Lenses are a part of everyday life—they help us focus words on a page, the light from stars, and the tiniest details of microorganisms. But making a lens for highly energetic light known as gamma rays had been thought impossible. Now, physicists have created such a lens, and they believe it will open up a new field of gamma-ray optics for medical imaging, detecting illicit nuclear material, and getting rid of nuclear waste.
Glass is the material of choice for conventional lenses, and like other materials, it contains atoms which are orbited by electrons. In an opaque material, these electrons would absorb or reflect light. But in glass, the electrons respond to incoming light by shaking about, pushing away the light in a different direction. Physicists describe the amount of bending as the glass's "refractive index": A refractive index equal to one results in no bending, while anything more or less results in bending one way or the other.
Refraction works well with visible light, a small part of the electromagnetic spectrum, because the light waves have a frequency that chimes well with the oscillations of orbiting electrons. But for higher energy electromagnetic radiation—ultraviolet and beyond—the frequencies are too high for the electrons to respond, and lenses become less and less effective. It was only toward the end of last century that physicists found they could create lenses for x-rays, the part of the electromagnetic spectrum just beyond the ultraviolet, by stacking together numerous layers of patterned material. Such lenses opened up the field of x-ray optics which, with x-rays' short wavelengths, allowed imaging at a nanoscale resolution.
There the story should have ended. Theory says that gamma rays, being even more energetic than x-rays, ought to bypass orbiting electrons altogether; materials should not bend them at all and the refractive index for gamma rays should be almost equal to one. Yet this is not what a team of physicists led by Dietrich Habs at the Ludwig Maximilian University of Munich in Germany and Michael Jentschel at the Institut Laue-Langevin (ILL) in Grenoble, France, has discovered.
ILL is a research reactor that produces intense beams of neutrons. Habs, Jentschel, and colleagues used one of its beams to bombard samples of radioactive chlorine and gadolinium to produce gamma rays. They directed these down a 20-meter-long tube to a device known as a crystal spectrometer, which funneled the gamma rays into a specific direction. They then passed half of the gamma rays through a silicon prism and into another spectrometer to measure their final direction, while they directed the other half straight to the spectrometer unimpeded. To the researchers' surprise, as they report in a paper due to be published this month in Physical Review Letters, gamma rays with an energy above 700 kiloelectronvolts are slightly bent by the silicon prism.
"Everything was wrongly predicted," explains Habs. "But we said, [the refraction] looks so marvelous for x-rays, why don't we have a look whether there is something? And suddenly we found there is a totally unexpected effect."
So what drives this new bending effect? Although he can't be sure, Habs believes it resides in the nuclei at the heart of the silicon atoms. Although electrons don't normally reside in nuclei because of the very strong electric fields there, quantum mechanics allows pairs of "virtual" electrons and antielectrons, or positrons, to blink briefly into existence and then recombine and disappear again. Habs thinks the sheer number of these virtual electron-positron pairs amplifies the gamma-ray scattering, which is normally negligible, to a detectable amount.
The bending in his group's experiment isn't much—about a millionth of a degree, which corresponds to a refractive index of about 1.000000001. However, it could be boosted using lenses made of materials with larger nuclei such as gold, which should contain more virtual electron-positron pairs. With some refinement, gamma-ray lenses could be made to focus beams of a specific energy.
Such focused beams could detect radioactive bomb-making material, or radioactive tracers used in medical imaging. That's because the beams would only scatter off certain radioisotopes, and stream past others unimpeded. The beams could even make new isotopes altogether, by "evaporating" off protons or neutrons from existing samples. That process could turn harmful nuclear waste into a harmless, nonradioactive byproduct.
"It is great to see that the advances x-ray optics have made … over the past 20 years might now even be moving into the [gamma ray] range," says Gerhard Materlik, chief executive of the Diamond Light Source, an x-ray facility in Didcot, U.K. "I hope that the predictions made by the authors about possible gamma ray optics can be realized to turn them into real optical components."