36 Chapter 1. Properties and Sources of Radiation
paths. Although conceptually they represent the same physical phenomenon, but
they can be distinguished by noting that Bremsstrahlung is a product of tangential
acceleration while synchrotron radiation is produced by centripetal acceleration of
charged particles.
The spectrum of synchrotron radiation is continuous and extends over a broad
energy range from infrared to hard x-rays. In general the spectral distribution
is smooth with a maximum near the so-calledcritical wavelength(see Fig.1.6.5).
Critical wavelength divides the energy carried by the synchrotron radiation into two
halves.
10 −1 100 101 102 103 104 1051010101010129630critical wavelengthPhoton Energy (eV)Relative Intensity (arbitrary units)Figure 1.6.5: Typical syn-
chrotron radiation spec-
trum. Critical wavelength
divides the total delivered
energy into two halves.The x-rays produced as synchrotron radiation are extremely intense, highly col-
limated, and polarized (most of the wave vectors oscillate in the same plane) in
contrast to conventional x-rays, which have very low intensities, are very difficult to
collimate, and are completely unpolarized. However the production of synchrotron
radiation is far more expensive than conventional x-rays and therefore dedicated
synchrotron facilities have been developed around the world where beam time is
made available to researchers.
LASER
LASER (Light Amplification by Stimulated Emission of Radiation) is generated
by exploiting a quantum mechanical phenomenon called stimulated emission of pho-
tons. Stimulated emission is an optical amplification process in which the photon
population is increased by allowing the incident photons to interact with atoms or
molecules in excited states. An excited atoms, when struck by an incident photon of
some frequency and phase, emits another photon of the same frequency and phase
to relax to the ground state. The initial photon is not destroyed in the process
and goes on to create more photons. The result is an intense, highly collimated,
and coherent beam of light. In essence, the trick of producing laser is to somehow
increase the population of atoms or molecules in the excited state and maintain it
through external means. If more atoms or molecules are in excited state than in