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 (E γ = hv) is absorbed by the atom and an electron of energy (E e ) 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.

Auger electron

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).

XXXXX Compton scatter

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