Russians fill gap in periodic table in search for more-stable heavy elements
Russian physicists have for the first time created atoms of the super-heavy element 117, filling a gap in the periodic table and providing further evidence that research is close to reaching a predicted “island of stability” of heavy elements that will persist more than a fraction of a second.
The same team at the Joint Institute for Nuclear Research in Dubna, Russia, had previously reported the production of elements 116 and 118, but the element between them, 117, had proved more difficult and its production eventually required the assistance of American researchers.
The Dubna team uses a particle accelerator called a cyclotron to launch ions of calcium-48 to high speeds, smashing them into a target to produce the desired elements. But producing element 117 required a target of the artificially produced element berkelium-249, which has a half-life of only 320 days.
Researchers at the Oak Ridge National Laboratory’s High Flux Isotope Reactor in Tennessee spent 250 days producing and isolating a mere 22.2 milligrams of berkelium-249 -- about the size of a fingernail clipping. They then shipped it to Dubna, where it was fashioned into a target.
The Dubna team then bombarded the target with calcium-48 around the clock for five months.
In a statement released Wednesday, Dubna director A.N. Sissakian said the team had observed six molecules of element 117 during the period, confirming their identity by monitoring the atoms produced when they decayed.
Theory says that super-heavy elements containing larger numbers of neutrons should be more stable than those with smaller numbers of neutrons, with the greatest stability reached with an element containing 120 or 126 protons and 184 neutrons. The more neutrons an element contains, the more stable it should be. The team produced five new atoms of element 117 with 117 protons and 176 neutrons and one atom with 177 neutrons.
These all decayed in a fraction of second. But atoms of elements 112 to 116 produced in that decay had more neutrons than atoms of elements 112 and 116 that had previously been produced artificially, and they survived longer -- seconds instead of fractions of a second.
That is “direct proof for the existence of the ‘Stability Island’ of super-heavy nuclei,” Sissakian said in his statement.
The findings were published Friday in the journal Physical Review Letters.