The Linac

Question 1

Where are photons made?

Answer 1

From the target!

Everything that hits the target is an electron.

Question 2

What does the electron gun do?

Answer 2

It supplied and injects electrons to the waveguide

Question 3

What does the waveguide do?

Answer 3

Conducts microwaves

Question 4

How does the electron gun work?

Answer 4

• 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

Question 5

What is a diode gun?

Answer 5

• 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

Question 6

What is a triode gun?

Answer 6

• 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.

Question 7

What does the magnetron/klystron do?

Answer 7

• 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

Question 8

What does the waveguide do?

Answer 8

• ## A series of cavities with irises which aid the acceleration of electrons

Question 9

What is the potential problem with the electron pathway?

Answer 9

• Electrons would repel one another as they have the same charge

Question 10

How are electrons prevented from repelling in the linac

Answer 10

• Using focussing and steering coils

Question 11

How do focussing coils work?

Answer 11

• A magnetic current is placed AROUND the waveguide. (A solenoid)
• Adjusting the electric current can vary the degree of focussing

Question 12

How do steering coils work?

Answer 12

• 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

Question 13

How does bending work?

Answer 13

• Elekta bend in incremental degrees

- Varian bends a full 270 at once - straight onto the target

Question 14

How does bending work?

Answer 14

• 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.

Question 15

Why do we need a vacuum system?

Answer 15

• All compartments need to be uninterrupted by particles in the air

Question 16

How is the vacuum maintained?

Answer 16

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

Question 17

What is the cooling system for?

Answer 17

• X-ray production is often inefficient, heat is produced in multiple stages
• Too hot = no photons