New glass 'lens' for intensifying X-rays.

When accelerated electrons in a laboratory X-ray source smash into a metal target, the resulting interaction sprays X-rays in all direction. To get a narrowly confined, precisely characterized beam, researchers typically use an arrangement of narrow slits to block the passage of all but a small fraction of the X-rays emerging from the target, thereby throwing away most of the X-rays produced and considerably reducing the beam's ultimate intensity.

Physicist Timothy M. Hayes and his co-workers at the Rensselaer Polytechnic Institute (RPI) in Troy, N.Y., have now succeeded in using a novel "lens" consisting of a bundle of glass fibers threaded with hundreds of microscopic channels to capture diverging X-rays and gradually steer them into an approximately parallel beam. This custom-built prototype "collimator" produces four times the X-ray output of a comparable slit system.

The idea of using such glass-fiber lenses or collimators to guide X-rays and neutrons originated with Muradin A. Kumakhov of the I.V. Kurchatov Atomic Energy Institute in Moscow (SN: 6/6/92, p.373). When Hayes and his colleagues expressed an interest in using such a lens to increase the intensity of X-ray beams available for studying various materials in the laboratory, Kumakhov and his co-workers fabricated a lens suitable for the purpose.

"It was made by the Russians to our specifications," Hayes says.

Brought to the United States by Vladimir E. Kovantsev, who worked with the RPI group for two months, the lens consists of 919 glass fibers packed together to form a bundle with a hexagonal cross section. Made from a specially formulated borosilicate glass containing traces of sodium, potassium and aluminum, each fiber contains 547 channels, 14 microns wide, with exceedingly smooth inner walls.

"The smoothness of the glass is the critical thing, and that depends on the composition," Hayes says.

Hayes expects that improvements in the alignment and positioning of the glass fibers alone could increase the intensity of a collimated X-ray beam by an additional factor or six. The availability of such intensities would greatly expand the types of materials studies that researchers could undertake without having to go to special facilities.