40 Mg found! Courtesy of National Superconducting Cyclotron Laboratory

Thursday, October 25th, 2007--

At the center of every atom are protons and neutrons and it's the number of protons that distinguishes one chemical element from another. But a single element can have different numbers of neutrons; these variants are called isotopes. This week, scientists reported two super-heavy (the actual term) never-before-seen isotopes. They found magnesium-40 (magnesium with 40 neutrons and protons) and aluminum-42 (aluminum with 42 neutrons and protons), according to a study published in the journal Nature.

To find new isotopes, scientists start with a neutron-heavy element, smash it to pieces and look through the remains. In this experiment, researchers from the National Superconducting Cyclotron Lab (NSCL) at Michigan State University started with a piece of calcium-48—a metal isotope that is big enough to include the magnesium isotope they were looking for. They heated the calcium so it evaporated and then accelerated the calcium isotopes to about half the speed of light, says Thomas Baumann, a physicist at the NSCL and the lead author on the paper. The researchers then shot the beam of atoms at a target and filtered out isotopes from the wreckage.

"It's very crude," says Baumann. He compares it to making a mosaic: "You want a specifically-shaped piece of ceramic for your mosaic. So you just take a plate and drop it, and then you pick out the pieces with the proper shape. But maybe it takes a lot of times to get the very rare exciting shapes," says Baumann.

In this experiment, Baumann and his colleagues smashed calcium-48 for five days, without seeing the magnesium-40 isotope they were hoping to find. Then one night they filtered out two magnesium-40s. By the end of the experiment they found another magnesium-40. “If you have three you can say, 'Yes, we saw it,'" Baumann says.

Elements can have more than one form of stable isotope--there are three stable isotopes for oxygen, for example. But as you add neutrons, an element becomes radioactive. The point of maximum neutron load is called the "drip-line." The magnesium-40 isotope may be near magnesium’s drip-line, the researchers think.

They also found 23 instances aluminum-42. This isotope has an odd number of neutrons and protons—even numbered nuclei tend to be more stable. This suggests that aluminum can get even heavier: "That means for the aluminum isotopes, the drip-line is much further out than previously thought," Baumann says. In fact, the researchers saw one instance of aluminum-43 during the experiment. Baumann says: "But, you know, one event is not a very convincing statement."

The drip-line “is kind of a fundamental property of nature," says David Morrissey, an author on the paper and a professor at Michigan State University. Researchers believe that the isotopes and chemical elements on Earth are the same everywhere else in the universe, says Morrissey. “So if you go to any other part of the universe and you look at what oxygen they have, they'll have the same stable isotopes of oxygen that we have here. And the other ones will be radioactive…wherever you go." This suggests that finding the drip-line means knowing a universal limit, so to speak.

To find heavier isotopes and the drip-line for these elements, the researchers say they will likely need an upgraded accelerator that can shoot out heavier isotopes than calcium-48. “We are at the end of the capabilities of our lab right now,” says Baumann. It is unclear whether a new facility with the needed upgrades will be commissioned in the U.S., Baumann says. “We have to wait and see if we can make advances in terms of technology to continue the quest of the neutron drip-line.”

--Flora Lichtman

Sources

Thomas Baumann
Physicist National Superconducting Cyclotron Laboratory Michigan State University East Lansing, Michigan

David Morrissey
Distinguished Professor of Chemistry National Superconducting Cyclotron Laboratory Michigan State University East Lansing, MI

Tools:

Elsewhere on sciencefriday.com

• 

  Crystal Ball
  Researchers have grown a crystal that bends in light. Under the right rays, the crystal can even catapult a gold ball ninety times heavier than itself.

• 

  Telling Time: Leap Year, Leap Day
  This Friday is a date that comes only once every 28 years - Friday, February 29th. We'll talk about leap years, leap days, and the finer points of telling time.

• 

  Looking forward to LHC
  Physicists around the world are looking forward to the startup of the Large Hadron Collider at CERN later this year. We'll talk about the project, and a legal challenge to the collider that argues the work could be too risky.

Explore More

• Physics
• Chemistry
• Nuclear, Quantum and Particle Physics