Beam Therapy Equipment 1 Linear Accelerator Basics Xray

Beam Therapy Equipment 1 Linear Accelerator Basics

X-ray production n 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, redirected with a magnet and hit a tungsten target to produce x-rays

Electron gun n n n 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

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

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

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

Waveguide n n n n 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

Standing wave n Microwaves reflected from distal surface to produce a standing wave

Travelling Wave n Microwaves absorbed at distal surface – travelling wave

Beam control n n n 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

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

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

Problems with 90° bending magnets