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Improved X-ray optics at home

Even with new synchrotron radiation beamlines in operation or under construction worldwide, there is still continued need for increasing the capability of equipment operating in the home laboratory from sealed-tube or rotating-anode sources. New developments in x-ray optics will have a significant impact on the power of laboratory x-ray sources. These developments come from steady improvement of the performance of x-ray optical components based on total reflection (mirrors, capillaries, & waveguides), refraction (refractive lenses), and diffraction (crystals & multilayers, Bragg-Fresnel lenses, zone-plates, etc.).

The successful fabrication of a replica ellipsoidal mirror ["Focusing Mirrors for Use with Microfocus X-ray Tubes", U. W. Arndt, P. Duncumb, J.V.P. Long, L. Pina, & A. Inneman, J. Appl. Cryst. 31 (1998), 733-741] and the development of polycapillary optics ["Applications of Polycapillary X-ray Optics in Protein Crystallography", P-W Li & R-C Bi, J. Appl. Cryst. 31 (1998), 806-811] are described in a recent issue of the Journal of Applied Crystallography.

[diffraction image]

Diffraction image of acidic phospholipase A2 taken in 480 seconds into MAR 300 mm detector using polycapillary optic on Rigaku RU200 rotating anode generator.

In conjunction with a new microfocus x-ray tube, a hollow needle-like ellipsoidal mirror can produce an intensity through a 0.5 mm2 aperture that is comparable to that produced with rotating anodes and Franks mirrors at 1/100th of the power dissipation. This suggests that much more compact and energy efficient instrumentation can be created for the home laboratory. The replica mirror technology has been developed by Hudec and his collaborators in the Czech Republic (and is similar to the replica mirror work of Ulmer and of de Korte). A mirror is made by coating a shaped mandrel with a gold reflecting surface followed by a thick electroplated nickel shell to retain the elliptical figure when released from the mold. There are also prospects for another 10-fold intensity gain if the smoothness and shape of the reflecting surface can be further perfected.

Crystallographers go to a synchrotron radiation source to collect a higher-resolution data set on protein single crystals because of the higher intensities available. With a 20-fold increase in intensity from a monolithic polycapillary x-ray optic, a higher resolution structure for acidic phospholipase A2 (2.2 Å, Rsym=8.6%, 83 Å unit cell) was observed when the Supper mirrors on a rotating anode were replaced by a polycapillary optic. Monolithic polycapillary optics are fabricated by pulling bundles of monocapillaries to various shapes and have been under active development by other groups including those of Kumakhov, Gibson, etc. These optics may well enhance the quality of work that can be done with laboratory sources when used with single crystals of modest-size unit cells.

Don Bilderback
Cornell U.
Ithaca, NY, USA