Particle Detectors | Research & Encyclopedia Articles

Particle Detectors

Particle detectors are instruments designed for the detection and measurement of sub-atomic particles such as those emitted by radioactive materials, produced by particle accelerators or observed in cosmic rays. They 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.

The Geiger counter is one of the oldest and simplest of the many particle detectors. The counter was developed in the early part of the twentieth century by Hans Geiger and Wilhelm Muller, shortly after the discovery of radioactivity. A schematic diagram of a Geiger counter is shown here. 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 which 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 which 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 which 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 counters are made from materials which 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 Ernest Rutherford 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 and the small flashes produced were observed by experimenters in a darkened room using only the human eye.

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 which is directly proportional to their energy over a fairly wide range. A schematic diagram of a gamma ray scintillation spectrometer is shown here. 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 pulseheight analyzer 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.

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 semi-conducting 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 which 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.

Since neutrons are uncharged, their detection must depend on an interaction with matter which produces energetic charged particles. There are several nuclear reactions initiated by neutrons which 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) which 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 whole 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 which produce a signal from the counter.

When a charged particle moves through a transparent material with a velocity v, greater than the velocity of light 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. It is named after the Russian physicist Pavel Cerenkov who discovered it in 1934. The special theory of relativity limits particle velocities to values less than c, the velocity 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.

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 and 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 C T R Wilson around the turn of the century and was useful in the early days of nuclear physics but suffered from several disadvantages such as the long time required to recycle and the low density of air. In 1932, Carl D. Anderson 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 the American physicist Donald Glaser 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 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 in physics in 1961.

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 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.

The ultimate in particle detectors are probably those being used and constructed at large national and international laboratories such as Fermilab in Illinois and CERN in Geneva, Switzerland. At these locations colliding beam accelerators have been built which 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 and the other DZero, 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 almost completely surround the collision points. They utilize most of the detection techniques discussed above including scintillators, solid state detectors and devices similar to wire chambers which provide much improved performance. These are called silicon microstrip detectors. They are made up of closely spaced strips of silicon detectors which 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.

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