Radiological Training for Accelerator Facilities
at a total material cost of only about $100. The machine consists of a polished metal sphere and
a moveable belt. The function of the belt is to carry an electrical charge up to the sphere where it
is stored. This process can be continued until a very high potential is developed in the sphere.
Once the sphere of a Van de Graaff machine is charged to its maximum voltage, the protons or
other particles to be accelerated are ejected from an ion source in the high-voltage terminal into
the accelerating tube. They are then accelerated by the potential difference between the sphere
and the grounded end of the tube.
Many machines have since been built following the general principles of the first Van de Graaff
mode. There are two factors that can limit the maximum potential of the sphere in a Van de
Graaff machine. One is the leakage current across the insulators and through the air (or other
gas) surrounding the sphere; the other is gaseous breakdown. The leakage limit is reached when
the leakage current, which increases with the voltage, becomes equal to the current carried by the
belt. The breakdown limit is reached if a spark in the gas discharges the sphere.
By placing accelerators in series (accelerators placed one after another), it is possible to achieve
much higher particle energies than with just one accelerator.
In present-day research, the Van de Graaff generator serves two purposes. One is as a source of
particles for research in low-energy nuclear physics; the second is to inject particles into much
A state-of-the-art accelerator, 25 MeV, was built and operated at the Daresbury Laboratory in the
In 1932 in England, John D. Cockcroft and Ernest T.S. Walton constructed what is called a linear
accelerator using a high-voltage source to accelerate protons through 700,000 volts (700 keV). In
1932, they first used these particles to bombard lithium and split it into two helium nuclei.
Cockcroft and Walton were awarded the Nobel Prize in 1951.