Lithium Lens Focuses X-Rays
Scott M. Aldis-Wilson
With a little extra care, researchers at the University of Michigan in Ann Arbor, Ecopulse Inc. in Springfield, Va., and the US Army Research Laboratory in Adelphi, Md., have expanded the materials from which a lens can be made. The team has reported the focusing of 10-keV x-rays with a prototype refractive lens constructed of lithium.
An experimental lithium refractive lens for synchrotron x-rays features a sawtooth geometry. Lithium degrades quickly in humid air, but a dry glove box makes handling much easier and enables the lens to remain viable for longer periods. Courtesy of Nino Pereira.
Previous work in focusing synchrotron x-ray sources had suggested that a compound refractive lens design could reduce the focal length from 100 to 1 m. Lithium is ideal for such lenses because it gives x-rays the largest phase shift per attenuation length of any material except hydrogen. However, the metal is uncommon in x-ray research because it is combustible in water; thus, lithium lenses degrade quickly in humid air.
The researchers' design accommodated these characteristics. They handled the lens in a plastic dry glove box fed by the boil-off from a Dewar vessel of liquid nitrogen. During focusing, an ion pump kept the lens under ~100-µPa pressure in a vacuum cross with two beryllium windows for the x-ray beam. Under these conditions, Nino Pereira of Ecopulse said, the lens has displayed the same efficiency for months.
The lens was constructed according to a sawtooth geometry developed by Björn Cederström, with 6.3-mm-wide, 0.75-mm-high teeth spaced 1.5 mm apart. The lens was located 49.2 m from the synchrotron source, and the focused x-rays were filtered through two aperture slits to avoid blurring of the focus.
To validate the performance of the lens, the researchers monitored the resulting fluorescence in a Ce:YAG crystal that was exposed to the focused 10-keV x-rays. The lens displayed an average transmittance of 0.74 and a gain of 3, compared with a theoretical gain of 4.5. The researchers attributed the deviation from theoretical performance to small-angle scattering caused by the surface roughness of the teeth.
Further work is under way to perfect the next lens, including investigations into intrinsic scattering in lithium. "Pure lithium, without contaminants, should not scatter, we still think," Pereira said.
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