"Ununseptium" or the superheavy element Z = 117 could finally be ready to be added to the periodic table. An international collaboration has produced four atoms of the elusive element, which was first spotted in 2010. The measured decay properties of these atoms match the previous data, which strengthens the case for official recognition of 117 as a new element along with its decay-chain elements 115 and 113. In the process, the researchers also discovered a new isotope "lawrencium-266".

Stable islands

While the majority of the periodic table is filled with naturally occurring elements, nuclear-physics experiments have added another 27 elements to the mix. Elements beyond atomic number (Z) 104 are referred to as superheavy elements, the most long-lived of which are thought to approach a so-called island of stability, where nuclei with extremely long half-lives should be found. Indeed, the shell model of the nucleus predicts that such superheavy elements become more stable as their neutron number goes up and they reach the "island", at Z = 184.

Although these elements are not found in nature, they can be produced by accelerating beams of nuclei and smashing them into targets of specific, very heavy nuclei. Ununseptium atoms are the heaviest ever observed – they are 40% heavier than an atom of lead. In 2010 a Russia–US collaboration working at the Joint Institute for Nuclear Research in Dubna, Russia, fired beams of the rare isotope calcium-48 at targets of berkelium-249 to catch the first glimpse of 117.

Pure target

In the latest experiment, the intense isotope beam from the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, was used to bombard the berkelium target, supplied by the Oak Ridge National Laboratory in Tennessee, US, to create the ununseptium atoms. A precious 13 mg of the highly purified berkelium (that itself has a half-life of only 330 days) was synthesized across 18 months at Oak Ridge and shipped to Johannes Gutenberg University in Mainz, where it was transformed into a target that could withstand the high-power calcium-ion beams.

Atoms of element 117 were then separated from huge numbers of other nuclear-reaction products at the TransActinide Separator and Chemistry Apparatus (TASCA) facility. They were identified via their radioactive-decay products, which included the lighter products 115–103, thereby adding to the proof of the observation of 117. Surprisingly, the team, headed by Christoph Düllmann of Johannes Gutenberg University, also identified a previously unknown alpha-decay pathway in Db-270 (dubnium – element 105) and the new isotope Lr-266 (lawrencium – element 103). With half-lives of about one hour and about 11 hours, respectively, they are among the longest-lived superheavy isotopes known to date. "This is of paramount importance as even longer-lived isotopes are predicted to exist in a region of enhanced nuclear stability," says Düllmann.

While the latest experiment might have confirmed the existence of element 117, it will still be some time for it is formally christened. The International Unions of Pure and Applied Physics and Chemistry will review both this and the 2010 results to decide whether further experiments are needed before acknowledging the element's existence. Only then will a particular institute be offered the naming rights.

"Making element 117 is at the absolute boundary of what is possible right now," says team member David Hinde, director of the Heavy Ion Accelerator Facility at the Australian National University. "That's why it's a triumph to create and identify even a few of these atoms."

The research is published in Physical Review Letters.