Detectors

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DETECTORS

The apparatus of particle physics has evolved from table-top experiments performed by a small group of people into detectors weighing thousands of tons, equipped with millions of channels of electronics and powerful computing systems, and staffed by collaborations of hundreds of scientists and engineers. As each generation of experiments brings new insights into the fundamental particles of matter, physicists strive to design more capable detectors to investigate the questions raised by new knowledge. This article is designed to give an introduction to some of the techniques involved.

The Detector as Camera

The apparatus for a modern experiment can be thought of as a digital camera taking pictures of individual interactions of elementary particles. The important questions about this camera are as follows: how accurate is the image, how well does it resolve objects that are close to each other, how long does it take to form the picture, and how quickly can it take consecutive pictures? An "intelligent" camera, moreover, would only take a picture when there was something interesting to be seen. Depending on the purpose of the experiment, the apparatus design concentrates on some or all of these aspects.

This camera, however, is not taking a picture of an object by reflected light; it is imaging the interactions of the tiniest elements of the subatomic world. The interaction could be the decay of a kaon into two pions, the annihilation of a high-energy electron with a positron, the collision of a 980-GeV proton with a 980-GeV antiproton, or the interaction of a neutrino with a proton. The experimenter typically wants to know the trajectories and energies of the particles produced in the interaction and to identify the type of particles (electron, muon, pion, etc.) produced in the interaction.

Experiments can be classified as colliding beam experiments where particles in counterrotating beams collide with each other or as fixed-target experiments where particles—from an accelerator, from the Sun, or from the depths of the universe— strike some material target. The detector challenges can be quite similar, although the geometry of the solutions may be rather different.

The Physical Basis of Detection Techniques

Ionization

The basis of most detection techniques is the fact that an energetic charged particle ionizes the material through which it passes, leaving a trail of positive ions and free electrons. Ionization chambers and silicon detectors measure the liberated charge directly. In proportional chambers, the liberated electrons seed a multiplication process in a strong electric field to produce a signal. In a scintillation counter, the molecules of the scintillator emit light as the electrons return to their ground state, and the light signal is then converted into an electrical signal by a photomultiplier. In bubble chambers and cloud chambers, the positive ions serve as nucleation centers for the formation of bubbles in the super-heated liquid of the bubble chamber and for the formation of droplets in the supersaturated vapor of the cloud chamber. Charged particles also leave a trail in photographic emulsion.

Cerenkov and Transition Radiation

Cerenkov and transition radiation are two other phenomena exploited for particle detection. These processes produce free photons rather than electrons. Cerenkov radiation is emitted by a charged particle passing through a medium if the particle speed exceeds the speed of light in the medium; transition radiation is emitted by fast-moving particles as they cross the boundary between materials of different refractive index. In both cases, the radiation can be used as a sensitive indicator of a particle's speed and is often used in distinguishing different types of particles. How these basic phenomena are used in systems of detectors will be described in the following sections.

Tracking Detectors

Tracking detectors, as the name implies, are used to determine the paths of particles produced in the event. A particle's trajectory may be used to derive its momentum and to determine whether it emerged directly from the interaction or was born in the decay of some other particle produced in the event. The momentum is determined by placing the tracking device in a known magnetic field and measuring the track's curvature. Cloud chambers and photographic emulsion were the earliest tracking devices, but bubble chambers were the detectors that presented some of the wonders of the elementary particle world most directly.

Bubble chambers, however, have the disadvantage that they can only take one picture every few seconds. Most current experiments rely on electronic detectors that may have an exposure time of a microsecond or less and need no recovery time.

Wire Chambers

A standard device for tracking particles over a large area is the proportional wire chamber; this invention (for which Charpak received the Nobel Prize in Physics) and its derivatives have allowed a thousand-fold increase in the sensitivity of experiments over previous detectors. A proportional chamber contains a number (from tens to tens of thousands) of thin wires, typically 0.02 millimeters in diameter and spaced by a few millimeters, enclosed in a volume of an appropriate gas. The wires are maintained at a high positive voltage with respect to some cathode. A charged particle traversing the gas liberates electrons along its path, which then drift in the electric field to the closest wire. Near the wire, the electric field is so strong that the drift electrons gain enough energy to ionize the gas, and a multiplication occurs inducing a signal on the wire. Particles from the interactions pass by several sets of wires, and by recording which wires produce signals, one can infer the trajectories of the particles. In a drift chamber, the wires are spaced further apart, and the time of the signal on the wire is recorded. This time depends on how far the particle passed from the wire and allows its position to be determined to 200 microns (0.2 mm) or better. Wire chambers range in size from a few cubic centimeters to many cubic meters.

In a Time Projection Chamber, the wires, rather than being distributed throughout the volume as above, are placed on the end walls, and a large electric field is applied across the gas volume. The tracks then appear as "projections" on these walls (see Figure 1).

Silicon Strip and Pixel Detectors

While wire chambers track particles as they fly away from the interaction, physicists want to study what happens right at the interaction point, and silicon detectors have been developed to meet this need. These detectors are capable of making measurements with a precision of 10 microns (0.01 mm) and distinguishing tracks separated by a hundred microns. They are made from pieces of silicon, several centimeters on a side and typically 300 microns (0.3 mm) thick. Readout strips that run the length of the silicon are spaced by 50 microns (0.05 mm). When a particle passes through the silicon, it generates a signal on the closest strip. Arrays of these detectors are placed close to the interaction point to reconstruct the event vertex in great detail.

