An international team led by Berkeley Lab's Heavy Element Group announced the production of superheavy element 116 using a titanium beam, a significant step towards making element 120. This discovery was shared at the Nuclear Structure 2024 conference, with the corresponding science paper to be available on arXiv and submitted to Physical Review Letters.
"This reaction had never been demonstrated before, and it was essential to prove it was possible before embarking on our attempt to make 120," said Jacklyn Gates, a nuclear scientist at Berkeley Lab leading the effort. "Creation of a new element is an extremely rare feat. It's exciting to be a part of the process and to have a promising path forward."
The team successfully produced two atoms of element 116, livermorium, over 22 days using the lab's heavy-ion accelerator, the 88-Inch Cyclotron. Creating an atom of element 120 is expected to be even rarer, but the rate of producing 116 suggests it is feasible to search for element 120 over several years.
"We needed for nature to be kind, and nature was kind," said Reiner Kruecken, director of Berkeley Lab's Nuclear Science Division. "We think it will take about 10 times longer to make 120 than 116. It's not easy, but it seems feasible now."
Element 120, if discovered, would be the heaviest atom ever created, residing in the eighth row of the periodic table, near the theorized "island of stability." Superheavy elements typically break apart almost immediately, but the right combination of protons and neutrons could form a more stable nucleus, providing researchers more time to study it. Understanding these elements can offer insights into atomic behavior, test nuclear physics models, and define the boundaries of atomic nuclei.
Making superheavy elements
Creating superheavy elements involves colliding two lighter elements to combine their protons into the desired final atom. However, this process is incredibly challenging, often requiring trillions of interactions for successful fusion. Additionally, not all elements can be feasibly turned into a particle beam or target.
Researchers select specific isotopes, variants of elements with the same number of protons but different numbers of neutrons, for their beam and target. The heaviest practical target is californium-249, which has 98 protons. To create element 120, researchers need a beam of titanium atoms with 22 protons, as opposed to the commonly used calcium-48 with 20 protons.
At the 88-Inch Cyclotron, experts verified that a sufficiently intense beam of titanium-50 could be produced over several weeks to create element 116, the heaviest element ever made at Berkeley Lab.
Until now, elements 114 to 118 had only been made with a calcium-48 beam, known for its "magic" configuration of neutrons and protons aiding in fusion. The experiment with titanium-50 confirmed the feasibility of using a "non-magic" beam for producing superheavy elements.
"It was an important first step to try to make something a little bit easier than a new element to see how going from a calcium beam to a titanium beam changes the rate at which we produce these elements," said Jennifer Pore, a scientist in Berkeley Lab's Heavy Element Group. "When we're trying to make these incredibly rare elements, we are standing at the absolute edge of human knowledge and understanding, and there is no guarantee that physics will work the way we expect. Creating element 116 with titanium validates that this method of production works and we can now plan our hunt for element 120."
The plan to create superheavy elements at Berkeley Lab is included in the Nuclear Science Advisory Committee's 2023 Long-Range Plan for Nuclear Science.
Feats of engineering
Generating an intense beam of titanium isotopes is complex. It begins with a special piece of titanium-50, a rare isotope constituting about 5% of all titanium in nature. This metal is heated in a tiny oven until it vaporizes at nearly 3000 degrees Fahrenheit.
This process occurs in an ion source called VENUS, a superconducting magnet that confines plasma. Free electrons in the plasma gain energy, knocking off electrons from titanium atoms, which are then maneuvered by magnets and accelerated in the 88-Inch Cyclotron.
"We knew these high-current titanium beams would be tricky because titanium is reactive with many gases, and that affects ion source and beam stability," said Damon Todd, an accelerator physicist at Berkeley Lab. "Our new inductive oven can hold a fixed temperature for days, keeping titanium output constant and aiming it right at VENUS' plasma to avoid stability issues. We are extremely pleased with our beam production."
Every second, about 6 trillion titanium ions strike the target (plutonium for element 116, californium for element 120), which is thinner than paper and rotates to dissipate heat. Operators fine-tune the beam's energy to ensure successful fusion without destroying the target nuclei.
When a superheavy element forms, it is separated from other particles by magnets in the Berkeley Gas-filled Separator (BGS) and detected by SHREC: the Super Heavy RECoil detector, which captures data to identify the element as it decays into lighter particles.
"We're very confident that we're seeing element 116 and its daughter particles," Gates said. "There's about a 1 in 1 trillion chance that it's a statistical fluke."
Plans for 120
Before attempting to create element 120, the 88-Inch Cyclotron must be prepared for a californium-249 target, which will be crafted by Oak Ridge National Laboratory.
"We've shown that we have a facility capable of doing this project, and that the physics seems to make it feasible," Kruecken said. "Once we get our target, shielding, and engineering controls in place, we will be ready to take on this challenging experiment."
The experiment could begin in 2025, potentially taking years to produce just a few atoms of element 120.
"We want to figure out the limits of the atom, and the limits of the periodic table," Gates said. "The superheavy elements we know so far don't live long enough to be useful for practical purposes, but we don't know what the future holds. Maybe it's a better understanding of how the nucleus works, or maybe it's something more."
Related Links
Lawrence Berkeley National Laboratory
Understanding Time and Space
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