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Particle Detectors

Particle Detectors

Geiger counter

Scintillation detector

Solid state detectors

Neutron detectors

Cerenkov detectors

Cloud chambers and bubble chambers

Wire chambers

Large layered detectors

Resources

Particle detectors, also called radiation detectors, are instruments designed for the detection and measurement of subatomic particles such as those emitted by radioactive materials, produced by particle accelerators or observed in cosmic rays. Such particles include electrons, protons, neutrons, alpha particles, gamma rays, and numerous mesons and baryons. Most detectors utilize in some way the ionization produced when these particles interact with matter.

Geiger counter

The Geiger counter is one of the oldest and simplest of the many particle detectors. German nuclear physicist Hans Geiger (18821945) and German-American physicist Erwin Wilhelm Müller (19111977) developed the counter in the early part of the twentieth century, shortly after the discovery of radioactivity. A schematic diagram of a Geiger counter is shown in Figure 1.

A wire electrode runs along the center line of a cylinder having conducting walls. The tube is usually filled with a monatomic gas such as argon at a pressure of about 0.1 atmosphere. A high voltage, slightly less than that required to produce a discharge in the gas, is applied between the walls and the central electrode. A rapidly moving charged particle that

gets into the tube will ionize some of the gas molecules in the tube, triggering a discharge. The result of each ionizing event is an electrical pulse that can be amplified to activate ear phones or a loud speaker, making the counter useful in searches for radioactive minerals or in surveys to check for radioactive contamination. The counter provides very little information about the particles that trigger it because the signal from it is the same size no matter how it is triggered. However, one can learn quite a bit about the source of radiation by inserting various amounts of shielding between source and counter to see how the radiation is attenuated.

Scintillation detector

Scintillation counters are made from materials that emit light when charged particles move through them. To detect these events and to gain information about the radiation, some means of detecting the light must be used. One of the first scintillation detectors was a glass screen coated with zinc sulfide. This sort of detector was used by New Zealand-British physicist Ernest Rutherford (18711937) in the early versions of his classic experiment in which he discovered the nucleus of the atom by scattering alpha particles from heavy atoms such as gold. The scattered alpha particles hit the scintillating screen. Experimenters in a darkened room using only the human eye observed the small flashes that were produced.

The modern scintillation counter usually uses what is called a photo multiplier tube to detect the light. Light incident on the photocathode of such a tube is converted into an electrical signal and amplified millions of times after which it can be sent to appropriate counters. Physicists working at particle accelerators often use transparent plastic materials like Lucite® or plexiglass to which are added materials to make them scintillate. These plastic scintillators can be cut to convenient shapes, mounted on a photomultipler tube and placed in particle beams to provide a very fast signal when charged particles pass through them.

A very useful scintillation detector, particularly for the measurement of gamma rays, utilizes a transparent crystal of NaI (sodium iodide) mounted on a photomultiplier tube. These crystals are particularly useful because charged particles produce in them an amount of light directly proportional to their energy over a wide range. A schematic diagram of a gamma ray scintillation spectrometer is shown in Figure 2.

Gamma rays have no charge and, thus, no detector is sensitive to them directly. Fortunately, gamma rays interact with matter and produce charged particles usually electrons. For the measurement of gamma ray energies, the two most important interactions are the photoelectric effect and the Compton effect. These two processes can combine to produce energetic electrons in the crystal, which scintillates to produce an amount of light directly proportional to the gamma ray energy. These light pulses are converted to electrical pulses in the photomultiplier tube. These are amplified and sent to a pulse height analyze, which sorts out the pulses and displays a pulse height spectrum. A particular gamma ray shows up as a fairly sharp peak in this pulse height distribution.

Solid state detectors

Similar results with much improved energy resolution, the sharpness of the peaks in the pulse height distribution, can be obtained using solid state detectors made from semiconducting materials such as silicon or germanium. When properly constructed, the electrical charges released in the material by the passage of charged particles can be collected directly producing a short electrical pulse that can be amplified and analyzed. Germanium detectors made for use with gamma rays can have peaks in the pulse height distribution almost 100 times narrower than the peaks from a sodium iodide detector. To obtain this improved resolution these detectors must be cooled to the temperature of liquid nitrogen: 77K (-320.8°F; -196°C).

