Alchemy With Laser Light

ATLANTA--Physicist Tom Cowan and his colleagues didn't intend to turn gold into platinum. But when you wield the world's most powerful laser, these things can happen. The team has produced the first nuclear physics reactions driven solely by laser light, Cowan reported here yesterday at a meeting of the American Physical Society. The intense light pulses, with a peak power that briefly exceeds the electrical generating capacity of the entire United States, also sparked fleeting bits of antimatter in their wake.

The laser, called Petawatt, is a souped-up arm of the giant Nova laser at Lawrence Livermore National Laboratory in Livermore, California. Its beam packs 1.5 quadrillion watts of power in pulses that last half a trillionth of a second. Physicists designed the laser to study "inertial confinement fusion," the ongoing attempt to ignite a pellet of hydrogen fuel and harness the energy that powers the sun. As a fringe benefit, Cowan's group placed thin foils of gold in Petawatt's path to observe its startling effects on matter.

The laser stripped electrons from gold atoms to create a short-lived plasma cloud. When the light plowed through the plasma, its intense electric fields accelerated the electrons to nearly the speed of light. Some of them collided with gold nuclei in the foil and emitted piercing gamma rays. The gamma rays knocked neutrons from one out of every 10 billion gold nuclei, making these nuclei so unstable that they decayed to platinum within several days. Other gamma rays split uranium nuclei in a layer behind the gold, splitting them into lighter elements. Still others spit out pairs of electrons and their antimatter counterparts, positrons. "These effects were previously the sole domain of particle accelerators," Cowan says.

Livermore scientists will dismantle Nova--used primarily for experiments that probe the subtleties of nuclear weapons explosions--in May to make way for the 192-beam National Ignition Facility. Until then, physicists will scour Petawatt's data for clues about how so-called relativistic plasmas behave in space. For example, the forces felt by electrons near the gold foil equal those felt 5 centimeters above a black hole. The matter-antimatter frenzy also may shed light on the dynamics of energy beams from gamma ray bursts and active galaxies. "This laser is the only experimental resource that gives us hints of what goes on there," says astrophysicist Yoshiyuki Takahashi of the University of Alabama, Huntsville.