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By
Dr: Nahla Nagy
Assistant Professor Radiological
Science
Interactions of X-Rays with matter
Radiation treatment is based on different kind of radiation and depends on the different kind of interaction between the radiation and matter (body tissue).
Interactions of X-Rays with matter
• X-rays traveling through matter can be transmitted, absorbed, or scattered.
Absorption
Scattering
Transmission
Energy deposition
Forms of x ray Interactions‐
X-Ray
Pair production
Coherent Scattering
Photoelectric Effect
Compton Effect
Coherent scatter
Coherent scatter occurs when a
low-energy x-ray photon excites
an atom but then passes
through without any net energy
transfer to the atom.
-
Coherent Scattering
Excitation of the total complement of atomic electrons
occurs as a result of interaction with the incident photon
No ionization takes place
Electrons in shells “vibrate”
Small heat is released
The photon is scattered in different directions
No loss of E
Pair production
Pair production occurs when a high-energy photon (> 1.02
MeV)interacts with the nucleus of an atom.
Pair production
The photon disappears,
and it’s energy is
converted into an
electron and a positron
of energy (511 keV)
Positron Annihilation
• Positron decay in matter
by annihilation with an
electron. Usually and
"atom" of positronium
(e+e-) forms which
annihilates to produce
two 511-keV photons.
Annihilation radiation
The two produced
photons that are emitted
at 180 degree to each
other .
Photoelectric effect (PE)
The PE effect occurs between tightly bound (inner-shell)
electrons and incident x-ray photons.
The PE effect occurs when a photon is totally absorbed by an
inner-shell electron and an electron (photoelectron)is emitted.
PHOTOELECTRIC ABSORBTION
IN THE PATIENT
The energy of the emitted photoelectron equals the difference between the incident photon energy and the electron binding energy (BE).
The incident photon energy (Eγ = hv) is absorbed by the atom
and an electron of energy (Ee) is ejected :
Ee= E γ ‐ BE
Outer-shell electrons then fill the inner-shell electron
vacancies to stabilize the atom, and the excess energy is
emitted as characteristic radiation or as Auger electrons.
An Auger electron is an outer-shell electron with a binding
energy less than the energy difference of the electron
transition.
Compton scatter
In Compton scatter, incident photons interact with loosely
bound valence (outer-shell) electrons.
A Compton interaction results in a scattered photon that has
less energy than that of the incident photon, and that travels
in a new direction (compare coherent scatter).
XXXXXCompton scatter
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