Short theory
Synchrotron radiation is emitted by charged particles traveling on a
curved path (as would happen while moving through a magnetic field).
Since the source of all electromagnetic radiation is the acceleration of
charge, synchrotron radiation is an example electromagnetic radiation
produced by centripetal acceleration (as opposed to bremsstrahlung,
which is produced by tangential acceleration). The wavelength of this
radiation is a function of the energy of the charged particles and the
strength of the magnetic field bending the charged particles. The
spectrum of the radiation is continuous and is characterized by its
critical wavelength, which divides the spectrum into two parts with
equal power (half the power radiated above the critical wavelength and
half below).
The critical wavelength can be found using the equation below
which reduces to the following equation when the charged particles are
electrons
Synchrotron radiation sources: rings, undulators, wigglers, National
Synchrotron Light Source doesn't produce light as its primary form of
electromagnetic radiation. Most research done at this facility uses the
x-rays and vacuum ultraviolet produced by the electron beam.
In 1945, the synchrotron was proposed as the latest accelerator for
high-energy physics, designed to push particles, in this case electrons,
to higher energies than could a cyclotron, the particle accelerator of
the day. An accelerator takes stationary charged particles, such as
electrons, and drives them to velocities near the speed of light. In
being forced by magnets to travel around a circular storage ring,
charged particles tangentially emit electromagnetic radiation and,
consequently, lose energy. This energy is emitted in the form of light
and is known as synchrotron radiation.
Synchrotron radiation is a nuisance in a particle accelerator as it
sucks energy out of the particles being accelerated, but it makes an
ideal source of high energy electromagnetic radiation. The beam produced
is composed of very nearly parallel rays (collimated) and is quite
intense.
The NSLS operates two electron storage rings: The VUV (vacuum
ultraviolet) Ring operates at an electron energy of 800 MeV designed for
optimum radiation at energies between 10 eV and 1 keV. The X-Ray Ring
operates at 2.5 GeV to optimize radiation between 1 keV and 20 keV.
• Synchrotron radiation can be produced for hours, maybe even days if
you were willing to pay the electrical bills and had some reason to work
around the clock. X-ray tubes can only operate for a few seconds or
maybe minutes. Run them too long and they'll burn out just like a light
bulb.
• Synchrotron radiation is "organized": the beam is highly polarized (most
of the waves are oscillating in the same plane) and collimated (most of
the waves are in the same direction). X-ray tubes produce "messy"
radiation that is completely unpolarized and may be focused only with
great difficulty. A synchrotron source is like an "x-ray laser", while
an x-ray tube is like an "x-ray floodlight".
• Syncrotron radiation can be "shared". A large synchrotron might have
upwards of 50 beamlines and run hundred if not thousands of experiements
in one year. Syncrotron facilities are expensive to build, but pay for
themselve in sheer volume of research |
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