X-ray concentrator will expand window on high-energy universe
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"The concept is pretty simple," said Dr. Marshall Joy, an X-ray astronomer at NASA's Marshall Space Flight Center. "You gather X-rays at this end and funnel them down to the other end. You can think of it as a lens for x-rays. Right now, there's isn't anything else that does that for X-rays at high energies." (interview with Dr. Joy, below.)
Right: Marshall Joy inspects a prototype capillary X-ray optic, built by X-Ray Optical Systems, comprising 2,500 fibers (like the one below); each fiber contains about 750 capillaries. Links to. (NASA/Marshall)
Joy is the principal investigator for the capillary X-ray optics project at NASA/Marshall. Professors Walter Gibson and Carolyn MacDonald at the University at Albany, State University of New York and researchers at X-Ray Optical Systems Inc., also in Albany, work closely with NASA on this project. The work had seed funding from NASA's Microgravity Research Program (see Blueprint for proteins, below).The x-ray portion of the spectrum covers a large energy range, 100 to 100,000 electron volts (100 eV to 100 keV) compared to about 2 to 4 eV for visible light. Astronomers have learned how to focus X-rays at low energies, but the middle and higher reaches of the X-ray spectrum are difficult to focus. It's as if we could only see the red portion of the visible light spectrum, and the universe darkened in the green spectrum and then went black altogether.
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Left: A scanning electron microscope image of a 0.5-mm wide bundle of about 750 capillaries. Links to. (X-Ray Optical Systems)
The closer you look, the more you see
No one is sure what will be seen at higher energies, but the scientists are going on more than blind faith. They have precedent in their own field.
"It's hard to know what you'll see until you see it," Joy said. "Until the 1970s, no one had a clue that the universe would be interesting in X-rays."
Then came the Uhuru small astronomy satellite, NASA/Marshall's three large High Energy Astronomy Observatories, and others.
"As it turned out, the sky was lit up in X-rays everywhere you looked," Joy said.
As with optical telescopes, astronomers discovered that whenever they looked with more sensitive telescopes, the more objects and the more detail they saw.
But X-rays can be focused by the same effect that can make a transparent piece of glass act as a mirror: If light strikes glass at a shallow angle, the light is reflected, as if by a mirror. This is called "grazing incidence reflection" since the light just grazes the surface.Above: Intensity plots for the prototype capillary optic shows that the X-ray throughput extends to high x-ray energies, and that the spot size in the focal plane decreases at high energies. AXAF and other X-ray telescopes can see no higher than 10keV. Links to . (NASA/Marshall and Albany/SUNY)
"Normally, X-rays refuse to be reflected unless the angle of incidence is very small," Joy said. "They just penetrate the material rather than reflecting off of it." As X-ray energies become higher, the angle of incidence becomes incredibly small, far less than 1/10th of 1 degree. "Grazing incidence optics" are used in a variety of X-ray astronomy telescopes, including the Chandra X-ray Observatory (formerly called the Advanced X-ray Astrophysics Facility) scheduled for launch in April 1999.
The Chandra X-ray Observatory observes in the 0.1 to 10 keV range. Capillary X-ray optics will extend the observable range from 10 to 80 keV range.
Interview with Dr. Joy
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Improved drugs to treat cancers and other diseases may be designed with the help of capillary X-ray optics.
"We expect this new technology to significantly accelerate the ability of researchers to gather the information necessary to design entire families of highly effective, disease-fighting drugs," said Dr. Daniel Carter of New Century Pharmaceuticals in Huntsville, Ala.
Proteins, enzymes, genetic materials, and other organic molecules have a mechanical side. Chemical reactions occur only if the other molecule has the right shape to join receptors and activators. It's like fitting a key into a lock.
