For decades, nuclear physicists have blasted record-breaking superheavy elements into existence, extending the periodic table step by step beyond uranium, the heaviest natural element. Such heavyweights tend to be unstable, but theory predicts “magic numbers” of protons and neutrons that confer extra stability, and finding a long-lived superheavy 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 magic number of protons. But researchers now report that it is no more stable than the superheavy elements near it on the periodic table. Element “114 is apparently not magic, or at least not as magic as classical predictions suggest,” says study leader Dirk Rudolph of Lund University.
The result focuses attention on the next candidate for a magic number of protons: element 120. Never before synthesized, element 120 is a goal 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 chase for 120 is on hold until researchers obtain the amount of californium—a rare element produced in high-flux nuclear reactors—needed for 120’s target. “A limited amount of target material poses technical problems that we need to solve in the near future,” says Yuri Oganessian of Russia’s Joint Institute for Nuclear Research (JINR), home of the SHEF. Oganessian is the namesake for oganesson, element 118, discovered in 2004 by his team at JINR and currently the heaviest ever made.
To explain why some nuclei are more stable than others, theorists believe protons and neutrons reside in “shells,” similar to the orbital shells of electrons that surround the nucleus and define each element’s chemistry. Just as a full electron shell makes a chemically inert noble gas, a full shell of protons or neutrons offers extra stability and longer lifetimes. 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 “doubly magic” nuclei—are among the most stable isotopes in nature.
But the theory can only approximate what the magic numbers are for superheavy elements. In 1998, when Oganessian’s team at JINR produced a solitary nucleus of element 114 for the first time, things looked promising for a magic shell of 114 protons: The atom appeared to survive for more than 30 seconds—an eternity for a superheavy element. But that long life was never replicated, and most of the half-dozen other confirmed isotopes of flerovium do not survive longer than 1 second.
So, last year, a team led by Rudolph and Christoph Düllmann of the University of Mainz took another look at the stability of flerovium with upgraded detectors at the GSI Helmholtz Centre for Heavy Ion Research in Germany. They fired a beam of calcium-48 ions at metal foils coated with plutonium-242 and plutonium-244. Most of the ions passed through the target, but over the course of a few weeks, a few collided with a plutonium nucleus 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 nuclei embedded in a particle detector, which timed and measured decay products to reveal the identity of the superheavy nucleus—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 Letters. They identified decay paths of the nuclei, including one never seen before, that wouldn’t be present in a stable nucleus with a full shell. These decay routes are so efficient, Rudolph says, that they concluded 114 is “not an outspoken magic number.”
Oganessian is not surprised. He says theorists believe the extra stability conferred by a full proton shell is “much weaker and blurred,” whereas a full neutron shell would have a much greater effect on stability. Frustratingly, the next full neutron shell, at 184, is currently out of reach: Researchers have never produced a nucleus with more than 177 neutrons.
But that doesn’t mean the search for magic stability is over. The GSI team’s improved data on element 114 will help theorists refine their models by providing “anchor points for theory,” Rudolph says. Newer versions of the nuclear shell model invoke shells shaped like rugby balls and other shapes instead of spheres and suggest the full proton shell actually lies at 120 or 126, not 114.
Getting there is a matter of the right beam and target materials plus beam intensity and long run times. “Brute force,” as Düllman calls it. He says elements 119 and 120 lie beyond the grasp of the current GSI facility, but they should be within reach of the RIKEN particle physics lab in Japan as well as SHEF. “I’m pretty convinced they will get us 119 and 120.”
sciencemag.org, 12 February 2021