Lecturer Radiological Science Interactions of X-Rays with matter By Dr: Mohamed Afifi Lecturer Radiological Science

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 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.

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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