X-Rays and their use in diagnostics: Synchrotron radiation
Spectroscopy consists in the study of
light-matter interaction. Indeed, light, depending on its wavelength, shows
different effects according to the different systems which it interacts with,
as they are characterized by precise electronic, vibrational (i.e. associated
to the reticular structure) and spin properties, that the radiation is able to
investigate. Ideally, in order to fully study a system by means of
spectroscopic methods, it would be necessary to have availability of radiation
sources at different wavelengths, from X-Ray to the infrared. Moreover, the use
of electromagnetic radiation allows to explore the reticular structure,
providing in several cases a microscopic image. For this purpose, the
diffraction of light with wavelengths comparable to atomic separation (around
0.1 nm) is a valid tool. This spectral range is equivalent to the typical
energies of X-Ray, whose use, in both spectroscopy and diffraction, allows us
to have information about structural and electronic properties of the system
under study.
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| Elettra: the Italian synchrotron in Trieste (https://www.elettra.trieste.it/it/index.html) |
The development of laser sources and the use of
nonlinear optics has encouraged the widespread of light sources with different
spectral range. However, synchrotron radiation becomes necessary in order to
have X-ray radiation, sufficiently intense and collimated in a wide spectral
range.
Synchrotron radiation is referred to the light
emitted by charged particles radially accelerated, i.e. whose acceleration is
orthogonal to their velocity. The energy lost in the form of light is
proportional to the fourth power of the particle speed and inversely
proportional to orbit radius. Actually, synchrotron radiation represents the
main drawback in the achievement of high energies in particles accelerators
such as LEP at CERN; however, it can be involved as light sources in several
experiments. Indeed, it shows many convenient properties: the possibility to
tune its wavelength within a large spectral range that covers from microwaves to
hard X-Rays; high brilliance, i.e. the emitted photons are spatially
collimated; high photon flux, i.e. the beam is highly intense and it can be
employed also to study sample with low concentration or low cross section; the
temporal duration of the pulses can be tens of picoseconds and they can be used
to performed time-resolved experiments.
The production of synchrotron radiation takes
place in specific machines, wherein electrons are accelerated to reach
velocities close to the light speed and the emitted radiation is sent to beamlines,
where several experiments are performed. Generally, electrons production takes
place at the LINAC; here, the particles create “bunches” and they are
accelerated before arriving at the booster. This pre-accelerator allows the
electrons to reach energies close to GeV before going to the accumulation ring.
Here, the charged particles are accelerated by accelerating cavities and they
are deviated by applying magnetic fields by means of ondulators, wrigglers and
quadrupoles. The diameters of these rings can vary from ten to several hundreds
meters. Typically, the highest possible energy for the electrons increases with
the size of the ring. The largest rings have a circumference of about 1 km:
ESRF (European synchrotron radiation facility) in Grenoble accelerates the
electrons up to 6 Gev; APS (Advanced photon source) in Chicago, up to 7 GeV; SPring8
(Super ring 8) in Tsukuba, up to 8 GeV.
These laboratories host several beamlines, that differ from each other in terms of the possible samples and experiments that can be performed. In particular, synchrotron radiation is a powerful tool for the diagnostics of cultural heritage. Indeed, its possible applications go from the X-Ray microtomography, which gives a non-invasive structural analysis of the system, to the photoemission spectroscopy, that studies the electronic levels of the sample.
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