Physicists have used a beam of titanium-50 to create the element livermorium. This is the first time that nuclei heavier than calcium-48 have been used to synthesize a superheavy element. The international team, led by Jacklyn Gates at Lawrence Berkeley National Laboratory (LBNL) in California, hopes that their approach could pave the way for the discovery of entirely new elements.
Superheavy elements are found at the bottom right of the periodic table and have atomic numbers greater than 103. Creating and studying these huge elements pushes our experimental and theoretical capabilities and provides new insights into the forces that hold nuclei together.
Techniques for synthesizing these elements have vastly improved over the decades, and usually involve the irradiation of actinide targets (elements with atomic numbers between 89–102) with beams of transition metal ions.
Earlier in this century, superheavy elements were created by bombarding actinides with beams of calcium-48. “Using this technique, scientists managed to create elements up to oganesson, with an atomic number of 118,” says Gates. Calcium-48 is especially suited for this task because of its highly stable configuration of protons and neutrons, which allows it to fuse effectively with target nuclei.
Short-lived and difficult
Despite these achievements, the discovery of new superheavy elements has stalled. “To create elements beyond oganesson, we would need to use targets made from einsteinium or fermium,” Gates explains. “Unfortunately, these elements are short-lived and difficult to produce in large enough quantities for experiments.”
To try to move forward, physicists have explored alternative approaches. Instead of using heavier and less stable actinide targets, researchers considered how lighter, more stable actinide targets such as plutonium (atomic number 94) would interact with beams of heavier transition metal isotopes.
Several theoretical studies have proposed that new superheavy elements could be produced using specific isotopes of transition metals, such as titanium, vanadium, and chromium. These studies largely agreed that titanium-50 has the highest reaction cross-section with actinide elements, giving it the best chance of producing elements heavier than oganesson.
However, there is significant uncertainty surrounding the nuclear mechanisms involved in these reactions, which have hindered experimental efforts so far.
Theoretical decrease
“Based on theoretical predictions, we expected the production rate of superheavy elements to decrease when beams beyond calcium-48 were used to bombard actinide targets,” Gates explains. “However, we were unsure about the extent of this decrease and what it would mean for producing elements beyond oganesson.”
To address this uncertainty, Gates’ team implemented a reaction that has been explored in several theoretical studies – by firing a titanium-50 beam at a target of plutonium-244. Based on the nuclear mechanisms involved, this reaction has been predicted to produce the superheavy element livermorium, which has an atomic number of 116.
To create the titanium-50 beam, the researchers used LBNL’s VENUS ion source. This uses a superconducting magnet to contain a plasma of highly ionized titanium-50. They then accelerated the ions using LBNL’s 88-Inch Cyclotron facility. After the reaction, the Berkeley Gas-filled Separator isolated livermorium nuclei from other reaction products. This allowed the team to measure the chain of products created as the nuclei decayed.
Altogether, the team detected two decay paths that could be attributed to livermorium-290. This is especially significant because the isotope is thought to lie tantalizingly close to and “island of stability” in the chart of the nuclides. This comprises a group of superheavy nuclei that physicists predict are highly resistant to decay through spontaneous fission. This gives these nuclei vastly longer half-lives compared with lighter isotopes of the same elements.
If the island is reached, it could be a crucial stepping stone for synthesizing new elements beyond oganesson. For now, Gates’ team is hopeful its result could pave the way for a new wave of experiments and plan to use their titanium-50 beam to bombard a heavier target of californium-249. If these experiments see similar levels of success, they could be a crucial next step toward discovering even heavier superheavy elements.
The research is described in a preprint on arXiv.
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