Storage rings

Work on EXAFS then progressed very little until the advent of the synchrotron radiation source (storage ring), described in Section 8.1.1.1. This type of source produces X-ray radiation of the order of 10 to 10 times as intense as that of a conventional source and is continuously tunable. These properties led to the establishment of EXAFS as an important structural tool for solid materials. 

The first—and central— point in this discussion concerns the pulsed X-ray source. The shortest time scales involved when chemical bonds are formed or broken are of the order of a few femtoseconds. An ideal X-ray source should thus be capable of providing pulses of this duration. Unfortunately, generating them represents a heavy technological problem. The best one can do at present is to use a pulsed synchrotron X-ray source (Fig. 2). Electrons are rapidly circulating in its storage ring at speeds close to the speed of light. X-rays are spontaneously... 

Figure 2. Synchrotron X-ray source (schematic). The electrons execute circular motions in the storage ring and emit intense X-rays along the tangent of the orbit. This radiation is enhanced by undulator magnets that are often placed inside the vacuum vessel for enhanced performance. The storage ring has a number of straight sections for undulators and wigglers (not shown).

Small angle X-ray scattering measurements were performed using the synchrotron radiation of the DCI storage ring at LURE (Universite d Orsay,... 

We will be rather brief in describing the principles of the experimental methods that are most commonly used in studies of dissociative recombination, afterglows and colliding beams (merged beams and ion storage rings). It should suffice to recount some of their generic properties. 

After publication of the ion-storage-ring data, new recombination mechanisms were proposed1 that do not require crossings between ionic and neutral potential curves. However, no detailed calculations have been made for Hj and it is not yet clear if such mechanisms are capable of explaining the experimental findings. 

Figure 6. Cross sections for the dissociative recombination of H3 (v = 0). Storage-ring results (ref. 28) dashed line. Merged beam results (ref. 42) for a deflector field of 3kV/cm (closed circles) and a deflector field of 200 V/cm (open circles).

An alternative and the most generally employed source of X rays for EXAFS experiments is that obtained from synchrotron sources based on electron (or positron) storage rings. 

No single development has influenced the field of EXAFS spectroscopy more than the development of synchrotron radiation sources, particularly those based on electron (or positron) storage rings. These provide a continuum of photon energies at intensities that can be from 103 to 106 higher than those obtained with X-ray tubes,... 

Finally, the pulsed time structure, useful for kinetic studies, arises from the fact that in a storage ring the electrons are orbiting in bunches. The specific energy, the number of bunches, and the circumference of the storage ring dictate the exact time structure. 

To test the feasibility of obtaining submicron size patterns in the resist films, an exposure source was used which consisted of the X-ray continuous spectrum produced by synchrotron radiation from the 5 0 MeV storage ring of the University of Orsay (ACO) since synchrotron radiation had been shown previously (2.,8.) to be a suitable source for providing very high resolution due to the small divergence of the beam. The maximum output flux of ACO... 

Table 4.1. Performance of available point-focus setup. DORIS is an older storage ring at HASYLAB in Hamburg. The ESRF in Grenoble is an advanced synchrotron radiation source... 

The traditional operation mode of synchrotron light sources is a discontinuous one particles are injected in the storage ring, the beam current is decaying exponentially, and after several hours the synchrotron radiation run is stopped for a new injection. 

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