Detectors on high-energy accelerators

Type of physical science: Elementary particle (high-energy) physics

Field of study: Techniques

Advances in the knowledge of elementary particles have been made possible as particle accelerators increase in energy and as the particle detectors used at these accelerators become more and more sophisticated. The typical modern detector system fills more than 2,000 cubic meters of space and consists of a large array of individual detectors to measure the position, energy, momentum, and mass of up to the several hundred particles that can be produced by particle collisions in an accelerator.

Overview

Beginning in the late 1950's, there has been a surge in scientists' knowledge about the subatomic world. This rapid increase has come about because of remarkable developments on two fronts: larger, more energetic accelerators and sophisticated detectors. Many of the accelerators that are in operation, or contemplated to be in operation in the future, operate as colliding-beam machines. Such machines typically have two counterrotating beams of particles, which are constrained to move in nearly circular trajectories by magnets that bend the paths of particles. These circular trajectories can be up to 28 kilometers in circumference. At several points along the circular paths, the beams are brought into collision. At these intersection points, the interactions of the beam particles can produce up to one hundred subatomic particles. A system of particle detectors surrounding the intersection region detects these particles and measures their positions and energies and, in some cases, determines their masses. From this information, the details of the collision can be reconstructed. The energies of produced particles can vary from 1 billion electronvolts (1 gigaelectronvolt) to several hundred gigaelectronvolts. In contrast, electrons in a television tube are accelerated to an energy of about 1,000 electronvolts.

One of the simplest detector components is a scintillation counter, which consists of a piece of scintillating plastic, approximately 1 centimeter thick, attached to a photomultiplier tube.

Scintillating plastic is a clear plastic such as Lucite or polystyrene in which is embedded a fluorescent chemical. The entire assembly is wrapped in black tape to keep out stray light. When a charged particle passes through the plastic, it gives up part of its energy by ionization; that is, the charged particle rips away electrons from the atoms that it encounters along its path of travel.

Because of the fluorescent chemical in the plastic, some of the energy transferred from the particle to the plastic by ionization appears in the form of light. This tiny amount of light is detected by the photomultiplier tube, which converts the light into an electrical signal. A scintillation counter is a fast device; it can respond to the passage of a charged particle in a few nanoseconds (a nanosecond equals one-billionth of a second). The "time resolution" of a scintillation counter is said to be a few nanoseconds. The time resolution of a detector means that, if two particles cross the detector separated by a time interval greater than the time resolution, then the detector recognizes the particles as distinct. Otherwise, the detector cannot tell if one or more particles are being detected. Because the scintillation counter is such a fast device, detector systems contain hundreds of scintillation counters. They provide the first stage in particle detection because the activation of other detectors depends on whether the scintillation counters have responded. This speed is crucial in modern accelerators, in which collisions can take place at the rate of a million collisions per second.

Another device commonly employed in detector systems is the multiwire proportional chamber (MWPC). In order to understand how an MWPC works, it is useful to understand the operation of a much simpler, but related device: the Geiger counter. The Geiger counter was invented in 1910 in the laboratory of famous British physicist Ernest Rutherford. It consists of a metal tube with a thin wire running through the center. The wire comes out of one end of the tube through an insulator. A voltage of about 1,000 volts is maintained between the wire (at a positive potential) and the tube, which is filled with a suitable gas. When a charged particle passes through the tube, the ionization mechanism rips away electrons in the gas. These electrons are accelerated toward the wire at the positive potential and, in the process, the electrons strip away more electrons. An avalanche of electrons falls on the wire, creating a current. This current can be amplified to produce a "click," for example. Anyone who has seen old science-fiction films has seen Geiger counters in action.

