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Interactions of X-Rays with Matter The dependence of the
X-ray attenuation on the atomic number relies on mainly on three
phenomena: photoelectric effect, Compton effect and pair
production;
The photoelectric effect is predominant at low X-ray
energies and with high atomic numbers. When a quantum of radiation
strikes an atom, it may impinge on an electron within an inner shell
and eject it from the atom. If the photon carries more energy than
is necessary to eject the electron, it will transfer this residual
energy to the ejected electron in the form of kinetic energy
The Compton effect occurs primarily in the absorption of
high X-ray energy and low atomic numbers. The effect takes place
when high X-ray energy photons collide with an electron. Both
particles may be deflected at an angle to the direction of the path
of the incident X-ray. The incident photon having delivered some of
its energy to the electron emerges with a longer wavelength. These
deflections, accompanied by a charge of wavelength are known as
Compton scattering.
Pair production is the formation or materialization of two
electrons, one negative and the other positive (positron), from a
pulse of electromagnetic energy traveling through matter, usually in
the vicinity of an atomic nucleus. Pair production is a direct
conversion of radiant energy to matter. It is one of the principal
ways in which high-energy gamma rays are absorbed in matter. For
pair production to occur, the electromagnetic energy, in a discrete
quantity called a photon, must be at least equivalent to the mass of
two electrons. The mass m of a single electron is equivalent to 0.51
million electron volts (MeV) of energy E as calculated from the
equation formulated by Albert Einstein, E = mc2, in which
c is a constant equal to the velocity of light. To produce two
electrons, therefore, the photon energy must be at least 1.02 MeV.
Photon energy in excess of this amount, when pair production occurs,
is converted into motion of the electron-positron pair. If pair
production occurs in a track detector, such as a cloud chamber, to
which a magnetic field is properly applied, the electron and the
positron curve away from the point of formation in opposite
directions in arcs of equal curvature. In this way pair production
was first detected (1933). The positron that is formed quickly
disappears by reconversion into photons in the process of
annihilation with another electron in matter.
Two less important (In diagnostic energy levels) effects
Thomson scattering (R), also known as Rayleigh,
coherent, or classical scattering, occurs when the x-ray photon
interacts with the whole atom so that the photon is scattered with
no change in internal energy to the scattering atom, nor to the
x-ray photon. Thomson scattering is never more than a minor
contributor to the absorption coefficient. The scattering occurs
without the loss of energy. Scattering is mainly in the forward
direction.
Photodisintegration (PD) is the process by which the x-ray
photon is captured by the nucleus of the atom with the ejection of a
particle from the nucleus when all the energy of the x-ray is given
to the nucleus. Because of the enormously high energies involved,
this process may be neglected for the energies of x-rays used in
radiography.
Absorption Edges
If the mass
absorption coefficient of a material is plotted against wavelength
as shown in Figure Y for a monochromatic x-ray beam,
mm
shows sharp discontinuities at particular wavelengths.
Fig Y
These correspond
to the ionisation energy of a K shell electron and indicate the
increased probability of photoelectric absorption, however this
drops sharply as the difference between the photon and electron
binding energy increases. The variation of
mm
with photon energy E and atomic number Z for the various scattering
and absorption processes is summarised in the following table and
shown graphically in figure X:
Summary of Main Attenuation
Mechanisms
|
Mechanism |
Variation of
mm
with E |
Variation of
mm
with Z |
Energy range
in tissue |
|
Rayleigh |
µ
1 / E |
µ
Z2 |
1 - 30 keV |
|
photoelectric
|
µ
1 / E3 |
µ
Z3 |
1 - 100 keV |
|
Compton
|
falls
gradually with E |
independent |
0.5 - 5 MeV |
|
pair
production |
rises slowly
with E |
µ
Z2 |
> 5 MeV |
The relative Importance of Attenuation processes
Only photoelectric effect and Compton effect are significant in the
production of diagnostic radiographic images
Figure X
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