Science Now – 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.
Bending the rules. Gamma ray lenses, which theory had suggested were impossible, could be made from heavy elements such as gold.Credit: Institut Laue–Langevin
For X-rays the real part of the refractive index, dominated by Rayleigh scattering, is negative and converges to zero for higher energies. For g rays a positive component, related to scattering, increases with energy and becomes dominating. The deflection of a monochromatic g beam due to refraction was measured by placing a Si wedge into a flat double crystal spectrometer. Data were obtained in an energy range from 0.18 – 2 MeV. The data are compared to theory, taking into account elastic and inelastic scattering as well as recent results on the energy dependence of the pair creation cross section. Probably a new field of g optics with many new applications opens up.
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.
They directed gamma rays 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.
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.