Study gives clues to increasing X-rays' power

In a paper to be published in an upcoming edition of Physical Review Letters, UNL Physics and Astronomy Professor Anthony Starace and his colleagues give scientists important clues into how to unleash coherent, high-powered X-rays.

"This could be a contributor to a number of innovations," Starace said.

Starace's work focuses on a process called high-harmonic generation, or HHG. X-ray radiation can be created by focusing an optical laser into atoms of gaseous elements - usually low-electron types such as hydrogen, helium, or neon. HHG is the process that creates the energetic X-rays when the laser light interacts with those atoms' electrons, causing the electrons to vibrate rapidly and emit X-rays.

But the problem with HHG has been around almost as long as the onset of the method in 1988: The X-ray light produced by the atoms is very weak. In an effort to make the X-rays more powerful, scientists have attempted using higher-powered lasers on the electrons, but success has been limited.

"Using longer wavelength lasers is another way to increase the energy output of the atoms," Starace said. "The problem is, the intensity of the radiation (the atoms) produce drops very quickly."

Instead of focusing on low-electron atoms like hydrogen and helium, Starace's group applied HHG theory to heavier (and more rare) gaseous atoms having many electrons - elements such as xenon, argon and krypton. They discovered that the process would unleash high-energy X-rays with relatively high intensity by using longer wavelength lasers (with wavelengths within certain atom-specific ranges) that happen to drive collective electron oscillations of the many-electron atoms.

"If you use these rare gases and shine a laser in on them, they'll emit X-Rays with an intensity that is much, much stronger (than with the simple atoms)," Starace said. "The atomic structure matters."

Starace said that unlocking the high-powered X-rays could lead one day, for example, to more powerful and precise X-ray machines. For instance, he said, heart doctors might conduct an exam by scanning a patient and creating a 3D hologram of his or her heart, beating in real time.

Nanoscientists, who study the control of matter on an atomic or molecular scale, also may benefit from this finding, Starace said. Someday, the high-intensity X-rays may be used to make 3D images of the microscopic structures with which nanoscientists work.

"With nanotechnology, miniaturization is the order of the day," he said. "But nanoscientists obviously could make use of a method to make the structures they're building and working with more easily visible."

X-Ray Rainbow

via Physics News Update #805

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 the Advanced Photon Source at Argonne National Lab, in Illinois, 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 (shvydko@aps.anl.gov, 630-252-2901) 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 monochomators (which fashion X-ray beams of pure wavelength, or color) and much higher-resolution X-ray spectrometers.

Shvyd'ko et al., Physical Review Letters, 8 December 2006

2D imaging conical x-ray spectrograph

We are using a Von Hamos x-ray spectrograph to observe the x-ray imession from laser cluster interaction, which only provides the spectral information. I think we can use this novel x-ray crystal spectrograph to measure the spectral resolved 2-D image in our experiment.

Extreme luminosity imaging conical spectrograph

S. A. Pikuz, T. A. Shelkovenko, M. D. Mitchell, K. M. Chandler, J. D. Douglass, R. D. McBride, D. P. Jackson, and D. A. Hammer
Laboratory of Plasma Studies, Cornell University, 439 Rhodes Hall, Ithaca, New York 14853

A new configuration for a two-dimensional (2D) imaging x-ray spectrograph based on a conically bent crystal is introduced: extreme luminosity imaging conical spectrograph (ELICS). The ELICS configuration has important advantages over spectrographs that are based on cylindrically and spherically bent crystals. The main advantages are that a wide variety of large-aperture crystals can be used, and any desired magnification in the spatial direction (the direction orthogonal to spectral dispersion) can be achieved by the use of different experimental arrangements. The ELICS can be set up so that the detector plane is almost perpendicular to the incident rays, a good configuration for time-resolved spectroscopy. ELICSs with mica crystals of 45×90 mm2 aperture have been successfully used for imaging on the XP and COBRA pulsed power generators, yielding spectra with spatial resolution in 2D of Z pinches and X pinches. ©2006 American Institute of Physics

Rev. Sci. Instrum. 77, 10F309 (2006)