Refraction returns to x-ray optics
Despite a huge and ever-growing body of experience with the x-ray portion of the spectrum, control of the high-energy radiation continues to be a challenge. Because of their short wavelength, x-rays tend to be absorbed by most materials or pass right through without much effect, unlike radiation in the visible to IR spectral region, which can be focused or directed quite easily. For many years, grazing-incidence mirrors provided the best means of controlling x-rays. Other alternatives include total-external-reflection-based components such as polycapillary optics or diffraction-based Fresnel zone plates.
Grazing-incidence optical systems by their nature have very long focal lengths, while polycapillary lenses tend to generate diffused light that is not very good for imaging applications. Fresnel zone plates, although placed in the optical path, create higher-order harmonics that require a limiting aperture or beam stop to reduce optical noise and absorb upwards of 50% of the radiation.
Inline x-ray lenses can contain up to several hundred individual lenses in a single package, shortening the focal length of an x-ray imaging system from several meters to several centimeters.
There is a refractive optics alternative for the x-ray region, though. As recent advances in the field show, by selecting the right material, carefully controlling the parabolic surface of the lens, and stacking many lenses in a row, the latest refractive "inline" x-ray lenses offer users relatively economical options that offer effective performance for certain applications.
Adelphi Technology Inc. (Palo Alto, CA), as well as Ecopulse Inc. (Springfield, VA) and Accel Instruments GmbH (Bergisch Gladbach, Germany), is making a play in this area. Adelphi has commercialized a manufacturing process that stacks between 50 and several hundred concave lenses in a single package to yield focal lengths on the order of a few centimeters, according to Adelphi vice president Charles Gary. "Imaging is where we're finding the most interest in these lenses," he says. "Medical images have a 50-µm resolution for [computed tomography] scanning. Our lenses can get resolution a few hundred nanometers, and that's a big benefit."
"There's certainly a nice parabolic [lens] approach at Adelphi," says Eric Dufresne, a researcher at the University of Michigan (Ann Arbor, MI) and beamline scientist at the Advanced Photon Source (Argonne, IL). "This has excellent potential for providing x-ray optics with wide acceptance at a reasonable cost. Refractive lenses have been relatively popular in Europe with synchrotron sources and in Japan."
Careful control of the lens shape is just one consideration when making an inline x-ray lens. "We custom-design the lens for given applications, taking into account whether the goal is imaging, focusing, or collecting x-rays and what energy of x-rays is to be used," says Gary. "We can then pick the material [lithium and beryllium for low energy, plastics such as Kapton for middle energies, aluminum or other materials for high-energy systems] and the aperture, radius of curvature, and number of lenses."
By carefully selecting the lens components, Adelphi can construct lenses with from 200- to 1000-µm apertures, compared with 500-µm apertures for zone plates, and handle x-ray energies from 1 KeV to 80 KeV. Mirrors, for example, typically work up to 10 KeV. And although capillary optics can work at higher energies, they cannot achieve a diffraction-limited focal spot as inline lenses can. "The spot size of capillary optics is typically larger than the individual capillary sizes due to spreading of the wave function after leaving the capillaries," says Gary. "However, capillary optics can capture a larger solid angle from an x-ray source."
