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.
In this article, 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 origins of the universe!
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.
Let's see how a particle accelerator works!
A Particle Accelerator
Did you know that you have a type of particle accelerator in your house right 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.
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 path 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.
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.
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).
Let's take a look inside a particle accelerator.
Inside a Particle Accelerator
All particle accelerators, whether linacs or circular, have the following basic parts:
- Particle source - provides the particles that will be accelerated
- Copper tube - the particle beam travels in a vacuum inside this tube
- Klystrons - microwave generators that make the waves on which the particles ride
- Electromagnets (conventional, superconducting) - keep the particles confined to a narrow beam while they are travelling in the vacuum, and also steer the beam when necessary
- Targets - what the accelerated particles collide with
- Detectors - devices that look at the pieces and radiation thrown out from the collision
- Vacuum systems - remove air and dust from the tube of the accelerator
- Cooling systems - remove the heat generated by the magnets
- Computer/electronic systems - control the operation of the accelerator and analyze the data from the experiments
- Shielding - protects the operators, technicians and public from the radiation generated by the experiments
- Monitoring systems - closed-circuit television and radiation detectors to see what happens inside the accelerator (for safety purposes)
- Electrical power system - provides electricity for the entire device
- Storage rings - store particle beams temporarily when not in use
In the next sections, we will examine these parts in detail, focusing on a linear accelerator like the one at SLAC.
Particle Source, Copper Tube and Klystrons
The particle source provides the particles that are to be accelerated. Particles can be electrons, protons, positrons (the first antimatter particle -- like an electron, but positively charged), ions, and nuclei of heavy atoms such as gold. At SLAC, an electron gun uses a laser to knock electrons off the surface of a semiconductor. The electrons then enter the accelerator portion of the linac.
At SLAC, positrons can be made by firing an electron beam at tungsten. In the collision, electron-positron pairs are made. The positrons can be accelerated by reversing the directions of the electric and magnetic fields within the accelerator.
The major structure of the particle accelerator is the copper tube. The copper tube has a strong vacuum inside through which the particles travel. The tubes are made of copper because copper conducts electricity and magnetism very well. At the SLAC linac, the copper tube is made of more than 80,000 copper cylinders brazed together for more than 2 miles (3.2 km)!
The copper tube is arranged to form a series of cells called cavities. The spacing of the cavities is matched to the wavelength of the microwaves. The spacing allows the electric and magnetic fields to repeat their pattern every three cavities. Electrons or positrons in the beam come through the cavities in small bunches. The arrival of each bunch is timed so that it gets a push from the electric field across the cavities.
Klystrons make microwaves, much like a kitchen microwave oven except that the klystrons' microwaves are about 1 million times more powerful. Klystrons produce microwaves by way of an electron gun. The electrons travel through the klystron in cavities, where their speed is regulated. As the electrons change speed in the klystron, they give off radiation in the form of microwaves. The microwaves are conducted through copper waveguides to the copper tube of the accelerator. Waveguides carry waves efficiently without losing intensity. The klystron and waveguides are kept under high vacuum to ease the flow of the waves.
Magnets, Targets and Detectors
Magnets, either conventional electromagnets or superconducting magnets, are placed along the accelerator tube at regular intervals. These magnets keep the particle beam confined and focused.
Imagine that the particle beam is like shot pellets fired from a shotgun shell. Typically, the pellets (electrons) tend to spread out. If the pellets are spread out, then they do not make many collisions within the narrow area of the target. However, if the pellets are confined by an external force (magnetism) to a narrow path, then they will make many collisions in the narrow target area. The more collisions, the more events that can be observed in any one experiment.
The magnets generate a field within their core. There is no magnetic force in the center where the electrons travel. If the electrons stray from the center, they will feel a magnetic push back into the middle. By arranging the magnets in a series of alternating poles, the electrons can remain confined down the length of the tube.
Targets vary with the type of experiment. Some targets can be thin sheets of metal foil. In some experiments, beams of different particles (electrons, positrons) collide with each other inside the detectors.
The detectors are one of the most important pieces of equipment in the accelerator. They see the particles and the radiation after the collision. Detectors come in many types, from bubble and cloud chambers to solid-state electronic detectors. A collider laboratory may have several types of detectors located at various parts of the accelerator. For example, a bubble chamber contains a liquid gas, such as liquid hydrogen. As the particles released from the collision pass through the chamber, they vaporize some of the liquid, leaving a bubble trail as shown below.
A cloud chamber detector has a saturated vapor inside the chamber. As an energetic particle passes through the vapor, the vapor is ionized, producing a trail much like the one made by a jet moving through a cloud (see "Why do those long white clouds form behind jets flying high overhead?" for details).
One detector at SLAC is the SLAC Large Detector (SLD). The SLD is a large, barrel-shaped, solid-state detector that stands more than six stories tall and weighs more than 4,000 tons!
