Putting it all on the table

The competition to synthesise heavy elements is well underway, but are we creating them just because we can; what can we learn from these superheavy additions to the Periodic Table? More than just a fixture in many a lab and classroom, the Periodic Table is a valuable tool detailing the chemical elements that make up everything from humans and our infrastructure to the stars in the night sky. It’s a useful source of information – but it’s far from finished. Chemists and physicists have been creating heavier and heavier elements since the 1940s, adding 26 new elements to the table – five since 2000. And we’re still working on adding more. The first 92 elements – from hydrogen to uranium – are naturally occurring on Earth. Anything past this has to be created in the lab. Neptunium (element 93) to lawrencium (element 103) are classed as heavy elements, while anything from rutherfordium (element 104) onwards are superheavy elements ­­– and it is these which are of particular interest. Their synthesis – albeit for short periods – offers an important insight into the nature of atomic bonding. “Each new element we discover provides more knowledge about the forces that bind nuclei and what helps them split apart,” says Dawn Shaughnessy from Chemical Sciences Division at Lawrence Livermore National Laboratory. “Synthesising superheavy elements tests our theories of how matter is produced, and the limits of the existence of matter.” “There were theories that originally said due to the forces inside the nucleus that elements heavier than 100 would never be created,” she says. “By experimentally producing these new elements, these theories have been rethought and we now understand that there are different forces at play in the nucleus that allow these superheavy elements to exist, even if only for a short time. The very nature of the stability of matter is constantly tested through these experiments.” Shaughnessy is part of the team who – with the Joint Institute for Nuclear Research in Dubna – discovered element 118, the heaviest element so far confirmed. Ununoctium, as it is temporarily named, was created in a cyclotron by bombarding a rotating sheet of californium-249 with a beam of calcium-48. Successful collisions saw calcium’s 20 protons, bind with californium’s 98, to create a fused atom with 118 protons. All superheavies are generally made by smashing one element into another in this way. Once created, that atom enters a separator – a dipole magnet that creates a homogenous magnetic field between two pole faces. The chamber between the poles is filled with hydrogen, which is picked up by the unstable atom until it reaches an equilibrium state. The magnetic field is set to recognise this equilibrium state so that only atoms with matching criteria are passed to the detector – the key to proving the new element’s momentary existence. The atom travels through a time-of-flight counter which detects the time the atom enters and its speed. The atom enters the detector and embeds itself into one of the silicon walls while the counter sends a signal to help scientists identify new and heavier atoms, which travel more slowly than the unwanted by-products. The element then almost immediately undergoes alpha decay and ejects a helium nucleus, forming a unique and distinctive decay pattern which proves its existence. “Chemically the limit of the periodic table is around element 170, but nuclear stability forces an end to the Periodic Table much sooner,” says Professor Rolf-Dietmar Herzberg, Head of the Nuclear Physics group at the University of Liverpool. “At the moment we have seen elements up to 118, and are confident that we can make 119 and 120, but thereafter nobody knows.” “As the nuclear charge gets bigger, the innermost electrons are drawn further and further in, around 170 they will be inside the nucleus, and thus recombine with the protons. Then it is clearly impossible to add another proton and make an element thus the end of the periodic table is reached.” As the Periodic Table began to extend, scientists noticed a pattern in the new elements; each new element was slightly heavier and more radioactive than the previous. In the 1960s, the idea of an Island of Stability emerged. Predicted by Glenn T Seaborg, this is a hypothetical region beyond the current Periodic Table where elements based on “magic numbers” are expected to be more stable and have half-lives ranging from minutes up to millions of years rather than seconds. “We know that nuclei like to have certain numbers of protons and neutrons that make them more stable, much like the noble gases have a certain number of electrons that make them less reactive.  The nuclear shell model can tell us what these “magic” numbers of protons and neutrons are, but as we go heavier and heavier, the situation gets more complicated because the shape of the nucleus can also make it more or less stable as well,” says Shaughnessy.

A hypothetical region beyond the current Periodic Table where elements based on "magic numbers" are expected to be more stable

“These magic numbers are 2, 8, 20, 28, 50, and 82 for both protons and neutrons and 126 for neutrons,” says Herzberg. “Magic numbers of protons are predicted at 114, 120 and 126. These are potential shell closures and are expected to have long-lived isotopes. These elements may have been created in stars and supernova and will probably not be found on Earth, but can be created in the lab and studied.” Shaughnessy says that there are different theories as to what the next “magic number” might be – it varies from 114, to 120 to 126. Element 114, Flerovium, was expected to be a relatively stable element because it has the “magic number” of protons, and indeed it has been created with 114 protons. Despite this, it was in fact found to be unstable.  The reason for this thinks Shaughnessey is all to do with neutons. “The problem is that we are still too neutron deficient to say for sure where the next magic proton number is. Because of the nature of the experiments we do, we cannot get to the centre of the so-called Island of Stability, which is the intersection of the next magic numbers of both protons and neutrons. We are only reaching the edges of this region, yet we still see effects in terms of longer nuclear lifetimes,” says Shaughnessy. “In order to truly map out this Island of Stability, we need to introduce more neutrons into the systems we are creating, which would require different accelerator beams than we currently have. When this technology comes along, we can get closer to the centre of the region and identify the next magic number of protons in the process.” It was once thought that the superheavies might have practical uses – possibly as nuclear fuel or in particle accelerators and neutron sources – but creating them has been much harder than expected, and they are not as long lived as we thought. “The superheavies have no direct practical uses. No-one will use them to make a better lightbulb,” says Herzberg. “Their uses are not obvious because they are so short-lived and it’s a huge challenge to make enough of them to be useful. Even with a long half-life, we might only end up with a few micrograms of the element.” They may not be proving useful in the way we once hoped, but the superheavies are unique and offer an opportunity to push the boundaries of our current knowledge of the Periodic Table. “We are able to use them to test theories to calculate further elements, and to understand how to incinerate nuclear fuel,” Herzberg says. “We can understand how stars generate elements by way of isotopes that lie far from stability using these models. The most sensitive tests to these models always come from the most extreme test cases, and the superheavies are one such case.” The superheavies provide a unique opportunity for study; an extremely sensitive laboratory in which to study shell structure at the extremes of mass and charge. Additions to the Periodic Table this century Element 116; Livermorium (2000) Element 113 and 115; Ununtritium and Ununpentium (2003) Element 118; Flerovium (2005) Element 117; Ununseptium (2010) Author Kerry Taylor-Smith Kerry is web editor on Laboratory News and has a degree in Natural Sciences from the University of Bath