Posters of the periodic table on the walls of science labs in schools around the world will need to be updated after the discovery of the newest superheavy element, element 117. With the temporary name of ununseptium, the temporary symbol Uus and the atomic number 117, it was the only missing element in row seven of the periodic table until its discovery by an international team of scientists from Russia and the U.S.
Scientists at the Flerov Laboratory of Nuclear Reactions at the Joint Institute of Nuclear Research in Dubna, Russia announced internally that they had succeeded in detecting the decay of a new element in January 2010, and the results have now been published in the journal Physical Review Letters.
The discovery of element 117 wasn’t easy. The two-year experimental campaign began at the High Flux Isotope Reactor in Oak Ridge with a 250-day irradiation to produce 22 mg of berkelium. This was followed by 90 days of processing at Oak Ridge to separate and purify the berkelium, target preparation at the Research Institute for Advanced Reactors in Dimitrovgrad, 150 days of bombardment at one of the world’s most powerful heavy ion accelerators at Dubna, data analysis at Lawrence Livermore National Laboratory and Dubna, and assessment and review of the results by the team. The entire process was driven by the 320-day half-life of the berkelium target material.
The experiment produced six atoms of element 117. For each atom, the team observed the alpha decay from element 117 to 115 to 113 and so on until the nucleus fissioned, splitting into two lighter elements. In total, 11 new “neutron-rich” isotopes were produced, bringing researchers closer to the presumed “island of stability” of superheavy elements.
"Island of stability" is a term in nuclear physics that refers to the possible existence of a region beyond the current periodic table where new superheavy elements with special numbers of neutrons and protons would exhibit increased stability. Such an island would extend the periodic table to even heavier elements and support longer isotopic lifetimes to enable chemistry experiments.
On course to the island of stability, researchers initially skipped element 117 due to the difficulty in obtaining the berkelium target material. The observed decay patterns in the new isotopes from this experiment, as close as researchers have ever approached the island of stability, continue a general trend of increasing stability for superheavy elements with increasing numbers of neutrons in the nucleus. This provides strong evidence for the existence of the island of stability.
"It fills in the gap and gets us incrementally closer than element 116--on the edge," said Ken Moody, one of the LLNL collaborators and a long term veteran of superheavy element research. "The experiments are getting harder, but then I thought we were done 20 years ago."
This discovery brings the total to six new elements discovered by the Dubna-Livermore team (113, 114, 115, 116, 117, and 118, the heaviest element to date). This is the second new element discovery for Oak Ridge (61 and 117). In addition, Oak Ridge isotopes have contributed to the discovery of a total of seven new elements.
Since 1940, 26 new elements beyond uranium have been added to the periodic table - no doubt proving a boon to the publishers of school textbooks.
"These new elements expand our understanding of the universe and provide important tests of nuclear theories," said Vanderbilt University Professor of physics Joe Hamilton. "The existence of the island of stability, a pure theoretical notion in the 1960s, offers the possibility of further expansion of the periodic table with accompanying scientific breakthroughs in the physics and chemistry of the heaviest elements."
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