|Physicists have always believed that it would be impossible to create a
practical lens that could focus gamma rays like light. Now, however, an
unexpected discovery suggests gamma-ray focusing is indeed possible.
When electromagnetic radiation travels through a medium, its speed is
given by the index of refraction of the material. When radiation goes
from one medium to another, the change in the index of refraction causes
its path to bend - and this forms the basis of classical optics. For
X-rays, the index of refraction is defined by Rayleigh scattering.
While physicists have used Rayleigh scattering to focus X-rays, the
strength of the effect drops off as the inverse square of the X-ray
energy. This means that at high X-ray energies - and on into low
gamma-ray energies - the radiation is not bent enough for a lens to work
effectively. One way round this is to put the radiation through a large
number of successive lenses. However, no lens is perfectly transparent
and at higher energies the large number of lenses needed would result in
practically all of the radiation being absorbed.
According to classical physics and conventional quantum physics, this
trend should continue at higher energies. This is what researchers at
Ludwig Maximilians University in Munich, Germany, and at the Institut
Laue-Langevin in Grenoble, France, set out to measure in silicon. But
instead they discovered that the exact opposite occurs - the index of
refraction starts to make a comeback at energies greater than about 700
keV. What is more, while the index of refraction is negative for X-rays,
it becomes positive for gamma rays.
Possible applications are medical imaging where gamma rays could be used
to track lithium in the brains of patients being treated for bipolar
disorders. The discovery could also result in a better fundamental
understanding of how light interacts with matter.