Flashcards in X-ray production Deck (78)
What is the process by which electrons are emitted from the cathode?
Thermionic emission (heat)
What is the heel effect?
The energy of the photons at the cathod cathode end of the beam is higher
How does the energy a Brehmsstrahlung photon relate to that of the incident electron?
photon = electron energy
In what direction are Brehmsstrahlung x-rays emitted when the incident electron is <100keV?
>100keV --> forward
What is the average x-ray energy in a photon beam?
What determines the Emax of a photon beam?
KVP (voltage change between cathode and anode)
kVp = Emax
What is the energy range of of orthovoltage therapy?
Where does the max dose occur in orthovoltage therapy?
Skin surface and bone (because photoelectric effect)
How and where are electrons accelerated towards the target?
Accelerated in the waveguide by microwaves traveling down tube
Which waveguide design allows change in output energy?
What are the features of a cobalt beam compared to a linac beam in terms of energy and penumbra?
What is the half-life of 60-Co, and how much does its' activity decline monthly?
A declines ~ 1% monthly
How is 60-Co produced?
Bombardment of non-radioactive Co with neutrons
What are the decay products of 60-Co?
60-Ni + B- particle + two g-rays
B- particle = 0.32MeV
g-rays = 1.17 & 1.33 MeV
What type of decay does 60-Co undergo?
What is the average energy of 60-Co g-rays?
What is the dose rate in free space of 60-Co?
240 cGy/min at 80cm SSD
What is a delta ray?
Ejected electron with sufficient energy to produce a secondary ionization track
What is fluence? What is energy fluence?
Fluence = # photons entering an imaginary sphere of cross sectional area a
Energy fluence = sum of all photon energies that enter the sphere
What is intensity?
Intensity = energy fluence over time
where energy fluence = sum of all photon energies that enter the sphere
T/F: Coherent scattering of photons refers to a change in direction without energy loss.
TRUE: Energy and wavelength of incident photon are retained, only thing that changes is direction of travel
What is the energy of an electron ejected by the photo electric effect?
hv (of incident photon) - Eb (electron binding energy)
How does photon attenuation related to Z for
a) photoelectric effect
b) compton scatter
photoelectric effect - proportional to Z^3
compton scatter - independent of Z
What is the threshold energy for pair production?
How does Z impact the probability of pair production?
Probability increases with increasing Z
What happens to the linear attenuation coefficient (u) as incident photon beam energy increases?
u decreases in photoelectric effect range as energy increases, then stabilizes in Compton range
What are do absorption edges and for which interaction do they occur?
Absorption edges are "jumps" on graph of photoelectric effect mass attenuation coefficient, correspond to electron binding energies of inner electron shells (L, K)
What characteristic of the absorbing material determines the mass attenuation coefficient for photoelectric effect? For Compton effect?
PE effect - Z^3
Compton - electron density
What is the approximate electron density of most elements/tissues? For which element does this differ?
Most tissues/ elements - ~3 x 10^23
Hydrogen - 6 x 10^23