Interaction of Radiation with Matter

Topics Radiation intensityAttenuation Linear attenuation coefficientDifferential absorption Half-value layerTransmission FractionX-ray Interaction Mechanism

Radiation IntensityThe intensity of radiation is defined as the rate of emitted energy from unit surface area through unit solid angle.

AttenuationThe total reduction in the number of x-rays remaining in an x-ray beam after penetration through a given thickness of tissue.

Attenuation is the product of absorption and scattering.

Linear Attenuation CoefficientThe attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles, or other energy or matter.

Differential Absorption

The difference in x-ray interaction.

Increases as the kVp is reduced.

Differential absorption and attenuation of the x-ray beam depend on the following factors:The atomic number (Z) of the atoms in tissue.

The mass density of the atoms in tissue.

The x-ray energy.

Half Value Layer (HVL)The Thickness of an absorber needed to reduce the intensity of the x-ray beam into half of its original value.

X-ray Interaction MechanismCoherent ScatteringCompton ScatteringPhotoelectric EffectPair ProductionPhotodisintegration

Coherent ScatteringEnergies below approximately 10 keV.

Sometimes called classical scattering or Thompson scattering, Rayleigh Scattering.

No ionization.

Coherent Scattering

Coherent scattering

Compton EffectIn the Compton effect, the incident x-ray interacts with an outer shell electron and ejects it from the atom, thereby ionizing the atom. The ejected electron is called a Compton electron or a secondary electron.

Compton EffectThe probability of the Compton effect is inversely proportional to x-ray energy (1/E) and independent of atomic number.

Compton Effect

Compton Effect

Photoelectric EffectInteracts with inner shell electrons

X-rays are absorbed

The electron removed from an atom is called photoelectron.

Photoelectric EffectThe probability if the photoelectric effect is inversely proportional to the third power of the x-ray energy (1/E)3.

The probability of photoelectric effect is directly proportional to the third power of the atomic number of the absorbing material (Z3).

Photoelectric Effect

Photoelectric Effect

Atomic Number and K-shell Electron binding energy of radiologically important elements

Effective Atomic Number of Materials Important to Radiologic Science

Pair ProductionIncident electron interacts with the nuclear field.

The interaction between the x-ray and the nuclear field causes the x-ray to disappear, and in its place, two electrons appear, one positively charged (positron) and one negatively charged (electron).

Pair Production

Pair ProductionOccurs above 1.02 MeV

PhotodisintegrationOccurs with x-ray energies above 10 MeV.

The nucleus is raised to an excited state and instantly emits a nucleon or other nuclear fragment.

Photodisintegration

Photodisintegration