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
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?