A new form of sodium packs a whacking 28 neutrons in its atomic nucleus, along with the 11 protons that define its chemical identity.
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.
Three years ago, an experiment with the RIKEN center’s particle accelerator, a superconducting cyclotron called the, 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.
“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.
The neutron drip line has served up surprises before. For example, it leaps from 16 neutrons for oxygen to 22 neutrons for fluorine . 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.