A new form of sodium—the element that combines with chlorine to make salt—packs a whacking 28 neutrons in its atomic nucleus, along with the 11 protons that define its chemical identity. With more than double the 13 neutrons in natural sodium, the neutron-rich isotope of the element is so extreme that few theoretical models predicted its existence. “It’s a surprise that these neutrons keep on hanging on,” says Katherine Grzywacz-Jones, a nuclear physicist at the University of Tennessee, Knoxville, who was not involved in the work.
Researchers at Japan’s RIKEN Nishina Center for Accelerator-Based Science created just a handful of sodium-39 nuclei. But their mere existence challenges physicists’ understanding of nuclear structure. It also suggests that tracing the process by which exploding stars forge many elements—a goal of a major new facility in the United States—may be more difficult than thought.
Three years ago, an experiment with the RIKEN center’s particle accelerator, a superconducting cyclotron called the Radioactive Isotope Beam Factory, produced a tantalizing hint of a single sodium-39 nucleus. “Therefore, we repeated the experiment with much higher beam intensity and a longer beam time,” says Toshiyuki Kubo, a RIKEN nuclear physicist.
Kubo’s 26-member team shot a beam of calcium-48 nuclei through a beryllium target to shred them and funneled the fragments through a snaking chain of magnets called BigRIPS. Researchers tuned that chicane so only sodium-39 or a nucleus with a similar mass-to-charge ratio could slalom through. The energy a nucleus deposited in a detector at the end revealed its charge. From the charge and mass, Kubo and colleagues could easily tally a nucleus’ protons and neutrons. Firing 500 quadrillion calcium-48 nuclei through the target, they spied nine sodium-39 nuclei, they report in a paper in press at Physical Review Letters.
Predicting which combinations of protons and neutrons will bind into a nucleus can be tricky. Protons and neutrons stick together by exchanging particles called pions, and a quantum mechanical effect favors nuclei with similar numbers of protons and neutrons. But the electrically charged protons repel one another, tilting the balance toward fewer protons. Nuclei also vary from a single proton to hundreds of protons and neutrons, and different theoretical approaches tend to work better in different mass ranges.
“By far most models did not predict that sodium-39 should be bound,” says Brad Sherrill, a nuclear physicist at Michigan State University and an author on the paper. However, 2 years ago, Witold Nazarewicz, a nuclear theorist at Michigan State, and colleagues tried to predict all possible nuclei by averaging model predictions, each weighted by its uncertainty. That gave a 50% probability that sodium-39 would exist. “Is the [RIKEN] result surprising?” Nazarewicz says. “No. Is it important? Yes.”
It adds an important detail to the nuclear landscape, he says. Physicists plot known and predicted nuclei on a checkerboard-like chart, with the number of protons climbing vertically and the number of neutrons increasing left to right. The nuclei form a wide swath diagonally across the chart, whose lower edge is called the neutron drip line. It marks the limit at which it becomes impossible to cram more neutrons into a nucleus with a given number of protons. And it’s known only up to neon, element 10.
The neutron drip line has served up surprises before. For example, it leaps from 16 neutrons for oxygen (element 8) to 22 neutrons for fluorine (element 9). To explain that jump, theorists had to include forces not just among pairs of protons and neutrons in a nucleus, but also among trios, Sherrill says. Some other bit of overlooked physics may explain why the drip line appears to leap by four neutrons from neon-34 to sodium-39.
The results could complicate a goal for physicists. Half of all elements heavier than iron emerge from supernova explosions, as nuclei quickly absorb neutrons gushing from the explosion even as they repeatedly undergo radioactive beta decay—in which a neutron in a nucleus spits out an electron and morphs into a proton. Precisely identifying the nuclei in the process is a priority for a new $730 million linear accelerator called the Facility for Rare Isotope Beams (FRIB) at Michigan State. If the drip line lies farther out, those nuclei may contain more neutrons and be harder to make, Sherrill says.
The first results from FRIB, which turned on in May, examine nuclei near sodium-39. Researchers there also shredded a beam of calcium-48 to create neutron-rich isotopes of magnesium, aluminum, silicon, and phosphorus—the elements following sodium—and measured how quickly they beta decay, the team reports in a paper in press at Physical Review Letters. In another tidbit to inform models, the half-life of magnesium-38 was surprisingly short, says Heather Crawford, a nuclear physicist at Lawrence Berkeley National Laboratory and lead author.
For Crawford’s experiment, FRIB produced a beam one-twelfth as intense as the one in the RIKEN study. In a few years, FRIB should increase its beam intensity 400-fold, making it possible to trace the neutron drip line farther up the chart, Crawford notes. “As FRIB ramps up in power, that’s one of the first things I expect will be pursued.”