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02/27/95

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SLAC scientists receive annual accelerator school prize

STANFORD -- Stanford Linear Accelerator Center physicist James Spencer and Herman Winick, deputy associate director of the Stanford Synchrotron Radiation Laboratory, have received the U.S. Particle Accelerator School Prize for Achievement in Accelerator Physics and Technology. The annual award is given by the American Physical Society's particles and beams division.

Spencer and Winick were recognized for work they did 15 years ago that has revolutionized the scientific use of high-energy X-rays, or synchrotron radiation, produced as a byproduct of the operation of large particle accelerators called storage rings. They designed and installed the first piece of hardware, called an insertion device, that allows scientists to produce synchrotron X-rays at a variety of frequencies and luminosities without interfering with the accelerator's basic operation.

Since then, synchrotron radiation has become big science. Among a number of other uses, this research has produced an improved way to image heart vessels, revealed the three- dimensional structure of a key HIV enzyme and created X-ray images of computer chips that provide unparalleled details of the impurities they contain.

Twenty years ago, however, synchrotron radiation, which is produced when a high-energy beam of charged particles are magnetically deflected, represented a major problem for accelerator designers because it drained energy from the particle beam. This loss increased rapidly with the amount of energy in the beam and the angle of deflection, and thus represented a fundamental limitation in storage ring design.

From the beginning, a group of scientists considered this radiation a valuable resource that they could use to probe the structures of atoms, surfaces and large biological molecules. In the early 1970s, however, the requirements of the particle and synchrotron physicists were at odds. The synchrotron radiation users needed X-rays with higher frequencies and energies than the accelerator produced when operated at the levels the particle physicists required.

Spencer and Winick realized that if the particle beam passed through a device called a wiggler - a line of magnets of alternating polarity - it could produce X-rays at the strength and frequency that the synchrotron users wanted. A series of laboratory tests convinced them that it would not interfere with the quality of the beam. Still, there was considerable debate before they were allowed to build and install one of these devices in SPEAR, SLAC's storage ring.

After getting the go-ahead in 1978, the scientists had the device installed and operating properly within a year. Not only did it produce 10 times as many X-rays, it also improved the beam quality, allowing particle physicists to complete their experiments 20 to 30 percent faster. In addition, the device allowed the sychrotron users to adjust the frequency and luminosity of the X-ray beam over a reasonable range.

SLAC's second storage ring, PEP, which was built one year later, was designed with three wigglers. Since then, nearly all of the storage rings built have been designed with insertion devices, and today there are about 30 such facilities operating worldwide.

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