New Opportunities in Synchrotron X-ray Crystallography

Several high-intensity synchrotron x-ray sources have been constructed over the past few years in the United States, West Germany, Great Britain, Japan, France, Italy, and the Soviet Union. Crystallographers have begun to use these facilities for experiments that take advantage of the characteristics of synchrotron radiation, namely, a broad distribution of wavelengths, high intensity, low divergence, strong polarization, and a pulsed time structure. In addition to more familiar diffraction experiments on single crystals and powdered samples, new types of crystallographic studies, for example, energy-dispersive and surface diffraction studies, have progressed rapidly with more general accessibility of synchrotron sources. These high-intensity sources allow diffraction experiments to be performed on very small crystals or on large biological molecules, and permit weak magnetic scattering to be detected Anomalous dispersion experiments can exploit he ability to vary the wavelength of the radiation, and the pulsed time structure of the beam makes possible fast time-resolved experiments. Because of the availability of synchrotron x-radiation, these and other kinds of experiments will be in the forefront of crystallographic research for the next several years.     

MATERIALS SCIENCE: Armenia Wants Second Mideast Synchrotron

Middle Eastern scientists who for years have been yearning for a synchrotron may wind up with two. Last spring, SESAME (Synchrotron Radiation for Experimental Science and Applications in the Middle East), an 11-nation consortium formed to install and operate a first-generation synchrotron now mothballed in Germany, selected Jordan as the site of the 0.8 giga-electron-volt BESSY-I synchrotron, disappointing Armenian officials who had hoped to snare the prize. But this month Armenia moved to the head of the line for a second, brand-new synchrotron after securing a $15 million down payment from the U.S. Congress as part of a foreign-aid spending bill that would funnel $90 million to the country.     

CHESS: The New Synchrotron Radiation Facility at Cornell

A new synchrotron radiation laboratory, CHESS, will soon be completed at Cornell University. The facility will operate in a mode parasitic to high energy physics experiments on the new 8-billion-electron-volt electron-positron storage ring (CESR) at Cornell. Electron and positron beams have already been stored and the first photons have been extracted. When completed, the laboratory will be available to the scientific community nationally and will provide the most intense tunable source of high energy x-rays in the country.     

LIII-Edge Anomalous X-ray Scattering by Cesium Measured with Synchrotron Radiation

Diffraction of monochromatized synchrotron radiation by crystals of cesium hydrogen tartrate has been used to measure the magnitude and phase of x-ray scattering for cesium near the LIII absorption edge. In this wavelength region the scattering amplitude of cesium is reduced by as much as 25 electrons per atom, compared to scattering of copper Kalpha x-rays. This change, which varies as a function of wavelength, affects the diffraction intensities in a manner similar to isomorphous substitution, and it is large enough to have promise for phase determination in the study of macromolecular structures. This experiment also demonstrates that accurate diffractometer measurements are possible with synchrotron radiation produced by an electron storage ring.