Published online 1 May 2008 | Nature | doi:10.1038/news.2008.794

News

The heaviest element yet?

Researchers claim to have seen an element with an atomic number of 122, but some are sceptical.

Does super-heavy and stable element 122 belong in the periodic table?

Could super-heavy elements be lurking in plain sight? One group of physicists says that they are, and claims to have seen the heaviest element yet found hiding amongst thorium atoms.

Some theories predict that some super-heavy elements might be unusually stable, thanks to a 'magic' number of protons and neutrons, and so could be lying around in nature. Several groups are now engaged in searches for them. If confirmed, this would be the first report of finding one.

But the team's claims1, which are not peer-reviewed, are being heavily criticised by other physicists, who fear that their technique is flawed. "I have grave doubts," says Rolf-Dietmar Herzberg, a nuclear physicist at the University of Liverpool, UK.

Extending the table

The heaviest naturally stable element is uranium, but over the years physicists have used accelerators to synthesize larger, heavier elements. In 2006, physicists in the United States and Russia created element 118. The repulsive force of 118's positively charged protons put enormous pressure on the nucleus, and it lasted for a little under a millisecond before splitting itself into lighter nuclei.

That splitting is typically used to detect new, heavy elements. By watching lightweight helium atoms fly out from the nucleus, researchers can infer the weight of the parent element, Herzberg says. But, he adds, "if you have something that lives months or longer, you would never see that decay".

To search for stable heavy elements requires a different technique, says Amnon Marinov of the Hebrew University in Jerusalem, who led the collaboration that claims to have seen the new element.

Atomic number 122

Marinov's team took a purified sample of thorium and used an electric field to accelerate the nuclei. They then passed them through a magnet, whose field bent lighter nuclei more than heavier ones. The technique, known as plasma-sector field mass spectrometry, separates the heaviest nuclei from the chaff.

The results clearly show the existence of an element with atomic number of 122, says Marinov. He thinks that such an element may be stable for hundreds of millions of years because its nuclei are 'hyper-deformed'. That means its 292 protons and neutrons are arranged in a shape more like a pill than a sphere, helping to prevent it from splitting itself apart.

Herzberg, however, is deeply sceptical about the claim. Marinov's detection puts the abundance of the element at roughly one in a trillion thorium atoms, a relatively weak signal. The mass spectroscopy technique employed, says Herzberg, could easily be picking up organic molecules, whose multiple nuclei can have the apparent mass of a single, super-heavy element. Such molecules could leak into the system from vacuum pumps and other equipment.

Unconvinced

Other, similar searches for stable elements have so far come up negative, Herzberg points out. "I would really like to see a lot more evidence in their favour," he says.

ADVERTISEMENT

Other physicists share this scepticism. "On the positive side, I can say it is 'potentially ground-breaking', but on the negative side, it may be 'wild speculation'," says Phil Walker, a nuclear physicist at the University of Surrey, UK. But in Walker's opinion, he says: "Based on my knowledge of what Marinov has published (and attempted to publish) in the past, I would put it in the latter category."

The results are "not very convincing" agrees Robert Eichler, who leads a heavy-element group at the University of Bern in Switzerland.

But Marinov remains adamant that he has seen the new element. The criticisms are easily refutable, and he intends to refute them, he says. "I hope to get published." 

  • References

    1. Marinov, A. et al. Preprint at http://arxiv.org/abs/0804.3869 (2008).
Commenting is now closed.