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Interaction of
Radiation with Matter
Topics
Radiation intensity
 Attenuation
 Linear attenuation coefficient
 Differential absorption
 Half-value layer
 Transmission Fraction
 X-ray Interaction Mechanism

Radiation Intensity
 The
intensity of radiation is defined
as the rate of emitted energy from
unit surface area through unit solid
angle.
Attenuation
 The
total reduction in the number of
x-rays remaining in an x-ray beam
after penetration through a given
thickness of tissue.
 Attenuation
is the product of
absorption and scattering.
Linear Attenuation Coefficient
 The
attenuation coefficient is
a quantity that characterizes
how easily a material or
medium can be penetrated by
a beam of light, sound,
particles, or other energy or
matter.
Differential Absorption
The
difference in x-ray
interaction.
Increases
reduced.
as the kVp is
Differential absorption and attenuation of
the x-ray beam depend on the following
factors:
 The
atomic number (Z) of the atoms
in tissue.
 The mass density of the atoms in
tissue.
 The x-ray energy.
Half Value Layer (HVL)
The Thickness of an absorber needed to
reduce the intensity of the x-ray beam
into half of its original value.
X-ray Interaction Mechanism
 Coherent
Scattering
 Compton Scattering
 Photoelectric Effect
 Pair Production
 Photodisintegration
Coherent Scattering
 Energies
below approximately 10
keV.
 Sometimes
called classical scattering
or Thompson scattering, Rayleigh
Scattering.
 No
ionization.
Coherent Scattering
Coherent scattering
Compton Effect
 In
the Compton effect, the
incident x-ray interacts with an
outer shell electron and ejects
it from the atom, thereby ionizing
the atom. The ejected electron is
called a Compton electron or a
secondary electron.
Compton Effect
The
probability of the
Compton effect is inversely
proportional to x-ray
energy (1/E) and
independent of atomic
number.
Compton Effect
Compton Effect
Photoelectric Effect
 Interacts
with inner shell
electrons
 X-rays
 The
are absorbed
electron removed from an
atom is called photoelectron.
Photoelectric Effect
 The
probability if the photoelectric
effect is inversely proportional to the
third power of the x-ray energy
(1/E)3.
 The
probability of photoelectric
effect is directly proportional to the
third power of the atomic number of
the absorbing material (Z3).
Photoelectric Effect
Photoelectric Effect
Atomic Number and K-shell Electron binding
energy of radiologically important elements
Effective Atomic Number of Materials
Important to Radiologic Science
Pair Production
 Incident
electron interacts with the
nuclear field.
 The
interaction between the x-ray
and the nuclear field causes the x-ray
to disappear, and in its place, two
electrons appear, one positively
charged (positron) and one
negatively charged (electron).
Pair Production
Pair Production

Occurs above 1.02 MeV
Photodisintegration
 Occurs
with x-ray energies above
10 MeV.
 The
nucleus is raised to an
excited state and instantly emits
a nucleon or other nuclear
fragment.
Photodisintegration
Photodisintegration