Heaviest element claim criticised
02 May 2008
Scientists claiming to have discovered the super-heavy element 122 have had their research dismissed by physicists who say their measurements are suspect.
A collaboration of researchers led by Amnon Marinov at the Hebrew University of Jerusalem say their mass spectrometer has detected something with an atomic mass around 292, hiding in piles of purified thorium. They think this is most likely a nucleus with 122 protons and 170 neutrons. The research is described in an article posted to the internet pre-print server, arXiv, but has not been peer reviewed.
If substantiated, the sighting would dramatically extend the limits of the periodic table, which currently stops at element 118. But physicists contacted by Chemistry World were critical of the team's findings. 'I doubt this will survive peer review: they will need more support and better evidence for their claims,' says nuclear physicist Rolf-Dietmar Herzberg, of the University of Liverpool, UK.
Among the criticisms levelled at Marinov's team are that their equipment may have spotted combinations of ions with the same mass as element 122, though Marinov says he is sure there was no such contamination.
Andreas Türler, who investigates superheavy elements at the Technical University of Munich, Germany, says the team have likely misused their mass spectrometer. 'We use an identical model, and it is not intended as an isotope separator - rather, to measure ultralow concentrations [of known elements],' he explains. 'Introducing highly concentrated solutions as Marinov's team did may cause all sorts of effects that are beyond the control of the operator. I seriously doubt that a reliable measurement was obtained under the conditions of the reported experiment.'
In response to that criticism, Marinov says his team calibrated their instrument to make sure that the thorium solution they injected was not so concentrated that it disrupted their readings.
But both Herzberg and Türler also point out that Marinov's identification of unbibium (122) is based on the assumption that the element behaves in a similar chemical fashion to thorium - doubtful, if relativistic effects affect the element's chemistry.
Perhaps the most surprising aspect of the 122 claim is that the element, the researchers calculate, has a half-life of 108 years - which is why it could be found lying around in thorium. Super-heavy elements are usually too unstable to exist for more than a few fractions of a second, so have to be made, fleetingly, by smashing nuclei together. Marinov thinks that element 122 is stable because its nucleus is 'hyper-deformed' from the usual spherical shape, preventing it from splitting apart. He has reported the discovery of long-lived isomers of hyper-deformed nuclei before, Türler says, but none of the claims have been verified by other experiments.
Researchers at Lawrence Livermore National Laboratory, California, who helped confirm the detection of element 118 in 2006, told Chemistry World they were too busy to check through Marinov's research.
'The popular vote is one of ridicule and I cannot fault that,' Herzberg sums up.
Marinov says he has submitted the article to the journals Nature and Nature Physics, but both turned it down without sending it for peer review. 'I am not changing the article and hope to get it published in another journal.'
Richard Van Noorden
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A Marinov et al, arXiv:0804.3869 (pre-print)
Also of interest
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Russian and American scientists claim to have discovered element 118, the newest and heaviest addition to the periodic table.
The periodic table is still expanding but there are probably not many elements left that can be synthesised. Dennis Rouvray investigates how much further we can go
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