Tuesday, June 10, 2008

Atom Smashers: What They Are & How They Work

Early in the 20th century, we discovered the structure of the atom. We found that the atom was made of smaller pieces called subatomic particles -- most notably the proton, neutron, and electron. However, experiments conducted in the second half of the 20th century with "atom smashers," or particle accelerators, revealed that the subatomic structure of the atom was much more complex. Particle accelerators can take a particle, such as an electron, speed it up to near the speed of light, collide it with an atom and thereby discover its internal parts.

Photo courtesy Brookhaven National Laboratory
End view of a collision of two gold beams in the Relativistic Heavy Ion Collider.

In this essay post, we will look at these amazing devices and how the results they obtain tell us about the fundamental structure of matter, the forces holding it together and the make-up of the universe!

Smashing Atoms

In the 1930s, scientists investigated cosmic rays. When these highly energetic particles (protons) from outer space hit atoms of lead (i.e. nuclei of the atoms), many smaller particles were sprayed out. These particles were not protons or neutrons, but were much smaller. Therefore, scientists concluded that the nucleus must be made of smaller, more elementary particles. The search began for these particles.

At that time, the only way to collide highly energetic particles with atoms was to go to a mountaintop where cosmic rays were more common, and conduct the experiments there. However, physicists soon built devices called particle accelerators, or atom smashers. In these devices, you accelerate particles to high speeds -- high kinetic energies -- and collide them with target atoms. The resulting pieces from the collision, as well as emitted radiation, are detected and analyzed. The information tells us about the particles that make up the atom and the forces that hold the atom together. A particle accelerator experiment has been described as determining the structure of a television by looking at the pieces after it has been dropped from the Empire State Building.

How A Particle Accelerator Works

Did you know that you have a type of particle accelerator in your house r ight now? In fact, you are probably reading this article with one! The cathode ray tube (CRT) of any TV or computer monitor is really a particle accelerator.

The CRT takes particles (electrons) from the cathode, speeds them up and changes their

direction using electromagnets in a vacuum and then smashes them into phosphor molecules on the screen. The collision results in a lighted spot, or pixel, on your TV or computer monitor.

A particle accelerator works the same way, except that they are much bigger, the particles move much faster (near the speed of light) and the collision results in more subatomic particles and various types of nuclear radiation. Particles are accelerated by electromagnetic waves inside the device, in much the same way as a surfer gets pushed along by the wave. The more energetic we can make the particles, the better we can see the structure of matter. It's like breaking the rack in a billiards game. When the cue ball (energized particle) speeds up, it receives more energy and so can better scatter the rack of balls (release more particles).

Particle accelerators come in two basic types:

  • Linear - Particles travel down a long, straight track and collide with the target.
  • Circular - Particles travel around in a circle until they collide with the target.

In linear accelerators, particles travel in a vacuum down a long, copper tube. The electrons ride waves made by wave generators called klystrons. Electromagnets keep the particles confined in a narrow beam. When the particle beam strikes a target at the end of the tunnel, various detectors record the events -- the subatomic particles and radiation released. These accelerators are huge, and are kept underground. An example of a linear accelerator is the linac at the Stanford Linear Accelerator Laboratory (SLAC) in California, which is about 1.8 miles (3 km) long.

Photo courtesy SLAC
Aerial view of the SLAC linear accelerator: The linac is underground and traced in white.

Circular accelerators do essentially the same jobs as linacs. However, instead of using a long linear track, they propel the particles around a circular track many times. At each pass, the magnetic field is strengthened so that the particle beam accelerates with each consecutive pass. When the particles are at their highest or desired energy, a target is placed in the pa t h of the beam, in or near the detectors. Circular accelerators were the first type of accelerator invented in 1929. In fact, the first cyclotron (shown below) was only 4 inches (10 cm) in diameter.

Photo courtesy Lawrence Berkeley National Laboratory
The first particle accelerator (cyclotron) developed by Ernest O. Lawrence in 1929

Lawrence's cyclotron used two D-shaped magnets (called Dee) separated by a small gap. The magnets produced a circular magnetic field. An oscillating voltage created an electric field across the gap to accelerate the particles (ions) each time around. As the particles moved faster, the radius of the their circular path became bigger until they hit the target on the outermost circle. Lawrence's cyclotron was effective, but could not reach the energies that modern circular accelerators do.

Photo courtesy SLAC
Schematic diagram of a cyclotron

Modern circular accelerators place klystrons and electromagnets around a circular copper tube to speed up particles. Many circular accelerators also have a short linac to accelerate the particles initially before entering the ring. An example of a modern circular accelerator is the Fermi National Accelerator Laboratory (Fermilab) in Illinois, which stretches almost 10 square miles (25.6 square km).

Photo courtesy Fermilab
Aerial view of the Fermi National Accelerator Laboratory (Fermilab)

Particle Physics Laboratories

  • U.S. Department of Energy: Exploring Matter and Energy
  • Fermi National Accelerator Laboratory (Fermilab)
  • CERN
  • Lawrence Berkeley National Laboratory
  • Brookhaven National Laboratory
  • Stanford Linear Accelerator Center (SLAC)
  • Argonne National Laboratory
  • Oak Ridge National Laboratory
  • Los Alamos National Laboratory
Further Reading:
  • The Physical Sciences: An Integrated Approach, by Robert M. Hazen and James S. Trefil
  • Atom: Journey Across the Subatomic Cosmos, by Isaac Asimov, D.F. Bach (Illustrator)
  • A Tour of the Subatomic Zoo: A Guide to Particle Physics, by Cindy Schwarz, Sheldon Glashow (Introduction)
  • Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher, by Richard Phillips Feynman, Paul Davies (Introduction), Robert B. Leighton (Editor)
  • A Brief History of Time, by Stephen Hawking
  • The Search For Superstrings, Symmetry, and The Theory Of Everything, by John R. Gribbin
  • The Second Creation: Makers of the Revolution in Twentieth-Century Physics, by Robert P. Crease, Charles C. Mann (Contributor), Timothy Ferris
  • The Quest for Unity: The Adventure of Physics, by Etienne Klein, Marc Lachieze-Rey, Axel Reisinger (Translator)

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