interaction processes Flashcards

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1
Q

attenuation

A

occurs when a beam of x-rays passes through matter. results in the reduction in intensity by scatter or absorption

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2
Q

transmission

A

the majority of the atom is empty space, therefore there is a possibility that a photon will pass through a medium without absorption or scatter occurring.
this is the process that contributes to the formation of a radio graphic image and exit does in EBRT.

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3
Q

total linear attenuation coefficient (u)

A
  • the fractional reduction in intensity of a parallel beam of radiation per unit thickness of the attenuation medium transversed.
  • as the probability of an interaction increases, so does the tlac
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4
Q

total mass attenuation coefficient (u/p)

A

the fraction of x-rays removed from a parallel beam of unit cross sectional area by the unit mass of the medium

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5
Q

Elastic Scatter

A
  • occurs when an incoming photon has a much lower energy than the binding energy of the electron
  • it may interact with an electron or be deflected, there is no loss of energy
  • occurs at very low photon energies, the energy of the incident photon at these low energies is insignificant in comparison to the binding energies of the electrons
  • the effect is largely irrelevant in radiotherapy because the atomic number of human tissue is low and radiographic energies are too high to make elastic scatter relevant
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6
Q

Photoelectric Absorption

A
  • occurs when an incoming photon has energy equal to or greater than the binding energy of an orbiting electron and interacts with it
  • the electron is ejected and a vacancy is created which is filled by an electron from a higher orbit.
  • kinetic energy of electron= the difference in energy before and after the transmission
  • differential blackening and contrast observed in radio graphic images occurs as a result of the photoelectric effect
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7
Q

Compton Scatter

A
  • occurs when the incident photon has a much higher energy than the BE of the electron
  • the incident photon collides with an electron which recoils, the ip is scattered and some energy is transferred in the forward direction relative to the incoming photon
  • it is not directly related to the z of attenuating material
  • low energy photons have a small reduction in energy and are scattered in all directions
  • high energy photons are scattered mostly in the forward direction
  • the probability of Compton scatter occurring is directly proportional to the electron density of the material
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8
Q

Pair production

A

e=mc2

  • photon interacts with electrical field of the nucleus and gives up its energy in the formation of a positron and a negatron
  • matter has been created from energy
  • the energy equivalent to the mass of one photon at rest is 0.511 meV, therefore the threshold photon energy needed for pair production is 1.022 meV
  • energy is divided between the two aprticles as KE
  • the particles are brought to rest by a series of interactions, once at rest the positron is very unstable and recombines with the negatron, resulting in annihilation of both particles and the creation of energy from mass
  • Two photons each with 0.511 meV of energy are created at 180’ of eachother
  • the probability of pp increases with rising energy above 1.022meV
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