japan’s RIKEN laboratory in Wako and Michigan State University (MSU) in East Lansing have produced a batch of new neutron-rich nuclei that suggest the drip line is farther off than many theories predict, they reported last week in Physical Review Letters . The team hunted in the neighborhood of calcium because its magic number of protons already imbues it with stronger binding, says Alexandra Gade, an experimenter at MSU.

Using RIKEN's Radioactive Isotope Beam Factory, researchers ripped apart heavy zinc nuclei by firing a beam of them through a beryllium target. They then used an extremely precise magnetic separator to sort through the vast array of nuclei in the wreckage. In all, the team produced eight new neutron-rich nuclei, including calcium-59 and calcium-60, with 39 and 40 neutrons, respectively. To produce two calcium-60 nuclei, researchers had to shoot 200 quadrillion zinc nuclei into the target.

The new results appear to trip up the ab initio models, which generally predict that calcium-60 should not exist. In fact, the data suggest it might be possible to make calcium nuclei with even more neutrons, Gade says. Of the 35 models the researchers compared, the two that best fit all the new data predict that calcium isotope exists up to calcium-70, which would have a whopping 50 neutrons.

Gade cautions against making any sweeping generalizations about the drip line. However, Phillips says he hopes the results will better constrain the drip line so experimenters don't have to simply feel it out. “I certainly hope it's not a matter of going element by element,” he says. In addition to its fundamental importance, the location of the drip line could have implications for the astrophysics of neutron stars. For example, processes in the crusts of these stellar remnants are thought to produce neutron-rich nuclei right out to the drip line, Gade says, so the precise properties and structure of the incredibly dense stars could depend on the details of the drip line.

Experimenters hope find even heavier isotopes of calcium and to make enough of the nuclei to study the properties, too. Such studies could become easier in 2022 when MSU completes its new $730 million accelerator, the Facility for Rare Isotope Beams (FRIB), which will be even more powerful that RIKEN's machine. "We looked at the calculations and [at the FRIB] we should be able to see calcium-68 and calcium-70," Gade says, "if they exist."