Smaller solid state detectors, usually made from silicon, are also used for measuring the energy of alpha particles, beta rays (electrons) from radioactive materials and x rays.

Neutron detectors

Since neutrons are uncharged, their detection must depend on an interaction with matter that produces energetic charged particles. There are several nuclear reactions initiated by neutrons that result in charged particles. One of the most useful for slow neutrons is the reaction in which a neutron is incident on a boron nucleus. This reaction produces a lithium nucleus and an alpha particle, both of which are rapidly moving. Note that it is the boron isotope of mass 10, with a natural abundance of about 20%, that is required for this reaction and that the alpha particle is simply the nucleus of the helium atom. The boron is usually incorporated in the gas molecule BF3(boron trifluoride) that can be used as the gas in a proportional counter, which is much like a Geiger counter. The difference is simply that the voltages used are lower so that the discharge does not spread disruptively along the entire central electrode with the result that the electrical signal coming from the tube is proportional to the number of ions produced. The signals are much smaller than from a Geiger tube and require more amplification but the signal produced by the lithium nucleus and alpha particle, both of which are heavily ionizing, is relatively large and easily distinguishable. For fast neutrons, the probability of this boron reaction becomes very low so that other methods are required. A useful technique is to use a proportional counter filled with hydrogen. Fast neutrons colliding with the protons in hydrogen produce energetic protons that produce a signal from the counter.

Cerenkov detectors

When a charged particle moves through a transparent material with a velocity v, greater than the

speed of light c in that material, it radiates light in the forward direction at an angle whose cosine is equal to c/vn, where n is the index of refraction of the material. This light is called Cerenkov radiation and can be detected with photomultiplier tubes, as was the case with scintillation detectors (Figure 3).

It is named after Russian physicist Soviet physicist Pavel Alekseyevich Cerenkov (19041990) who discovered it in 1934. The special theory of relativity limits particle velocities to values less than c, the speed of light in a vacuum. Cerenkov detectors can be of two types. A threshold detector merely detects the fact that light is emitted and indicates that the velocity of the particle passing through it is greater than c/n. Other more complicated detectors can actually determine the velocity v by measuring the angle at which light is emitted.

Cloud chambers and bubble chambers

A cloud chamber utilizes an enclosed volume of clean air saturated with water vapor. If this volume of air is enclosed in a cylinder with a piston, and the volume is suddenly expanded, the temperature of the air falls, causing the mixture to become supersaturated. If a charged particle passes through the volume at this time the vapor tends to condense on the ions produced, leaving a trail of water droplets on the path of the charged particle. With proper illumination and timing, these trails can be photographed. If a magnetic field is applied, the radius of curvature of these tracks can be measured. This information, combined with the density of droplets along the trail, can be used to measure the energy of the particle. The cloud chamber was first used by Scottish physicist Charles Thomson Rees (C.T.R.) Wilson (18691959) around the beginning of the twentieth century, and was useful in the early days of nuclear physics. However, it suffered from several disadvantages such as the long time required to recycle and the low density of air. In 1932, American experimental physicist Carl David Anderson (19051991) discovered the positron, the antiparticle of the electron while using a cloud chamber to observe cosmic rays.

A rather similar device called the bubble chamber was developed using liquids rather than a gas. Liquefied gases such as hydrogen, xenon, and helium have been used. Pressure is applied to the liquid to keep it a liquid above its normal boiling point at atmospheric pressure. If the pressure is suddenly reduced, the liquid is superheated but will not boil spontaneously, at least for a short time. In order to boil, the liquid must have small irregularities on which bubbles of vapor form and they can be provided in the bubble chamber by the ions left by charged particles passing through the chamber. Thus, tiny bubbles form along the tracks of particles passing through the chamber just after the pressure has been reduced. The bubbles grow very quickly but if the tracks are photographed at just the right time after expansion, they are revealed as a thin trail of tiny bubbles.

