Radiation synchrotron

Synchrotron radiation is the only emission mechanism that is uniquely dependent on the presence of magnetic fields. A relativistic electron spiraling in an external magnetic field radiates photons at a characteristic frequency a c = /me) = where y = E/mc, which depends only on the local field strength. An ensemble of electrons with energy spectrum N E) radiates with an intensity I oS) while even for an op- 

In contrast, an optically thick thermal spectrum varies as oP at low frequencies. The emission is polarized, since the electron radiates orthogonally to the local field direction circular polarization results from viewing the source longitudinally. The synchrotron spectrum thus reflects the exponent of N E), so the identification of this radiation is 

The half-angle of the cone of radiation produced (Acp) can be shown to be related to y as  

Synchrotron applications in archaeology date back to 1986 (Harbottle et al. 1986), and have been subsequently documented on a dedicated website at Daresbury [http //www.srs.ac.uk/srs/]. The majority of the early applications use SR as a source for X-ray fluorescence microanalysis on a variety of materials, including glass (Schofield et al. 1995, Janssens et al. 1996), ink and paper (Mommsen et al. 1996), dental calculus (Capasso et al. 1995), and bone (Janssens et al. 1998b). The repertoire has been expanded to include X-ray 

The source size varies for the different synchrotrons. At SRS it is 9.9 mm (horizontal) and 0.3 mm (vertical). Since the sample is about 20 m away, careful attention to monochromatisation and focussing optics is required so that the source size matches the crystal size. A curved crystal monochromator which is based on a triangular shaped crystal plate and uses germanium (111) reflection was first described by Lemonier et al. [225] at LURE. Similar focussing monochromators are now in use 

By far the greatest use of synchrotron radiation in protein crystallography has been to obtain data from crystals that either are very small, are radiation sensitive or have large units cells [77,228,230]. The brilliant source allows short data collection times. For example, 2.7 A data for crystals of phosphorylase b (some 120000 measurements 23 000 unique reflections) have been obtained in less than 2 hours. Such rapid data collection was advantageous in measurements of activity in the crystal [232]. 

As an example of large unit cell size, data have been recorded from crystals of cowpea mosaic virus to 4.3 A resolution at LURE [234]. The hexagonal unit cell has dimensions a=451 A, c = I038 A. The oscillation range was 0.4°. 

Synchrotron radiation also permits time-resolved studies, as for example in the fibre diffraction studies on muscle [235]. The feasibility of this work with protein crystals was demonstrated by the studies of Bartunik [236], who followed the time course of reflection intensities of carbonmonoxy-myoglobin on a time scale of 500 seconds before and after photodissociation of the ligand by a laser pulse. Thus, synchrotron radiation allows much greater scope in the study of biological molecules. 

The inelastic interaction between synchrotron radiation and solid matter is also utilized by crys-tallographers, although spectroscopic transitions rather than diffraction phenomena are involved (EXAFS. XANEvS. UPS, XPS). These techniques measure the fine structure of absorption edges, to yield information about the bonding of the atom under study. 

The present article concentrates on atomic physics experiments involving hard synchrotron radiation. Selected experiments are reviewed in order to elucidate basic principles as well as experimental possibilities and achievements. No attempt is made to cover the whole rapidly expanding field or to speculate on future developments. The discussion starts with instrumental details, e.g., the properties of synchrotron radiation, and a brief review of x-ray monochromators and detectors. Thereafter x-ray absorption studies are described that are experimentally very simple (at least in principle). More detailed information on atomic structure may be obtained if one observes not only the x-ray absorption, but additionally the induced fluorescence or emitted electrons. Finally, studies of photon scattering are sketched. 

The theoretical results will be reviewed for a simple case. Consider a single electron moving in a homogeneous magnetic field with field direction 

A closer inspection of Eq. (1) reveals the following behavior of the intensity 

The X-ray sources we have just described in the previous sections emit X photons by de-exdtation of atoms that underwent electronic transitions. The origin of synchrotron radiation is different. 

As we have aheady mentioned, a charged particle in motion with a decreasing speed emits an electromagnetic radiation called braking radiation. On the contrary, when a charged particle is accelerated, the electromagnetic radiation that is emitted is called synchrotron radiation [SCH 49]. 

