Physicists make giant leap in antimatter production

Physicists make giant leap in antimatter production
Rice’s Liang is part of US team creating billions of positrons with laser blasts

BY JADE BOYD
Rice News staff

In the novel “Angels & Demons,” author Dan Brown’s famous protagonist from “The Da Vinci Code,” Harvard symbologist Robert Langdon, races to thwart a secret society’s plot to destroy Vatican City by exploding a quarter-gram of antimatter.

When Rice physicist Edison Liang went home one day last summer, ecstatic with the news that he and his collaborators in California had just found a way to make more antimatter — about 100 times more — than ever before, his wife stopped him cold.

	Lawrence Livermore National Laboratory
	Physicist Hui Chen sets up targets for an antimatter experiment at Lawrence Livermore National Laboratory.

“She said, ‘Wait a minute. Are you talking about lots of positrons?'” Liang recalled. “She was reading Dan Brown’s book, and she wanted to know how futuristic was the fictional scenario.”

Liang put her fears to rest. Even though he and his colleagues at Lawrence Livermore National Laboratory had found a way to make hundreds of billions of particles of antimatter called positrons, they’re still far from being able to make the quantities Brown wrote about — much less store and transport them.

Antimatter was discovered in 1932 and captured the imagination of science fiction writers almost immediately. It’s the stuff that’s supposed to power the starship Enterprise in “Star Trek,” for example.

Antimatter is so dramatically compelling because particles of antimatter and matter annihilate when they meet. Their entire mass is converted into pure radiation energy, and pound for pound, the reactions yield more than 100 times the energy released by the fusion reactions that power the sun. That means that just one gram of antimatter could power an explosion larger than the nuclear blast at Hiroshima.

Physicists have never had the technology to make very much antimatter. Scant amounts are made in the high-energy collisions inside atom smashers and from the decay of radioactive isotopes, but scientists wishing to study antimatter have struggled to make sufficient quantities for meaningful experiments.

In 1993, Liang made a proposal to create copious amounts of positrons — the antimatter equivalent of electrons — by blasting gold targets with intense lasers. But it was not until the summer of 2008 that a Livermore team led by Hui Chen and Scott Wilks, together with Liang, succeeded in making an estimated 100 billion positrons by blasting a millimeter-thick gold target with one of the world’s most powerful lasers.

The laser ionizes and accelerates electrons, which are driven right through the gold target. On their way, some of the electrons strike gold nuclei hard enough to throw out a pair of sister particles — an electron and a positron. The laser blasts, which last only a trillionth of a second, produce positrons more rapidly and in greater density than ever before in the laboratory.

”By creating this much antimatter, we can study in more detail whether antimatter really is just like matter and perhaps gain more clues as to why the universe we see has more matter than antimatter,” said Livermore physicist Peter Beiersdorfer.

Quarks and electrons combine in various ways to form matter. Antimatter is similar in most respects to matter, save for having the opposite electrical charge. Thus, the antiparticle of the negatively charged electron is a positively charged positron, and quarks and the particles that they make — the protons and neutrons found inside atomic nuclei — each have their own corresponding antiparticles.

Physical laws are nearly symmetrical with respect to matter and antimatter, and one of the great mysteries in physics is why the observable universe consists almost entirely of matter. Normal matter and antimatter are thought to have been in balance at the beginning of the universe, but for some unexplained reasons, the universe evolved “asymmetrically,” with some matter remaining after the initial stock of antimatter was annihilated.

High-energy events throughout the universe create particles of antimatter all the time. For example, some cosmic rays produce tiny amounts of antimatter when they strike the Earth’s atmosphere, and larger quantities are produced in violent supernovas, pulsars and around black holes near the center of galaxies. Astronomers can detect these by looking for gamma rays, waves of energy that are given off when positrons and electrons collide and annihilate.

Liang, one of the world’s leading experts on gamma-ray bursts and other high-energy astrophysical phenomena, said he hopes that with a bit of refinement, the team can produce enough positrons to simulate, in the lab, some of the conditions found in black holes and other cosmic events. In addition, dense positrons may one day be used to create a gamma-ray laser. 

“This is a very exciting time,” Liang said. “We appear to be on the verge of being able to conduct antimatter experiments that have never before been possible. There is no doubt that many new applications and innovative technologies will emerge from this.”