linear accelerator

Question 1

what is a linear accelerator

Answer 1

• used for EBRT
• single or dual energy
• energies range from 4mv to 20mv
• modifications allow for a range of x-ray and external beam therapies

Question 2

how does the linear accelerator work?

Answer 2

• electrons are accelerated in a straight line
• radio frequency waves are generated my a magnatron or a kylstron and pass through a wave guide, establishing an electric field
• electrons are accelerated towards a target for x-ray beam production

Question 3

pulse modulator

Answer 3

• supplies high voltage negative pulses
• 380-440V
• supplies the electron gun and the source of microwaves

Question 4

microwaves

Answer 4

• fr waves are generated by a magnatron or kylystron
• generated at a frequency of 3000 MHz
• passed from a microwave source to the accelerating wave guide by the feed wave guide
• pass through an isolator to prevent reflection back to magnatron or kylstron
• a quartz window seperates the feed wave guide from the evacuated accelerating wave guide

Question 5

electron gun

Answer 5

• cathode spiral of tungsten wire
• heated by a current
• produces electrons by thermionic emission
• high voltage used (40kv)
• gives the electrons an initial velocity of 40% of the speed of light
• electrons are generated in pulses and injected into a wave guide to be further accelerated

Question 6

accelerating wave guide

Answer 6

two types;
TRAVELLING WAVE GUIDE- simple in operation but only allows acceleration to low energies (6mv)
STANDING WAVE- more complex but allows acceleration to higher energies (10mv) more efficient acceleration allows wave guide to be reduced in length

Question 7

basic construction of the travelling wave guide

Answer 7

• circular in cross section
• evacuated hollow copper tube to allow electrons to be accelerated without collision with gas molecules
• tube allows transmission of microwaves
• at the end of the wave guide, microwaves are diverted and absorbed while electrons enter a field free area prior to striking the target

Question 8

speed of electrons vs. speed of microwaves

Answer 8

electrons enter the wave guide at 40% of the speed of light

microwaves travel at the speed of light

Question 9

slowing down microwaves

Answer 9

• microwaves are slowed down to 40% of the speed of light by a series of washer like structures known as irises
• at the entrance to the wave guide the irises are placed close together in bunching section
• the bunching section allows the electrons to align themselves with the microwaves ready to be accelerated
• electrons gain KE by being accelerated and also gain mass (e=mc2), therefore they become heavier and harder to accelerate so only achieve 98% of the speed of light

Question 10

focusing coils

Answer 10

• the electrons are focused by the focusing coils to stop them from colliding with the walls
• they produce magnetic fields which run parallel to the ais along the wave guide