The power radiated by a particle moving along a circular trajectory with radius p is given by the relation  

This general relation enables us to emphasize two essential features of synchrotron radiation  

2 Synchrotron radiation has been used in X-ray diffraction for over 50 years [HEL 98]. Over the past 20 years, the use of this type of source has become more and more widespread. However, the apparatus created for synchrotron radiation sources are specific prototypes. We will not go into the details of these instrumental developments in this book. This section on synchrotron radiation is intentionally brief and those readers interested in knowing more can refer to specialized works (see, for example, [WIE 03]). 

The maximum of the intensity distribution is close to A. . The characteristic wavelength for typical, large machines is around 1 nm. It is possible to express the distribution of any electron synchrotron radiation in normalized intensity units using the characteristic wavelength as the wavelength unit. The universal distribution function is shown in Fig.6.11. 

NSLS I (Brookhaven, USA) UVSOR (Okazaki, Japan) SUPERACO (Orsay, France) BESSY (Berlin, FRG) ALADDIN (Wisconsin, USA) ADONE (Frascati, Italy) 

Thus time-resolved experiments can be performed [6.51,52]. Data for a number of facilities producing synchrotron radiation are given in Table 6.1. With synchrotrons of resonable sizes the intensity per Doppler width that is achievable is comparable to that which is obtained from efficent line light sources. However, the intensity increases towards the extreme UV (XUV) and X-ray regions, where no comparable continuum light sources exist. 

Synchrotron radiation has many applications in a large number of scientific and industrial areas physics, chemistry, biology, medicine and electronics. With regard to atomic and molecular spectroscopy synchrotron 

Layout of the MAX storage ring (Lund) for the generation of synchrotron radiation (Courtesy P. Rojsel, MAX Synchrotron Radiation Laboratory) 

The discrete line sources described above for XPS are perfectly adequate for most applications, but some types of analysis require that the source be tunable (i.e. that the exciting energy be variable). The reason is to enable the photoionization cross-section of the core levels of a particular element or group of elements to be varied, which is particularly useful when dealing with multielement semiconductors. Tunable radiation can be obtained from a synchrotron. 

In a synchrotron, electrons are accelerated to near relativistic velocities and constrained magnetically into circular paths. When a charged particle is accelerated, it emits radiation, and when the near-relativistic electrons are forced into curved paths they emit photons over a continuous spectrum. The general shape of the spectrum is shown in Fig. 2.4. For a synchrotron with an energy of several gigaelectronvolts and a radius of some tens of meters, the energy of the emitted photons near the maximum is of the order of 1 keV (i.e., ideal for XPS). As can be seen from the universal curve, plenty of usable intensity exists down into the UV region. With suitable mono- 

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Ultraviolet photoelectron spectroscopy (UPS) is a variety of photoelectron spectroscopy that is aimed at measuring the valence band, as described in sectionBl.25.2.3. Valence band spectroscopy is best perfonned with photon energies in the range of 20-50 eV. A He discharge lamp, which can produce 21.2 or 40.8 eV photons, is commonly used as the excitation source m the laboratory, or UPS can be perfonned with synchrotron radiation. Note that UPS is sometimes just referred to as photoelectron spectroscopy (PES), or simply valence band photoemission. 

Flimpsel F J, McFeely F R, Morar J F, Taleb-lbrahimi A and Yarmoff J A 1990 Core level spectroscopy at silicon surfaces and interfaces Proc. Enrico Fermi School on Photoemission and Adsorption Spectroscopy and Interfaces with Synchrotron Radiation vo course CVIII, eds M Campagna and R Rose (Amsterdam Elsevier) p 203... 

Margaritondo G 9QQ introduction to Synchrotron Radiation (New York Oxford University Press)... 

Smith N V and Himpsel F J 1983 Photoelectron spectroscopy Handbook on Synchrotron Radiation ed E E Koch (Amsterdam North-Holland)... 

Flendrickson W A 1991 Determination of macromolecular structures from anomalous diffraction of synchrotron radiation Soienoe 254 51-8... 

