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THERAPY MACHINES
BLOCK DIAGRAM OF THE LINEAR ACCELERATOR
Power supply - supplies power to the modulator
Modulator - is a pulse-forming network that supplies pulsed DC power to the
klystron/magnetron and sends a signal to the electron gun to fire electrons
Klystron/magnetron - converts pulsed DC power into microwave radiation that is
supplied to the accelerator tube
Electron gun - supplies the electrons to the accelerator tube on command from the
modulator
Wave-guide system - copper tubes that efficiently transport microwave energy from the
klystron/magnetron to the accelerator tube
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Transfers high frequency microwaves
More efficient than wire
Encloses a vacuum (Freon)
Accelerator tube: copper tube with washers placed 1/4 wavelength apart of the incoming
microwave wave frequency. (Cannot use a regular x-ray tube because of the high voltage
we use / we could not insulate at these high voltages)
2 types
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traveling wave ( old / low energies)
standing wave (used in today's LINAC's / high energies)
TREATMENT HEAD:
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contains the equipment to produce an electron beam (scattering foil) or a photon
beam (tungsten target)
also contains the necessary equipment needed to monitor and collimate the
radiation beam
Bending magnet - bends the electron beam as it emerges from the accelerator tube (90 or
270 degrees)
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90 degree causes larger penumbra because of electron spread
270 degree causes less penumbra because electron beam will become focused
Primary fixed collimators - determine the maximum extent of the radiation field (cannot
be moved)
Carousel - rotating disk that houses the x-ray beam flattening filters and the electron
beam scattering foil
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flattening filters - give a flat beam profile across the field of the patient
2 ionization chambers - placed at each side of the beam to maintain desired dose (if one
fails the other is a backup)
Adjustable collimators (secondary) - used to give appropriate field sizes for the
treatment fields
Magnetron:
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made from a solid copper disk with resonant cavities cut into it
at the center of the disk is a cathode and the outer rim acts as the anode
electrons spirals past the resonant cavities creating microwave radiation
(generates oscillating vibration as the electron passes by the resonant cavity)
KLYSTRON:
1. The electron gun produces a flow of electrons
2. The bunching cavities regulate the speed of the electrons so that they arrive in
bunches at the output cavity by the process of velocity modulation. (they travel
through the drift tube)
3. The bunches of electrons excite microwaves in the output cavity of the klystron.
4. The microwaves flow into the waveguide, which transports them to the
accelerator
5. The electrons are absorbed in the beam stop.
Velocity modulation - is the variation in electrons speeds through the drift tube that is
caused by microwaves that causes all of the electrons to arrive at the catcher at the
same time.