Photon Interactions and Attenuation

These flashcards cover fundamental concepts regarding photon interactions and attenuation with matter, important for understanding health physics and medical radiation physics.

Photoelectric effect

An interaction where a photon disappears and an orbital electron is emitted from an atom. Abrupt jumps when hv=Eb.

Compton scattering

An interaction in which a photon transfers part of its energy to a loosely bound electron, causing it to be ejected.

Pair production

A process where a photon disappears and creates an electron-positron pair in the Coulomb field of a nucleus.

Linear attenuation coefficient (μ)

An important parameter that characterizes how x-ray or gamma-ray photons interact with matter, measured as probability per unit path length. Depends on energy hv of the photon beam and atomic number z of the absorber.

Half-value layer (HVL)

The thickness of an absorber required to reduce the intensity of radiation to half its original value. (ln2/u)

Mass attenuation coefficient (μm)

A measure that indicates how easily a material can attenuate radiation, expressed in cm²/g.

Tenth-value layer (TVL)

The thickness of an absorber needed to reduce the intensity of radiation to one-tenth of its original value. (ln10/u)

Rayleigh scattering

Elastic scattering of photons by atoms in which the photon changes direction without losing energy.

Thomson scattering

Elastic scattering of low-energy photons by free electrons, resulting in no energy loss but a change in direction.

Photonuclear reaction

An interaction that occurs when a high-energy photon interacts with the nucleus of an atom, leading to the emission of particles.

Similarities between gamma and x-rays

electromagnetic waves, 0 rest mass, no net electric chgarge, travel at c

Differences between gamma and x-rays

gamma originates in nucleus, x-rays originate in electron cloud

Photodisintegration

direct photon-nucleus interaction

Pair Production

Interaction between photon and electrostatic field where photon disappears. Followed by the annihilation of the positron with a “free” electron producing 2 annihilation quanta.

Mean free path

Absorber thickness which attenuates the beam intensity to 1/e.

Atomic Attentuation Coefficient

Fraction of an incident gamma-ray beam that is attenuated by a single atom.

Absorption Edges

hv<Eb, photons can’t undergo photoelectric effect.

Recoil Electron

Emitted when part of incident photon energy is transferred to the orbital electron. Energy hv’ is lower than incident photon energy hv.

Major Importance Groups

Photoelectric, compton scattering, pair production

Minor Importance Groups

Rayleigh Scattering, thomson scattering, photonuclear reactions

Nuclear Coulomb Field

Produced electron-positron pair with combined kinetic energy.

Regions of Relative Predominance

Photoelectric effect is predominant at low photon energies. Compton effect is predominant at intermediate photon energies. Pair production is predominant at high photon energies.