With such tiny spacing, it takes a large number of elements to cover a sensible area, and such detectors are presently only feasible where the location

FIGURE 1

of the interactions is known in advance, as in colliding beam experiments. Even then, the Babar detector at the Stanford Linear Accelerator Center (SLAC) has 150,000 channels, and the collider experiments at the Fermilab Tevatron in Batavia, Illinois, each have silicon systems with approximately 750,000 channels. Not to be deterred, physicists are exploiting new technologies in microelectronics to develop pixel detectors in which the individual element is not a strip 50 microns by several centimeters but instead is a rectangle typically 100 microns by 150 microns. Detectors with tens of millions of pixel elements are currently in the prototyping stage.

Scintillation Counters

The term "scintillation" was used at the turn of the twentieth century to describe the faint flashes of light visible under a microscope produced by alpha particles hitting a zinc-sulphide screen, and a scintillation "counter" was a scientist such as Ernest Rutherford looking through a microscope. A modern scintillation counter is made of a transparent material that emits light when a charged particles passes through it; the light is viewed by a photomultiplier that produces an electrical signal. Scintillation counters range in size from a few square centimeters to a square meter or more. Figure 2 shows a scintillator

FIGURE 2

wall containing approximately 700 scintillation counters. Scintillation counters do not usually provide precise spatial information, but they can yield accurate information on the time of an event and are often used to indicate the occurrence of an interesting event. The development of plastic scintillating fibers 1 millimeter in diameter and smaller and of solid-state photomultipliers with 80 percent quantum efficiency has led to the use of scintillating material for tracking detectors.

Particle Identification

Particle identification is intended to determine the types of particles produced in the event. Nondestructive identification techniques depend on measuring both the speed of the particle and its energy or momentum, thus deriving its mass. The direct determination of speed by measuring the flight time over a known path is only useful at low energies. Another technique is to measure the ionization produced by the particle per unit distance—this is also only useful at low energies. For medium and high energies, the more powerful current technique measures the angle of the Cerenkov light produced as the particle passes through some suitable radiator and combines this with a momentum measurement to determine the particle type.

Calorimeters

A calorimeter is a device used to measure the energy of a particle or a set of particles. For neutral (uncharged) particles like photons and neutrons, calorimetry is the only direct way of measuring their energies and trajectories. Despite its name, the technique does not usually involve measuring a temperature rise, although it does mean absorbing the energy of the incident particle(s). The quantity measured is typically the total amount of ionization or light produced, with the assumption that this is proportional to the incident energy.

Calorimeters can be divided into electromagnetic calorimeters, used to measure the energies of electrons, positrons, and photons, and hadron calorimeters, used to measure the energies of hadrons (pions, kaons, protons, neutrons, etc.). The distinction arises because electrons and photons are fully absorbed in 30 centimeters of material of high atomic number, whereas it takes 2 meters or more of steel to fully absorb the energy of a hadronic particle.

The sequence of interactions produced when a particle strikes a calorimeter is called a cascade or shower. Sampling calorimeters use plates of a dense passive material that initiate and absorb most of the cascade energy, alternating with detectors between the plates that sample the cascade. The sampling detectors may be scintillation counters, ionization chambers filled with liquid-argon, or wire-chambers. Fully active calorimeters are sensitive to all the charged particles in the cascade.

Electromagnetic calorimeters use materials containing elements with high atomic number. Any photon, electron, or positron striking such a material produces a cascade of photons, electrons, and positrons. In sampling electromagnetic calorimeters, the passive material is a metal such as lead, tungsten, or uranium. Fully active calorimeters are made from scintillators such as cesium iodide, lead tungstate, or sodium iodide; from lead glass in which the electrons produce Cerenkov light; or from liquid krypton used as an ionization medium. Water and oil are also used as a Cerenkov radiator at lower energies.

Unlike electromagnetic calorimeters, hadron calorimeters extend a few meters in length to ensure that the incident hadron interacts and that the ensuing cascade of hadronic particles is fully absorbed. Because of the amount of material needed to absorb this cascade, hadron calorimeters are all of the sampling style. Uranium, brass, iron, and lead have all been used as the passive material.

Muon Identification and Measurement

The system for identification and measurement of muons is the last detector a particle may encounter. Muons play an important role in the search for new phenomena, and the muon system is an important aspect of many experiments. Muons are distinguished by their ability to penetrate meters of matter, and the standard technique for muon identification is to intersperse tracking detectors between plates of magnetized iron. Only muons will penetrate the iron, and their deflection (due to the magnetic field in the iron) can be used to estimate their momentum. In high-energy collider experiments, the muon identifier is the outermost and often the most massive system in the experiment. In neutrino experiments exposed to a muon-neutrino beam, the functions of target, hadron calorimeter, and muon identification are often combined (see Figure 3). In both types of experiment, the muon system may weigh thousands of tons.

When the discovery of the top quark, the most massive elementary particle ever observed, was announced, the experiments reporting findings on it were described as "5,000-ton three-story marvels of

FIGURE 3

FIGURE 4

sophisticated circuitry and engineering with tens of thousands of electronic channels to scan and record the products of tens of thousands of collisions every second." Figure 4 shows the CDF apparatus being assembled for the second run of the Tevatron. The structure of the experiment collaborations, however, was equally remarkable. Participating scientists came from fifteen different countries, detector components were developed and fabricated on four continents, and the experimental data were analyzed collectively, with no regard for national origin. Just like the detectors, this is a characteristic feature of the large experiments in particle physics. They are true international collaborations, united in the quest to learn more about the fundamental nature of matter.