Gold lenses that Dietrich Habs' s team will use in September for focusing gamma rays

Karl Gruber

Tue 22 May

Light comes in many forms, some of it is visible some of it is not. And a good example of our ability to use light in different ways is the use of lenses, which focus light waves for specific applications. Lenses help us read, and are loaded in microscopes and telescopes, but they all only work for visible, low energy, light. Gamma rays are high-energy light waves, which due to their properties, were thought to be impossible to focus on using lenses. Now Munich researchers have changed this long-held idea.

In an article published on May 4th in the journal Physical Review Letters, a team of physicists led by Dietrich Habs at the Ludwig Maximilian University of Munich and Michael Jentschel at the Institut Laue-Langevin (ILL) in Grenoble, France, have been able to deflect gamma rays with a prism, a first step towards the creation of gamma lenses. Their findings open a new page in the field of physics, and promise myriad of new applications in areas like medical imaging, radioactive waste management and high-resolution nuclear spectroscopy, among others.

So, how does it work?

First let's think about light again, and how it goes through a piece of glass. Unlike other materials, which reflect or absorb light, glass does neither. It refracts light, or, in plain English, it bends light in a specific direction. This bending occurs on a very small scale, within the realm of atoms where the electrons repulse light in a specific direction. This property of glass and other materials is known as the refraction index, where an index of 1 means there is no repulsing or bending of light.

Lenses work by having different refraction indices, each one giving the glassware a specific property. But most current applications are based on visible, low energy light, because here light interacts well with the electrons found in conventional lenses. However, when gamma rays come into play, conventional lenses don't perform so well because of the high energy waves of gamma rays and the inability of electrons to respond accordingly. A similar problem occurred with X-rays, which are more energetic than visible light (but less than gamma rays) and this was solved by using stacks of many X-ray lenses. This was done in the late 90s, and resulted in X-ray lenses and the creation of the field of x-ray optics.

As Dietrich Habs explains, "All high energy light is very similar to X-rays...it was assumed that it was not possible to build lenses for X-rays. Then in 1996, A. Singirev built the first lenses for X-rays. X-rays are used in many studies of atoms in matters like physics, chemistry, geoscience, and environmental physics. Now we can do the same thing with gamma rays, but we look for different isotopes. Gamma rays are much more penetrating than X-rays, they are much more sensitive because the nuclear levels have a smaller width and can be addressed individually after monochromatisation."

So most physicists thought that this was the end of the road for high-energy light, as it was considered impossible to manage anything like gamma rays. But Dietrich Habs has proved them wrong. In their research, Dietrich's team bombarded neutrons in chlorine and gadolinium to produce gamma rays, which were then directed into a crystal spectrometer, used to harness the gamma rays into a specific direction. Then the gamma rays were split into two halves, one which went through a silicon prism and the other, the control, went into a spectrometer, which would measure its direction. The idea was to detect if there was any bending of the gamma rays when they went through the prism. To everyone's surprise, gamma rays indeed bent after passing through the silicon prism.

Gamma rays do bend after all.

Their results show that gamma rays with an energy above 700 kiloelectronvolts are slightly bent after passing through the silicon prism. The bending itself was rather small, instead of showing a refraction index of 1, gamma rays that went through the silicon prism had an index of 1.000000001. This may seem very small, but researchers think it can be improved upon by using other materials for lenses, such as gold. The important issue is that there was a bend, however small.

And what is so special about these silicon lenses that accomplished this feat? According to Dietrich Habs the key is found in the nuclei of the silicon atoms. These atoms have peculiar properties, like the existence of so-called "virtual" electrons and anti-electrons, or positrons, which according to the uncertainty principle of quantum mechanics occur in the nuclei, for fractions of a second. Habs thinks these elusive particles are responsible for the detected refraction of gamma rays.

Further work is needed to amplify this bending of the gamma rays, and Habs's team is already working on this. They will soon be using gold lenses, which hold promise of improving their results.

The potential applications of these findings could serve important applications such as in the design of effective approaches to detect radioactive materials or explosives, or to design radioactive tracers for medical imaging. What is even more amazing is that these harnessed gamma rays could be used to make brand new isotopes, which could, for example, convert toxic nuclear waste into harmless non-radioactive material.

And more applications are likely to come. Dietrich Habs already has some big plans for his discovery which he shared with The Munich Times:
"We have patented 50 new medical radioisotopes for cancer therapy and diagnostics. We can also produce very brilliant secondary beams like brilliant neutron beams or moderated positron beams which will have many applications in material science."