Let There Be Aluminum-42

Experiment creates surprise isotope

By Davide Castelvecchi
From Science News, October 27th, 2007; Vol.172 #17 (p. 260)

Physicists have created the heaviest isotope yet of magnesium, but in their experiments an unexpected isotope of aluminum also showed up. The findings could help astrophysicists understand occasional X-ray emissions from neutron stars that are growing in mass.

The 7-day-long experiment took place at the National Superconducting Cyclotron Laboratory (NSCL), an atom smasher at Michigan State University in East Lansing. Hoping to test the limits of how many extra neutrons will bind to an atomic nucleus, researchers were trying to create magnesium-40, a heavyweight element with 18 more neutrons than the most common isotope, magnesium-22. Standard theory says that magnesium-40 should be the heaviest isotope of the element that can exist, if only for a fleeting instant, before decaying.

NSCL’s Thomas Baumann and his colleagues shot nuclei of calcium-48—the heaviest naturally occurring calcium isotope—at a tungsten foil at about half the speed of light. Atomic collisions created all sorts of debris, including fragments from both calcium and tungsten nuclei, out of which new atomic nuclei occasionally formed.

Like Adam in the book of Genesis, the heavy magnesium nuclei started appearing on the fifth day of the experiment. The researchers picked up three of them among the quadrillion particles produced. And it was very good, but then something even more interesting happened (think Eve). The detector recorded 23 particles whose charge and mass marked them as aluminum-42, the researchers report in the Oct. 25 Nature.

According to Baumann, most theories had predicted that aluminum-42 wouldn’t exist. While physicists know that the strong nuclear force keeps atomic nuclei together, they cannot calculate exactly the complex interplay of forces among neutrons and protons. Several competing models aim to approximate this interplay. “The range of predictions is pretty broad,” says Baumann.

The discovery of aluminum-42 suggests that even heavier aluminum isotopes could exist, says Paul-Henri Heenen of the Free University of Brussels in Belgium. And other elements, higher in the periodic table, might also be able to accommodate more neutrons than expected.

“It’s interesting, but also worrisome,” says Hendrik Schatz, an NSCL physicist who was not involved in the experiment. In particular, he says that the results complicate physicists’ efforts to understand how stars and supernova explosions forge neutron-rich isotopes as intermediate steps toward creating elements heavier than iron.

Schatz says that the new results are even more directly relevant to another astrophysical scenario. When matter falls onto a neutron star and starts sinking into its crust, pressures 10 trillion times as high as those at the sun’s center force electrons and protons to merge, forming neutrons. The transient formation of isotopes such as aluminum-42 and magnesium-40 during this process could help explain certain anomalous flashes of X rays that astronomers have observed coming from neutron stars, Schatz speculates.

The experiment was a “tour de force,” says Michael Pearson of the University of Montreal, adding that the results should help discriminate between different nuclear models. However, he says that existing theories might still apply to elements that are heavier than aluminum.

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