Lenses are a part of every-day 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
material, and getting rid of nuclear waste.
On
the first platform, the gamma-ray lens in its two-axes gimbal.
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 –ultra-violet 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.
Gamma
– Ray Lens could be made to focus beams of a specific energy.
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 ben them at all and the
refractive index for gamma rays should be almost equal to one. Yet this not
what a team of physicists led by Dietrich Habs at the Ludwig Maximilian
University of Muchich in Germany and Michael Jentschel at the InstitutLaue -
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.
Such
focused beams could detect radioactive bomb-making material, or radioactive
tracers used in medical imaging.
“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.
A
photograph of gold lenses · The gold standard for gamma-ray lenses
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 pas
others unimpeded. The beams could even make new isotopes altogether, by
“evaporating” off protons or neutrons from existing samples. That process could
trun 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.”
