An Introduction To Particle Accelerators A-Level Physics

A Question Q. Where is the nearest particle accelerator to this classroom? A.

Accelerating with high voltages Particles can be accelerated to high speeds and thus high energy The energy is measured in electron-Volts 1 eV = 1.602x J An industrial sized Van de Graaff generator can accelerate electrons up to a few Mega electron-Volts

The Van de Graaff Generator This shows Robert Van de Graaff’s original high voltage generator at MIT in 1933Robert Van de Graaff’s original high voltage generator

Why do we need particle accelerators? If particles have large velocity, the wavelength decreases. So they can be used to study atomic spacing. High energy particles can be smashed into each other, allowing other particles to be studied!

How do accelerators work? Electric fields (creating electromagnetic waves) attract charged particles and speed them up. Magnetic fields can be used to change their direction.

Obtaining particles to accelerate Electrons: Heating a metal causes electrons to ‘boil off’ – thermionic emission. Protons: They can easily be obtained by ionizing hydrogen. Antiparticles: High energy particles are collided with solid materials and antiparticles are ultimately produced.

Accelerating Particles In a Linear Accelerator (Linac), a carefully selected frequency of alternating voltage is placed across a series of cylindrical electrodes to accelerate a particle. Linac Game

Accelerating Particles 2 The particles can also be considered to be ‘surfing’ the electromagnetic wave…

Accelerator Design There are two main types of accelerator experiment:  Fixed Target - e.g. Rutherford’s alpha scattering experiment.  Colliding Beams - can use much more energy!

Linac or Synchrotron?! Linear accelerators (Linacs) are used for fixed-target experiments, as injectors to circular accelerators, or as linear colliders. A Linac at the University of California

Linac or Synchrotron?! The largest Linac is at Stanford University, USA. It can accelerate particles up to 50 GeV

Linac or Synchrotron?! The particle beams from a circular accelerator (synchrotron) can be used for particle colliding experiments or extracted from the ring for fixed-target experiments. This is the European Synchrotron Radiation Facility (ESRF) in Grenoble, France Particle energy at Fermilab, USA:Fermilab Vaan de Graaf:H - ions to about 1 MeV Linac:H - ions to about 500 MeV Booster synchrotron:protons to about 10 GeV Main injector:protons to about 150 GeV Tevatron synchrotronprotons (and p - ) to 1 TeV

CERN Accelerators at CERN LINAC2+3 – Hadron Linacs PSB – Proton Synchrotron Booster PS – Proton Synchrotron SPS – Super Proton Synchrotron LHC – Large Hadron Collider Other things AD – Antiproton Decelerator CMS – Compact Muon Solenoid ALICE – A Large Ion Collider Experiment

What can they do? Linacs and Synchrotrons can be used together or alone:

Accelerating Particles 3 Heat energy provides the work function for thermionic emission. Electric fields accelerate particles:

Accelerating electrons PE lost = KE gained  qV= ½ mv 2 e.g. A thermionic diode has a p.d. of 5kV placed across its electrodes. Calculate the maximum velocity of the electrons. v = (2qV/m) ½ v = (2 x 1.6x x 5000 / 9.1x ) ½ v = 4.2x10 7 ms -1

Accelerating electrons The greater the acceleration of the electron, the greater its energy. KE = ½ mv 2 Q. What happens when velocity approaches the speed of light? A.Mass increases! (Velocity can never equal the speed of light). KE = ½ mv 2 still applies.

Bending Beams of Particles For a beam of particles to move through a circular path, they must each experience a centripetal force. Q. Does this force increase the energy of the particles? The centripetal force is provided by a magnetic field perpendicular to the direction of the particles. Demo: Bending a beam of electrons in a CRT monitor or Teltron tube.

The Cyclotron Cyclotrons accelerate particles in a circular path up to around 10MeV. They are relatively small and so can be used in hospitals to provide beams of (deuterons creating) neutrons for cancer therapy. ech/accelerators/index.ht ml