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Home | Publications | EMU Notes in Mineralogy series | EMU volume 8, chapter 6

EMU Notes in Mineralogy - volume 8

Nanoscopic approaches in Earth and planetary sciences (F. Brenker and G. Jordan, eds)

Chapter 6: Synchrotron radiation micro- and nano-spectroscopy

L. Vincze, G. Silversmit, B. Vekemans, R. Terzano and F. Brenker

Introduction

Synchrotron radiation (SR) is generated when highly relativistic charged particles (typically electrons or positrons) are forced to follow a curved trajectory in strong magnetic fields. As a result of the radial acceleration of these high-velocity charged particles, orbiting at speeds (v) of nearly the speed of light (c), electromagnetic radiation is generated which covers a wide wavelength (energy) range and has unique properties for spectroscopic studies. Synchrotron radiation is emitted tangentially to the electron path, in the form of a narrow cone of intense electromagnetic beam (Fig. 1).

This type of radiation is generated in so-called electron (or positron) storage rings, which consist of an evacuated, quasi-circular vacuum chamber coupled with a lattice of magnets, in which electrons/positrons can circulate freely in a closed orbit (Fig. 2). The path of the charged particles within the storage ring is determined by the magnetic lattice within the ring, which both focuses and bends the beam of charged particles, keeping it in a closed trajectory.

The so-called first generation synchrotron storage rings were built for particle physics experiments, high-energy particle accelerators, in which the generated synchrotron radiation was considered to be an unwanted by-product, resulting in an energy-loss for the accelerated particles. In the 1960s, scientists began to use synchrotron radiation from several of these first generation accelerators in a ‘parasitic mode’, realizing that the emitted synchrotron radiation has very advantageous properties for many types of spectroscopic applications.

These unique properties are as follows:

(1) High intensity and high brightness (intensity normalized by source area), which are many orders of magnitude greater than that achievable by conventional X-ray tubes.

(2) High brilliance, exceeding other natural and artificial light sources by many orders of magnitude: 3rd generation sources typically have a brilliance greater than 1018 photons/s/mm2/mrad2/0.1%BW. This property is associated with the low emittance of these sources, i.e. the product of source cross section and solid angle of emission is small.

(3) High natural collimation, or small angular divergence of the beam.

(4) Widely tunable in energy/wavelength by monochromatization (from infrared to hard X-ray regime).

(5) High level of polarization (linear or elliptical).

(6) Pulsed light emission with pulse durations of 1 ns.

 The second generation of synchrotron radiation facilities, such as the Photon Factory in Japan, DORIS ring upgrade in Germany, or the NSLS in the USA, were constructed expressly to provide synchrotron X-rays for scientific research. In these second generation synchrotron facilities, the main sources of SR are the so-called bending magnets, located at the end of each of the straight sections within the storage ring. These bending magnets change the direction of the passing electron beam in their (vertical) magnetic field, thereby, on the one hand, forcing the electron beam to follow a closed orbit and, on the other hand, generating a narrow cone of SR by the acceleration associated with the changing electron beam direction.

A third generation of synchrotron facilities has been completed. The first, and one of the largest, third generation synchrotron storage ring was the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Its construction was followed by the completion of the 7 GeV Advanced Photon Source (APS) in the USA and the 8 GeV storage ring SPRING-8 in Japan. These large 3rd generation storage rings are providing even greater-brilliance X-ray beams, ~10,000 times greater than those of the second generation.

In these facilities (see Fig. 3), the main sources of SR are no longer the above-mentioned bending magnets, which mainly serve here for keeping the electron beam in a closed orbit and as secondary sources for less demanding experiments. Here, the primary sources of SR are periodic magnetic structures located in the straight sections of the ring, commonly referred to as insertion devices, i.e. wigglers or undulators.

These insertion devices force the electrons to an oscillating, sinusoidal path, generating SR-beams which are order(s) of magnitude greater in intensity than achievable by single bending magnets. The SR generated is guided to the experimental stations by so-called beamlines, illustrated schematically in Fig. 4. The term beamline refers to the instrumentation that guides the beam of electromagnetic radiation produced to an end station which uses the synchrotron radiation produced by the bending magnets or insertion devices (wigglers/undulators) in the storage ring of a synchrotron radiation facility. Important components of a typical beamline are: (1) the monochromator to select a well defined wavelength (or wavelength range) from the incident spectrum; and (2) various types of X-ray optics to confine/focus the synchrotron beam onto the sample under investigation (Fig. 4).