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Hologram: new dimensions for X-rays.

Hologram: New dimension for X-rays

With the electron microscope, scientists can see the hairs on a housefly's feet and uncover other marvels of the microscopic universe. But the world imaged in electron micrographs is largely a dead and unnatural one because samples must be chemically fixed, thinly sliced, dehydrated or altered in other ways. Biologists have hoped for X-ray microscopes, which, while using a longer wavelength and hence having a lower resolution than their electron counterparts, would enable scientists to study unaltered, living samples in detail.

In the last few years, researchers have made significant strides toward this goal, and some X-ray microscopes that produce two-dimensional images are attracting biological customers. Now two research teams report in the Oct. 23 SCIENCE that they have passed important milestones in X-ray holography, the main microscopy technique for producting three-dimensional images. One group, using the National Synchrotron Light Source (NSLS), has produced a hologram of unprecedented resolution; the other has made the first X-ray-laser hologram.

The synchrotron group, led by Malcolm Howells at the Lawrence Berkeley (Calif.) Laboratory, made holograms of rat pancreas granules. The smallest resolvable feature in these holograms is 400 angstroms, 25 times smaller than the best previous holograms and equal to the resolution of the best X-ray microscopes. Howells attributes this increased resolution in part to an improved X-ray source: NSLS last year added an undulator--a series of magnets that deflect synchrotron electrons side to side, creating a brighter, more coherent X-ray beam. The improved resolution is also due to the group's use of high-resolution resist, rather than film, for recording the hologram.

"The key step preventing X-ray holography from being a useful technique was the inability to record the hologram,' says Howells, "and now we've accomplished that step with the undulator and resist.'

At the moment, however, their holograms appear two-dimensional because the depth of focus is about the same as the thickness of their sample. They hope to achieve three-dimensionality by pushing the resolution down to 100 angstroms. One potential difficulty is the technique's long exposure time--80 minutes in their recent work. To avoid blurred images from moving samples, says Howells, "we definitely have to find a way to hold the sample still.'

In contrast, the X-ray laser that James E. Trebes and his colleagues at Lawrence Livermore (Calif.) National Laboratory used to make holograms is so bright and coherent that hologram exposures on film take less than a nanosecond. In principle, this will enable researchers to freeze the action of a moving sample without blurring the image. So far, Trebes's group has demonstrated the feasibility of X-ray laser holography by making, again, two-dimensional-appearing holograms of a gold bar and carbon fibers, with a resolution of a few microns.

The researchers plan to develop a laser source that will image biological samples at much higher resolutions, which will, among other things, help achieve three-dimensionality. "That's the nice thing about Howells's work,' says Trebes. "He's shown that you can really make a high-resolution hologram. This was in some doubt before.'

One of the key ingredients in the system devised by Trebes's group is a multilayered X-ray mirror that can separate X-rays from other wavelengths made in the lasing process and is flat enough to maintain the beam's coherence. Another critical part is the X-ray laser, which is produced when Livermore's Nova laser zaps selenium foil, creating a plasma that produces X-rays.

"For the last three years, X-ray lasers have been a lab curiosity, a research topic in themselves,' says Trebes. "This paper announces that X-ray lasers have arrived and it's time to start using them [for imaging].'
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Author:Weisburd, Stefi
Publication:Science News
Date:Oct 31, 1987
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