Research at the Center for X-ray Optics


Why x-ray optics?

X rays have been very important in many areas of science and technology since their discovery. The first medical radiograph was made in 1896, just one year after their discovery by Roentgen.  Most techniques of generating x rays are inefficient, and create x rays which diverge away from the source like light from a light bulb, so that it would be desirable to capture and collimate or focus x-rays, as with the lens in a flashlight.  However, because x rays easily penetrate most objects (even people), they simply pass through normal lenses, hardly bending at all.  Similarly, normal mirrors don't work because the ray simply goes through the mirror without reflecting.

Reflective optics

Refraction The bending of light by a lens is described by Snell's law, which tells us that when light travels from a material of low index, like air, to a material of higher index, like glass, it bends, or refracts, away from the glass surface. However, for x rays, the index for glass is actually very slightly less than that of air or vacuum. This means that the ray bends very slightly toward the glass surface. If the x ray hits the surface at a special, very small angle called the critical angle, the ray in the glass will be parallel to the surface. For incident angles less than the critical angle, there is no way to have a ray in the glass, so the x ray is totally reflected. This principle has been used for many years to make x-ray mirrors. However the critical angle is usually less than one tenth of one degree, and so the mirror has to be very large and very flat to reflect a large x-ray beam. The same principle has been used since the 1920s to bounce x rays down hollow glass tubes. Of course a single tube can only catch a very small part of the x-ray beam.


In the early 1980's polycapillary optics, arrays of thousands of hollow glass tubes, were invented in Russiaby Kumakhov and coworkers.  In 1991, the Institute for Roentgen Optics in Moscow, and the Center for X-ray Optics in Albanywere jointly founded to purse the study of these and other x-ray optics.  The Center for X-ray Optics now houses a half dozen x-ray beam enclosures, including several microfocus sources, with copper, molybdenum and tungsten tubes, high resolution energy sensitive and imaging detectors, and two high power rotating anode systems used for both imaging and crystallography experiments.  Theoretical development and extensive computer modeling are also a large part of the research program, as well as collaborations with several synchrotron beam lines and neutron sources.


Because x rays, like all light, are also waves, there can be interference effects.  The wavelength of x-rays is on the order of 0.1 nanometer, similar to the spacing between atoms in solids, so there are complex interference effects, called diffraction, when x rays are passed into solids.  This is routinely used to reflect x rays at much high angles than can be achieved by grazing incidence total reflection.  However, because it is an interference effect, a crystal "mirror" only works for a single wavelength.  This is a disadvantage if it is desirable to keep most of the intensity from a typical source, which emits a large range of wavelengths, but an advantage for applications in which it is desirable to use a beam of a single wavelength (called monochromatic, since for visible light a single wavelength has a single color).  The Center for X-ray Optics has made use of flat crystal optics since its inception and has been studying curved crystal optics since 2001.