Edexcel International A Level Physics

Revision Notes

4.34 Particle Accelerators & Detectors

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Particle Accelerators & Detectors

Linear Accelerators

  • A linear accelerator (LINAC) is a type of particle accelerator that accelerates ions (charged particles) to very high speeds in straight lines
  • LINACs use electric fields within and between metallic tubes which act as oppositely charged electrodes
  • The high-energy ions produced are used in collider experiments
    • These experiments enable the internal structure of atoms and subatomic particles to be investigated

  • LINACs are comprised of a series of hollow cylindrical tubes 


Linacs accelerate ions through progressively longer tubes, connected to an alternating power supply. This ensures they are always accelerating from one tube to the next

  • An AC power supply is connected across each tube to ensure ions are always accelerated from one to the next 
    • The ions will be attracted to the midpoint of a tube
    • At this point, the AC supply will switch such that the ions are repelled to the exit, and attracted to the next tube
    • This process continues in a straight line all the way to the end of the accelerator

  • The frequency of the AC supply is fixed
    • This means the polarity (positive or negative charge) of each tube switches at a constant rate

  • Therefore, each tube must be built successively longer
    • This is because the ions are speeding up
    • Hence, this ensures ions spend the same amount of time under acceleration in each tube


  • A cyclotron is a type of particle accelerator that accelerates ions from a central entry point around a spiral path
  • They are used for medical research such as:
    • Producing medical isotopes (tracers)
    • Creating high-energy beams of radiation for radiotherapy

  • Cyclotrons are comprised of:
    • Two hollow semicircular electrodes called 'dees'
    • A uniform magnetic field applied perpendicular to the electrodes
    • An AC power supply applied across each dee, which creates an electric field in the gap between them

Cyclotron, downloadable AS & A Level Physics revision notes

A cyclotron uses magnetic fields and electric fields to accelerate charged particles, like protons. The magnetic fields keep protons in a circular path, and the electric field increases their speed

  • The process of accelerating an ion in a cyclotron is:
    • A source of charged particles is placed at the centre of the cyclotron and they are fired into one of the dees
    • The magnetic field in the electrode makes them follow a circular path, since it is perpendicular to their motion until they eventually leave the electrode
    • The potential difference applied between the electrode accelerates the ions across the gap to the next dee (since there is an electric field in the gap)
    • In the next dee, the ions continue moving in a circular path within the magnetic field
    • The potential difference is then reversed so the ions again accelerate across the gap
    • This process is repeated as the particles spiral outwards and eventually have a speed large enough to exit the cyclotron

  • The alternating potential difference is needed to accelerate the particles across the gap between opposite electrodes
    • Otherwise, the ions would only speed up in one direction

Particle Detectors

  • When charged particles move through any medium, such as a gas, they transfer energy to it
  • This is usually through the process of ionisation
    • High-energy ions transfer some of their energy to surrounding atoms, removing electrons
    • The ions and electrons produced are then accelerated by applied electric fields
    • Once these are discharged they form pulses of electric current

  • Each pulse of electricity is counted by electronic counters connected by electrodes
    • 'Counts' are then interpreted as detection of individual particles

  • Ionisation is the principle by which many particle detectors operate, such as in: 
    • Geiger-Muller tubes
    • Spark chambers
    • Gas and cloud chambers

  • The particles are sometimes deflected meaning they are also  scattered
    • This can cause multiple scattering of the particle in the material

Exam Tip

Make sure you can distinguish between the two types of particle accelerator: remember, LINACs only use electric fields (to accelerate ions in straight lines) whereas cyclotrons use both electric fields and magnetic fields. 

In particle detectors, you only need to describe the two key principles which allow scientists to detect particles: ionisation and deflection (by applied electric fields).

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