X-ray Attenuation Mechanisms (OCR A Level Physics)

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Katie M


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X-ray Attenuation Mechanisms

  • X-ray attenuation is defined as:

The reduction in energy, or intensity, of a beam of X-rays due to their interaction with matter

  • There are four main methods in which X-rays can be attenuated:
    • Simple scattering
    • Photoelectric effect
    • Compton scattering
    • Pair production
  • These mechanisms occur within the material the X-rays are travelling in

Simple Scattering

  • Simple scattering occurs when:

A low-energy X-ray photon encounters an electron in an atom causing it to be scattered without a change in energy

  • Simple scattering occurs with lower-energy X-ray photons
    • In this scenario, 'low-energy' means the energy of the X-ray photon is not sufficient to cause ionisation
  • During simple scattering, photons are deflected from their initial path by interaction with the atoms of the material. However, there are:
    • No change in energy of the X-ray photon
    • No absorption of the X-ray photon 
  • This mechanism causes blurring or 'noise' in X-ray imaging
    • This is because scattered X-rays arrive at the detector from several angles as well as from the main beam


Photoelectric Effect

  • The photoelectric effect occurs when:

An X-ray photon is absorbed by an inner shell electron causing it to be ejected from the atom as a photoelectron

  • As a result of the photoelectric effect, the X-ray photon is completely absorbed and all its energy is imparted to the photoelectron
  • Since energy is always conserved, the energy of an incident X-ray photon is equal to:

The work function + the maximum kinetic energy of the photoelectron

  • The energy within a photon is equal to hf
    • This energy is transferred to the electron to release it from a material (the work function) and the remaining amount is given as kinetic energy to the emitted photoelectron
  • This equation is known as the photoelectric equation:

E equals h f equals ϕ plus 1 half m v squared subscript m a x end subscript

  • Where:
    • h = Planck's constant (J s)
    • f = the frequency of the incident radiation (Hz)
    • Φ = the work function of the material (J)
    • ½ mv2max Ek(max) = the maximum kinetic energy of the photoelectrons (J)


Compton Scattering

  • The Compton Effect is when:

An X-ray photon is deflected by an interaction with an orbital electron causing the wavelength of the photon to increase and the ejection of the electron from the atom at a high speed

  • This process is similar to simple scattering, except the X-ray photon imparts some of its energy to the orbital electron
  • Because of this exchange of energy:
    • The X-ray is deflected from its initial path
    • The X-ray’s wavelength increases, as its energy decreases
    • The electron involved is ejected from the atom involved in the interaction
  • The electron and X-ray are deflected in different directions due to conservation of momentum


Pair Production

  • Pair production occurs when:

A high energy X-ray photon passes close to the nucleus of an atom causing the production of an electron-positron pair

  • This arises as a consequence of Einstein's mass-energy equivalence principle:

E = mc2

  • Where:
    • E = the energy of the X-ray photon (J)
    • m = the mass of the electron and position = 2me (kg)
    • c = the speed of light (m s−1)
  • Pair production can, therefore, only occur with high energy X-rays
    • This is because the energy of the X-ray photon must be above a certain value to provide the total rest mass energy of the electron-positron pair
  • The minimum energy, Emin, for a photon to undergo pair production is the total rest mass energy of the particles produced:

Emin = hfmin = 2mec2

  • As a result of pair production, the X-ray photon is completely absorbed and all its energy is imparted to the electron-positron pair


2.2.5 Pair Production

When a photon with enough energy interacts with a nucleus it can produce an electron-positron pair

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Katie M

Author: Katie M

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