Two atoms of element 116 demonstrate path to hunt for element 120 and extend the periodic table
BY ROBERT F. SERVICE
BERKELEY, CALIFORNIA—At 2:47 p.m. on 27 April, a computer connected to an atom smasher here at Lawrence Berkeley National Laboratory (LBNL) registered a single blip, followed almost immediately by three more. An automated Slack message of a “thinking” face emoji pinged its way to Jacklyn Gates, head of LBNL’s superheavy element team, who soon discovered that the blips—signals spit out after atoms crashed into the bull’s eye of a detector—represented evidence for an atom of element 116 followed by its decay into daughter products. In 2000, scientists in Russia first created element 116, the third heaviest atom known in nature, by smashing a beam of calcium atoms into a target made of curium. But LBNL used a beam of titanium atoms and a plutonium target, a rival approach that sets the stage for the lab to hunt for element 120, which would be the heaviest element ever created.
The single atom of 116 and another LBNL created a few weeks later were announced this week at the Nuclear Structure 2024 conference and are detailed in a paper submitted to Physical Review Letters. The two atoms, on their own, represent a “very exciting result,” says Witold Nazarewicz, chief scientist at the Facility for Rare Isotope Beams at Michigan State University. But he says they also show that LBNL is back in the global quest to extend the periodic table.
From 1936 through 1976, LBNL used its atom smashers to discover 16 elements, from element 43 (technetium) to 106 (seaborgium). But in the decades that followed the axis of superheavy research shifted to facilities in Germany, Japan, and Russia, which collectively discovered the last 12 elements.
“It’s really important that the U.S. is back in this race, because superheavy elements are very important scientifically,” Nazarewicz says. These elements test the limits of nuclear theory by showing how many protons and neutrons can coexist in an atomic nucleus before cracking up. And because electrons in the inner shells of these atoms are pulled so tightly by the abundance of oppositely charged protons, they whip around at close to the speed of light, distorting the shape of the nucleus. Physicists are eager to capture the details, in part to understand whether superheavies could be forged in space, for example in the explosive mergers of pairs of neutron stars.
In principle, synthesizing superheavy elements is simple. Researchers fire beams of ions of a lighter element into a thin target of a heavier element and hope the two nuclei fuse. The most recent superheavies, 114 through 118, were discovered using a beam of calcium-48, whose “magic” number of protons and neutrons lend it stability and a higher probability of merging with target nuclei. But that approach concluded with calcium ions fusing with californium to produce element 118, because elements heavier than californium cannot be made in large enough quantities for viable targets. So, physicists began firing beams of heavier but less stable ions, such as titanium and chromium.
In the 2000s, Germany’s Helmholtz Center for Heavy Ion Research (GSI) was the first to hunt for element 120 using a titanium beam that fired 4 trillion ions per second. But researchers estimated the experiment would need to run for at least a year before creating a single atom of 120. So, it was somewhat unsurprising that a 6-week run in 2011 produced no signal for it. A similar search for element 119 also came up empty. GSI has since suspended the efforts, revamping its facility toward particle astrophysics research.
LBNL’s new titanium beam is more powerful, generating some 6 trillion ions per second. It begins with a ceramic oven the size of a peanut that vaporizes titanium at temperatures of 1800°C. The atoms are confined by superconducting magnets that must be cooled to near absolute zero. Microwaves strip away about half of each atom’s electrons, creating ions that are injected into a cyclotron that accelerates them to 11% the speed of light. They are then fired at a plutonium disk, which spins 30 times per second to help dissipate the heat of the collisions.
Most of the ions pass through the target untouched. But on rare occasions a direct hit fuses the titanium and plutonium, and additional magnets steer the resulting compound nucleus toward electronic detectors. In the recent experiments, the LBNL team’s detectors twice registered atoms of 116, livermorium, that within milliseconds shed alpha particles (two protons and two neutrons) to form flerovium (114) and then copernicium (112) before that breaks up. “That decay chain makes it unambiguous” the lab created 116, says LBNL nuclear physicist Erich Leistenschneider.
