1.6. General Properties and Sources of Particles and Waves 39
the mid 1980s. Since then a large number of these so called x-ray lasers have been
developed in different laboratories around the world. A more recent advancement
has been the development of the so called free electron lasers. These highly powerful
lasers (kW range) produce low wavelength coherent light in brief bursts and are
generally tunable to a precise wavelength within their range of operation.
Lasers have found countless applications in many diverse fields. From precision
heavy metal cutting to the delicate eye surgery, from CD burners to range finders,
lasers are now an essential part of our everyday lives.
Most of the lasers currently in use have spectra that fall within or around the
visible light spectrum. A lot of work is now being directed towards production
of new laser materials and apparatus that could produce lower wavelength lasers.
These lasers would deliver more power to the target in shorter periods of time. Some
success has already been achieved in developing these so called, x-ray lasers (their
wavelengths lie within the range of conventional x-rays).
Although very useful, laser can be extremely hazardous specially to skin and
eye. It can cause localized burning leading to permanent tissue damage and even
blindness. Since the possibility and degree of harm depends on its wavelength (or
energy), intensity and time of exposure, therefore the regulatory commissions have
classified lasers in different classes: Class-I laser is known to be safe and would not
cause any damage to eye even after hours of direct exposure. Class-IV laser, on the
other hand, is extremely dangerous and can cause irreversible damage such as per-
manent blindness. The lasers between these two classes are neither absolutely safe
nor extremely dangerous and the workers are allowed to directly work with them
provided they use appropriate eye protection equipment.
Radioactive Sources of Photons
There are a large number of radioactive elements that emitγ-rays. Theseγ-rays
are often accompanied byα-andβ-particles. Besides naturally occurring sources it
is possible to produce these isotopes in laboratory as well. This is normally done by
bombarding a source material by neutrons. The nuclei, as a result, go into unstable
states and try to get rid of these extra neutrons. In the process they also release
energy in the form ofγ-rays. The two most commonly used radioactive sources of
γ-rays are iridium-192 (^19277 Ir) and cobalt-60 (^6027 Co).
The easiest way to produce cobalt-60 is by bombarding cobalt-59 with slow neu-
trons as represented by the following reaction.
59
27 Co+n→60
27 Co+γ (7.^492 MeV) (1.6.3)
For this reaction we need slow neutrons. Californium-252 is an isotope that is
commonly used as a source of neutrons.^25298 Cfis produced in Nuclear Reactors
and has a half life of approximately 2.64 years. It can produce neutrons through a
number of fission modes, such as
252
98 Cf→94
38 Sr+154
60 Nd+4n. (1.6.4)However the neutrons produced in this way have higher kinetic energies than needed
for them to be optimally captured by cobalt-59. Therefore some kind ofmoderator,
such as water, is used to slow down these neutrons before they reach the cobalt
atoms. The resultant cobalt-60 isotope is radioactive and gives off 2 energeticγ-rays