Crude rays

By modifying diffraction, the fundamental problem of focusing X-rays could be resolved

Published: Friday 31 January 1997

 Compound diffraction of X-ray although most of us have only encountered x-rays at the radiologists clinic, they are among the most useful tools for physicists. From x-ray diffraction, which is still the most important tool for studying crystal properties, to the study of electronic properties of various chemical and biological systems, x-rays find immense use in science. But a simple and convenient way of focusing these beams was what was needed.

A Snigirev and his colleagues at the European Synchrotron Radiation Facility, Grenoble, France have demonstrated a crude refractive lens for focusing high energy x-rays.

The fundamental problem in the development of a refractive lens for x-rays is the choice of material. The difference in the refractive index (a measure of the amount of deviation the beam undergoes in a medium) of ordinary materials and air for x-rays is very small.

Heavier materials have a larger refractive index but then they absorb very much of the beam to be of any use. The light materials, because of the small difference in refractive index will have too large focal lengths to be of any practical use in the experiments. Snigirev and his colleagues have come up with an ingenious method to circumvent this problem. They have demonstrated a compound lens made up of aluminium. Aluminium has a small absorption of x-rays but a single lens will have a large focal length. A compound lens, made up of a linear array of tens or even hundreds of lenses has a total focal length which is much smaller than the focal length of each individual lens. Snigirev and his group have done just this. They drilled 30 holes of radius 0.3 mm each in a block of aluminium. They then used 14 keV x-ray beam and focused it on a region of size about eight micrometers at a distance of about two meters.

The demonstration proves that the idea, admittedly crude in this form, is workable. With more sophisticated techniques one can envisage compound refractive focusing systems which provide variable and accurate focusing of the x-ray beams. That will prove to be a real boon to the researchers in the field (Nature, Vol 384, No 6604).

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