8.4 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

Cyclotrons

• 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

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