Electron Characteristics Flashcards
Are collisional loses (electrons) more predominant in Low or High Z materials?
Low Z materials
What are collisional loses dependent on?
Electron Density of Medium
Is MSP greater in low or high Z materials?
Low Z materials
Radiation loses of electron interaction (bremsstrahlung) is proportional to ______
Proportional to E - as E increases, bremsstrahlung increases;
Proportional to Z^2 - as Z increases, bremsstrahlung increases
Define MCS
Multiple Coulomb Scattering; form of electron scattering
Low energy e’ and higher Z result in:
more scattering
What type of Z material is used for LINAC scattering foils?
High
What is a delta ray?
Scattered electrons that can cause ionization
What do Inelastic interactions with electrons cause?
Ionization and Excitation
What is the principle process of energy transfer?
Interactions
What does Energy Transfer lead to?
Ionization and Excitation
Inelastic interactions with Nuclei produces what?
Bremsstrahlung x-ray
Formula for Mean Energy of electron:
Eo = 2.4 MeV/cm x R50
Formula for Most Probable E of electron:
Epo = C1 + C2Rp + C3Rp^2 *Rp = practical range (where electrons stop)
Formula for E at a given depth of electron:
Ez = Eo x (1 - z/Rp) z = depth
Formula for Max/Practical Range of electron:
Max Range = E / 2cm
Formula for dose at 90% isodose line for electron:
d90 = E / 3.2cm
Formula for dose at 80% isodose line for electron:
d80 = E / 2.8cm
For electrons, how is surface dose related to energy?
As E increases, surface dose increases
For electrons, what type of E causes isodoses to “bulge out”
Low E e’
An increase in FS has what effect on dose for electron treatments?
Dose Increases
Formula for output factor of e’ block
(Dxx * Dyy)^1/2
Formula for electron E loss
2MeV / cm
Electron Characteristics in relation to E:
- Surface dose increases with E
- Dmax increases with E
- Dmax plateau broadens with E
- Dx increases with E
- Slope is less steep with E
X-Ray contamination in electrons is what type
bremsstrahlung
X-Ray contamination _____ with e’ energy, by how much for 5MeV, 10MeV, and 20MeV?
increases;
- 5 MeV: 1%
- 10 MeV: 2%
- 20 MeV: 4%
Effects of beam obliquity for e’ beams:
- Increases side scatter
- dmax becomes shallower
- decreased depth of penetration
- Surface dose increases
Characteristics of electron backscatter factor (EBF)
- EBF increases with lower E
2. EBF increases with increasing Z material
Formula for Electron Output with distance:
((VSD + dmax) / (VSD + dmax + gap))^2
- VSD = virtual source distance
- gap = difference of SSD and nominal, ie. if SSD = 111, then gap is 11
What is VSD dependent on
- electron E
2. Cone size
What is the Surface dose, dmax of 6E
75%, 1.3cm
What is the Surface dose, dmax of 9E
80%, 2.0cm
What is the Surface dose, dmax of 12E
85%, 2.8cm
What is the Surface dose, dmax of 15E
90%, 2.8cm
What is the Surface dose, dmax of 18E
93%, 2.2cm - 3.2cm
