Synchrotron experiments

Therefore, these synchrotron experiments show that the repeat distance for the smectic layer lies very close to, but is distinct from, that of the crystal. Moreover, the smectic peak disappears totally when the transformation is complete, indicating that the mesophase has been destroyed in the semicrystalline sample. 

The financial support of the CICYT (project no. IN89-0066), DGICYT (project no. PB94-1529), and Consejerfa de Educacidn de la Comunidad de Madrid is gratefully thanked. We also acknowledge the support of NATO (grant CRG 920094) and the assistance of Dr. W. Bras and the Daresbury Laboratory (UK) in the synchrotron experiments. 

Fig. 5. An early fixed-target accelerator comprised of a laige main-ring synchrotron with four stages of acceleration (MRA) a booster synchrotron a linear accelerator (linac), and a Cockcroft-Walton generator. Pistons aie accelerated to 0, 5 MeV in the Cockcroft-Walton generator to 300 MeV in the linac to 8 GeV in the booster synchrotron, and to 400-500 GeV in the mam-ring synchrotron. Experiments are not limited to accelerated protons, but also can be conducted with beams of secondary particles (mesons and neutrinos) which are knocked out of the target by impacting protons...

Finally, we must consider the recent availability of synchrotron x-ray sources for industrial use. The high resolution now made possible by these sources produces zeolite XRD powder patterns with features perhaps calculated, but never observed before. Figure 14 shows the enhanced resolution from a synchrotron experiment versus that obtained from a conventional one. Here the XRD patterns of ZSM-5 clearly demonstrate the enhanced resolution and wealth of diffraction information that can be obtained at such sources as the... 

The time resolution obtainable using a laboratory source is in many cases sufficient to follow chemical reactions and to study materials in a given environment. Many chemical reactions and synthesis may take hours to complete, and here good results may be obtained using the laboratory experiment. In addition it is a valuable tool when preparing for synchrotron experiments. 

The synchrotron experiments were performed at beamline UE52-PGM at BESSY, Berlin. Substrates and films were prepared at the end station in a UHV preparation chamber at a base pressure of 5 x 10mbar and transferred into the measurement chamber without exposure to air. The NEXAFS spectra on sapphire substrates were recorded by measuring the partial electron yield with a dedicated channeltron detector, the spectra on Au substrates were measured using the sample current. Energy calibration and intensity normalisation were carefully done according to [18]. 

Current PCRAM cells have dimensions of the order of a few 10 s of nanometers (although further down-size scaling seems to bepossible, since PC switching between c- and a-states, and vice versa, has been demonstrated down to dimensions of 2 nm [5]), with transformation times of the order of nanoseconds. It is extremely challenging to perform experimental smdies of PC transformations in situ in actual memory cells, at such very small dimensions and very short times, although synchrotron experiments are now approaching this capability. Thus, computer simulations can play an... 

Because of the unique features of the x-ray radiation available at synchrotrons, many novel experiments ate being conducted at these sources. Some of these unique features are the very high intensity and the brightness (number of photons per unit area per second), the neatly parallel incident beam, the abihty to choose a narrow band of wavelengths from a broad spectmm, the pulsed nature of the radiation (the electrons or positrons travel in bunches), and the coherence of the beam (the x-ray photons in a pulse are in phase with one another). The appHcations are much more diverse than the appHcations described in this article. The reader may wish to read the articles in the Proceedings of the Materials Research Society Hsted in the bibhography. 

In diffraction experiments a narrow and parallel beam of x-rays is taken out from the x-ray source and directed onto the crystal to produce diffracted beams (Figure 18.5a). The primary beam must strike the crystal from many different directions to produce all possible diffraction spots and so the crystal is rotated in the beam during the experiment. Rotating the crystal is much easier than rotating the x-ray source, especially when it is a synchrotron. 

Figure 1 Schematic view of a typical EXAFS experiment at a synchrotron radiation facility. Note that it is possible to record transmission and fluorescence EXAFS simultaneously with reference EXAFS.

Unlike traditional surface science techniques (e.g., XPS, AES, and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one s own laboratory than at a Synchrotron radiation focility, which is therefore a significant drawback to the EXAFS technique. These focilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. 

The only X-ray source with sufficient intensity for surface measurements is synchrotron radiation. Synchrotron radiation is white light, including all wavelengths ftom the infrared to X rays. A spectroscopy experiment needs a particular wavelength (photon energy) to be selected with a monochromator and scanned through... 

The advantages of SEXAFS/NEXAFS can be negated by the inconvenience of having to travel to synchrotron radiation centers to perform the experiments. This has led to attempts to exploit EXAFS-Iike phenomena in laboratory-based techniques, especially using electron beams. Despite doubts over the theory there appears to be good experimental evidence that electron energy loss fine structure (EELFS) yields structural information in an identical manner to EXAFS. However, few EELFS experiments have been performed, and the technique appears to be more raxing than SEXAFS. 

ReflEXAES can be used for near-surface structural analysis of a wide variety of samples for which no other technique is appropriate. As with EXAES, ReflEXAES is particularly suited for studying the local atomic structure around particular atomic species in non-crystalline environments. It is, however, also widely used for the analysis of nanocrystalline materials and for studying the initial stages of crystallization at surfaces or interfaces. ReflEXAES was first proposed by Barchewitz [4.135], and after several papers in the early nineteen-eighties [4.136, 4.168-4.170] it became an established (although rather exotic) characterization technique. Most synchrotron radiation sources now have beam-lines dedicated to ReflEXAES experiments. 

If the object of a synchrotron is to accelerate electrons to the highest possible energy, synchrotron radiation is a serious obstacle that limits the energy attainable. On the other hand, the electromagnetic radiation from a synchrotron can be useful for experiments on the properties of solids and for other purposes. For tins reason, some electron synchrotrons are built primarily for the synchrotron radiation they emit. 

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