The Russian Superheavy Element Factory will attempt to synthesize element 120 – a potential island of stability now that element 114 has been ruled out.
BICKEL /SCIENCE
By Daniel Clery
For decades, nuclear physicists have been creating record-breaking supermassive elements, with the periodic table step-by-step moving beyond uranium, the heaviest natural element. Such heavyweights are usually unstable, but the theory predicts ‘magical numbers’ of protons and neutrons that provide extra stability, and finding a long-lived supermassive has long been a holy grail for researchers.
Element 114, known as flerovium and first created in 1998, was considered the best candidate for extra stability, as theorists believed 114 was a magical number of protons. But researchers now report that it is no more stable than the supermassive elements close to it on the periodic table. Element “114 is apparently not magic, or at least not as magical as classical predictions suggest,” says study leader Dirk Rudolph of Lund University.
The result calls attention to the next candidate for a magical number of protons: element 120. Never synthesized before, element 120 is a target of the Superheavy Element Factory (SHEF), a new facility in Russia that began its first experiments in November 2020. Researchers there have already made 60 atoms of moscovium, element 115, by firing ion beams at a thin layer of target material. But the hunt for 120 is delayed until the researchers obtain the amount of californium – a rare element produced in high-flux nuclear reactors – needed for the target of 120. “A limited amount of target material poses technical problems that we will discuss in the future. in the near future, ”said Yuri Oganessian of Russia’s Joint Institute for Nuclear Research (JINR), home of SHEF. Oganessian is the namesake of oganesson, element 118, discovered by his team at JINR in 2004 and currently the toughest ever made.
To explain why some nuclei are more stable than others, theorists believe that protons and neutrons reside in ‘shells,’ similar to the orbital shells of electrons that surround the nucleus and determine the chemistry of each element. Just as a full electron shell makes a chemically inert noble gas, a full shell of protons or neutrons provides extra stability and longer life. Nuclei with full shells of both protons and neutrons, such as helium-4 (atomic number 2), oxygen-16 (atomic number 8), and lead-208 (atomic number 82) – known as ‘double magic’ nuclei – are among the most stable isotopes in nature.
But the theory can only approximate what the magic numbers are for supermassive elements. In 1998, when Oganessian’s team at JINR first produced a solitary core of element 114, it looked promising for a magical shell of 114 protons: the atom seemed to survive for more than 30 seconds – an eternity for a supermassive element. But that long lifespan was never replicated, and most of the six other confirmed isotopes of flerovium survive no more than 1 second.
Last year, a team led by Rudolph and Christoph Düllmann from the University of Mainz looked again at the stability of flerovium with improved detectors at the GSI Helmholtz Center for Heavy Ion Research in Germany. They fired a beam of calcium-48 ions on metal foils covered with plutonium-242 and plutonium-244. Most of the ions passed the target, but over the course of a few weeks, some collided with a plutonium core and fused into flerovium.
After being ejected from the foil, the fresh flerovium nuclei were separated from beam ions and other debris by a magnetic field that deflects ions according to their mass. The cores are embedded in a particle detector, which timed and measured decay products to reveal the identity of the supermassive core – and how long it lived.
The researchers created two atoms of flerovium-286 and 11 of flerovium-288, the team reported last month in Physical Review LettersThey identified decay paths of the cores, including one never seen before, that would not be present in a stable, full-shell core. These decay routes are so efficient, Rudolph says, that they concluded that 114 is “not a distinctly magical number.”
Oganessian is not surprised. He says theorists believe that the extra stability provided by a full proton shell is “much weaker and hazier,” while a full neutron shell would have a much greater effect on stability. Frustratingly, the next full neutron shell, at 184, is currently out of range: Researchers have never produced a nucleus with more than 177 neutrons.
But that doesn’t mean the quest for magical stability is over. The improved data from the GSI team on element 114 will help theorists refine their models by providing “anchor points for theory,” Rudolph says. Newer versions of the nuclear scale model conjure up shells in the shape of rugby balls and other shapes instead of spheres and suggest that the full proton shell is actually at 120 or 126, not 114.
Getting there is a matter of the right blast and target materials plus blast intensity and long run times. “Brute force”, as Düllman calls it. He says elements 119 and 120 are beyond the scope of the current GSI facility, but must be within the reach of the RIKEN particle physics lab in Japan and SHEF. “I’m pretty sure they’ll get us 119 and 120.”