# Accelerators

To investigate the structure of the nucleus, and even that of sub-nuclear particles, one particle is usually fired at another at high energy – a method that has been compared with trying to find out what a watch contains by throwing two watches together and seeing what bits fly out!

A particle composed of smaller particles requires a certain amount of energy to split it apart and this can be provided by a high-energy projectile such as another particle. The acceleration of particles to the high energies needed in the collisions is a very important part of nuclear physics.

All accelerators are for charged particles, the kinetic energy of the particle being increased by an electrostatic field.

The electron gun in the cathode ray tube used in a television (not the LCD type) or a cathode ray oscilloscope is a simple type of accelerator – the electrons being accelerated through a potential difference of some 10 000 to 20 000 V and reaching speeds between 107 and 108 ms-1. (relativistic effects being ignored).

You can check this using the formula: ½ mv2 = eV where v is the maximum electron velocity, m the mass of the electron, V the potential difference and e the electron charge.

The Van de Graaff generator was 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.

###### (See: Van de Graaff generator for school version and safety recommendations)

A diagram of a Van de Graaff generator is shown in Figure 1.

A continuous belt made of insulating material runs on motor driven rollers past two sets of points A and B. The belt at A is given a negative charge either by friction or by a power pack. The charge is carried up to B and so onto the dome. The charge on the dome steadily builds up and so acts as a high voltage source from which a particle beam can be produced.

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS CD

© Keith Gibbs 2016