Scientists have for the first time used laser light to crack open the nucleus of an atom. Two groups have harnessed lasers to split uranium apart, knock neutrons out of gold, and perform other nuclear feats, they report in the February Physical Review Letters. The experiments show that you don't need huge accelerators to do nuclear physics, and they could lead to a convenient source of protons for treating cancer.
All atoms envy iron, which has the most stable nucleus of all the elements. Given a sufficient energy kick, uranium and other heavy atoms fission into smaller, more stable atoms. Traditionally, scientists have pelted uranium nuclei with neutrons to provide the extra energy needed for fission. But a decade ago, physicists proposed that high-energy photons might pack enough of a wallop to split atoms. Back then, however, lasers weren't powerful enough to do the job.
No longer. A team in the United Kingdom, using the VULCAN 50-terawatt laser, and a second group in the United States, using the 20-times-stronger NOVA petawatt laser, each split uranium nuclei. In both experiments, a relatively weak pulse of photons dislodged electrons from a small chunk of tantalum or gold, creating a plasma of swarming electrons just above the metal's surface. Then a second, far more powerful pulse slammed into the plasma. "The electrons absorb an enormous amount of energy from that light pulse," explains U.K. team member Kenneth Ledingham, a physicist at the University of Glasgow. The energetic electrons then crash back into the metal target. As they knock into the metal nuclei, the electrons emit powerful gamma rays that bathe a uranium target nearby, causing some of the atoms to fission.
"We have gotten to the point where lasers have crossed the energy frontier," says Lawrence Livermore National Laboratory's Thomas Cowan, who is with the U.S. team. Ledingham notes that using a thin film of Mylar or aluminum instead of uranium creates proton beams by knocking loose hydrogens out of the material. Such a beam could be used to zap tumors more effectively than x-ray-based radiation therapy can, as protons interact more strongly with tissue than photons, so they dump their energy in a more concentrated area. "We're very excited about this," he says.