SLD is a multi-layered detector. Each layer sees a different event:
- Vertex detector - detects position of tracks of particles
- Drift chamber - detects positions of charged particles at several points along their tracks. Curved tracks reveal the momentum of the particle (related to its mass and velocity).
- Cerenkov detector - sees radiation given off by rapidly moving particles and determines the particles' velocity
- Liquid argon calorimeter - stops most of the particles and measures their energy
- Warm iron calorimeter - detects muons (one of the subatomic particles)
- Magnetic coil - separates the two calorimeters
For details on the workings of each part, see SLAC Virtual Visitor Center: Detectors.
Vacuum and Cooling Systems
Vacuums must be kept in accelerators for two reasons:
- to prevent sparking caused by microwaves in air, which would damage waveguide and accelerator structures
- to prevent loss of energy that would occur if the beam collided with air molecules
A combination of rotary pumps and cold traps are used to maintain the low vacuum (one-millionth of an atmosphere). Rotary pumps work like fans to remove air. Cold traps use liquid gases (usually nitrogen) to cool the surface of the trap. Any air or dust molecule will be attracted to the cold surface and removed from the tube. Cold traps must be kept cold or else they will release the collected dust and air molecules.
The electric currents passing through the copper tubing in the accelerator produce vast amounts of heat. This heat must be removed for two reasons:
- to prevent the copper tubing from melting - this would destroy the structure
- to prevent the copper tubing from expanding - this would break the vacuum seals
The SLAC linac has tubes of water to cool the copper tubing of the accelerator structure and the magnets. The cooling water is circulated to cooling towers above ground to remove the heat. Any superconducting magnets get cooled with liquid nitrogen or liquid helium. Because the linac is underground, there is less chance of seasonal heating and cooling.
Atom Smasher Computers and Electronics
Computers and electronic systems do several tasks in the operation of a particle accelerator:
- control the particle source, klystrons and magnets used in accelerating the particles
- monitor the beam
- collect and record the data from the experiments
- analyze the data
- monitor the safety systems
- shut down the system in the event of an emergency
Particle accelerators have many computers that control the system. These computers generally have the highest-speed microprocessors available, with large amounts of computer memory and data storage. These computers are often networked together. In some cases, computer data analyses may be done by on- or off-site supercomputers.
Shielding, Monitors, Power and Storage
Because accelerated particles are forced to change speeds, change directions or hit targets, they lose energy. This energy is often in the form of ionizing radiation like x-rays or gamma rays. In addition to radiation, energized particles themselves present a hazard to human health. To prevent leakage of radiation while the accelerators are in operation, they are shielded. Accelerator structures are often located in concrete tunnels underground. The concrete and the earth shield the environment. Technicians are not in the tunnels when the accelerators are operating, and control rooms are shielded with concrete. In addition, workers wear radiation badges and are monitored constantly. Particle accelerators in the United States fall under the jurisdiction of the Nuclear Regulatory Commission, which licenses their use and regularly inspects them for safety. If the accelerator is affiliated with a university, the university's radiation safety office also participates in this process.
The tunnels are often equipped with closed circuit televisions to monitor the equipment and gauges within the accelerator. Radiation detectors are located throughout the accelerator structures to monitor for leakage in the shielding and protect the workers.
Electrical Power Systems
As you can guess from our description of the equipment, particle accelerators use a lot of electricity. In some places, this is supplied through the local power company. Some accelerators have their own electric generators on-site.
Because it takes so much effort to accelerate particles for an experiment, many accelerators have storage rings. Storage rings maintain a beam that has already been accelerated. For example, if you are colliding an electron beam with a positron beam, you may have to keep one beam in storage while you accelerate the other. A storage ring has the same components as the main accelerator but with fewer klystrons. The particles travel around the ring at the accelerated speed, needing only one or two klystrons to compensate for any lost energy as the beam changes directions.
Now that we've seen what's inside an accelerator, let's see what can we learn from these devices.
With all of this technology, what have we learned about the structure of matter? When physicists first began using accelerators in the 1950s and1960s, they discovered hundreds of particles smaller than the three well-known subatomic particles -- protons, neutrons and electrons. As bigger accelerators were built, ones that could provide higher energy beams, more particles were found. Most of these particles exist for only fractions (less than a billionth) of a second, and some particles combine to form more stable composite particles. Some particles are involved in the forces that hold the nucleus of the atom together, and some are not. In examining this complicated picture, a standard model of the atom has emerged.
According to this model, matter can be divided into the following building blocks:
- Fermions - subatomic particles that make known matter and antimatter matter leptons - elementary particles that do not participate in holding the nucleus together (examples - electron, neutrino) quarks - elementary particles that do participate in holding the nucleus together anti-matter - counter-particles of quarks and leptons (anti-quarks, anti-leptons)
- Hadrons - composite particles (examples - proton, neutron)
- Bosons - particles that carry forces (four known types)
In the next section, we'll delve into the details of each of these subatomic particles.
