CA05 (2010) - Question C1

Many modern linear accelerators can generate electron treatment beams of a variety of different energies.
Briefly describe how the production of different energies is achieved.
(A detailed explanation of the physical principles is not expected).

Curriculum Reference: 3.1 - Generation of Electron Beams

This question is different to the 2009 version of C1, although along similar lines. I recommend asking your physicist about this one - the best answer I could find was 'There is a switch, which adjusts everything in the linac to produce electrons of the right energy', which to be honest is about all we need to know.

Production of different electron energies

All components of the linear accelerator are involved in the production of beams of different energy. This includes:

  • The electron gun
  • The accelerating waveguide (and RF generator)
  • The beam transport system
  • The scattering foil

Choice of electron beam energy is made on the computer system controlling the linear accelerator. Each energy has a preloaded set of variables for each component of the linac. A series of interlocks prevents treatment unless all the correct settings have been applied for the required beam energy.

  • The voltage and heat applied to the electron gun determine the initial velocity of the electrons
  • The RF supplied to the waveguide can influence the acceleration the electrons receive
  • The length of waveguide used for acceleration has a significant influence on electron energies
  • The beam transport system must be configured to 'bend' the electrons of a particular energy by applying the correct voltage through the electromagnets
  • Each beam energy has an individual scattering foil which must be in place to produce as useful beam

I can't think of any diagrams that would suit this question.


Back to CA05
Over to the 2009 version of question C1.