Scientists create a superheavy element
An international research team has created and observed several atoms of the superheavy element with atomic number 117. The measured properties of the atoms strengthen the case for official recognition of 117 as a new element.
The periodic table includes every known chemical element from the lightest to the heaviest, with each element defined by the number of protons in its nucleus. Elements beyond atomic number 104 are referred to as superheavy elements, weighing nearly 300 times more than hydrogen. The most long-lived of these are expected to be situated on a so-called ‘island of stability’, where nuclei with extremely long half-lives should be found.
Although superheavy elements have not been found in nature, they can be produced by accelerating beams of nuclei and shooting them at the heaviest possible target nuclei. Fusions of two nuclei occasionally provide the right number of protons to produce a superheavy element, though this is a very rare event and they generally only exist for a short time.
Initial reports about the discovery of an element with atomic number 117 were released in 2010 from a Russia-US collaboration working at the Joint Institute for Nuclear Research in Dubna, Russia. The latest experiment was conducted at the GSI Helmholtz Centre for Heavy Ion Research, Germany, by 72 scientists and engineers from 16 institutions in Australia, Finland, Germany, India, Japan, Norway, Poland, Sweden, Switzerland, the UK and the US. Their research has been published in the journal Physics Review Letters.
The experiment required close coordination between the accelerator and detection capabilities at GSI in Germany and the actinide isotope production and separation facilities at Oak Ridge National Laboratory (ORNL) in the US. The rare isotope berkelium-249, essential for the synthesis of element 117, was produced over an 18-month campaign which took it from ORNL to the Johannes Gutenberg University of Mainz and from there to the GSI accelerator. More than 1019 extremely rare calcium-48 nuclei, with 20 protons and 28 neutrons, were fired at the berkelium-249 target, which had 97 protons.
Atoms of element 117 were separated from huge numbers of other nuclear reaction products in the GSI’s TransActinide Separator and Chemistry Apparatus (TASCA) and were identified through their characteristic radioactive decay, which occurs within a tenth of a second. These measured chains of alpha-decays produced isotopes of lighter elements with atomic numbers 115 to 103, whose registration added to the proof for the observation of element 117.
In the decay chains, both a previously unknown alpha-decay pathway in Db-270 (dubnium - element 105) and the isotope Lr-266 (lawrencium - element 103) were identified. With half-lives of about one hour and about 11 hours, respectively, they are among the longest-lived superheavy isotopes known to date. As unwanted background events are present in all superheavy element experiments, the longer lived an isotope is, the harder is its reliable identification. But the TASCA was upgraded for the experiment to better separate unwanted background products, proving that the reliable identification of superheavy nuclei is now possible.
“This is of paramount importance as even longer lived isotopes are predicted to exist in a region of enhanced nuclear stability,” said Professor Christoph Düllmann, who led the research.
Professor Horst Stöcker, Scientific Director of GSI, added: “The successful experiments on element 117 are an important step on the path to the production and detection of elements situated on the ‘island of stability’ of superheavy elements.”
A committee comprising members of the International Union of Pure and Applied Physics and Chemistry (IUPAC) will review these new findings, along with the original ones, and decide whether further experiments are needed before acknowledging element 117’s discovery. In the event of acceptance, a name may be proposed by the discoverers.
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