The MWPC was invented by George Charpak in 1969. It consists of many parallel wires stretched across an insulating frame. The wires can be as close as 1 millimeter. This plane of wires, called the anode plane, is placed between two other planes, called cathode planes, made of foil, wire mesh, or wires. The space between the planes is filled with a suitable gas and the entire assembly is made gas-tight. The cathode planes and anode plane are maintained at a potential difference of a few thousand volts. When a charged particle passes through the gas of the MWPC, it ionizes the gas, which causes a small current on the anode wire closest to the point of passage of the charged particle. This current can then be amplified. The anode wires each act as individual detectors. The time resolution of an MWPC is a few hundred nanoseconds. The "spatial resolution" of an MWPC is about equal to the separation of the wires. The spatial resolution of a detector means that, if two particles cross a detector within the time resolution of the detector, then they are recognized as distinct particles only if they are separated in space by a distance greater than the spatial resolution. Clearly, one must be careful in quoting the spatial resolution of an MWPC. For example, if the wires are oriented vertically, then spatial resolution only makes sense for the horizontal separation of particles. Indeed, two particles can have a large vertical separation and still strike the MWPC near the same wire. MWPCs can have several hundred wires, which can be as long as 1 or 2 meters. The usual arrangement of MWPCs in a detector is to have a number of MWPC planes oriented at different directions. In this way, the three-dimensional coordinates of each charged particle passing through the planes can be determined.

Silicon microstrip detectors are used when very precise spatial measurements must be made. A silicon microstrip detector consists of strips of junctions of n-type and p-type semiconductors. This forms a p-n junction diode if a voltage of correct polarity is applied. When a charged particle passes through such a device, electron-hole pairs are generated and a signal is detected at the strip nearest the passage of the particle. Such devices are small but have excellent spatial resolutions of tens of microns (one micron equals one-thousandth of a millimeter). Like MWPCs, silicon microstrip detectors can be arranged so as to provide the three-dimensional coordinates of the passage of charged particles.

Detector systems are often immersed in a large volume of magnetic field, which is provided by an electromagnet. If arrays of position-measuring devices are placed in this magnetic field, then the particle trajectories can be determined and the momentum of particles inferred. For a given mass (m) and velocity (v), a particle has momentum p = mv. If the particle with a given charge (q) is moving in a direction perpendicular to a magnetic field of strength (B), then it moves in a circle of radius (R) where p = qBR.

Another device employed in detector systems is the calorimeter. A calorimeter is constructed by alternating layers of absorbing material with layers of scintillating plastic. The absorber material is usually iron or lead. Typical thicknesses of these layers is between 0.5 centimeter and 1 centimeter. When a particle impinges on this stack, it first encounters a layer of absorbing material. The particle is likely to interact in this layer and produce a number of particles which pass through the first layer of scintillating plastic. These particles pass through the next absorbing layer and produce more particles which pass through the next layer of scintillating plastic. The process continues, producing a "shower" of particles. Because the total energy of all the particles in the shower cannot exceed the energy of the particle initiating the shower, the shower eventually dies off. No more particles are created, and those in the shower eventually lose their energy in the absorbing material and are stopped. The charged particles in the shower produce light in the scintillator, which is collected and sent to a photomultiplier tube.

The output of the photomultiplier is, on average, proportional to the energy of the particle initiating the shower. A calorimeter measures time, position, and energy. The time resolution is a few hundred nanoseconds, and the spatial resolution is determined by the transverse dimensions of the layers of material and scintillator.

Cherenkov counters are used to determine the mass of particles. When a charged particle moves through a medium faster than the speed of light, then the particle emits light, called Cherenkov radiation, which can be measured using photomultipliers. The amount of light emitted depends on the velocity of the particle. If the momentum of the particle has been measured, then a measurement of the amount of Cherenkov light determines the mass of the particle.

Applications

There are dozens of large detector systems located throughout the world. Because they have many common features, it is possible to describe a generic detector. Most of the detector is immersed in a magnetic field supplied by an electromagnet with superconducting coils. The colliding beams are contained in an evacuated pipe to reduce unwanted interactions between the beam particles and atoms in the air. Surrounding the beam pipe is a silicon microstrip detector, which spatially distinguishes particles produced in the collision. As these particles move through the detector, they tend to separate, and this separation is enhanced by the presence of the magnetic field. Surrounding these detectors are wire chambers, which determine particle positions. Since points along the charged-particle trajectories are determined, the curvature can be ascertained and the momentum determined. Surrounding these detectors are Cherenkov light detectors. Charged particles, the momenta of which have been determined, emit Cherenkov light depending on the particle mass. Next, calorimeters detect charged particles and all neutral particles except for neutrinos, which pass through the calorimeters unscathed. (Neutrinos carry away energy, but none of this energy is transferred to the detectors so they escape "unseen.") The silicon microstrip detectors, the wire chambers, the Cherenkov detectors, the calorimeters, and the coil of the electromagnet are cylindrical in their arrangement and concentric with the beam pipe. Surrounding the coils of the electromagnet are large plates of iron and still other wire chambers. Interspersed throughout the detector system are scintillation counters that quickly determine if an interaction has taken place.

