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Scientists closer to a more stable superheavy element

Six new isotopes of man-made element 114 are found at Berkeley.

October 29, 2010|By Thomas H. Maugh II, Los Angeles Times

Berkeley researchers have produced a brand-new version of the man-made element 114 that decayed into five more novel atoms, a feat that brings them closer to their ultimate goal of making a superheavy element that can last for more than fractions of a second.

Researchers have so far made elements with atomic numbers as high as 118 in their search for the so-called Island of Stability, whose residents might have unusual and useful properties. But so far, all of these elements have been short-lived.

It now appears that a stable superheavy element will need to have an atomic number in the 120s — meaning that it must contain at least 120 protons. Creating such a huge atom is beyond the scope of today's technology.

In the meantime, researchers hope to refine their theories by studying new versions, or isotopes, of superheavy elements that have already been created, such as the as-yet-unnamed element 114.

"With each new isotope one can discover and get information about, we get more points to compare experiment with theory," said physicist Paul A. Ellison of the Lawrence Berkeley National Laboratory, lead author of the paper describing the six new isotopes in Friday's edition of the journal Physical Review Letters.

The theory governing decay of superheavy elements "did pretty well … but there were some discrepancies," he said. "It's possible the model needs to be revised."

Element 114 was the starting point for the new experiment. It was first created 11 years ago by researchers at the Joint Institute for Nuclear Research in Dubna, Russia. Its existence was confirmed last year by the Berkeley team, who bombarded plutonium isotopes containing 148 neutrons with calcium isotopes that had 28 neutrons using their 88-inch cyclotron.

They identified four different isotopes of element 114 that contained 172 to 175 neutrons, with lifetimes ranging from a tenth of a second to two seconds. Each of these atoms decayed by spontaneous fission — breaking down into two more or less equally sized fragments — or went through one or two alpha decays before fissioning. An alpha decay, which provides more information to researchers, involves the release of two protons and two neutrons — essentially, a helium nucleus.

The ideal isotope would have 114 protons and 184 neutrons. Theory predicts that such an atom would be unusually stable, able to survive long enough for scientists to study its properties.

But to make it, scientists need to bombard plutonium atoms (which contain 94 protons) with radioactive isotopes of calcium (provider of the remaining 20 protons). A particle accelerator powerful enough to do that is being built at Michigan State University, but it will not be ready for some time.

For the new experiment, Ellison and his colleagues reasoned that they could learn more about element 114 by producing a version with fewer neutrons that would be forced to go through alpha decays. To do that, they bombarded the plutonium with the calcium at a higher energy so that a resulting collision would have to bleed off five neutrons. They knew that their odds of success were only 20% as good as they were in their prior experiments to confirm the existence of element 114, he said.

They ran the cyclotron for a full month, achieving precisely one positive result.

That collision produced an atom of element 114 with the desired atomic weight of 285 (171 neutrons), which decayed in less than a fifth of second and became copernicium-281. That decayed in less than a fifth of a second to darmstadtium-277, which lived a mere 0.008 seconds before becoming hassium-273. That, in turn, lasted a third of a second before becoming seaborgium-269, which made it three minutes and five seconds before emitting an alpha particle to become rutherfordium-265. That final atom lasted 2.5 minutes before fissioning.

Although there was only one event, "it all happened very rapidly in the very same spot of the detector and is a fingerprint-like signature, so we are very sure it is real," Ellison said.

The heaviest new isotopes — those of element 114 and copernicium — showed smaller energies associated with the alpha decay than theory had predicted, Ellison said. Those discrepancies will help refine current theories about the Island of Stability, he concluded.

thomas.maugh@latimes.com

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