The Russian Superheavy Element Factory will attempt to synthesize element 120 – a potential island of stability now that element 114 has been ruled out.
C. BICKEL /SCIENCE
By Daniel Clery
Nuclear physicists have been creating record-breaking super-heavy elements for decades, expanding the periodic table beyond uranium, the heaviest natural element. Such heavyweights tend to be unstable, but the theory predicts ‘magic numbers’ of protons and neutrons that provide extra stability, and finding a long-lived super-heavy has long been a holy grail for researchers.
Element 114, known as flerovium and first created in 1998, is considered the best candidate for extra stability, as theorists believe that 114 is a magical number of protons. But researchers now report that it is no more stable than the super-heavy elements nearby on the periodic table. Element “114 is apparently not magic, or at least not as magical as classical predictions indicate”, says study leader Dirk Rudolph of Lund University.
The result focuses on the following candidate for a magical number of protons: element 120. Element 120 has never been synthesized before, and is a target of the Superheavy Element Factory (SHEF), a new facility in Russia launched in November 2020 started with his first experiments. Researchers there have already made 60 atoms of moscovium, element 115, by firing ion beams onto a thin layer of target material. But the chase for 120 is delayed until researchers obtain the amount of californium – a rare element produced in high-flood nuclear power reactors – needed for the target of 120. “A limited amount of target material presents technical problems we encounter in the near future must resolve, “said Yuri Oganessian of Russia’s Joint Institute for Nuclear Research (JINR), home of the SHEF. Oganessian is the namesake for oganesson, element 118, which was discovered by his team at JINR in 2004 and is currently the heaviest ever made.
To explain why some nuclei are more stable than others, theorists believe that protons and neutrons are in ‘shells’, similar to the orbital shells of electrons that surround the nucleus and define the chemistry of each element. Just as a complete electron shell makes a chemically inert noble gas, a complete shell protons or neutrons provide extra stability and longer life. Nuclei with full shells of 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 the nature.
But the theory can only approximate what the magic numbers are for super-heavy elements. In 1998, when Oganessian’s team at JINR first produced a single nucleus of element 114, things looked promising for a magical shell of 114 protons: the atom appears to survive longer than 30 seconds – an eternity for a super-heavy element. But that long life was never repeated, and most of the half-dozen other confirmed isotopes of flerovium survive no longer than 1 second.
Last year, a team led by Rudolph and Christoph Düllmann from the University of Mainz re-investigated the stability of flerovium with upgraded detectors at the GSI Helmholtz Center for Heavy Ion Research in Germany. They fired a beam of calcium-48 ions onto metal foils coated with plutonium-242 and plutonium-244. Most of the ions moved 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 are separated from balkions and other debris by a magnetic field that emits ions according to their mass. The nuclei embedded in a particle detector, which set up and measured decay products to reveal the identity of the super-heavy nucleus – and how long it lived.
The researchers created two atoms of flerovium-286 and 11 of flerovium-288, the team reported last month Physical overview letters. They identified decay paths of the nuclei, including one that had never been seen before, that would not occur in a stable nucleus with a complete shell. Rudolph says that these decay routes are so efficient that they conclude that 114 ‘is not a pronounced magic number’.
Oganessian is not surprised. He says theorists believe the extra stability that a full proton shell provides is “much weaker and vague”, while a full neutron scale will have a much greater impact on stability. Frustratingly, the following complete neutron scale, at 184, is currently out of reach: researchers have never produced a nucleus with more than 177 neutrons.
But that does not mean that the search for magical stability is over. The enhanced GSI team 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 evoke shells in the form of rugby balls and other shapes instead of spheres, indicating that the full proton shell actually lies at 120 or 126, not 114.
Getting there is a matter of the right beam and target material plus beam intensity and longevity. ‘Brute force’, as Düllman calls it. He says elements 119 and 120 are beyond the reach of the current GSI facility, but should be within reach of the RIKEN particle physics lab in Japan as well as SHEF. “I’m pretty sure they’ll get 119 and 120 for us.”