The Need for Large Accelerators

An Article Written Originally for Midlevel Teachers


In order to study small particles, a high energy beam of particles must be generated. The reason is that the higher the energy, the more finely penetrating and discriminating a particle probe can be and the smaller the structure that can be studied. Also, the more energy (mass) available, the more new or more massive particles can be created in a collision of the particle with a target particle.

Fermilab produces charged particle (proton) beams with billions of electron volts of energy to study the make up of particles in the tiny, dense nuclei of atoms. Fermilab's 1985 modification to the one-mile diameter main ring accelerator allow protons to reach 1,000 billion electron volts (GeV, G for giga meaning 109), or one trillion electron volts (TeV, T for teva meaning 1012). The modified accelerator is called the Tevatron. The most recent (1993) improvement is the addition of the Main Ring Injector. This will increase the intensity (number of collisions per second) by a factor of five and allow both fixed target and collider experiments to be run simultaneously.

Particles were first used to probe the inside of atoms in about 1910. Ernest Rutherford used naturally emitted alpha particles from a radioactive source to bombard thin gold foil. He found that most alpha particles passed through the foil undeflected, while a few bounced back at sharp angles apparently due to hitting tiny solid objects. This was the first experimental evidence that there was a small, heavy, positively charged core to the atom and that the rest of the atom was mostly empty space.

In the 1930s, 40s, and 50s the study of the nucleus (nuclear physics) grew and included the details of the patterns of radioactive decay of nuclei and the forces that hold the nucleus together. Particle physics, also known as high energy physics, developed as a branch of nuclear physics to investigate the structure of nuclear particles using high speed (high energy) particle probes.

The first circular particle accelerators were small instruments ranging in diameter from a few inches to a few feet. Two fundamental limitations on particle speed required that larger accelerators be built to create the higher energy particle-probes necessary to study nuclear particles.
Since forcing the charged particles to follow curves seems to be the source of problems in accelerating a particle, why not accelerate them in a straight line? This is done in the first stage accelerator, called the Linac, at Fermilab and on a larger scale at the Stanford Linear Accelerator Center in California. However, the advantage of the circular accelerator is that each time around the circle the particles can be given a new push, similar to the way a playground merry-go-round can be given many pushes by a person standing in one place. To gain the equivalent number of pushes, a linear accelerator would have to be incredibly long and expensive.

Each bunch of protons in the Fermi accelerator is pushed 50,000 times each second by passing through just one "pushing station" on each four-mile trip around the circle. The fully accelerated proton travel at more than 99.999% the speed of light and has more than 800 times its original rest mass. The distance the proton travels in one second is four miles times 50,000 which is 200,000 miles, or the equivalent of eight trips around the earth.