Interactions with Matter Flashcards
How do photons interact with electrical and magnetic fields?
They don’t
Describe coherent scatter. What energies does this occur at? What influences the probability?
A photon transfers all of its energy to an orbital electron but is not enough to eject it. The electron then ejects this excess energy as a photon with the same energy, but different direction.
Occurs at <10 keV (remember ionization happens 10-30 keV).
p(Coherent) is proportional to Z
Describe photoelectric effect. What energies does this occur at? What influences the probability?
A photon transfers all of its energy to an orbital electron and is high enough to eject it. The electron leaves with the excess kinetic energy. An open shell allows drop down of another electron –> characteristic x-ray or auger electron.
This occurs at energies 10-26 keV (remember ionization happens at 10-30 keV).
p(PE) ~ Z3/E3. Note that increasing energy makes this less likely to happen (more likely to have compton)
Describe compton scatter. What energies does this occur at? What influences the probability?
The photon transfers some of its energy to an orbital electron, with still enough energy to eject it, the photon scatters as well. This is analogous to a pool cue ball.
This occurs at energies 26 keV - 24 MeV.
p(Compton) ~ electron density.
If a photon is deflected 90 degrees in a compton interaction, what is the resultant energy? What about 180 degrees?
At 90 degrees the energy is 0.511 MeV. At 180 degrees it is 0.255 MeV.
Describe pair production. What energies does this occur at? What influences the probability?
The high energy photon interacts with the electric field around a nucleus, causing shearing of the photon into an electron and a positron (thus happens at E > 1.022 MeV). The positron can then travel further before annihilating with an orbital electron, releasing two 0.511 MeV photons.
Occurs at energies > 10 MeV
p(PP) ~ Z2
What is a challenge with pair production as relates to dose in a patient?
The travel of the positron means dose can be accumulated out of field.
Describe triplet production. What energies does this occur at? What influences the probability?
Similar to pair production, but the photon interacts with an electrical field around an orbital electron. The orbital electron is ejected in addition to the positron/electron created.
The threshold energy here is double that of pair production = 2.044 MeV.
p(TP) ~ Z
Describe photonuclear disintegration. What energies does this occur at?
The high energy photon bombards a nucleus, ejecting neutrons or larger particles. This happens at energies above 10 MeV and is responsible for neutron contamination of high energy beams.
What is the difference in photons and particles as they interact with matter?
Photons will attenuate in matter, meaning they lose an absolute # of photons but keep the same speed (that of light). Particles will lose velocity but fluence remains the same (stopping power).
What is the W value?
W is the average energy needed to produce an ion pair, which is 33.97 eV in air. This is very small compared to the energy of charged particles (typically in MeV range = lots of ion pairs produced)
What are ‘stopping power’ and ‘LET’?
Stopping power is the amount of energy a particle loses per unit path length. LET is the amount of energy deposited in local ionizations per path length. Essentially ‘drag’ vs. ‘damage’. Both increase as a particle slows down.
What is the relationship of charge (Q) and velocity (v) to LET?
LET is proportional to Q2, thus particles with more charge (C=+6) are higher LET (vs. H=+1)
LET is inversely proportional to V2, as a particle slows it releases more energy into the medium.
What is the relationship of a medium with LET?
Higher density = more LET
Higher atomic number = less LET (harder to ionize high Z material)
What are the components of stopping power?
Collisional (Sc) = collisions deposit energy as dose
Radiative (Sr) = brehmsstrahlung, does not typically contribute to dose as it is radiated away
