Discovering Particles

Particle accelerators

The studies that can be performed using particles as they occur naturally are limited by two factors: particle energy and collision frequencies. Higher energies are needed to be able to produce and study particles with larger masses, and morefrequent collisions are needed to investigate interactions and decays that occur rarely, but can have important consequences. The solution is to use particle accelerators. These take advantage of the fact that charged particles can be accelerated to higher energies using electric fields, and are defl ected by magnetic fields. With an appropriate arrangement of magnets, beams of charged particles can be focused in much the same way as light can be focused using optical lenses.

Particle energies are usually measured in multiples of the electronvolt (eV), the energy gained by an electron when accelerated through an electric potential of 1 volt. For comparison, commonly used household batteries, when new, produce electric potentials of between 1.5 volts and 9 volts. Useful multiples include the megaelectronvolt (1 MeV = 1,000,000 eV), the gigaelectronvolt (1 GeV = 1,000,000,000 eV) and the terraelectronvolt (1 TeV = 1,000,000,000,000 eV). Although the electric potential through which an electron must pass to gain an energy of 1 TeV is enormous, the actual amount of energy is tiny – about enough to operate a 10-watt low-energy light bulb for 0.00000001602 seconds (16.02 nanoseconds).

There are two mains types of particle accelerator in use today: the linear accelerator, where particles pass once along a straight-line track, and the synchrotron. The latter uses synchronised electric and magnetic fields to accelerate particles around circular paths, and to keep them orbitting, at a fixed energy, until they’re made to interact. The design was suggested by Marcus Oliphant, Professor of Physics at the University of Birmingham, in 1943. The world’s first two proton synchrotrons both began operation in 1953: a 3.3 GeV machine at the Brookhaven National Laboratory, USA; and a 0.97 GeV machine at the University of Birmingham. They continued in use until 1968 and 1967 respectively.

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