This array of detectors responds in a unique way to different types of particles. Only charged particles will be detected by the silicon microvertex detectors and wire chambers. If the velocity of a charged particle is large enough, then the particle will be detected by the Cherenkov counter. The photon, a quanta of light which carries no electrical charge, will deposit all of its energy in a characteristic way: It forms a shower of particles which is well contained in space.

Electrons (which are charged) deposit their total energy in the calorimeter in the same way as photons. Protons (charged), neutrons (neutral), and π-mesons (both charged and neutral) deposit all of their energy in the calorimeter in a way distinct from photons and electrons. They form showers which are much more spread out in space. A particle called the muon (a heavy electron) is charged. A muon passes through the calorimeter, the coil of the electromagnet, and the iron around the coil and is detected by the outermost wire chambers. Neutrinos are neutral and not detected; they show up as missing energy.

The detectors require electronics. The signals from each of thousands of phototubes (from scintillation counters, Cherenkov detectors, and calorimeters) and from each of the several hundred thousand wires (from MWPCs or equivalent devices) pass through huge bundles of cables which worm their way to buildings housing the electronics. The cables are long enough to "store" the signals (the signals travel down the cables with a speed near the speed of light) while "fast" electronics decide whether to keep the data from this collision. The "fast" electronics consist of modules which perform logic, that is, give an affirmative or negative answer depending on the precise combination of inputs. Other electronics convert analog signals to digital information. Finally, if the decision is to keep the information about the collision, then millions of bits of information are transferred to a computer and stored on a recording medium.

All the while, computers monitor every device looking for subtle changes in output which might signal deteriorating or broken electronics. Sophisticated computer displays show detailed three-dimensional representations of the scattering process, and sophisticated computer programs analyze the millions of interactions recorded on magnetic tape to extract the physics. This process can take hundreds of man-hours of work.

A typical large detector system takes years to build, involves hundreds of physicists, engineers, and technicians, and costs millions of dollars. Many of the detector components require extensive research and development work, which is usually carried out at universities collaborating on the construction of the detector system. One of the crowning achievements of this type of effort was the discovery, in 1983, of weak bosons, the carriers of the weak force, at the Centre Europeen de Recherche Nucleaire (CERN).

Context

Detectors of elementary particle physics are used not only in elementary particle physics research but also in such diverse applications as medicine, geophysics, and space sciences. In medicine, radioactive nuclides are used to determine the size of organs in the body by concentrating active nuclides in selected regions. These nuclides emit photons that are detected by a sodium iodide detector, which is a sodium iodide crystal attached to a phototube.

Sodium iodide detectors, which function as calorimeters, are often used to detect photons and electrons in large detector systems at accelerators. Three-dimensional "scans" of body organs can be obtained by having the emitted photons viewed by three sodium iodide detectors.

In geophysical applications, particle detectors are used for searching for uranium-rich minerals near the surface of the earth or for underground deposits of petroleum. To search for uranium near the surface, sodium iodide detector arrays are loaded aboard low-flying airplanes.

The radioactivity of the uranium-rich minerals emits photons at discrete energies with intensities which give a measure of the uranium concentrations. To study the makeup of geological structures underground, which may indicate the presence of petroleum, a detector is lowered into a bore hole. One technique is to lower a radioactive source shielded from a sodium iodide detector. The photons from the source scatter off of the surrounding geological structure into the sodium iodide detector. The amount of scattering depends on the geological makeup of the site.

The first U.S. satellite, Explorer 1, was launched in 1958. On board was a Geiger counter which established the existence of the so-called Van Allen belts, two belts of radiation surrounding the earth at distances of 5,000 kilometers and 20,000 kilometers from the earth's surface. These radiation belts are protons (inner belt) and electrons (outer belt) from the solar wind trapped by the earth's magnetic field. Particle detectors have also flown in other satellites and space probes and aboard the space shuttle.

