Coaxing Skittish Ions Into an Atomic Jar

Electrons are harder to catch than hopped-up bugs on a summer day. Now researchers have created a method for capturing electrons by using a pulsed electric field in a series of steps similar to the way a child traps an insect in a jar. Physicists might someday use this technique to build antimatter and thus answer questions that divide some of physics' strongest theories.

The universal shortage of antimatter is a constant frustration to physicists. It should be fascinating stuff--atoms with negatively charged nuclei surrounded by clouds of positive charge, and with who knows what bizarre properties--but no one has ever made enough of it to tell. A few years ago physicists forged 108 atoms of antihydrogen, each sporting a positron, or antiparticle of hydrogen, dancing around a negatively charged antiproton. But all perished within billionths of a second when the particles collided with the walls of the particle accelerator within which they were formed.

Physicists led by Bart Noordam of the FOM Institute for Atomic and Molecular Physics in Amsterdam think their new technique could make more antimatter and keep it from self-destructing. Instead of using positrons and antiprotons, they tested the set-up by shooting electrons at rubidium ions. The speeding electrons swerve toward the oppositely charged rubidium ions. Before the electrons can swing past the ions and escape, an electric field decelerates them and turns them back. Then, suddenly, the field is switched off, leaving some of the stalled electrons easy prey for the rubidium ions to capture into wide orbits. Researchers compare the method to the way a child coaxes a lightning bug into a jar, then clamps on the lid just as it turns around to escape.

The system is about 100 times more efficient than other methods of assembling atoms, the researchers report in the 24 April issue of Physical Review Letters. They plan to test it next with hydrogen nuclei instead of rubidium ions and then move on to assembling antiprotons and antielectrons into antimatter. Because the nuclei start out motionless instead of hurtling down an accelerator shaft, they are less likely to run into ordinary matter and annihilate themselves.

"A few years ago, I would have thought this [technique] is completely nuts," says Thomas Gallagher of the University of Virginia in Charlottesville. "I think it's a nifty trick," he says. If the trick works for antiatoms, the material could give physicists much to think about. For example, general relativity holds that an atom and its antimatter counterpart should have identical masses, but some versions of string theory disagree.

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