Bubble chambers work very well with particle accelerators that are pulsed. The expansion of the chamber can be timed so that particles from the accelerator pass through just after the chamber is expanded. As with the cloud chamber, application of a magnetic field permits measurement of the curvature of the tracks and when this information is combined with the density of bubbles along the track the energy, momentum, charge (sign), and mass of the particle can be determined. The bubble chamber was invented in 1953 by American physicist and neurobiologist Donald Arthur Glaser (1926) who used a small glass device containing about 30 cubic centimeters of diethyl ether. The use and size of bubble chambers grew during the following decades culminating in the discovery of the omega minus particle in the 80 in (203 cm) bubble chamber at Brookhaven National Laboratory in 1964; and the construction of the 3168 gal (12,000 l) Gargamelle chamber at the CERN (European Organization for Nuclear Research) laboratory in Geneva, Switzerland, in the early 1970s. In recognition of the great importance of this device to particle physics research, Glaser was awarded the Nobel Prize for physics in 1960.

Wire chambers

In many nuclear and particle physics experiments, beam lines are constructed along which secondary particles of interest, produced by an accelerator, are maintained in a beam by a series of focusing and

KEY TERMS

Anti-proton The anti-particle of the proton. Identical to the proton except that its charge is opposite in sign.

Gamma ray Energetic electromagnetic radiation emitted by radioactive nuclei, produced by particle accelerators and present also in cosmic rays.

Mesons and baryons Sub-atomic particles, usually with very short lifetimes, believed to be composed of quarks in various combinations.

Omega minus particle A short lived baryon believed to be made up of three quarks called strange quarks.

Photomultiplier tube An electronic tube, sensitive to very small amounts of light. The tube converts a light signal into an electrical signal of useful size.

Positron The anti-particle of the electron. Identical to the electron except that its charge is opposite in sign.

Quarks Believed to be the most fundamental units of protons and neutrons.

bending magnets. Wire chambers are used along these beam lines to actually track individual particles as they move along the beam line. The chambers are similar in a general way to the Geiger counter since they are gas counters. Instead of one wire, the chambers have many parallel wires spaced at distances of a few millimeters. The position of charged particles passing through the chamber can be measured with uncertainties even less than the wire spacing, using fast timing circuits. These chamber measurements facilitate identification of the particle and the measurement of its momentum.

Large layered detectors

The ultimate in particle detectors are probably those being used and constructed at large national and international laboratories such as Fermilab (Fermi National Accelerator Laboratory) in Batavia, Illinois, and CERN in Geneva, Switzerland. At these locations, colliding beam accelerators have been built that produce collisions of fundamental particles, such as electrons and positrons at CERN, and protons and anti-protons at Fermilab. At various points around these large circular accelerators, the counter rotating beams cross, and head-on collisions can take place making large amounts of energy available for the production of other particles. Huge detectors costing millions of dollars and requiring hundreds of physicists to run them are constructed surrounding these collision points.

At Fermilab, two of these large devices, one called CDF (Collider Detector) and the other DZero (D0 Experiment), have recently reconstructed events, produced in these collisions, which provide strong evidence for the existence of the long sought top quark. To do this, the detectors are designed to detect as many of the millions of particles produced in these collisions as possible. At DZero, about 400,000 proton-anti-proton collisions occur per second. The detectors, weighing thousands of tons, are constructed in layers and surround the collision points. They utilize most of the detection techniques discussed above including scintillators, solid-state detectors and devices similar to wire chambers that provide much improved performance. These are called silicon micro-strip detectors. They are made up of closely spaced strips of silicon detectors that give very fast position measurements of particles accurate to about 0.01 mm. The thousands of individual detectors and detector systems are connected to computers which help select the very special events that might involve the top quark from the millions that do not.

Resources

BOOKS

Das, Ashok, and Thomas Ferbel. Introduction to Nuclear and Particle Physics. River Edge, NJ: World Scientific, 2004.

Folan, Lorcan M. Modern Physics and Technology for Undergraduates. River Edge, NJ: World Scientific, 2003.

Green, Dan. The Physics of Particle Detectors. Cambridge, UK, and New York: Cambridge University Press, 2000.

Serway, Raymond, Jerry S. Faughn, and Clement J. Moses. College Physics. 6th ed. Pacific Grove, CA: Brooks/Cole, 2002.

