BBC Home

Explore the BBC

Front Page

Life | The Universe | Everything | Advanced Search
Front PageReadTalkContributeHelp!FeedbackWho is Online

Click here to complete your registration.

3. Everything / Maths, Science & Technology / Physics

Particle Accelerators

A Brief History of Bangs

In 1919, Rutherford splits the atom. The next year he realises that what was produced in his experiment was a fundamental particle, the proton. He may have worked it out sooner had it not been for that fact that it looked suspiciously like the hydrogen nucleus. It took, then, another eight years for the resonance linear accelerator to be created.

This device was used by Rolf Wideröe, a German physicist, to accelerate potassium and sodium to an energy of 710 keV to split the lithium atom. The reason why he did this is unclear but in the same year a Russian physicist George Gamow showed that an atom could be split using low energy ions. This began the race to build a particle accelerator.

An English man and an Irish man go into a physics lab and build the world's smallest peashooter. Ernest Walton and Douglas Cockcroft built the first particle accelerator just one year after it had been predicted. It then took only two years for two Americans to build a cyclotron, which had to be better because it was curved. These accelerators were mainly being used to create fission reactions, and nothing mind blowing was being discovered.

Then, small particles started being invented as appropriate excuses for atomic properties. The first to be predicted was the meson, this was put forward to explain nuclear forces. Then the muon, an electron-like particle, was found, and no one knew what it was so they said it was a meson. (In fact the muon is a very big electron-like particle and it decays after 2 millionths of a second into electrons and neutrinos.) There was then a brief period of time in which nuclear physicists became greatly in demand to do fireworks displays for foreign visitors. Faced with the choice of a lot of money or a blunt object, physicists dropped their research and did their duty for their country. Or the country they were in at the time and felt the safest in. After America showed how to throw a proper fireworks' display, the first synchrotrons were built. Then the world of particle physics went mad.

Types of Accelerators

Linear Accelerators

The linear accelerator is the simplest type of accelerator. Fundamentally it is a long line of coils (or drift tubes) which charged particles are accelerated through. However, there are two types of linear accelerator. One type of accelerator is the standing-wave linear accelerator; particles travel along a cylindrical vacuum tank through a series of drift tubes, separated by gaps. As the particles cross the gaps, electromagnetic waves, called standing waves, accelerate them. (Or, more simply put, as the particle passes through the drift tube, the current through it is swapped. If the current was kept it would pull the particle back towards the tube when it leaves. Changing the current repels the particle from the end of the tube.) The waves provide an electric field that speeds up the particles by acting on their electric charges.

This type of accelerator can only manage to accelerate particles to 200 MeV. (Ah, if you don't know what this is you should try and find out.) Physicists mainly use them as a primary accelerator that feeds into a synchrotron. In industry and medicine they are used as powerful X-ray machines.

The other type of linear accelerator is the travelling-wave linear accelerator. This speeds particles through a single long pipe by an electromagnetic wave that travels with the particle. This high-frequency wave is called a travelling wave. As long as the wave speed matches the particles' speed, the particles will continue to gain energy.

This type of accelerator can accelerate particles to 30 GeV, this is the Stanford Linear Collider, the longest accelerator in the world at 3.2km. The SLC is used to smash electrons and positrons into each other at 50 GeV to create uncharged weak bosons (the particle for the nuclear weak force).


The more advanced type of particle accelerator is the cyclotron. The idea behind these accelerators relies on the understanding of the effects of fields on charged particles. A cyclotron is made of two magnets and two D-shaped electrodes, which physicists like to call 'Dees'.

The particles are forced into a circular path by the magnetic field; the electrodes are supplied with an alternating current that attracts and repels the particle, thus accelerating the particle. This type of accelerator is much easier to make than a few miles of linear accelerator. However, it is not perfect. As the electrodes accelerate the particles they increase the radius of their path. This means that as the need to produce a faster moving particle the larger the radius of the cyclotron has to be. This is not really a problem for the electrodes but it is a huge problem for the magnets!


With most complicated problems it is often the simplest solution that works best. The linear accelerator is very simple, and does not require a huge magnet. The problem is it has to be very long. How could there be a simple way of making it shorter and more useful? Well, make it circular. The problem with this is that you have to force the electrons into a circular path using magnets. Unlike having a single magnet providing the force to push the particle into a circular path this system requires a series of magnets positioned so as to deflect its path into a circle.

This type of accelerator again suffers the same problem as the cyclotron, yet it's problems are more easily overcome. As the particle travels faster the mass increases, and so the force needed to pull it around in a circle increases. In the synchrotron this is done by increasing the strength of electromagnets used to turn the particle. As soon as the particle is accelerated it needs the force applied to it to increase. In this case the energy required to keep the particle in a circular path instantly becomes too much for any conventional system. This is the main reason that CERN is 27 kilometres long.

Rather worryingly the largest particle accelerator in the world is not the most powerful. CERN can produce particles of 50GeV, which is fairly impressive but not as impressive as the Fermi National Accelerator Laboratory (Fermilab). Fermilab is home to the Tevatron, a particle accelerator capable of producing 1TeV. This is mainly due to the fact that all the magnets used in the accelerator are super conductive, meaning no loss of energy and interesting magnetic properties! If this was not enough the accelerator is a storage ring collider accelerator. This means two sets of particles are rotate in opposite directions around the ring, then collides the two sets of particles. This effectively means a collision of at least 2TeV will occur when two particles collide.

Last Words

The science of particle accelerators and the results obtained from them are at the cutting edge of physics. The instruments used to detect particles are at least three stories high. The data they produce when particles collide is equivalent to 10,000 copies of the Encyclopaedia Britannica per second. This means mammoth computers that don't run Windows. The scientists analysing the data are so amazingly imaginative that they come up with names like 'weakon' and 'koan'. Most worryingly they have already worked out the properties of undiscovered particles.

Discuss this Entry  People have been talking about this Guide Entry. Here are the most recent Conversations:

The CERN particle accelerator game
(Last Posting: Oct 10, 2005)

A few remarks
(Last Posting: Jun 19, 2001)

Tourist opportunities
(Last Posting: Aug 4, 2000)

Ultimate truth?
(Last Posting: Feb 11, 2003)

(Last Posting: Oct 20, 2000)

(Last Posting: Aug 4, 2000)

Add your Opinion!

There are tens of thousands of h2g2 Guide Entries, written by our Researchers. If you want to be able to add your own opinions to the Guide, simply become a member as an h2g2 Researcher. Tell me More!

Entry Data
Entry ID: A305407 (Edited)

Written and Researched by:

Edited by:

Date: 20   June   2000

Text only
Like this page?
Send it to a friend

Referenced Guide Entries

Most of the content on this site is created by h2g2's Researchers, who are members of the public. The views expressed are theirs and unless specifically stated are not those of the BBC. The BBC is not responsible for the content of any external sites referenced. In the event that you consider anything on this page to be in breach of the site's House Rules, please click here to alert our Moderation Team. For any other comments, please start a Conversation below.

Front PageReadTalkContributeHelp!FeedbackWho is Online

Most of the content on h2g2 is created by h2g2's Researchers, who are members of the public. The views expressed are theirs and unless specifically stated are not those of the BBC. The BBC is not responsible for the content of any external sites referenced. In the event that you consider anything on this page to be in breach of the site's House Rules, please click here. For any other comments, please start a Conversation above.

About the BBC | Help | Terms of Use | Privacy & Cookies Policy