An X-ray Rainbow
JANUARY 10, 2007
From Physics News Update 805, December 2006
by Phil Schewe, Ben Stein, and Davide Castelvecchi
In 1670 Isaac Newton demonstrated the composite nature of sunlight when he sent a carefully collimated sunbeam through a prism, which spread out the light into a rainbow of colors; by sending a beam of single color through a second prism (with no further spreading) Newton showed that the color was not being imposed by the prism but was intrinsic to the light itself. Now, physicists using XOR beamline 3-ID the U.S. Department of Energy's Advanced Photon Source at Argonne have spread out a beam of x-rays (which are, after all, just a more energetic version of visible light) into a rainbow of colors.
Trying to reflect x-rays from a surface is difficult because x-ray wavelengths are some 10,000 times shorter than those for visible light. Glancing reflection of only a few tenths of a degree is normally possible, and even then the beam of x-rays will suffer very little wavelength-dependent spreading. However, another phenomenon, Bragg diffraction, allows for scattering of x-rays from a crystal through large angles; in this case the incoming x-rays scatter not merely from a top layer of atoms in the crystal but from numerous atomic planes. Furthermore, if the atomic planes are not parallel to the crystal surface the diffracted x-ray beam will be spread out prismatically into a range of component wavelengths (or colors).
In the Argonne experiment an incoming beam of 9-kiloelectronvolt x-ray photons with angular spread of only 1 micro-radian (two-tenths of an arcsecond) was backwards scattered and spread out into an x-ray rainbow with an angular dispersion of 230 micro-radians (see figure).
Argonne physicist Yuri Shvyd'ko says that his rainbow is not just a novelty but will have many practical applications in x-ray optics. These include compression of x-ray pulses in time and the development of x-ray monochromators (which fashion x-ray beams of pure wavelength, or color) and much higher-resolution x-ray spectrometers.
Contact: Yuri Shvyd'ko (firstname.lastname@example.org)