In a groundbreaking achievement, nuclear physicists at RIKEN have achieved the impossible by successfully producing an extremely neutron-rich isotope of sodium, known as 39Na, which was previously believed to be non-existent. This remarkable discovery has far-reaching implications for our understanding of atomic nuclei structure and the astrophysical processes involved in creating heavier elements found on Earth.
The newly created isotope, 39Na, breaks the boundaries set by many atomic nuclei models, which predicted its impossibility. If this super-heavy version of sodium were used to create table salt along with the most neutron-rich isotope of chlorine, it would taste and behave like regular salt, except it would be roughly 1.6 times heavier, according to nuclear physicist Toshiyuki Kubo.
Each of the 118 known elements has a fixed number of protons, but the number of neutrons in their nuclei can vary. The stable form of sodium typically contains 12 neutrons; however, the recently discovered form, 39Na, has a striking 28 neutrons, which is more than double the previous record-holder, 37Na. This previous isotope of sodium was discovered over 20 years ago.
Neutrons do not influence an atom’s electrons and therefore have no impact on the element’s chemistry, as they are electrically neutral. As a result, different isotopes of the same element with varying neutron counts are chemically indistinguishable.
The motivation to search for 39Na arose from a previous experiment where Kubo and his team at the RIKEN Nishina Center for Accelerator-Based Science stumbled upon what they believed to be a 39Na nucleus. “We were very surprised at this one event,” Kubo recalls. “And so, we decided to revisit the search for 39Na in our present experiment.”
To eliminate any doubts, the team conducted a recent experiment at RIKEN’s Radioactive Isotope Beam Factory, one of the few nuclear facilities worldwide capable of producing such nuclei. In just two days, they successfully created nine nuclei of the 39Na isotope, confirming its existence.
This breakthrough holds vital significance in unraveling the mysteries of atomic nuclei structure. The newfound knowledge has the potential to enhance our comprehension of the complex astrophysical processes involved in the formation of heavier elements on Earth.
In nuclear theory, this discovery serves as a crucial reference point for refining models of neutron-rich nuclei and assessing their accuracy. The study of neutron-rich nuclei involves intricate calculations, and so far, theoretical physicists have only been able to accurately model stable nuclei with fewer neutrons. Identifying the 39Na isotope could pave the way for more precise calculations of nuclei with a higher neutron count.
Moreover, this achievement sheds light on the origins of heavier elements. It is widely believed that the creation of heavy metals on Earth is the result of immense energy released during the merger of two neutron stars or the collision of neutron stars with black holes. The subsequent release of gas and dust contributes to the formation of rare materials found on planets like Earth. However, the specific processes involved in the production of heavy metals have been the subject of ongoing debate.
By creating the most neutron-rich form of sodium, the team of scientists at RIKEN has pushed the boundaries of scientific knowledge. This exceptional isotope, with 39 neutrons and 11 protons, designated as 39Na, has added to the known existence limit called the neutron ‘drip line.’ This discovery holds significant implications for future research on neutron-rich elements and the exploration of their properties.
Lead physicist Toshiyuki Kubo, with over four decades of experience in isotope discovery, is thrilled about this breakthrough. The existence of 39Na challenges multiple nuclear models and theories, providing a crucial benchmark for validating and enhancing nuclear mass models. Kubo highlights the accuracy of a Japanese model from 2020, which successfully predicted the existence of 39Na and has gained credibility due to its accurate predictions for other isotopes.
Determining the neutron ‘drip line,’ which marks the maximum number of neutrons an element can possess before leaking neutrons, is a formidable task. Scientists have only determined this limit up to neon, the tenth element on the periodic table, leaving 108 more elements to investigate. Achieving this feat is difficult due to the minuscule possibilities involved in creating nuclei close to the limits of stability and the challenge of ruling out the existence of other nuclei with even greater neutron counts. Researchers believe it may be possible to create 41Na, which would become the drip line for sodium. However, the 2020 Japanese model suggests that 39Na could be the limit.
Moving forward, Kubo and his team plan to experimentally determine the drip line for magnesium, the element following sodium, and further explore the structure of 39Na. Their goal is to directly study the nuclear structure that enables the existence of this remarkable isotope. “We would like to directly study the nuclear structure that allows 39Na to exist,” Kubo explains.
In the ever-evolving field of nuclear physics, the discovery of the highly neutron-rich sodium isotope 39Na is a groundbreaking achievement that challenges existing knowledge and paves the way for further advancements in our understanding of atomic nuclei structure and the formation of heavier elements.
