In search of the island of stability
New observations may help extend the periodic table.
Research unveiled this week may help pinpoint the 'island of stability' a theoretical region of relatively stable but very heavy elements beyond the limits of the current periodic table.
Rolf-Dietmar Herzberg from the University of Liverpool, UK, heads a large team who have examined high-energy states of the element nobelium using the particle accelerator of the University of Jvyskyl, Finland. The results, published in Nature1, will constrain theorists' view of exactly where the island of stability lies.
Since the 1940s, scientists have tried to expand the periodic table by smashing atomic nuclei together to create heavier elements. This is tough, because the protons and neutrons packed into a nucleus resist the fusion of two nuclei. And even if the nuclei stick, the fused nucleus may not last: the heavier it is, the more rapidly it falls apart again by radioactive decay to form smaller, more stable atoms.
Physicists claim to have made and detected elements with up to 118 protons (the periodic table lists elements according to the number of protons in their nuclei) although only 111 elements are officially acknowledged. But many of the very-high-number elements last just fractions of a second before decaying.
Yet physicists still hope to find some 'superheavy' nuclei that are stable for minutes or hours. That's because theoretical models suggest that nuclei with particular numbers of protons (Z) and neutrons (N), called 'magic numbers', are unusually stable.
A simple version of the theory behind magic numbers visualises protons and neutrons inside a nucleus filling up levels of increasing energy, like the layers of an onion. But these layers are not evenly spaced: many levels are close together and form 'shells', with relatively large energy gaps between shells.
A 'magic' number is the number of particles it takes to perfectly fill up a shell once a shell is filled, it would take a large jump in energy for one of those particles to cross the gap to the next highest level. A nucleus with a full outer shell is unusually stable, just as the noble gases, which have the right number of electrons to exactly fill electron shells, are unusually unreactive.
For lighter elements, the energy levels and the magic numbers are well known (see box). And the higher magic numbers for neutrons are well predicted by theory. But the theory is less certain for protons: for them, the picture is more complicated because their positive charges repel each other.
We would be a step closer to finding out the magic Z numbers if researchers could measure the exact energy levels in the higher shells. Then theorists could work out the Z numbers that should make heavy nuclei stable in other words, pinpoint the fabled 'island of stability' beyond the current limits of the periodic table.
Now Herzberg's team has given theorists some data to chew on, working out an energy level so energetic that it could be filled only in elements beyond position 114 in the periodic table.
The researchers did this by studying nobelium-254 (element 102), the heaviest element they could make in relatively large quantities. They produced excited versions of nobelium-254, in which protons and neutrons are bumped up to higher energy levels than normal. They then carefully watched as the atoms relaxed back to a non-excited state. By monitoring the energy of every particle or wave produced by this relaxation, they could calculate the exact energy levels in the higher shells before the relaxation. The highest level they could pin down turned out to be equivalent to that occupied by non-excited elements that have 115 protons or more.
A separate team working at Argonne National Laboratory, Illinois, will shortly publish similar results.
Theorists will now need to account for these energy levels in the models they use to calculate high magic numbers.
On the beach
But there could be bigger problems ahead. Recent research suggests there may not be an exact island of stability at all.
The whole notion of distinct shells and therefore magic numbers in superheavy nuclei is rather fragile, explains Witold Nazarewicz, a physicist from the University of Tennessee in Knoxville. Theorists now think that the highest energy levels in the superheavy elements may all be equally spaced, with medium-sized gaps between them: the big energy jump that would make a shell very stable is not there. "This makes a general region of 'magicity'," Nazarewicz says. This semi-stable 'beach' region should occur in atoms with around 120 to 126 protons.
"Our theorist friends have thrown a wrench in the works," says Ken Moody, of the Lawrence Livermore Laboratory in Berkeley, California.
Even supposing some superheavy elements are theoretically more stable, actually making them will prove a huge challenge. One problem is that elements existing at the 'island of stability' will probably have to be neutron-rich, and researchers cannot easily make neutron-rich elements: the lighter elements available for fusion experiments have a higher percentage of protons in their nuclei than the exotic heavier elements. This leads to a shortfall of neutrons when atoms of the lighter elements are fused.
Nonetheless, Herzberg is excited by his team's measurements. "It's like a message in a bottle from the island of stability," he says.
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- Herzberg R. D., et al. Nature, doi: 10.1038/nature05069 (2006).