Block Diagram of the Linear Accelerator

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 asignal 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

• 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

• traveling wave ( old / low energies)
• standing wave (used in today's LINAC's / high energies)

TREATMENT HEAD:

• 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)

• 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

• 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:

• 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 cavitiesregulate the speed of the electrons sothat 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.