Picking the right ion energy level for fusion remains more art than science. “Until you see an event you don’t know if your decisions are correct,” Gates says. In the end, the LBNL team required just 22 days of beam time to spot two atoms of 116, less than what they expected, and a sign that titanium and plutonium fuse more readily, with a higher “cross-section” than what some theorists predicted. “We needed for nature to be kind, and nature was kind,” says Reiner Kruecken, director of LBNL’s nuclear science program.
Still, theories suggest the cross-section for a titanium-californium combo will likely be 10-fold lower, meaning it could take close to a year of beam time to see a single atom of 120. “It’s not easy,” Kruecken says. “But it seems feasible now.”
LBNL is not alone in the race to bag a new superheavy. In 2018, nuclear scientists at RIKEN, Japan’s national R&D agency, launched an effort to synthesize element 119 by firing vanadium ions into a curium target. Disruptions from the pandemic and other obstacles limited the beam time to 95 days through 2021, with no events to show for it. Since then, the effort has proceeded in fits and starts, and is currently on hold as researchers replace a burned-out component that helps steer superheavies toward detectors, says Krzysztof Rykaczewski, a physicist at Oak Ridge National Laboratory (ORNL) who collaborates with the RIKEN team.
Superheavy hunters
At the dawn of the nuclear age, Lawrence Berkeley National Laboratory led the quest to fill out the periodic table, discovering 16 elements. Leadership then shifted elsewhere. The lab now thinks it could be the first to bag element 120.
INSTITUTION | GOAL | ION BEAM | TARGET | STATUS | ELEMENT DISCOVERIES |
---|---|---|---|---|---|
LBNL (U.S.) | 120 | Titanium | Californium | 2025 start | 16 |
JINR (Russia) | 120 | Chromium | Curium | Unknown | 6 |
GSI (Germany) | 119, 120 | Titanium | Berkelium, californium | 2011–12, unsuccessful | 5 |
RIKEN (Japan) | 119 | Vanadium | Curium | 2018, ongoing | 1 |
Meanwhile, researchers at Russia’s Joint Institute for Nuclear Research (JINR) have their eyes set on 120. With the help of ORNL, which provided the radioactive targets, JINR discovered the last five superheavies, including 118, oganessian, named after the institute’s leader, Yuri Oganessian, now 91 years old. Although the U.S. collaboration was suspended after Russia’s invasion of Ukraine, last year the JINR team also synthesized two atoms of 116, by firing a beam of chromium atoms at a uranium target, according to an institute press release. In an email, Oganessian says the experiment is ongoing to determine the cross-section of the chromium-uranium fusion, and the results remain unpublished. According to a 2023 article on the Russian Academy of Sciences’s website, JINR is planning to make element 120 by firing chromium ions at a curium target. Oganessian declined to describe the status of that effort beyond noting that Russian research reactors can now produce the needed targets.
If discovered, 119 and 120 would start a new, eighth row of the periodic table. Researchers want to see whether they follow the chemical rules of the corresponding elements in lighter rows of the periodic table or if their relativistic electrons alter their behavior.
Who will get to 120 first is unclear. GSI physicist Alexander Yakushev says the LBNL setup has better odds than JINR’s chromium-curium approach. “The reaction with titanium should be more promising, because the fusion probability is higher,” he says. But JINR may have a significant head start.
LBNL faces other challenges. The team relies on ORNL to make the highly radioactive californium targets, which it does every other year at one of its research reactors. The team will also have to add radiation shielding around the target and instruments to operate the experiment remotely, without human supervision, Gates says. And the team will need money to commandeer lengthy runs on LBNL’s 88-Inch Cyclotron, a workhorse instrument since the 1960s that has other scientific users.
LBNL has plans for such upgrades. But they must be approved and funded by the Department of Energy (DOE), meaning the start of an element 120 hunt is still at least a year away. “I cannot say there is funding sitting on a shelf for this effort,” says Sharon Stephenson, who directs nuclear physics research within DOE’s Office of Science. “However, we are very aware of their plans and our planning is supportive of their efforts.” Now, the LBNL researchers will need just a little more luck if they are to reclaim their heavyweight title. (ScienceAdviser)