Lecture 2.pdf - Medical Linear Accelerators Physics 404 November 9 2017 Dr Alanah Bergman FCCPM Department of Medical Physics Vancouver Cancer Centre

Lecture 2.pdf - Medical Linear Accelerators Physics 404...

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Physics 404 November 9, 2017 Medical Linear Accelerators
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Dr. Alanah Bergman, FCCPM Department of Medical Physics Vancouver Cancer Centre 604-877-6000 x672781 [email protected]
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Great Resource for Radiation Therapy Physics
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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
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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
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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
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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)
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Medical Linear Accelerator (Linac) waveguide bending magnet Carousel with flattening filters /scattering foils monitor ion chamber target X-ray beam
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Linacs…
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Linac – no covers
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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
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Magnetron in your Microwave!
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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
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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
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  • Summer '19
  • Electron, Radiation, physicist, electron beams, Medical Linear Accelerators, Electron Accelerating Waveguide

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