Casting Light on Material Structures

Abstract

Synchrotron radiation is generated whenever electrically charged particles moving at speeds near that of light are forced into bent paths by magnetic fields. It is electromagnetic radiation, which can be visible light, ultraviolet or X-rays, depending on the energy of the particles and the strength of the magnetic field. The radiation is named for the class of accelerators in which it first became a serious problem, electron synchrotrons. Because electrons have less than 1/1800th as much mass as protons, they reach relativistic velocities at much lower energies than do protons. At high energies synchrotron radiation robs so much energy from electrons moving in circular paths that when what is now the world's most energetic electron accelerator the 20-billion-electron-volt machine of the Stanford Linear Accelerator Center -was planned, the builders decided to make it a straight line two miles long. It may be considered somewhat ironic that that linear accelerator now supplies accelerated electrons to a facility that uses synchrotron radiation for research in several different scientific fields. Synchrotron radiation used to be considered a dead loss by accelerator operators. In recent years it has suddenly become a very important new research field. In the words of Herman Winick, deputy director of the Stanford Synchrotron Radiation Laboratory, There is an explosive growth of interest in its applications. At the moment there are about 10 storage rings and 11 synchrotrons around the world at which synchrotron radiation experiments are done. The SSRL gets its synchrotron radiation from the SPEAR storage ring. A smaller storage ring at the University of Wisconsin, the Deutsches Elektronen-Synchrotron at Hamburg, and more than one ring at Novosibirsk in the USSR are among those in the world now supplying synchrotron radiation for experiments. The attitude of the U.S. scientific establishment and its government funders has undergone a total turnaround from three or four years ago, Winick says. At that time Winick was working at the now defunct Cambridge Electron Accelerator in Cambridge, Mass. An attempt to get $1 million a year to keep the CEA going as a facility for synchrotron radiation was unsuccessful. Over the next three years, the National Science Foundation plans to give about $7 million to the SSRL alone. At the same time, plans have been announced for enlargement of the facility in Wisconsin and for another national synchrotron radiation facility at Brookhaven National Laboratory, where an accelerator will be built to be solely a source of synchrotron radiation. Other sources of X-rays, of which the best are 60-kilovolt rotating anode tubes, do not provide the broad spectrum or high power of synchrotron radiation. As an example of the difference, Winick cites a group of scientists from the Bell Telephone Laboratories who came to SSRL to do a spectrum that they had previously done by other methods. With synchrotron radiation it took them 20 minutes to do a spectrum that had previously taken two weeks. Another advantage of synchrotron radiation is that it comes in bursts that correspond to the bunches of electrons circulating in the storage ring. This gives experiments a built-in time resolution and makes possible the study of chemical and biological processes over time. A study of contracting muscle fibers is one possible experiment. Other biological possibilities include studies of membrane action, which are a particular interest of Sebastian Doniach, who has just completed his The two-mile linear accelerator feeds electrons to the SPEAR storage ring (lower right of top picture), where they generate synchrotron radiation used by the SSRL (building at right of ring). Mirrors permit several experiments to share one radiation beam (bottom).