BASIC PRINCIPLES OF RADIATION PHYSICS
S.PANNEER SELVAM,
Associate Professor of Medical Physics,
Sri Ramachandra Medical College & Research Institute, Porur,
Chennai.
Electromagnetic radiation (often abbreviated E-M radiation
or EMR) is a phenomenon that takes the form of self-propagating
waves in a vacuum or in matter. It comprises electric and
magnetic field components, which oscillate in phase perpendicular
to each other and perpendicular to the direction of energy
propagation.
EM radiation carries energy and momentum that may be
imparted to matter with which it interacts.
Electromagnetic waves are produced by the motion of electrically
charged particles. These waves are called as "electromagnetic
radiation" because they radiate from the electrically charged
particles. They travel through empty space as well as through air
and other substances.
Electromagnetic radiation has a dual "personality." Besides acting
like waves, it acts like a stream of particles (called "photons")
that have no mass.
WAVE CONCEPT
• Wavelength : Distance between the peaks ( λ )
• Frequency : Number of cycles passes per second , a fixed
point ( ν )
• Velocity : Distance travelled per second by a point ( V )
V=νxλ
Lower frequency
:
Longer wavelength
Higher frequency :
Shorter wavelength
PARTICLE CONCEPT
▪ Electromagnetic waves, may react with matter as if they
were particles rather than waves
▪ These particles are discrete bundles of energy
▪ Each of these bundles of energy is called a Quantum or
Photon
▪ Frequency doubled , energy of the photon doubled
▪ The photons with the highest energy correspond to the
shortest wavelengths
Electromagnetic radiation is classified into several types according
to the frequency of its wave; these types include (in order of
increasing frequency and decreasing wavelength): radio waves,
microwaves, infrared radiation, visible light, ultraviolet
radiation, X-rays and gamma rays.
SOURCES OF EMR
There are both natural and man-made sources of
electromagnetic radiation
▪ Natural Sources
Ultraviolet rays , Visible light
▪ Man made sources
X-Rays , Microwaves and Radio waves
They also classified electromagnetic radiation into Ionizing
and Non Ionizing radiation
▪ Non Ionizing
Visible radiation, infrared radiation, low-energy radio
waves and microwaves
▪ Ionizing
X-Rays & Gamma Rays
COMMON PROPERTIES OF EMR
▪ It travels through free space as straight lines
▪ The speed of EMR is constant in space. All forms of EMR
have the same speed of 299,800 kilometers/second (~ 3 x
108 m/s) in space
▪ According to quantum theory of EMR, the energy associated
with a photon is given as
E = hυ
▪ Electromagnetic radiation on passing through a medium gets
attenuated
▪ EMR obeys Inverse square law
Binding Energy
▪ Electrons held in their orbits by electrostatic attraction
▪ This force of attraction is Binding Force
▪ Binding force is > in K Shell
Bound / Free electrons
▪ In high atomic number : Electrons in the orbits close to
the nucleus are Bound electrons and electrons far from the
nucleus are Free electrons
▪ In low atomic number : All the Electrons considered to be
Free Electrons
IONIZATION
• An atom as a whole is normally Electrically Neutral
• When one or more of the orbital electrons are removed from
the Atom, the remainder of the atom is left positively
charged and is known as Positive ION
• This process of removal of orbital electrons is known
Ionization
• The energy required to produce ionization may be supplied
to the atom by means of Ionizing Radiation
– e.g. x-ray, alpha, beta, gamma, etc.,
EXCITATION
• Sometimes energy given to the atom is less than the binding
energy & an orbital election receives insufficient energy to
enable it to leave the atom as in ionization
• Instead, electron may move from its original shell to the
outer shell
• Then the atom has more energy than normal state
• It is said to be in excited state and this process is known as
excitation
IONIZING RADIATION
• Ionizing radiation is the radiation that can produce charged
particles (ions) in materials
that it strikes.
Types of Ionizing Radiation
▪ Alpha radiation
▪ Beta radiation
▪ Gamma radiation
▪ Neutrons
▪ X-rays – discovered by W. C. Roentgen, 1895
RADIOACTIVITY
•
It is the property possessed by certain nuclides of
undergoing spontaneous transformation of their nuclei
accompanied by emission of particles or radiation
• Isotopes are nuclides which have the same number of
protons but different number of neutrons in their nuclei
Types of Radiation
• Alpha ( Helium nucleus )
• Beta ( electron )
• Gamma Rays ( Photon )
Half Life
▪ Is the time taken for the radioactive nuclide to decay to one
half of its original value
Units of Radioactivity
▪
Becquerel : one disintegration / sec
▪
Curie :
3.7 X 1010 disintegrations / sec
Inverse Square Law
• The intensity of the radiation from a point source varies
inversely as the square of the distance from the source,
provided there is no absorption (or) scattering by the
medium.