27 Jan 1999
Physicists have taken a major step toward the creation of the ultimate "light": a laser beam made of gamma rays. In last week's Physical Review Letters, researchers report that bombarding an artificial form of the element hafnium with x-rays causes it to produce gamma rays.
Ever since the invention of laser light in 1961, theorists have realized that the new technology might go all the way upward through the electromagnetic spectrum, into gamma-radiation territory. A gamma laser could pack a powerful enough punch to blow missiles out of the sky or simulate conditions near stars. But it would work differently from all existing lasers, which excite electrons in a gas, liquid, or solid, then stimulate them to emit coherent radiation. For atoms to emit gamma-caliber photons, scientists must achieve the same trick with atomic nuclei--pumping a large population of them into deformed, excited states called isomers--rather than merely titillate electrons.
With most nuclei, the gamma-emitting isomers emit their energy too quickly for a large population to develop, no matter how fast energy is pumped in. But a few isomers are much longer lived. Physicist Carl Collins of the Center for Quantum Electronics at the University of Texas, Dallas, and his colleagues in five countries first managed to coax gamma emission from tantalum-180, an isotope whose nuclei exist naturally in an excited isomeric state. However, the photons they gave off had about the same energy as the blast of x-rays needed to provoke them. Calculations showed that a long-lived, high-energy isomer of hafnium-178 that is found as a waste product of the production of radioisotopes for medicine could emit gamma photons 30 times more energetic.
Now Collins and his colleagues have aimed x-rays from a dentist's machine at their own thin, crusty, plastic-encased hafnium-178 sample and detected an answering blast of gamma photons. The waves were not coherent, like in true lasers. "This is more like a gamma ray flashbulb," remarks physicist Paul Kepple of the Naval Research Laboratory in Washington, D.C. But if researchers can harness the gamma rays into a coherent beam, he says, they could be on their way to the world's most powerful laser. Says Kepple: "Who knows where it is going to go?"