interaction processes Flashcards
1
Q
attenuation
A
occurs when a beam of x-rays passes through matter. results in the reduction in intensity by scatter or absorption
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.
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
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
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
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
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
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