Fermions: Matter and Anti-matter
Fermions distinguish between matter (leptons and quarks) and anti-matter.
Leptons are extremely small particles (less than 10-15 m radius) that have no known size or internal structure. They have tiny masses, travel very fast and are best described by wave functions. The best known examples of leptons are the electron and the neutrino. The leptons have been classified into:
- electron-electron neutrino
- muon-muon neutrino
- tau-tau neutrino
Quarks are extremely small particles (less than 10-15 m radius) that participate in the strong nuclear force. Isolated (single) quarks have never been found, probably because they combine with each other so quickly. Quarks also have fractional electric charges. They are classified as follows:
- down (d) - charge = -1/3
- up (u) - charge = +2/3
- strange (s) - charge = -1/3
- charm (c) - charge = +2/3
- bottom (b) - charge = -1/3
- top (t) - charge = +2/3 (most massive, discovered in 1995)
As of now, quarks are thought to be the most fundamental particles.
Not much is known about antimatter. The first anti-matter particle discovered was the positron, which has a mass similar to an electron but with a positive charge. This area of particle physics is currently under investigation.
Hadrons, Bosons and the Big Bang
These particles are combinations of quarks. They have mass and reside in the nucleus. The two most common examples of hadrons are protons and neutrons, and each is a combination of three quarks:
- Proton = 2 up quarks + 1 down quark [+1 charge on proton = (+2/3) + (+2/3) + (-1/3)]
- Neutron = 2 down quarks + 1 up quark [0 charge on neutron = (-1/3) + (-1/3) + (+2/3)]
These particles are thought to be exchanged when forces occur. A force is defined as a push or pull. But that does not tell us what it really is or how it is mediated. Richard Feynman suggested that forces occur when two particles exchange a boson, or gauge particle. Think of two people on roller skates: If one person throws a ball and the other one catches it, they will be pushed in opposite directions. In this analogy, the skaters are the fundamental particles, the ball is the force carrier and the repulsion is the force. In the case of particles, we see the force, the effect, but not the exchange.
There are four known bosons:
- Gluon - mediates the strong force, but only operates over distances of 10-13 cm
- W and Z - mediate the weak force (1/10,000 strong force), but only operate over distances of 10-15 cm
- Photon - mediates electromagnetic force (1/137 strong force) and operates over an infinite distance
A fifth gauge particle (graviton) has been proposed, but has not yet been found. The graviton is thought to mediate gravity, which is 10-39 strong force and operates over an infinite distance.
Historically, James Clerk Maxwell unified electricity and magnetism in the19th century. As physicists have constructed more powerful accelerators with higher temperatures and energies, they have seen that certain forces come together, or unify. Particle-accelerator experiments have shown that the electromagnetic force and the weak force can be combined into the electroweak force. Many physicists believe that all of the forces stem from one force that existed long ago. Theories attempting to unite the forces are called unified theories or grand unified theories (GUT). It is hoped that GUTs will tell us what the universe may have been like in its beginnings. Because accelerator experiments have simulated what are thought to be the conditions that existed just fractions of a second after the Big Bang, they may provide evidence to support or refute various GUTs.
According to the Big Bang theory:
- Prior to the Big Bang, the universe was extremely hot and small and matter existed only as free quarks.
- Once the explosion happened: Rapid inflation occurred and the universe cooled. Quarks combined into hadrons. The forces separated. Matter (atoms) formed. Matter condensed into galaxies, stars, etc.
By making bigger and bigger particle accelerators, physicists can simulate the conditions that existed within 10-43 seconds of the Big Bang!
Future Directions in Particle Physics
Several questions still remain unresolved with respect to the standard model:
- Why are there three pairs of quarks when it appears that only one pair is needed to make matter?
- What gives particles (also atoms and matter) mass?
- Why is the top quark (which is 35 times bigger than the bottom quark) so massive compared to the others?
These are but a few questions that are being pursued in the world of particle physics.
For more information on particle accelerators and particle physics, check out the links on the next page.
Related HowStuffWorks Articles
More Great Links
Particle Physics Laboratories
- U.S. Department of Energy: Exploring Matter and Energy
- Fermi National Accelerator Laboratory (Fermilab)
- Lawrence Berkeley National Laboratory
- Brookhaven National Laboratory
- Stanford Linear Accelerator Center (SLAC)
- Argonne National Laboratory
- Oak Ridge National Laboratory
- Los Alamos National Laboratory
- Particle Accelerators Around the World
- American Institute of Physics Exhibit: A Look Inside the Atom
- Journey into the Atom - learn about the atom and particle physics
- The Particle Adventure
- PBS A Science Odyssey: Atom Builder - You Try It!
- The Subatomic Zoo
- ABCs of Nuclear Science
- Fermilab: The Science of Matter, Space and Time
- American Institute of Physics Exhibit: The Discovery of the Electron
- 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)