Another application of particle detectors is in nonaccelerator-based physics. A dramatic example of such an experiment is the Irvine-Michigan-Brookhaven (IMB) detector, which looks for the decay of the proton. The detector is a volume of 7,000 tons of ultra-pure water in a container located 660 meters underground in the Morton salt mine below Lake Erie. This container is surrounded by 2,408 photomultipliers. The rock above the detector shields the detector from cosmic-ray background. One of the decay modes that is searched for is the breakup of the proton into an antielectron and a neutral π-meson. These decay products would cause showers of particles in the water, which in turn would create Cherenkov light to be detected by photomultipliers. The result of this experiment is that the lower limit on the lifetime is 3 x 10 to the power of 32 years, quite a remarkable achievement since the lifetime of the universe is estimated at 15 x 10⁹ (15 billion) years.

Principal terms

CALORIMETER: a device for measuring the energies of particles which consists of alternate layers of absorbing material and scintillating plastic

CHERENKOV RADIATION: the radiation, or light, emitted by a charged particle moving through a medium faster than the speed of light

ELECTROMAGNET: a coil of wire (usually copper but sometimes superconducting) carrying electrical current to establish a magnetic field

GEIGER COUNTER: a detector which consists of a wire (at a high voltage) within a metal tube filled with gas; the passage of a charged particle ionizes the gas and causes an electrical signal on the wire

IONIZATION: the process that occurs when a charged particle passes through matter, ripping away electrons from atoms along its path

MULTIWIRE PROPORTIONAL CHAMBER (MWPC): a device for measuring the position of particles, consisting of planes of wires in a gas-filled environment; its operation is similar to a Geiger counter

PARTICLE ACCELERATOR: a device for imparting energy to a charged particle, bringing it to high energies and velocities that are close to the speed of light

PHOTOMULTIPLIER: a device which produces an electrical current proportional to the intensity of incident light; sometimes called an "electric eye"

SCINTILLATION COUNTER: a light-tight assembly of a clear plastic which is attached to a photomultiplier and treated with a fluorescent chemical

SUBATOMIC PARTICLES: the particles that make up atoms, such as protons, neutrons, electrons, and other elementary particles including the photon (quantum of light), neutrino, π-meson, and K-meson

Bibliography

Calder, Nigel. THE KEY TO THE UNIVERSE. New York: Viking Press, 1977. Written by the former editor of THE NEW SCIENTIST, this book is a complement to and an elaboration of an NET-BBC TV coproduction of the same name.

Carrigan, Richard A., and W. Peter Trower. PARTICLES AND FORCES AT THE HEART OF MATTER. New York: W. H. Freeman, 1990. This book is a collection of SCIENTIFIC AMERICAN articles, many of which explicitly show how large detector systems have been used to make fundamental discoveries.

Close, Frank, Michael Marten, and Christine Sutton. THE PARTICLE EXPLOSION. New York: Oxford University Press, 1987. This book, which is very well written and superbly illustrated, provides many photographs of large detector systems. Explores the physics of the subatomic world and the complex machines that are used to study it in a clear manner. Close is respected for his research in this field. Intended for the layperson.

Kleinknecht, Konrad. DETECTORS FOR PARTICLE RADIATION. Cambridge, England: Cambridge University Press, 1986. This is a technical but readable treatise covering the basic operation of every type of particle detector. The last chapter deals with the application of particle detectors in medicine, geophysics, and space sciences.

Lederman, Leon, and David Schramm. FROM QUARKS TO THE COSMOS. New York: Scientific American Library, 1989. Lederman is an experimental physicist who won the 1988 Nobel Prize in Physics for the discovery of the muon neutrino. Both authors try to explain science for the general public. This book examines many of the experiments that were crucial to the understanding of the subatomic world. The book has great illustrations which clearly and simply show how a number of particle detectors operate.

Leo, W. R. TECHNIQUES FOR NUCLEAR AND PARTICLE PHYSICS EXPERIMENTS. New York: Springer-Verlag, 1987. This book is a textbook which covers all types of detectors and their applications. It is not as technical as the book by Kleinknecht, and it stresses the how-to approach. For example, there is a section, with detailed photographs, on the steps to take in assembling a scintillation counter.

Myers, Stephen, and Emilio Picasso. "The LEP Collider." SCIENTIFIC AMERICAN 263 (July, 1990): 54-61. This article is about the 28-kilometer electron-positron collider located in Europe. Includes photographs and discussions of the large detectors in use at this machine.

Radiation: Interactions with Matter

Storage Rings and Colliders