Interaction of Radiation with Matter

Topics Radiation intensity Attenuation Linear attenuation coefficient Differential absorption Half-value layer Transmission Fraction X-ray Interaction Mechanism

Radiation Intensity The intensity of radiation is defined as the rate of emitted energy from unit surface area through unit solid angle. The radiation from a surface has different intensities in different directions. The intensity of radiation along a normal to the surface is known as intensity of normal radiation, In. By using Lambert's cosine law and Stefan-Boltzmann law for a surface at absolute temperature.

Attenuation The 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. Absorption is an all-or-none condition for an x-ray

Linear Attenuation Coefficient The 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. -The attenuation coefficient is also called linear attenuation coefficient, narrow beam attenuation coefficient, or absorption coefficient. -A large attenuation coefficient means that the beam is quickly "attenuated" (weakened) as it passes through the medium, and a small attenuation coefficient means that the medium is relatively transparent to the beam. -http://en.wikipedia.org/wiki/Attenuation coefficient

Differential Absorption The difference in x-ray interaction. Increases as the kVp is reduced.

The atomic number (Z) of the atoms in tissue. 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 Mechanism Coherent Scattering Compton Scattering Photoelectric Effect Pair Production Photodisintegration 10x10-8 to 10 10x10-9 10x10-9 to 10x10-10

Coherent Scattering Energies below approximately 10 keV. Sometimes called classical scattering or Thompson scattering, Rayleigh Scattering. No ionization. -Classical scattering is an interaction between low-energy x-rays and atoms. The x-ray loses no energy but changes direction slightly. The wavelength of the incident x-ray is equal to the wavelength of the scattered x-ray.

Coherent Scattering

Coherent scattering

Compton Effect In 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. -The Compton Effect occurs between moderate-energy x-rays and outer-shell electrons. It results in ionization of the target atom, change in direction, and reduction in x-ray energy. The wavelength of the scattered x-ray is greater than that of the incident x-ray. -The x-ray continues in a different direction with less energy.

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

Compton Effect

Compton Effect

Interacts with inner shell electrons Photoelectric Effect Interacts with inner shell electrons X-rays are absorbed The electron removed from an atom is called photoelectron. Photoelectric effect is total x-ray absorption.

Photoelectric Effect The 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 Production Incident 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 Production Occurs above 1.02 MeV Annihilation radiation

Photodisintegration Occurs 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