The Linac Flashcards
1
Q
Where are photons made?
A
From the target!
Everything that hits the target is an electron.
2
Q
What does the electron gun do?
A
It supplied and injects electrons to the waveguide
3
Q
What does the waveguide do?
A
Conducts microwaves
4
Q
How does the electron gun work?
A
- Provides a cloud of electrons which are given an initial acceleration with an electric field.
- They are synchronised by microwaves.
- A heated filament releases thermionic emission.
- An elekta gun is diode type
5
Q
What is a diode gun?
A
- An electron gun which produces electrons at the same time as RF waves.
- The gun is IN the vacuum.
- Electrons are fired into the waveguide
- The gun is pulsed at the same time as the RF source
6
Q
What is a triode gun?
A
- Has three parts.
- Anode, cathode and GRID.
- Grid can be charged positively or negatively.
- When charged negatively, there is a build up - when change to +ve they are able to move.
- Triode, always producing electrons but stopping them entering.
7
Q
What does the magnetron/klystron do?
A
- Produce electromagnetic waves to release into the waveguide.
- It is not part of the vacuum so does not need to disrupt the vacuum when being replaced
8
Q
What does the waveguide do?
A
- ## A series of cavities with irises which aid the acceleration of electrons
9
Q
What is the potential problem with the electron pathway?
A
- Electrons would repel one another as they have the same charge
10
Q
How are electrons prevented from repelling in the linac
A
- Using focussing and steering coils
11
Q
How do focussing coils work?
A
- A magnetic current is placed AROUND the waveguide. (A solenoid)
- Adjusting the electric current can vary the degree of focussing
12
Q
How do steering coils work?
A
- A pair of coils at either side of the waveguide will steer the electrons
- Make sure that any drift is picked up and the centre of the target is hit
13
Q
How does bending work?
A
- Elekta bend in incremental degrees
- Varian bends a full 270 at once - straight onto the target
14
Q
How does bending work?
A
- Elekta bend in incremental degrees
- Varian bends a full 270 at once - straight onto the target
- Electrons will be deflected and photons will not.
- Energies need to be variable to direct different energy beams.
15
Q
Why do we need a vacuum system?
A
- All compartments need to be uninterrupted by particles in the air
16
Q
How is the vacuum maintained?
A
Two options:
- Seal the whole system, fairly secure - but if leaks the whole system needs to be replaced
- OR High energy vacuum ion pumps, necessary to maintain vacuum - difficult to detect leaks
17
Q
What is the cooling system for?
A
- X-ray production is often inefficient, heat is produced in multiple stages
- Too hot = no photons
