interaction processes

Question 1

attenuation

Answer 1

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

Question 2

transmission

Answer 2

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.

Question 3

total linear attenuation coefficient (u)

Answer 3

• 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

Question 4

total mass attenuation coefficient (u/p)

Answer 4

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

Question 5

Elastic Scatter

Answer 5

• 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

Question 6

Photoelectric Absorption

Answer 6

• 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

Question 7

Compton Scatter

Answer 7

• 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

Question 8

Pair production

Answer 8

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