What Makes an Element? The Frankenstein of Sodium Holds Clues

A few years ago, a group of physicists created an unusual, never-before-seen subatomic particle. Using a particle accelerator at Riken, a Japanese research institute, they slammed streams of calcium nuclei against a metal disk, over and over, for hours at a time. Then, sifting through the aftermath of the collisions, they found their coveted particle. They named their creation: sodium .That’s right, sodium. Don’t let the familiar name fool you; you’ll never find this object in ordinary table salt. Almost all sodium on Earth is sodium-23, where the number refers to the 11 protons and 12 neutrons that make up its nucleus. Yet those 23 particles do not encompass all that can or could be sodium. Technically, any nucleus with 11 protons is sodium. The periodic table, after all, organizes the elements by the number of protons in their nuclei, and sodium is element #11. That says nothing about the number of neutrons the particle harbors inside.
What the physicists in Japan had created was a kind of Frankensodium, an 11-proton particle with a whopping 28 neutrons stuffed into its nucleus. This sodium-39 was the most massive isotope of sodium known to exist.It took eight hours and hundreds of quadrillions of collisions—that’s 1017—to produce one single sodium-39. And it fell apart almost immediately. “The production rate is very small for these isotopes,” concedes Riken physicist Toshiyuki Kubo.

Physicists create their unusual particles by accelerating a beam of calcium nuclei near the speed of light and colliding it against the edge of a silver beryllium disk (pictured). Photograph: Toshiyuki Kubo/Riken
The specimen served its purpose, though. It set a new record for what sodium could be, a longstanding quest of a certain subgroup of scientists. Over several decades, physicists have gone down the periodic table—hydrogen, helium, lithium, and so on—to find the heaviest isotope of each element permitted by the laws of physics. Publishing this Monday in Physical Review Letters, the Riken physicists and their team confirmed that the limit for a fluorine nucleus is 22 neutrons, and a neon nucleus can contain up to 24. Sodium’s limit remains uncertain, but from this experiment, it appears to be at least 28 neutrons. Physicists call the limit the “neutron drip line” because if you try to push a nucleus’s limit by adding another neutron, that neutron simply slips off without any resistance.It took some 20 years to confirm the nuclear limits of fluorine and neon, because the experiments are so difficult, says physicist Artemis Spyrou of Michigan State University, who was not involved in the work. To prove a particle is the heaviest of its kind, it’s not enough just to create it. You have to show that nothing heavier exists. “That’s the hard part,” says Spyrou. “If you don’t see it, is it because it doesn’t exist? Or is it because your experiment wasn’t good enough?”

After slamming calcium nuclei at a metal target, the physicists use a football-field-long machine (pictured) that sorts the debris for interesting particles with magnets.Photograph: Toshiyuki Kubo/Riken
Kubo and his team spent years preparing for the task. They had to upgrade their accelerator power. Kubo also built a sophisticated particle filter, a machine nearly the length of a football field, which uses magnets to separate atomic nuclei from one another. Then, to show that fluorine-31, the version with 22 neutrons, was the heaviest type of fluorine, the team performed particle collisions that theoretical models predicted should produce fluorine-32 and fluorine-33. When they didn’t see these heavier fluorines, they could confirm with near certainty that fluorine-31 would prevail. (Neon-34 gained champion status via a similar protocol.) The team did not make these official pronouncements lightly: They analyzed their results for nearly five years before publishing them this week.