A prism for x rays

With far more energy than visible light, x rays reflect from surfaces only at glancing angles and with little wavelength-dependent spreading. Bragg diffraction in a crystal allows for scattering of x rays through larger angles because the incoming x rays scatter from numerous atomic planes beneath the surface. With a normally cut crystal, though, there is still little dispersion. Physicists using the Advanced Photon Source at Argonne National Laboratory have now spread out a beam of x rays into a rainbow of colors with the help of an asymmetrically cut crystal, whose atomic planes are not parallel to the crystal surface. The effect is strongest when the reflecting atomic planes are nearly perpendicular to the surface; then exact backscattering occurs not at normal incidence to the planes but at an angle that depends on the x-ray energy. The diffracted x-ray beam is spread out prismatically into its component wavelengths. In the experiment, an incoming beam of 9-keV x rays with angular spread of only 1 microradian was backscattered into a rainbow with an angular dispersion of 230 μrad. According to team leader Yuri Shvyd’ko, the new technique can lead not only to a new type of x-ray monochromator and to very-high-resolution x-ray spectrometers but also to compression of x-ray pulses in time. (Y. V. Shvyd’ko et al., Phys. Rev. Lett. 97 , 235502, 2006 .)