Ion chamber - Electron Gun - Ion Vaccum (Linear Accelerators)

The fundamentals of linac operation

The modulator converts 3-phase input AC power to DC, which drives transformers and a thyratron-switched pulse forming network (PFN). The pulsed power drives a magnetron or klystron and the electron gun. A high peak power, mechanically tunable magnetron or klystron is pulsed at high pulse repetition frequency (PRF) and current. The magnetron or klystron generates pulsed RF power, which is coupled to the linac structure through an RF window, while the electron gun injects a stream of electrons down the beam center-line. The frequency-matched sinusoidal resonance of the tuned accelerator structure causes injected electrons to `bunch' and accelerate towards the tungsten target. As they near the velocity of light, the electrons gain in energy, and as they impinge on the target, a Bremsstrahlung shower of X-ray photons is produced. The shape and direction of the X-ray are controlled by an external beam collimator. The dose rate and integral dose are measured by ion-chamber dosimetry units. A linac control console provides circuits and readouts for control and monitoring of all elements, as well as a comprehensive system of safety interlocks.

ION Chamber

Ionisation chambers make use of the fact that radiation of sufficiently high energy, such as the xray and electron beams used in radiotherapy, can knock electrons off atoms and molecules, which is the process of ionisation. This creates ion pairs of opposite charge in what was an electrically neutral environment.

?A voltage is applied across the volume, with oppositely charged electrodes. The negative ions are attracted towards the positive electrode (anode) and the positive ions to the negative electrode (cathode), so the ion pair is split and the electrodes receive a tiny signal (current) for each pair created. The current per pair is very small, but it is still an order of magnitude easier to measure than the temperature rise in a calorimeter from the same radiation

The ion chamber incorporates two, independent, dose monitoring chambers, and a chamber with plates for monitoring changes in the flatness and symmetry of the useful beam. For more information on the ion chamber

The X-ray mode, the power in the electron beam is approximately constant, so any change in beam loading produces a corresponding change in energy. Thus, in X-ray mode, the gun emission is servo-controlled to maintain constant beam energy by using beam energy data from the ion chamber

GUN

The source of electrons is the electron gun in the RF system. Electrons emitted from its filament are focused into a fine beam by the electrostatic action of the gun focusing electrode. The electrons then pass through a hole in the anode which leads into the accelerating waveguide. Within the waveguide, the electron beam is influenced by the focusing action of a static axial magnetic field provided by focus coils that surround the waveguide over the first half of its length.

Ion Vacuum

This is a description of the vacuum pumps that evacuate the waveguide of the digital accelerator, the ion pumps which keep a constant pressure in the vacuum system of the digital accelerator. There are two ion pumps on the digital accelerator, which keep the pressure in the vacuum system ,One ion pump is installed at the gun end of the waveguide and the second ion pump is installed at the target end

three purposes for Ion Vaccum :

• To avoid oxidation of gun filament from burning out due to poor vacuum.
• To prevent collision of gas molecules with high-velocity electrons, being accelerated across the accelerator guide.
• To inhibit microwave arcing which causes damage to accelerator structures.

The vacuum system of Linac includes electron gun, accelerator waveguide, flight tube (comprises of target/electron window assembly), ion pumps, input and output mode transformers. The key vacuum component is the ion pumps which serve to maintain high vacuum by gas ionization, and measure vacuum pressure by means of ion current within the entire vacuum system.

In case of vacuum breakdown due to outgassing (release of trapped gas atoms), vacuum leak or ion pumps fault, vacuum control interlock will stop Linac irradiation.

References

• nshcs.hee.nhs.uk
• Beam Physics Manual - Cooling, Gas and Vacuum Systems Manual( Elekta Digital Linear Accelerator )
• https://aapm.onlinelibrary.wiley.com/doi/10.1118/1.3681967
• IPEM Report 81, Physical Aspects of Quality Control in Radiotherapy
• doi.org/10.1016/S0963-8695(98)00042-5
• https://www.hkamp.org/