By David Hinde, Australian National University
THE hunt for long-lived superheavy elements has taken another leap forward now we’ve confirmed the existence of Element 117, also known as ununseptium.
It was first seen briefly by a team of US and Russian researchers back in 2010. Its existence has now been confirmed by an international team of researchers including myself, with details published this month in the Physical Review of Letters.
The new evidence should set the stage for Element 117 to become the heaviest named element in the periodic table.
The current temporary name indicates that the atom contains 117 protons. For comparison, uranium is considered the heaviest naturally occurring element, having 92 protons. Its most stable isotope U-238 is radioactive with a half-life of 4.5 billion years, similar to the age of the Earth.
The name for this new element is sometimes written as ununseptium, which simply means 117 in Latin. This name was proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1979 as a temporary name – but it’s never used by superheavy element researchers.
How to make a superheavy element
Elements beyond 104 are referred to as superheavy, and the most long-lived ones are expected to be situated on a so-called “island of stability” where neutron-rich nuclei with extremely long half-lives should exist. Currently, we are exploring the “shore” of this island, since we cannot yet reach the centre of the island.
Superheavy atoms such as Element 117 have not been found in nature but they can be made by nuclear fusion. That involves bringing together the nuclei of smaller atoms that combine to give the right number of protons.
Four atoms of Element 117 were formed by nuclear fusion at the German accelerator laboratory GSI, where more than ten billion billion rare calcium-48 nuclei (each with 20 protons and 28 neutrons) were fired at a target (pictured right) made of the even rarer isotope berkelium-249 (having 97 protons and 152 neutrons).
The Bk-249 material was created at the High Flux Isotope Reactor, at Oak Ridge in the USA, then shipped to the University of Mainz in Germany, where the expertise existed to make it into the thin target. The Bk-249 has a half-life of only 330 days, so the experiment had to be done quickly.
In the experiment, atoms of Element 117 were separated from huge numbers of other nuclear reaction products in the TransActinide Separator and Chemistry Apparatus (TASCA) and were identified through their radioactive decay which occurred in less than a tenth of a second.
The observed chains of alpha-decays produced isotopes of the slightly lighter Elements 115 to 103, whose observation and identification provided proof that Element 117 had indeed been produced.
The difficulty of making such elements
There is a huge problem in forming atoms of superheavy elements, not least the extremely low probability of fusion of the two nuclei which are smashed together.
Instead of fusing, the two nuclei usually stick together for a while, maybe one hundredth of a billion billionth (10-20) of a second, then come apart. The longer they stick, the better the chance of forming an atom of the new superheavy element.
The research team at the Australian National University Heavy Ion Accelerator Facility has developed a unique capability to investigate how long two nuclei stick together in the nuclear fusion reactions used to attempt to create superheavy elements.
With the challenge of forming even heavier elements in mind, a couple of years ago the German team leaders invited the ANU group to join their collaboration, to carry out focused measurements to help choose the best nuclear fusion reactions.
Already two collaborative experiments (investigating indications of different fusion behaviour from isotopes of sulphur and titanium) have taken place at ANU.
Preparations at the ANU and in Germany are currently underway to extend the measurements towards the reactions that might be the most favourable to create superheavies up to Element 120.
All we need is some magic
The creation of superheavy elements, and the measurement of their properties, contributes to a wide range of associated fields including quantum and nuclear physics, astrophysics and chemistry.
Neutrons and protons exist in quantum energy levels in the nucleus, in a similar way to the quantum energy levels of electrons in atoms. In atoms these lead to the unreactive noble gas elements which have 2, 10, 18, 36, 54 or 86 electrons in total, which all result in closed (full) outer electron shells.
In a similar way, “magic numbers” of protons and neutrons give additional stability.
Superheavy atoms only exist because their nuclei are more stable than otherwise as a result of these magic numbers. The magic numbers are predicted within different models to be at 114, 120 and 126 protons, while all predict 184 is the magic neutron number.
Correctly predicting the masses and decay properties of isotopes of superheavy elements in this region provides a severe test of models of the atomic nucleus, for which the magic numbers are a key component. By testing the models outside their “comfort zone”, they can be refined, or rejected.
These models in turn are used in nuclear astrophysics to describe the nuclear reactions that drive our universe. Element abundances and supernova dynamics are critically dependent on properties of as yet unobserved neutron-rich nuclei. Models are currently the only way to estimate how they behave.
In chemistry, for the heaviest elements the inner electrons are so strongly attracted to the highly charged nucleus that relativistic effects are very important. This affects the chemical properties of the elements, so measuring the chemical properties, such as achieved for Element 114 (discovered in 1999 and known as flerovium), tells us how well we understand these relativistic effects.
The fusion process involves transforming two separate many-body quantum systems (the colliding nuclei) into one. The huge repulsion between 117 protons packed into the tiny volume of the superheavy nucleus makes this a very delicate operation.
Detailed quantum properties of the two colliding nuclei seem to affect how easily they can fuse into one, including the presence of magic numbers, and the shapes and relative orientations of the two nuclei. Similar orientation effects have been seen in reaction rates of some molecules.
The ANU group has been working to understand this quantum dynamics, as well as the transition from an initial situation involving quantum probabilities to a definite irreversible outcome. This should be of general relevance to interactions of other many-body quantum systems such as atoms and molecules.
David Hinde receives funding from the Australian Research Council for research into nuclear reactions forming superheavy elements.
[Lead Photo Flickr/Larry, CC BY-ND]