Gold lenses used to create gamma optics
A fundamental assumption of physics is toppled: even extremely high-energy radiation is refracted in suitable materials such as silicon or gold
Scientists at Ludwig-Maximilians-Universität in Munich and the Max Planck Institute of Quantum Optics in Garching have opened up a new chapter in optics: in experiments with gamma rays at the Institut Laue-Langevin (ILL) in Grenoble they have proven that these extremely high-energy electromagnetic waves can be focused by lenses like conventional light - the researchers have thus refuted a fundamental assumption of theoretical physics that had been valid for decades. Their discovery will make a great many new applications possible in medicine and materials research, for example.
Optical instruments like telescopes and microscopes are based on the refraction of light: in a medium such as glass the electromagnetic waves propagate more slowly than in air or in a vacuum, and are therefore diffracted - for example, onto the focal plane of a photo camera. The refractive index, which depends on the lens material and the frequency of the waves, describes how large this effect is: the more it deviates from 1, the stronger the diffraction of the light beams.
Until now, physicists had assumed that electromagnetic radiation with far greater energy than that in the visible spectrum could not be diffracted with lenses. They had calculated that the refractive index in this region of the spectrum is almost precisely 1 for all materials. However, back in the mid-1990s it turned out that X-rays are also diffracted by beryllium or carbon lenses, and thus that X-ray optics were possible. When Dietrich Habs, professor at Ludwig-Maximilians-Universität in Munich and Fellow of the Max Planck Institute of Quantum Optics in Garching, and his team were making their measurements at the ILL they discovered that this also applies to the even higher energy gamma-rays - after X-ray optics, the era of gamma optics is now beginning.
Short-lived electron-positron pairs diffract gamma rays
“I was fascinated by the X-ray lenses,” Habs recalls. “So I asked myself whether something comparable could also exist in the region of gamma radiation.” Habs’ team started out by using prisms made of silicon rather than glass for its experiments. “Silicon atoms have 14 protons in their nucleus and generate a very strong electric field there,” explains Dietrich Habs. “An extremely high number of electron and positron pairs are constantly being created in this field. And although they exist for only a short time, they can still interact with the gamma radiation.” This Delbrück diffraction is the cause of the unexpected diffraction of the gamma rays.
This discovery is indeed surprising: decades ago, theoretical physicists had already supposedly proven that this effect cannot occur. Today it is clear that their mathematical approximation methods must fail due to the strong fields in the vicinity of the atomic nuclei - the physicists’ task for the future will therefore be to develop new computational methods for this area.
Gamma lenses could improve depression therapy and lithium batteries
Habs is also looking at concrete applications of his discovery: “Patients suffering from manic depression often take lithium medication – but nobody knows exactly how these drugs work in the brain,” says the nuclear physicist. “In the future, we will be able to use gamma rays and gamma lenses to make three-dimensional images with a resolution in the micrometer range, and to look where the lithium accumulates and why it has an effect on the psyche at all.” The element also plays a crucial role in rechargeable batteries for notebooks and electric vehicles - if they are overloaded, small lithium dendrites form, which drastically reduce the life of the energy storage systems. Habs wants to observe this effect in detail with gamma rays in order to also optimise rechargeable batteries - an important contribution to electric vehicles with greater range.
“In future it will also be possible to detect radioactive materials or explosives with precision on the basis of our findings,” believes the researcher. “I see a further application field in the diagnosis and therapy of cancer: we could produce new medical radioisotopes that doctors could use to detect and treat tumours earlier.”
In the summer his team will conduct further measurements at the European Synchrotron Radiation Facility (ESRF) in Grenoble - this time with gold lenses. “Gold has 79 protons in its nucleus and will diffract the gamma rays even more strongly than silicon,” explains Habs.