The next generation of pharmaceuticals, already starting to emerge, is coming from "rational drug design" where scientists deduce the structures of proteins and enzymes in viruses, bacteria, and our own bodies, and then tailor drugs for specific functions, like hitting a virus's "off" switch. For example, pharmaceutical companies are developing drugs which inhibit the neuraminidase enzyme (above) which is crucial to the function of the flu virus. Links to. (Center for Macromolecular Crystallography)
But first, you have to learn the structures of these complex chemicals. NASA/Marshall has taken the lead in this promising field of biotechnology by growing hundreds of protein crystals aboard the Space Shuttle and Russia's space station Mir so the crystals could be examined on Earth. The International Space Station, now under construction, will expand on this work.
Once returned to Earth, scientists examine the protein crystals by X-ray crystallography. The crystals have internal facets that reflect X-rays to form dot patterns that are unique to the structure of that protein at that particular viewing angle. Using a computer, scientists can turn the dot pattern into a model of the protein's structure - like the neuraminidase enzyme shown here - revealing where the atoms are located. From this, they can decipher the key-and-lock structure and learn how to change how the protein works.
However, the process takes many hours and dozens of crystallograms, partly because the X-rays are relatively weak. Now the x-ray optics produce much brighter beams on the tiny organic samples, greatly decreasing the time needed to collect enough data to obtain a complete structure.
"This new capillary X-ray technology will allow us to pursue more challenging research problems in our own laboratory with a speed and effectiveness never before possible," Carter said. "These and future applications should have a profound impact on many areas of science and medicine."
Capillary optics don't focus X-rays into images, but merely act as light guides to concentrate X-rays from a large area onto a small one.
Capillary optics start out as bundles of glass tubes that are heated and drawn until the bundles are just 300 to 600 microns wide and contain hundreds to thousands of channels 3 to 50 microns wide. That's so small that the tip of an eyelash would jam inside the opening of one capillary. It also means that even the smallest speck of contamination is like having a pebble stuck inside a straw.
If the glass tubes are bent ever so gently, the X-rays graze the surface at just less than the critical angle. Imagine a race car bouncing back and forth between the guard rails as it barrels down the track. That's pretty much what X-rays do inside a glass capillary. And the capillaries can be bent so they concentrate the X-rays into a small area of the detector, making a smaller but brighter image. That's important in any detector system since noise is always present. A brighter image means that the signal-to-noise ratio is greater.
Up to half the X-ray energy may be absorbed by the capillary concentrator or lens, but no optical system passes all of the energy that arrives at the aperture.
The technology of capillary optics is relatively new, having been conceived in the Former USSR in the 1980s.
"Any new technology takes a while to become commonplace," Joy said. That is starting to happen, and NASA/Marshall is working with various companies to encourage development of X-ray capillary optics.
Somewhat like the large condenser lens in front of a slide projector's lamp, capillary optics can capture the X-rays from a wide angle and concentrate them into a narrow beam aimed at the center of the crystal being studied. This produces a brighter dot pattern in a shorter period time, thus helping scientists deduce the molecular structure faster and with less uncertainty.
Finally, capillary optics can also be used to produce sharper images in medical X-rays. Medical X-rays are really shadow pictures made by projecting the X-rays through a patient's body and looking, for example, for dense shadows (which appear white in the negative film) that indicate potential tumors.
A problem is that dense tissue will also scatter the X-rays. This scattering can blur the image of tumors when they are small and easiest to fight. Placing a capillary optic lens against the patient's skin can capture the X-rays as they leave the body and reduce the blurring, increasing the chances of early detection.
"Any hard X-ray detector will work with it," he said. "It's kind of like a lens in a camera."
Left: X-rays illuminate a film plate immediately behind a capillary lens to show the bundled nature of the fibers. Variations in brightness are normal and would be measured in calibration tests. Links to. (NASA/Marshall)
|Center for X-ray Optics at the University at Albany, State University of New York.
X-Ray Optical Systems, Inc. in Albany is the industry partner in this project.
NASA's Microgravity Research Program sponsors the capillary X-ray optics work and other advanced technologies.
Initially, capillary lenses won't be used to produce images.
"These are basically concentrators," he continued. "They don't produce images in the conventional sense, so you want to use them for spectroscopy." That means studying the energy levels to understand more about what is powering a neutron star, supernova, or causing some other violent event.
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Author: Dave Dooling
Curator: Linda Porter
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