Linacs Flashcards
What is the photon target in an accelerator made out of?
E <15 MV High Z (tungsten or tantalum)
E >15 MV low Z (aluminum or copper)
What is the electron scattering foil made out of?
Dual scattering foil:
primary = high Z (tantalum)
secondary = low Z (aluminum)
What is the on-time for a pulsed linac?
6-7 microseconds
What is the deadtime for a pulsed linac?
1.5 ms
What are flattening filters made out of?
Often low Z, like aluminum
How much leakage does the primary collimator have?
~0.1%
How much leakage is there through the jaws?
2% by IEC spec, in practice <0.5%
How much leakage is there through MLCs?
5% per IEC spec, in practice intraleaf <1%, interleaf 1-2%
How efficient is photon production in a linac?
in the MV range, ~30-95%. In the kV range, only about 1%.
What is the typical resonating frequency of a linac accelerating waveguide?
3 GHz
How are wavelength and frequency related?
c = lambda* f
How does the Linac modulate its output?
The monitor chamber tracks output and feeds back to the gun heater or grid voltage to modulate output as needed
How does the linac modulate symmetry?
the monitor chamber compares the charge collected in the various sections of the chamber and modulates the steering magnets to control symmetry (servos)
How much can the 2 monitor chambers differ before the interlock is thrown?
5%
What is flatness?
measure of the difference between the maximum and minimum dose over the central 80% of the beam profile
What is symmetry?
measure of the maximum difference between points at equal distance from the central axis within the central 80% of the beam profile
Where (in a phantom) are flatness and symmetry measured?
10 cm for photons, dmax for electrons
Where would you find the exact equation used to calculate flatness and symmetry?
usually in your machine’s purchase agreement
What are some typical flatness magnitudes?
3-5%
What are some typical symmetry magnitudes?
0-5% (mostly 0-4%)
What was the SAD of most Co-60 units and why?
80 cm due to dose-rate limitations
What was a typical spot size for a Co-60 beam?
1-2 cm
How much of the usable beam does the flattening filter absorb?
50-90%
What is the SAD for helical tomotherapy?
85 cm
what is the standard msr for tomotherapy?
5x10 cm field
How does an EPID work?
Most common has:
1) copper plate (~1 mm) to attenuate beam, thereby producing electrons
2) a phosphor screen to convert electrons into scintillation photons
3) amorphous silicon panel to convert scintillation photons to electrical signal to be read out as an image
What is a thyratron?
high voltage switch (switches the high voltage on and off)
It is a tube filled with an insulating gas.
Why do linacs produce pulsed beams?
mainly to manage heat dissipation
The pulsed nature due to the wave acceleration leads to nanosecond-scale pulses. The pulses we care about are on micro/millisecond scale
What is the beam current for a photon beam?
100 mA/pulse
What is the beam current for an electron beam?
1 mA/pulse
What are some differences between flat and FFF beams?
FFF are:
- softer energy spectrum (no hardening in the FF)
- more constant E spectrum across field
- higher dose rate (don’t lose usable beam in FF)
- less electron contamination (most comes from FF)
- less head scatter
How does dmax for a FFF beam compare to a flattened beam?
about the same - softer beam pulls it shallower but reduced electrons from the head push it deeper