Taylor, John R., Chris D. Zafirates, and Michael A. Dubson. Modern Physics for Scientists and Engineers. Pearson Prentice Hall, 2004.

Young, Hugh D. Sears and Zemanskys University Physics. San Francisco, CA: Pearson Addison Wesley, 2004.

PERIODICALS

Litke, Alan M., and Andreas S. Schwarz. The Silicon Microstrip Detector. Scientific American (May, 1995): 76.

Robert L. Stearns

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Radiation Detectors

Radiation Detectors

Optical detectors

Electrical detectors

Resources

Radiation detectors are devices that sense information about ionizing radiation. Although light and radio waves are technically forms of electromagnetic radiation, devices that detect them are not considered radiation detectors.

Though the term radiation detectors may bring to mind images of nuclear power plants, these devices have found homes in such fields as medicine, geology, physics, and biology. The term radiation refers to high-energy electromagnetic waves or particles given off by radioactive matter or other sources. Most commonly, radiation takes the form of alpha particles (fast helium nuclei), beta particles (fast electrons), neutrons, gamma rays, and x rays. Some of these are more easily detected than others, but all are invisible to the human eye. Since people cannot sense radiation, they need mechanical devices to observe and understand it.

People are always subjected to a certain amount of radiation because Earth contains radioactive minerals and cosmic rays bombard Earth from space. These omnipresent sources of radiation are called background radiation, and all radiation detectors have to cope with it, which they often do by shielding it out. Some detector applications subtract off the background signals, leaving only the signals of local radioactive sources.

In general, radiation detectors do not witness the radiation itself. The detectors look for footprints that it leaves behind. Each type of radiation leaves specific clues; physicists often refer to these clues as signatures. The goal in detector design is to create an environment in which the signature may be clearly written.

For example, if someone wants to study nocturnal animals, it might be wise to consider the ground covering. Looking at a layer of pine needles by day, one finds few, if any, tracks or markings. However, one can choose to study a region of soft soil and find many more animal prints. The best choice yet is fresh snow. In this case, one can clearly see the tracks of every animal that moved during the night. Moreover, the behavior of an animal can be documented. Where the little prints of a fox are deep and far apart, it was probably running, and where its prints are more shallow and more closely spaced, it was probably walking. Designing a radiation detector presents a similar situation. Radiation can leave its mark clearly, but only in special circumstances.

Clues are created when radiation passes too close to, (or even collides with), another objectcommonly, an atom. What detectors eventually find is the atoms reaction to such an encounter. Scientists often refer to a single encounter between radiation and the detector as an event. Given a material which is sensitive to radiation, there are two main ways to tell that radiation has passed through it: optical signals, in which the material reacts in a visible way; and electrical signals, in which it reacts with a small, but measurable voltage.

Optical detectors

One type of optical detector is the film detector. This is the oldest, most simple type and one that closely resembles the analogy of tracks in snow. The film detector works much like everyday photographic film, which is sensitive to visible light. A film detector changes its appearance in spots where it encounters radiation. For instance, a film detector may be white in its pure form and subsequently turn black when hit by beta particles. Each beta particle which passes through the film will leave a black spot. Later, a person can count the spots (using a microscope), and the total number reveals the level of beta radiation for that environment.

Since film detectors are good at determining radiation levels, they are commonly used for radiation safety. People who work near radioactive materials can wear pieces of film appropriate for the type of radiation. By regularly examining the film, they can monitor their exposure to radiation and stay within safety guidelines. The science of determining how much radiation a person has absorbed is called dosimetry. Film detectors do have limitations. Someone studying the film cannot tell exactly when the radiation passed or how energetic it was.

An optical radiation detector more useful for experiments is a scintillation detector. These devices are all based on materials called scintillators, which give off bursts of light when bombarded by radiation. In principle, an observer can sit and watch a scintillator until it flashes. In practice, however, light bursts come in little packages called photons, and the human eye has a hard time detecting them individually. Most scintillator detectors make use of a photo multiplier, which turns visible light (i.e., optical photons) into measurable electrical signals. The signals can then be recorded by a computer. If the incoming radiation has a lot of energy, then the scintillator releases more light, and a larger signal is recorded. Hence, scintillation detectors can record both the energy of the radiation and the time it arrived.

