Generation of Electron Beams in a Linear Accelerator
Electrons are generated in a similar way to photons in a linear accelerator, with the major difference being:
- Lack of a tungsten target
- A scattering foil instead of a flattening filter
- An electron applicator or electron cone which provides added collimation of the beam
Production of Electrons
Electrons are produced in an electron gun. A hot cathode emits electrons, which are accelerated towards an anode, passing through an aperture to reach the accelerating waveguide. A negatively charged focussing electrode narrows the electrons into a fine beam which then passes through the aperture in the anode. A grid allows for synchronisation of electron generation to match the acceleration in the waveguide by preventing electrons from travelling through the gun.
Acceleration of Electrons
Upon entering the waveguide, the electrons are accelerated by a magnetic field generated by microwaves. These microwaves are produced in either a klystron or a magnetron and are supplied to the waveguide.
The length of waveguides capable of generating high megavoltage photon beams (> 6 MV) makes a straight beam treatment head unfeasible. High energy linacs use a beam transport system to deliver the electrons to the treatment head. This is accomplished using strong electromagnets which bend the beam through 270o. The three turns cause the electrons to initially diverge and then converge upon the scattering foil as a pencil beam.
Electrons are initially scattered by a scattering foil, a thin layer of high atomic number that scatters electrons in many directions. This converts the beam from a pencil beam to a usable wide beam.
The electrons pass through ionisation chambers which are important in calculating monitor units.
Electrons are not collimated by the secondary or tertiary collimators. This is due to the lateral scatter of electrons would cause a significant geometric penumbra at the target surface.
An electron cone is fitted to the treatment head, and is available in several preset sizes. This applicator has several collimators which attenuate any lateral scatter. It usually extends to just several centimetres from the surface. The most distal aperture may have an electron cutout fitted. This allows the field to be shaped appropriately, and is located in this position to prevent lateral scatter causing blurring of the beam edge.
Alteration of electron beam energy
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