A trip to nuclear island of inversion
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The structure of nuclei of Mg-36 is odd The experiment helps understanding of matter
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Protons and neutrons that comprise a nucleus array themselves in shells, each shell with a different energy level. The phenomenon is described by the nuclear shell model. According to the model, specific numbers of protons and neutrons lead to shell structures that are especially stable — except, that is, for nuclei of elements in the so-called ‘island of inversion.’
There, ground-state nuclei that otherwise would have fairly typical shell structures adopt weird and strongly deformed structures.
Mapping out which nuclei are within or outside the island of inversion helps researchers extend the usefulness of the nuclear shell model.
Kirby W. Kemper, the Robert O. Lawton Distinguished Professor of Physics at Florida State University, took part in an experiment at the National Superconducting Cyclotron Laboratory.
Unexpected
In the experiment, Kemper and his colleagues found that the structure of atomic nuclei of one radioactive isotope in particular — magnesium-36, or Mg-36 — is odd and unexpected, according to a Florida State University press release.
Far from the everyday world occupied by such common elements such as gold and lead lies the little-understood realm inhabited by radioactive, or unstable, elements.
The recent experiment by Kemper who collaborated with other scientists from the United States, Japan and England illustrated how the ‘normal’ rules of physics do not apply for some of these radioactive elements.
“Ten years ago, complicated experiments like this one were a dream,” Kemper said. “Five years ago, we thought that in the next 10 years we would be able to carry it out. Now we have done one and so are much further along in experimental capability than even our wildest hopes.”
In the experiment a beam of calcium-48 nuclei was generated and directed at a beryllium target. This generated a variety of reaction products, including silicon-38, or Si-38. A large scientific instrument known as a fragment separator then was tuned to allow Si-38 to pass through and continue down the beam line.
Downstream, these Si-38 isotopes struck a second beryllium target, resulting in the creation of a smattering of new nuclei, including Mg-36. The beam was turned up into the focal plane of a three-storey-tall spectrograph — a giant analytical tool — that was set to accept only Mg-36. When analyzed, the spectroscopic data indicated that Mg-36 is in fact within the island of inversion.
Contemporary theoretical models suggested that its nucleus, with 12 protons and 24 neutrons, should exist just within the island of inversion. But until the team’s result, which appears in Physical Review Letters, experimental
ists had not made the necessary measurements of the rare magnesium isotope to know for sure.
“Gamma-ray spectroscopy for Mg-36 has never been done because this nucleus is incredibly hard to reach,” said Alexandra Gade, an assistant professor at the National Superconducting Cyclotron Laboratory and lead author of the
Physical Review Letters paper. “It’s not just another nucleus.”
Just one nucleus
For every 400,000 Si-38 nuclei impacting the second target, just one Mg-36 nucleus was produced.
“To the average person, this might seem like a lot of work for not a whole lot of benefit,” Kemper said. “But experiments like this are really all about broadening our understanding of matter — how it is formed, how it behaves under extreme conditions, and what universal rules apply to it.
This is fundamental to increasing our understanding of all matter in the universe. After all, even common elements such as gold and lead had to come from somewhere.” — Our Bureau
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