Physics 404
November 9, 2017
Medical Linear Accelerators
Dr. Alanah Bergman, FCCPM
Department of Medical Physics
Vancouver Cancer Centre
604-877-6000 x672781
[email protected]
Great Resource for Radiation Therapy Physics
Particle Accelerators
•
a machine that uses EM fields to linearly propel charged
particles (e.g. e-) to near the speed of light along a fine beam
•
Often used in nuclear/particle physics research either as a
stand-alone system (e.g. Stanford Linear Accelerator -SLAC)
or as an “energy-boost” component preceding particle
injection into a cyclotron.
3km long SLAC
Particle Accelerators
•
comprised of a linear array of microwave cavities to which an
alternating (RF) high-energy EM field is applied.
•
Bunches of particles
experience + and – EM
fields timed such that they
ride an accelerating ‘wave’ from one end
of the waveguide to the
other.
•
> 1GeV for research
applications
Medical Linear Accelerators
•
Medical linear accelerators are modified from their original
research counterparts
(a 3 km long, 50 GeV accelerator (ie. SLAC) is not practical
in a typical downtown cancer clinic!)
•
Waveguides much shorter
(0.3 – 1 m)
•
e- max energy lower
(4 MeV – 20 MeV)
•
Usually mounted on
rotatating gantry
Medical Linear Accelerators
•
Capable of treating in “electron mode” or “photon mode”
•
Electron mode:
-
scattering foil introduced to create a wide beam of
electrons (up to 20 x 20 cm).
-
multiple energies available (e.g. 4, 6, 9, 12, 15 MeV)
•
Photon mode
:
-
introduce a metal (e.g. tungsten) target and “smash” e-
beam into it –> bremsstrahlung x-ray photons
-
usually 2 photon energies available (e.g. 6 & 15 MV)
Medical Linear Accelerator (Linac)
waveguide
bending magnet
Carousel with
flattening filters
/scattering foils
monitor ion
chamber
target
X-ray beam
Linacs…
Linac – no covers
1. Magnetron / Klystron
•
Source of microwave power to accelerate e-
•
This is done by amplifying introduced RF electromagnetic waves
•
Magnetron or Klystron is used
•
Both operate in a vacuum
•
Frequency ~ 3GHz (10
9
cycles/s)
!
“S band”
1
Magnetron in your Microwave!
2. RF Waveguide
•
Made of copper, brass or aluminum
•
Filled with pressurized gas (sulfur hexafluoride SF
6
)
!
prevents arcing
!
service personnel must take precautions
(5x heavier than air – may displace oxygen if working in a pit
!
asphyxiation!)
2
4. Electron Gun
•
e- produced by thermionic emission (heated filament cathode)
•
Injects pulses “bunches” of 1-80 keV e- into accelerator
•
Pulses are 2-4
μ
s long, ~ 300 pulses per second (pps)
4
You've reached the end of your free preview.
Want to read all 65 pages?
- Summer '19
- Electron, Radiation, physicist, electron beams, Medical Linear Accelerators, Electron Accelerating Waveguide