Materials used in scintillation detectors include certain liquids, plastics, organic crystals, (such as anthracene), and inorganic crystals. Most scintillating materials show a preference for which type of radiation they will find. Sodium iodide is a commonly used inorganic crystal that is especially good at finding x rays and gamma rays. In recent years, sodium iodide has received increasing competition from barium fluoride, which is much better at determining the exact time of an event.

Electrical detectors

Electrical detectors wait for radiation to ionize part of the detector. Ionization occurs when incoming radiation separates a molecule or atom into a negative piece (one or more electrons) and a positive piece (i.e., the ion, the remaining molecule, or atom with a plus electrical charge). When a material has some of its atoms ionized, its electrical characteristics change and, with a clever design, a detecting device can sense this change.

Many radiation detectors employ an ionization chamber. Fundamentally, such a chamber is simply a container of gas that is subjected to a voltage. This voltage can be created by placing an electrically positive plate and an electrically negative plate within the chamber. When radiation encounters a molecule of gas and ionizes it, the resulting electron moves toward the positive plate and the positive ion moves toward the negative plate. If enough voltage has been applied to the gas, the ionized parts move very quickly. In their haste, they bump into and ionize other gas molecules. The radiation has set off a chain reaction that results in a large electrical signal, called a pulse, on the plates. This pulse can be measured and recorded as data. The principles of the ionization chamber form the basis for both the Geiger-Müller detector and the proportional detector, two of the most common and useful radiation-sensing devices.

A Geiger-Müller counter in its basic form is a cylinder with a wire running through the inside from top to bottom. It is usually filled with a noble gas, like neon. The outside of the metal cylinder is given a negative charge, while the wire is given a positive charge. In this geometry, the wire and the cylinder function as the two plates of an ionization chamber. When electrons are knocked from the gas by radiation, they move to the wire, which can then relay the electrical pulse to counting equipment. The voltage applied to a Geiger-Müller detector is quite high and each ionization creates a large chain reaction. In this way, it gives the same-sized pulse regardless of the radiations original speed or energy.

One version of the Geiger-Müller detector, the Geiger counter, channels the electrical pulses to a crude speaker which then makes a popping noise each time it detects an event. This is the most familiar of radiation detectors, particularly in films which depict radioactivity. When the detector nears a radioactive source, it finds more events and gives off a correspondingly greater number of popping sounds. Even in a more normal setting, such as the average street corner, it will pop once every few seconds because of background radiation.

A proportional detector is very similar to the Geiger-Müller detector, but a lower voltage is applied to the ionization chamber, and this allows the detector to find radiation energies. More energetic radiation ionizes more of the gas than less energetic radiation does; the proportional detector can sense the

KEY TERMS

Background radiation The ambient level of radiation measured in an otherwise non-radioactive setting.

Dosimetry The science of determining the amount of radiation that an individual has encountered.

Event A detected interaction between radiation and the detector material.

Ionization chamber A detector in which incoming radiation reacts with the detector material, splitting individual atoms or molecules into electrically negative and positive components.

Scintillation A burst of light given off by special materials when bombarded by radiation.

Signature The distinctive set of characteristics that help identify an event.

difference, and the sizes of its pulses are directly related to the radiation energies. A large pulse corresponds to highly energetic radiation, while a small pulse likewise corresponds to more lethargic events. Since it can record more information, the proportional counter is more commonly found in scientific experiments than the Geiger-Müller detector, which, like the film detector, is primarily used for radiation safety.

Physicists who search for rare subatomic particles have utilized the principles of ionization chambers. They have developed many types of exotic detectors that combine ionization chambers with optical detection.

Resources

BOOKS

Delaney, C.F.G., and E.C. Finch. Radiation detectors: Physical Principles and Applications. New York: Oxford university Press, 1992.

Del Guerra, Alberto. Ionizing Radiation Detectors for Medical Imaging. Hackensack, NJ: World Scientific Publishing Co.

Brandon R. Brown

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