Colliders

DOE-HDBK-1108-97

Radiological Training for Accelerator Facilities

Student's Guide

Colliders

Until 30 years ago, all accelerators were so-called fixed-target machines in which the speeding

particle beam was made to hit a stationary target of some chosen substance. But in the early

1960s, physicists had gained enough experience in accelerator technology to be able to build

colliders in which two carefully controlled beams are made to collide with each other at a chosen

point. Several colliders exist around the world today, and the technology for them is by now

well established.

Colliders are more demanding to build, but the effort pays off handsomely. For example, in a

fixed-target machine, most of the energy of the projectile particle is locked up after impact on

the target, in continued forward motion of the debris. In a collider, on the other hand, two

particles of equal energy coming together have no net motion, and collision makes all their

energy available for new reactions and the creation of new particles.

Most metals conduct electric current more readily as they are cooled. But a few special metals

and alloys lose all trace of electrical resistance at very low temperatures. At the very low

temperatures in superconductors, currents flow unhindered and will persist forever once started.

The enormous potential of this discovery, made by the Dutch physicist Kamerlingh-Onnes in

1911, is easy to see. In electrical devices, the barrier to higher efficiency is power loss.

Immersion in cryogenic liquid is the only way to achieve the extremely low temperatures needed

for superconductivity to occur, and for this reason it remained merely a laboratory curiosity until

recently.

The rewards of this new technology for high-energy physics were so great that an enormous

effort was made to realize superconductivity on a large scale. The effort paid off in 1983, when

protons first passed through liquid-helium-cooled magnets of the Fermilab Tevatron.

Superconducting magnets are not only more efficient than conventional ones but, with higher

currents, can generate stronger magnetic fields. This allows accelerators to achieve huge

energies using rings that are not impracticably large.