A tabletop waveguide delivers focused x rays

Despite the widespread use of x rays as a fundamental tool for visualizing interior features of solid objects, bright beams that are emitted in a single direction onto the target of interest are difficult to come by in a laboratory setting. Unlike large-scale accelerators, which emit highly collimated beams, conventional small-scale sources generate x-ray radiation in all directions. Once they’re emitted, x rays cannot easily be manipulated with mirrors or lenses.

To obtain bright x rays in a clearly defined path, Malte Vassholz and Tim Salditt of the University of Göttingen have now developed and demonstrated an approach for generating the radiation directly within a waveguide structure. The layered material that makes up the waveguide emits x rays within a nanometers-wide channel, and the resulting beam’s brilliance exceeds that of a conventional µ-focus x-ray tube by two orders of magnitude. The method could lead to a tool for soft-matter imaging and coherent scattering experiments in laboratories.

Laboratory-scale sources produce x rays by hitting a metal anode with electrons accelerated by a high voltage. Radiation is emitted at all angles when the atoms in the metal deflect and slow those electrons as well as when the electrons excite the metal atoms. To better control the angles at which a metal emits x rays, Vassholz and Salditt built a sandwich-like structure, illustrated in the figure, that was made up of a fluorescent metal layer embedded between guiding and cladding layers. Using a high-energy electron beam that was generated by an instrument adapted from an x-ray tube, the researchers excited the central metal layer, which caused it to emit x rays that were funneled into the guiding layers. Those beams traveled through the guiding layers and were emitted through the waveguide exit. A detector placed across from the exit showed sharp emission peaks corresponding to the waveguide modes, indicating that the device had effectively channeled x rays of up to 35 keV onto a target.

Additional experiments and calculations suggested that the brightness of the emitted x rays could be further enhanced by using different metals or by varying the thickness of the layers. The researchers propose that the design could enable benchtop measurements of microscale structures that until now have only been accessible using synchrotron radiation. (M. Vassholz, T. Salditt, Sci. Adv. 7, eabd5677, 2021. )