1. Beam Therapy Equipment
1 Linear Accelerator Basics

2. X-ray production Treatment beams produced by linear accelerators
Consists of an electron gun, RF source, waveguide, bending magnet and treatment head
Electrons emitted from the gun, accelerated down the waveguide using microwaves, re-directed with a magnet and hit a tungsten target to produce x-rays

4. Electron gun Made of thermionic material e.g. Thorium
Cathode – negative charge, electrons accelerated from it
Electrons emitted when heated – ‘boil’ off
Gun often gridded – has a positive grid in front to accelerate the electrons
Grid is pulsed so get pulses of electrons
Have pulses of electrons entering waveguide

5. Microwave production
Two devices invented in 1939 for WWII radar – Magnetron (GEC) and Klystron (Varian brothers)
Operated at frequency of 3 GHz – wavelength of 10cm
Required for high power radar systems

6. Magnetron – self oscillator
Cathode in centre of evacuated, lobed chamber
Magnetic field perpendicular
Electrons spiral out inducing resonant radio-field
Sampled and fed to waveguide

7. Klystron - amplifier Electron gun produces stream of electron
Pulsed by low energy microwaves
Pulsed electrons induce high energy microwaves in tuned cavity

8. Waveguide
Hollow, pressurised, rectangular tube – carries RF to linac waveguide
Circular, linac waveguide under vacuum
Made of copper – to conduct charge
Electric fields from microwaves produce electric currents produce new magnetic and electric fields that travel along pipe
Electrons enter waveguide accelerated by microwave radiation into bunches
Varian – Standing wave
Elekta – Travelling wave

9. Standing wave
Microwaves reflected from distal surface to produce a standing wave

10. Travelling Wave
Microwaves absorbed at distal surface – travelling wave

11. Beam control Linacs have focusing magnets and steering magnets
Electric currents through wires produce magnetic fields
These magnetic fields are used to move the electron beam (-ve charge)
They focus the electrons into thin beam
Also used to steer the beam vertically and horizontally in the waveguide for flatness

13. When electron beam exits accelerator it is bent (90°/270°) and strikes target producing x-rays
These get flattened, measured and shaped and exit treatment head

14. Bending magnet Only required if need to redirect electron beam
Doubly achromatic
Needs to be cooled
Change field strength and thus beam energy by adjusting current

15. Problems with 90° bending magnets