New metliods appear regularly. The principal challenges to the ingenuity of the spectroscopist are availability of appropriate radiation sources, absorption or distortion of the radiation by the windows and other components of the high-pressure cells, and small samples. Lasers and synchrotron radiation sources are especially valuable, and use of beryllium gaskets for diamond-anvil cells will open new applications. Impulse-stimulated Brillouin [75], coherent anti-Stokes Raman [76, 77], picosecond kinetics of shocked materials [78], visible circular and x-ray magnetic circular dicliroism [79, 80] and x-ray emission [72] are but a few recent spectroscopic developments in static and dynamic high-pressure research. 

Cakmak M, Teitge A, Zachman FI G and White J L 1993 On-line small-angle and wide-angle x-ray scattering studies on melt-spinning poly(vinylidene fluoride) tape using synchrotron radiation J. Polym. Sc/. 31 371- 81... 

Ocko B M, Wang X J, Adzic R and Wandiowski Th 1998 Surface x-ray scattering studies of Eiectrosorption Synchrotron Radiat. News 11 23-30... 

Tyliszczak T, Hitchcock A, Wu S, Chen A, Szymanski G and Lipkowski J 1998 X-ray absorption studies of mixed overlayers formed by copper adatom co-adsorbed with anions at the Au(111) electrode surface Synchrotron Radiat. News 11 31-8... 

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. 

Figure 8.38 Curve fitting of Mo extended X-ray absorption fine structure (EXAFS) for Mo(SC6H4NH)3, taking into account (a) sulphur and (b) sulphur and nitrogen atoms as near neighbours. (Reproduced, with permission, trom Winnick, H. and Doniach, S. (Eds), Synchrotron Radiation Research, p. 436, Plenum, New York, 1980)...

Eland, J. H. D. (1983) Photoelectron Spectroscopy. 2nd edn, Butterworth-Heinemann, London. Huffier, S. (2001) Photoelectron Spectroscopy Principles and Applications. 3rd edn. Springer, Berlin. Prince, K. C. (1995) Photoelectron Spectroscopy of Solids and Suifaces Synchrotron Radiation Techniques and Applications, World Scientific Publishing, Singapore. 

Winnick, H. and Doniach, S. (Eds) (1980) Synchrotron Radiation Research, Plenum, New York. 

It was estabhshed ia 1945 that monolayers of saturated fatty acids have quite compHcated phase diagrams (13). However, the observation of the different phases has become possible only much more recendy owiag to improvements ia experimental optical techniques such as duorescence, polarized duorescence, and Brewster angle microscopies, and x-ray methods usiag synchrotron radiation, etc. Thus, it has become well accepted that Hpid monolayer stmctures are not merely soHd, Hquid expanded, Hquid condensed, etc, but that a faidy large number of phases and mesophases exist, as a variety of phase transitions between them (14,15). 

Unlike for synchrotron radiation, the maximum iatensity of x-rays from an x-ray tube is limited by how fast heat can be removed from the target to prevent its melting. In a conventional sealed tube, the target is stationary, relatively small, and must be continually cooled with water. In a rotating anode tube, the target is larger and is continually rotated so that the heat can be distributed over a larger surface. With such a tube the amount of heat, and hence. 

The analysis was performed by XRF method with SR. SRXRF is an instrumental, multielemental, non-destructive analytical method using synchrotron radiation as primary excitation source. The fluorescence radiation was measured on the XRF beam-line of VEPP-3 (E=2 GeV, 1=100 mA), Institute of Nuclear Physics, Novosibirsk, Russia. For quality control were used international reference standards. 

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

Walter, R.L., et al. High resolution macromolecular structure determination using CCD detectors and synchrotron radiation. Structure 3 835-844, 1995. 

An aerial view of the European Synchrotron Radiation Facility at Grenoble, France, an advanced source of synchrotron x-ray radiation for use in the study of protein structure, as well as for use in the physical and material sciences. The synchrotron radiation is produced in the circular building in the lower left of the photograph. (Courtesy of ESRF.)... 

Typically a few pm but material dependent mono-layer sensitivity with synchrotron radiation... 

Material dependent, but "3% in a two phase mixture with synchrotron radiation can be -0.1%... 

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