# Accelerators

Physicists are trying to probe the structure of matter! To find what is in a particle they fire
one particle at another and detect the resulting particles. It's rather like trying to find out what is in a watch by throwing two watches at each other and seeing what bits fly out!

This requires energy and the acceleration of particles to the high energies needed in the collisions is a very important part of nuclear physics. The machines used to do this are called accelerators.

Accelerators give energy to charged particles by accelerating them in an electric field.

Many of us have at least one small-scale accelerator in our own homes – our televisions (as long as it is the 'old' cathode ray tube type and not a plasma screen or LCD). Electrons in the TV are accelerated using an electron gun through 10 000 to 20 000 V and reaching speeds between 10 million and 100 million m/s. (The third photograph.)

Two early accelerators are shown in the first two photographs – an electrostatic generator designed by Cockcroft and Walton and the Van de Graaff generator devised by the American scientist Robert Jamieson Van de Graaff in the early nineteen thirties and patented in 1935. His early machines were capable of generating voltages of 80 000V but this was later raised to over 5 million volts by improved versions of the machine! Most Van de Graaff generators that are used in schools give voltages of between 50 000 and 150 000V. Always take care when you use it.

### schoolphysics: Cyclotron animation

To see an animation of the operation of a cyclotron click on the animation link.

## High energy accelerators

To get the particles, whether protons or electrons to accelerate to higher and higher energies you need bigger and bigger machines. One of the largest in the world is the LHC or Large Hadron Collider at CERN in Geneva. This giant machine has an accelerating ring with a circumference of nearly 27 km and lies buried in a 3.8 m diameter tunnel that crosses the France - Switzerland border at the foot of the Jura Mountains. It can accelerate protons to speeds close to that of light. One proposed experiment is to direct two beams of protons so that they travel round the ring in opposite directions finally smashing together and producing a spray of sub-nuclear particles.

The energy involved is about that of a flying mosquito but as CERN's website says this energy is concentrated into a space about a million million times smaller than the mosquito!

One very important thing about CERN – it was here that scientists developed the world wide web (www) or what we now know as the Internet. If you are reading this on line it is because of CERN. You can see an aerial view of CERN in the following photograph – Geneva airport is in the foreground and the dotted ring shows the position of the huge accelerator.(Photo credit: CERN)

International cooperatipon

Accelerators of this size are so expensive to build that they are usually funded by many countries – the physicists 'booking' time for their own research group to use the facility. To use the machine at CERN effectively requires great organisation and cooperation between the scientists in Europe.

Similar international cooperation is involved in the JET fusion project outside Abingdon in Oxfordshire. This is a European centre for the study of nuclear fusion. (Photo credit: JET)

### schoolphysics: Synchrotron animation

To see an animation of the operation of a cyclotron click on the animation link.

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB