Detectors

Detectors

The interactions of alpha, beta, and gamma radiations with matter produce positively charged ions and electrons. Radiation detectors are devices that measure this ionization and produce an observable output. Early detectors used photographic plates to detect "tracks" left by nuclear interactions. The cloud chambers, used to discover sub-nuclear particles, needed photographic recording and a tedious measurement of tracks from the photographs. Advances in electronics, particularly the invention of the transistor, allowed the development of electronic detectors. Scintillator-type detectors use vacuum tubes to perform the initial conversion of light to electrical pulses. The amplification and storing these data follow the advances in transistor electronics. Miniaturization in electronics has revitalized types of gas-filled detectors. These detectors were developed as "single element" detectors and now have been revived into "multiple element" detectors with more than one thousand elements. Advances in materials, particularly ultra-pure materials, and methods of fabrication have been critical to the creation of new and better detectors.

As the requirements for greater accuracy, efficiency, or sensitivity increases, so does the complexity of the detector and its operation. The following list presents some types of commonly used detectors and includes comments on each of them:

Geiger Counter: The detector most common to the public is the Geiger-Mueller counter, commonly called the Geiger counter. It uses a gas-filled tube with a central wire at high voltage to collect the ionization produced by incident radiation. It can detect alpha, beta, and gamma radiation although it cannot distinguish between them. Because of this and other limitations, it is best used for demonstrations or for radiation environments where only a rough estimate of the amount of radioactivity is needed.

Scintillation detectors: Scintillators are usually solids (although liquids or gases can be used) that give off light when radiation interacts with them. The light is converted to electrical pulses that are processed by electronics and computers. Examples are sodium iodide (NaI) and bismuth germanate (BGO). These materials are used for radiation monitoring, in research, and in medical imaging equipment.

Solid state X-ray and gamma-ray detectors: Silicon and germanium detectors, cooled to temperatures slightly above that of liquid nitrogen (77 K), are used for precise measurements of X-ray and gamma-ray energies and intensities. Silicon detectors are good for X-rays up to about 20 keV in energy. Germanium detectors can be used to measure energy over the range of >10 keV to a few MeV. Such detectors have applications in environmental radiation and trace element measurements. Germanium gamma ray detectors play the central role in nuclear high-spin physics, where gamma rays are used to measure the rotation of nuclei. Large gamma-ray detection systems, such as Gammasphere and Eurogam are made of these detectors.

Low-energy charged particle detectors: Silicon detectors, normally operated at room temperature, play a major role in the detection of low-energy charged particles. Singly, they can determine the energy of incident particles. Telescopes (combinations of two or more Si detectors) can be used to determine the charge (Z) and mass (A) of the particle. This type of detector is used in environmental applications to look for alpha-particle emitters (such as radium) in the environment.

Neutron detectors: Neutrons are much harder to detect because they are not charged. They are detected by nuclear interactions that produce secondary charged particles. For example, boron trifluoride (BF3) counters make use of the 10B(n,a)7Li reaction to detect neutrons. Often one uses a moderator, such as paraffin, to slow the neutrons and thus increase the detection efficiency. These detectors are used to monitor the neutron fluxes in the vicinity of a reactor or accelerator. Liquid scintillators can measure both neutrons and gamma rays. By carefully measuring the shape of the electronic signal, scientists can and distinguish between these two types of particles.

Neutrino Detectors: Neutrinos interact very weakly with matter and are therefore very hard to detect. Thus, neutrino detectors must be very large. The Sudbury Neutrino Observatory in Canada, was developed to understand the solar neutrino problem (too few neutrinos come out of the Sun than expected) and contains an active volume of 1000 tonnes (metric tons) of deuterium oxide (heavy water). This is a Cerenkov counter in which the interaction of the neutrino with the heavy water produces an electron moving faster than the speed of light in the water. The moving electron generates a cone of light that can be observed with photomultiplier tubes. Information from these tubes will be used to determine the energy and direction of the incident neutrino.

High-energy charged particle detectors: As the energy increases, large and even more complex detection systems are needed, some involving thousands of individual detectors. These detectors typically involve the "tracking" of large numbers of particles as they pass through the detector. Large magnets are required to bend the paths of the charged particles. Multi-wire detection systems with nearly a quarter of a million channels of electronics provide information on these tracks. High speed computer systems process and store the data from these detectors. Similarly, powerful computer systems are needed to analyze these data so that a scientific discovery can be made.

Table 12-1. A partial list of detectors used in Nuclear Science. Some detectors can be used only in a limited energy range.

Particle Type Detector Type Features Charged     protons, nuclei, electrons, or pions Geiger-Müller counters

gas ionization counters

multiwire chambers

semiconductor detectors

portable radioactivity detector

gas-filled chamber in an electric field

good position resolution

good energy resolution

  magnetic spectrometers

scintillators and photomultipliers

Cerenkov detectors

good momentum resolution

good timing resolution

good particle identification

Neutral     photons scintillators and photomultipliers

germanium semiconductor crystals

good timing, moderate energy resolution

good energy resolution

neutrons liquid scintillator or BF3 tubes via fission, capture gamma rays, or proton collisions neutrinos Cerenkov detectors

nuclear reactions

via neutrino-electron interactions

detect resultant radiation

Table 12-1 summarizes the information that is presented in this section. It shows the different types of detectors that are suitable for measuring specific particles. When an experiment is designed, first a scientist chooses a particular detector based on the particles and their properties (such as energy, position, or time) that must be measured. Some detectors, such as scintillators, can make accurate time measurements but only a fair position determination. A scientist designs an experiment using an optimum choice of detector system. Cost is a major factor in modern detector design, especially for large systems consisting of a multitude of detectors and associated electronics.