Chemistry Reference
In-Depth Information
In 1904, Barkla discovered the polarization of X-rays, which were emitted
from an X-ray tube in a particular direction. He showed that X-rays are linearly
polarized if the X-ray beam is perpendicular to the electron beam of the X-ray
tube [5]. In this case, the electric field vector only oscillates in the plane spanned
by both beams. Furthermore, if X-rays are scattered from a paraffin block they
are linearly polarized since it can be shown that a second block can scatter these
X-rays only in a particular direction [1].
The polarization has given evidence for the wavelike character of X-rays and
moreover for the transversal kind of X-ray waves. Much later, synchrotron
radiation was shown to be linearly polarized in the plane of the storage ring.
The polarization of X-rays can be used to reduce the spectral background and
thereby to improve the detection limits in X-ray fluorescence analysis.
1.3.4SynchrotronRadiationasX-RaySource
The first synchrotrons were constructed by Edwin Mattison McMillan in the
United States and by Vladimir Iosifovich in the former Soviet Union in 1945,
Synchrotron radiation (SR) was discovered in 1947 by General Electric in
New York [51] when a bright arc of visible light was observed for the first time
at an electron accelerator. Its closed electron tube was partly covered by a
transparent instead of an opaque coating so that radiation became visible.
Today, SR is obtained from a storage ring in which charged particles like
electrons are stored in several bunches and maintained at high constant velocity
or kinetic energy. The particles come from the actual accelerator ring called
booster where they are accelerated by electric fields to almost light velocity.
The relativistic particles are forced into a fixed circular orbit by several strong
magnets. They are accelerated radially by the magnetic fields and hereby
produce a brilliant radiation. Extended descriptions can be found in the
literature [52-57] and online in the Internet [58-61].
Originally, the first users of synchrotrons constructed such large accelerators
for particle physics. These machines called colliders were applied to high-
energy collisions, for example, of electrons and positrons, as a new branch of
science. SR was regarded as an undesired loss of energy that had to be
compensated. Only several years later, scientists used the highly brilliant
radiation emerging from those machines and recognized its incomparable
potential for research offered to physicists, chemists, geologists, physicians,
biologists, engineers, and art historians. The benefits of SR in all disciplines of
application are unequalled.
A synchrotron facility usually consists of an electron or positron source, a
first linear accelerator, a second circular accelerator, called a booster-synchro-
tron, and a storage ring that consists of a metallic tube with circular and straight
sections with a total length from some meters up to several kilometers
(Figure 1.15). In the booster-synchrotron, the electrons are accelerated by
high-frequency (HF) amplifiers or clystrons to nearly light velocity. Around the
storage ring, several dipole magnets, so-called bending magnets (BMs), are
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