Sodium on steroids: a breakthrough in nuclear physics thought to be impossible

Illustration of the model of atomic matter

RIKEN nuclear physicists have successfully created an extremely neutron-rich sodium isotope, 39Na, previously predicted by many models of atomic nuclei to be non-existent. This discovery has significant implications for our understanding of the structure of atomic nuclei and the astrophysical processes that form the heaviest elements on Earth.

Nuclear physicists have created the most neutron-rich form of sodium, which will help reveal more about the complex world of nuclei.

RIKEN physicists have created an exceptionally neutron-rich isotope of sodium, 39Na, which was previously thought to be impossible. This breakthrough has important implications for understanding the structure of atomic nuclei and creating the heaviest elements on Earth.

The extremely neutron-rich form of the element sodium, which many models of atomic nuclei predict should not exist, was first created by nuclear physicists at RIKEN[1].

If you made table salt from this super-heavy version of sodium and the more neutron-rich isotope of chlorine, the other components of the salt would taste and behave like regular salt, except it would be about 1.6 times heavier. , says nuclear physicist Toshiyuki Kubo.

But much more than a scientific curiosity, this discovery has important implications for theories of the structure of atomic nuclei. This knowledge in turn informs our understanding of the astrophysical processes that form the Earth’s heavier elements.

In terms of nuclear theory, the discovery provides a vital reference point for modifying models of neutron-rich nuclei and for evaluating their[{” attribute=””>accuracy, explains Kubo. Theoretical studies of neutron-rich nuclei involve extremely complicated calculations, and theoretical physicists have so far only been able to precisely model more stable nuclei with few neutrons. This finding could help refine calculations for nuclei with more neutrons.

This in turn has implications for our understanding about the origins of heavier elements. For example, the nuclear astrophysical processes that create Earths heavy metals are thought to be the result of the huge amounts of energy produced by the merger of two neutron stars or collisions of neutron stars and black holes. The gas and dust released eventually contribute to the rare materials of planets, such as Earth. However, the exact processes that produce heavy metals have long been debated.

New Square Dripline

A new square on the drip line: Each square indicates an isotope, with the number of protons increasing as the squares move vertically upward and the number of neutrons increases horizontally to the right. The known existence limit, the neutron drip line, is indicated by a thick blue line. Sodium-39 (39Na) in red has 11 protons and 28 neutrons, giving it a mass number of 39. Its recent discovery by RIKEN researchers has seen it added to the drip line. Credit: 2023 RIKEN

Packing neutrons into sodium

Each of the 118 known elements has a fixed number of protons (11 in the case of sodium), but the number of neutrons in its nuclei has can vary, notes Kubo. The only stable form of sodium contains 12 neutrons, whereas the newly discovered one has more than double at 28, which is two more neutrons than the previous record holder for the most-neutron-rich isotope of sodium, 37Na, which was discovered more than 20 years ago.

Since neutrons are electrically neutral, they dont influence an atoms electrons and hence have no effect on the elements chemistry. Thus, atoms of the same element that contain different numbers of neutronsknown as isotopesare chemically indistinguishable.

The impetus to search for the new form of sodium (called 39Na because its nucleus contains 39 neutrons and protons) came from a previous experiment, when a team led by Kubo at the RIKEN Nishina Center for Accelerator-Based Science stumbled upon what appeared to be one nucleus of 39Na. We were very surprised at this one event, recalls Kubo. And so, we decided to revisit the search for 39Na in our present experiment.

In the latest experiment, they put the existence of 39Na beyond all doubt by creating nine nuclei of the isotope in a two-day run at RIKENs Radioactive Isotope Beam Factoryone of only about three nuclear facilities in the world currently capable of producing such nuclei.

Isotope hunter

Its far from the first time that Kubo has helped to create a new isotope during his four-decade-long career. Actually, Ive been involved in discoveries of about 200 new isotopes or so, he says. I really enjoy creating and observing what nobody has ever seen before.

But the discovery of 39Na, has special significance for him, not least because many nuclear models predict that it shouldnt exist. The discovery makes a significant impact on nuclear mass models and nuclear theories that address the edge of the nuclear stability, because it provides a key benchmark for their validation, explains Kubo. For example, Kubo notes that a model developed by a Japanese team in 2020 correctly predicted the existence of 39Na and its predictions for other isotopes have been on target[2]increasing its credibility.

Drip line monitoring

One of the reasons discovery is important is because 39Na may be the most neutron-rich version of sodium that can be produced. Nuclear physicists are particularly interested in determining the maximum number of neutrons an element can have before it starts losing neutrons, an amount known as the neutron drip line when plotted on a table of nuclei. The location of this limit provides a key reference point not only for nuclear theories, but also for nuclear mass models that play a key role in theories of nucleosynthesis.

But it is extremely difficult to ascertain an element’s drip line, so far nuclear physicists have only been able to determine it down to the tenth element of the periodic table, neon, meaning they still have 108 more elements to go.

One of the reasons dripline is difficult to measure is because of the tiny possibilities involved in creating cores that are close to the limits of stability. Another difficulty is that it is extremely difficult to rule out the existence of other nuclei that have even more neutrons. Kubo says it might be possible to do that 41Na, in which case it would become the dripline for sodium, although he notes that the 2020 Japanese model does 39Na is the drip line.

Next, Kubo and his team plan to attempt to experimentally determine the dripline for magnesium, a superior element to sodium. They also want to probe the structure of 39n/a. We would like to directly study the nuclear structure that allows it 39Na exist, explains Kubo.


  1. Discovery of 39Na by DS Ahn et al., November 14, 2022,

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