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Scientists create new isotopes, perhaps clues to atom binding

Times Staff Writer

Attempting to understand how heavy elements are formed inside stars and supernovas, Michigan scientists have created three unusually heavy isotopes of magnesium and aluminum, including one that has been sought for a decade and a second that current theory says should be impossible to produce.

The isotopes existed for only fractions of a second in the aftermath of atomic collisions in the target area of the National Superconducting Cyclotron Laboratory at Michigan State University, but studying them will help physicists better understand the so-called strong force that holds the nucleus of atoms together.

The discovery “is a major advance in our understanding of the limits of nuclear binding,” said physicist Rick Casten of Yale University, who was not involved in the study.

The 118 known elements are defined by the number of positively charged protons in their nuclei. Carbon, for example, has six; magnesium has 12; and aluminum has 13.

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But the nuclei also contain uncharged neutrons that stabilize the atom. Carbon can have four to 16 neutrons, but only the isotopes with six or seven neutrons -- carbon-12 and carbon-13 -- are stable. The rest are radioactive.

About 300 stable isotopes are known and, so far, more than 3,000 unstable ones are thought possible.

There is no good theory to predict the maximum number of neutrons, so it is up to experimentalists to determine the limits. Such a limit is known as the dripline because a plot of proton number versus neutron number has the heaviest isotopes hanging down like icicles.

Physicist Thomas Baumann and his colleagues at Michigan State reported Thursday in the journal Nature that they had bombarded a tungsten target with a beam of calcium-48 ions accelerated to half the speed of light. They observed the formation of magnesium-40, aluminum-42 and aluminum-43.

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Magnesium-40, which has 12 protons and 28 neutrons, has been sought unsuccessfully since 1997, and researchers had begun to believe that it was beyond the bounds of stability.

Aluminum-42, with 13 protons and 29 neutrons, lies outside the theoretical bounds of the dripline, and its discovery will force a reexamination of the basic theory.

It is unusual, moreover, in that aluminum-42 is the first near-dripline isotope discovered with an odd number of neutrons. Generally, the pairing of neutrons stabilizes nuclei, and physicists had not expected to find such a heavy isotope with an odd number of neutrons.

Aluminum-43, with 13 protons and 30 neutrons, is also outside the dripline, but the team observed it only once, so it cannot formally claim its discovery. But the fact that the isotope has an even number of neutrons should make it more stable than aluminum-42, giving credibility to the claim, the team said.

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thomas.maugh@latimes.com


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