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This image represents a false-color contour intensity distribution around the (11-1) diffraction peak of a nanocrystal of gold,
measured under coherent conditions at the Advanced Photon Source at Argonne National Laboratory.

A group of Illinois researchers led by Professor Ian K. Robinson is developing a coherent X-ray diffraction method as a potentially important new form of X-ray microscopy. While a conventional diffraction experiment would record a featureless blob, the coherence reveals the detailed fringes that are due to the faceted shape of the crystal. Because this pattern is a continuous function, it can be oversampled, and hence inverted, to obtain an image of the crystal under investigation.

Inverse problems appear frequently in physics. The magnitude of the Fourier transform of some function may be measurable, but not its phase. The "phase problem" in crystallography arises because the number of discrete measurements (Bragg peak intensities) is only half the number of unknowns (i.e. the electron density points in space.) Oversampling of diffraction data allows a solution, and the Illinois group has successfully phased an oversampled hard x-ray diffraction pattern measured from the single nanocrystal of gold illustrated above, as reported in this week's [5 November 2001] Physical Review Letters (I. K. Robinson, I. A. Vartanyants, G. J. Williams, M. A. Pfeifer, and J. A. Pitney, Phys. Rev. Lett. 87, 195505 (2001)).

More information about this exciting work is available from Professor Robinson or Dr. Ivan Vartanyants (Department of Physics and the Frederick Seitz Materials Research Laboratory).