Particle detector is a device that physicists use to observe subatomic particles, units of matter smaller than an atom. Most particle detectors are installed in large machines called particle accelerators. An accelerator speeds up a beam of subatomic particles to almost the speed of light. It then directs the beam into a stationary target or forces it to collide with a beam moving in the opposite direction. The resulting impacts produce other subatomic particles. Detectors near the impact points reveal details about the mass, energy, electric charge, and other properties of the particles.
Physicists have conducted experiments with particle accelerators and detectors since the early 1930’s. Scientists also use particle detectors to study cosmic rays, subatomic particles that originate in outer space.
How particle detectors work
All particle detectors work by recording transfers of energy. Subatomic particles transfer this energy to atoms that are part of the particle detector.
A particle that carries an electric charge transfers small amounts of its energy electrically. As the particle moves through the detector, its charge interacts with charges in atoms of the detector. The resulting changes in the atoms trace out the path of the particle.
Certain particles cannot transfer energy electrically. The neutrino, for example, cannot do so because it carries no electric charge; and the neutron has internal electric charges that cancel one another. Such particles do, however, interact nonelectrically with atoms in the detector. These interactions produce charged particles that then transfer energy electrically. By studying these secondary particles, physicists can determine properties of the uncharged particles.
Photographic materials and devices called cloud chambers, bubble chambers, and spark chambers detected particles in early accelerators, but are not used in modern machines. Instead, today’s accelerators use tracking chambers, sampling calorimeters, scintillators, and combined systems of detectors.
Types of particle detectors
Photographic films and plates
were among the earliest particle detectors. A particle passing through a photographic material “exposed” the material. Developing the film revealed the place where the particle passed through.
Cloud chambers
used an ion trail left by a subatomic particle passing through a gas. The particle created this trail by transferring energy to electrons in the gas. An electron is a negatively charged particle that orbits an atomic nucleus. When a particle transfers a certain amount of energy to an electron orbiting a nucleus, the electron leaves its orbit. The negative charges in an atom are usually balanced by an equal number of positive charges. So when an electron leaves its orbit, it leaves behind a positively charged atom–that is, a positive ion.
British physicist Charles T. R. Wilson invented the cloud chamber in 1911. In his invention, which became known as the Wilson cloud chamber, a container held a mixture of a gas and a vapor. Vapor condensed around the ions in the ion trail to form droplets. The droplets were visible as streaks that represented the particle paths, and were photographed.
Bubble chambers
caused a subatomic particle to produce a trail of small gas bubbles in a liquid. The liquid was first heated above its boiling point, but held under high pressure to prevent it from boiling. The pressure was then reduced rapidly so the liquid would boil at the slightest disturbance. Subatomic particles disturbed the liquid, causing gas bubbles to form along their paths. Donald A. Glaser, an American physicist, invented the bubble chamber in 1952. He won the 1960 Nobel Prize for physics for this invention.
Spark chambers
caused an electrical discharge, somewhat like a small lightning strike, along an ion trail produced by a particle. A spark chamber was a box containing a series of thin metal plates mounted parallel to one another. The chamber was filled with neon gas or another noble gas (a gas that does not normally react with other substances). The gas atoms that were ionized could conduct electric current, but the normal atoms could not.
To detect a particle, the chamber applied voltage to the plates. As a result, current in the form of sparks jumped from plate to plate along the particle path. Japanese physicists Shuji Fukui and Sigenori Miyamoto built the first practical spark chamber in 1959.
Tracking chambers
trace particle paths by producing an electric signal, rather than a visible trail. Some of these detectors use an electric field to gather ions generated by subatomic particles onto a wire, a strip, or a pad. The ions produce pulses of electric charge. Electronic circuits enlarge the pulses so that they can be measured and recorded as digital data on magnetic tape. Georges Charpak, a Polish-born French physicist, received the 1992 Nobel Prize for physics for his invention of the multiwire proportional chamber, the first modern tracking chamber.
Sampling calorimeters
collect ions produced by a subatomic particle as it passes through a gas or, more commonly, a liquid. The main use of a sampling calorimeter is to determine a particle’s energy of motion, rather than to determine its position extremely accurately–as a tracking chamber does. This detector is based on a relationship between energy of motion and ionization: The number of ions collected for a particle that stops within the detector is approximately proportional to the particle’s energy of motion.
Scintillators
produce light when particles transfer energy to them. The light, in turn, strikes devices called phototubes or photodiodes, which then produce electric signals.
Scientists often build particle detectors between or around the poles of an electromagnet. The magnet causes charged particles to move along curved paths. From these curves, scientists can determine the momentum of particles.
Combined systems.
Modern detectors are usually arranged in groups that work together. A combined system can measure properties of hundreds of subatomic particles at the same time. Physicists need such systems because modern accelerators commonly produce hundreds of particles in a single interaction. Generally, scientists can understand the interaction only by measuring a large number of these particles.
The number of interactions occurring in a combined system is often so large that only a small fraction of them can be recorded. The selection of interactions to record and the interpretation of the recorded data require tremendous amounts of computing.
A combined system may weigh thousands of tons and cost tens of millions of dollars to build. It may fill a large room and contain hundreds of thousands of individual wires, scintillators, and other components. It may take hundreds of physicists 10 years or more to design and build a large detector and to use it to gather and analyze data.
