Lecture 20 Photon Interactions with matter pt 1

What are the three things that can happen when a photon interacts with matter?

Absorption (Attenuation); Scatter; Transmission

Which photon interaction is most common?

Transmission

What is attenuation?

Progressive decrease in the initial number of photons.

What is differential attenuation?

Comparison of attenuation rates of the body tissues; also known as subject contrast.

What are the three patient factors that affect attenuation rate?

Tissue density; Tissue thickness; Average atomic number of tissue (all are direct).

What is scatter radiation?

Photons that have undergone a change in direction after interacting with atoms.

What is secondary radiation?

Characteristic radiation emitted by atoms after absorbing x-ray photons; also described as scatter or leakage radiation.

When is a photoelectric interaction most likely to occur?

When photon energy is slightly greater than the K-shell binding energy.

What happens during a photoelectric interaction?

Absorption of the incident photon occurs.

What happens during a photoelectric effect interaction?

Incident photon strikes an inner shell electron and ejects it; all the photon’s energy is given to the electron.

What are the results of the photoelectric effect?

1. Atom is ionized;

2. Ejected electron (photoelectron) can ionize other atoms;

3. Outer electron fills the inner shell hole, emitting characteristic (secondary) radiation;

4. This process continues with further electron transitions.

How is the probability of a photoelectric effect related to atomic number?

Directly related — higher atomic number = higher probability.

What mathematical relationship describes photoelectric effect probability?

Z³ (atomic number cubed).

Why is the photoelectric effect important in radiography?

It contributes to radiographic contrast.

What is coherent scatter also called?

Classical, Thomson, or Unmodified scatter.

At what photon energies does coherent scatter occur?

At very low energies, 10 keV or less.

What happens during coherent scatter?

Photon interacts with an atom and changes direction slightly, with no ionization.