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PHYS4005: From Accelerators To Medical Imaging > Lecture 2: Linear Accelerators > Flashcards

Lecture 2: Linear Accelerators Flashcards

1
Q

When were cathode ray tubes invented?

A

1857

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2
Q

How were cathode ray tubes invented?

A

They were developed from Geissler tubes.

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3
Q

What is a cathode ray tube?

A

A tube of low pressure gas with electrodes at each end. When a voltage is applied to the electrode, electrons and ions generated by direct ionisation diffuse towards the electrodes, ionising more gas atoms and causing a current to flow. When electrons recombine with the ions, photons of light are emitted.

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4
Q

How were Crookes tubes invented?

A

They were developed from cathode ray tubes.

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5
Q

What are Crookes tubes?

A

Tubes consisting of a higher voltage cathode and an anode in a lower pressure tube (than the CRT). Ions collide with the cathode, releasing electrons which are accelerated to give a cathode ray. They can ionise the atoms in the glass of the tube which causes it to glow.

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6
Q

When did Wilhelmina Roentgen investigate the cathode ray tube?

A

1895

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7
Q

What did Wilhelmina Roentgen discover when investigating the cathode ray tube?

A

That he could use the cathode ray tube to create an x-ray image.

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8
Q

What is the purpose of an electrostatic accelerator?

A

To generate high voltages to accelerate charged particles.

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9
Q

Give two examples of electrostatic accelerators

A
  • Van de Graaff generator
  • Cockcroft-Walton generator
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10
Q

What is the maximum voltage that a Van de Graaff generator is capable of producing?

A

~ 15 MV

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11
Q

What is the maximum particle energy that a Van de Graaff generator can achieve?

A

~ 15 MeV

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12
Q

What is the maximum particle energy that a Crockcroft-Walton generator can achieve?

A

A few MeV

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13
Q

What electrical component is a Van de Graaff generator similar to?

A

A capacitor

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14
Q

What is the equation for capacitance?

A

V = voltage
Q = charge
C = capacitance

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15
Q

What is the equation for capacitance when voltage is changing?

A

V = voltage
t = time
I = current
C = capacitance

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16
Q

Give two problems with the use of high voltages in electrostatic accelerators

A
  • They are dangerous
  • They tend to arc to ground
17
Q

What are the two main types of linear accelerator?

A
  • Standing wave LINAC
  • Travelling wave LINAC
18
Q

Describe how standing wave LINACs work

A

Particles are accelerated from the negative end of one tube to the positive end of an adjacent tube. This works because the adjacent tubes have out-of-phase voltages and the voltages in each tube are reversed whilst the particles are in the tubes.

19
Q

Describe how the voltage changes for the tubes in a standing wave LINAC

A

Adjacent tubes are out of phase with one another.

20
Q

What are the 7 things that a standing wave LINAC needs to operate?

A
  • A voltage gap to pull the particle through the series of tubes
  • Tubes connected to an RF voltage ~ 200 MHz
  • Tubes that act as Faraday cages (constant velocity inside the tube)
  • Synchronised voltage sources for all tubes
  • Potentials that reverse whilst the particle is inside the tube.
  • A positive voltage in the tube that the particle is accelerating towards.
  • Increasing tube length of successive tubes.
21
Q

What is the equation for the time in a single tube for a standing wave LINAC?

A

t = time in a single tube
f = frequency
l_i = length of tube
v_i = velocity of particle in the tube

22
Q

Describe how travelling wave LINACs work

A

Electrons are accelerated towards a positive E-field (standing waves in cylindrical wave guides) and ‘ride’ the wave close to the speed of light.

23
Q

Why can’t electromagnetic waves in free space be used in a travelling wave LINAC?

A

They are transverse so don’t propagate in the correct direction and they travel at the speed of light so the electrons cannot keep up with the wave.

24
Q

What energy are particles injected into a standing wave LINAC at?

A

~ 1 MeV

25
Q

What energy are electrons injecting into a travelling wave LINAC at?

A

~ 100 keV

26
Q

What is the main medical use of LINACs

A

Radiotherapy

27
Q

How do medical LINACs work for radiotherapy?

A

1) An electrode gun (cathode ray tube with a hole in the anode) produces electrons.
2) A Klystron microwave electron accelerator (travelling wave LINAC) causes electrons to bunch and accelerates them to ~0.6c.
3) Standing wave RF accelerator brings the electrons to ~c.
4) 270 degree bending magnet focuses the electrons on the target plane.
5) Electrons are fired at the target to produce gamma rays.
6) A collimator focuses the rays.

28
Q

Why do RT beams rotate when being used for medical purposes?

A

They rotate so that healthy cells aren’t damaged as heavily as cancerous cells. A smaller dose can be delivered at a greater number of angles.

PHYS4005: From Accelerators To Medical Imaging (20 decks)

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