Synchrotron Radiation Gwyn P. Williams

Topics: X-ray, Electron, Particle accelerator Pages: 18 (2587 words) Published: June 23, 2013
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Synchrotron Radiation
Gwyn P. Williams National Synchrotron Light Source, Brookhaven National Laboratory

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Synchrotron Radiation
Gwyn P. Williams National Synchrotron Light Source, Brookhaven National Laboratory Today synchrotron radiation is used for a number of applications. The Na­ tional Synchrotron Light Source at Brookhaven National Laboratory (BNL) serves the needs of a large spectrum of university and industrial users. The range of applications covers such diverse subjects as catalysis and corrosion. Here I'll concentrate on x-ray lithography, which is an especially interesting ap­ plication. A fascinating sidelight of this application is the production of com­ mercial accelerators for x-ray lithography. Synchrotron radiation is synonymous with x-rays. From the initial invention of the x-ray tube in 1896 until about 1960, the brightness (the log of brilliance in Fig. 1) didn't change very much. About that time rotating anode tubes ap­ peared. Then, starting in the Sixties, synchrotron radiation facilities began pop­ ping up. They appeared at Madison and Stanford and at various places in Europe, including England, where I became involved about 20 years ago. Soon, these accelerator x-ray sources produced dramatic increases in the x-ray bright­ ness until they are now a trillion times brighter than a conventional x-ray machine. When a charged particle accelerates it radiates photons or x-rays. For example, in a conventional x-ray tube the electron beam accelerates from a fila­ ment to an anode, where it decelerates rapidly at the anode and emits x-rays (shown schematically in Fig. 2). A different approach is to accelerate (or decelerate) the particle by passing it through a magnetic field. This is what really happens in an accelerator. When the particles curve in the magnetic field, x-rays are emitted. These x-rays are very collimated. The effective source is very small because the electrons can be focused down. There's also no heating involved, so it's possible to make an ex­ tremely bright source of x-rays. This is the essence of synchrotron radiation. The Tevatron at Fermilab uses protons. It's much harder to make an ac­ celerator like the Tevatron for electrons because electrons radiate much more as

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they go around a circle. As a result, it is difficult to put enough power in to keep a beam in the storage ring. A proton is 2000 times heavier than an electron. Emission of x-rays goes as the inverse fourth power of the mass. As a result, protons don't radiate nearly as much synchrotron radiation. Actually, that's why very high-energy machines like the Tevatron use protons. Figure 3 shows a schematic of the synchrotron radiation distribution. Elec­ trons circulate around a storage ring. These rings come in various sizes. The radiation is emitted tangentially. It fills the horizontal plane and is typically one milliradian in divergence. The angular cone is similar to a helium neon laser. Figure 4 illustrates the location of the storage ring in the Brookhaven com­ plex. Along with a reactor for neutrons, the AGS, an accelerator similar to Fer­ milab's, and the...
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