By the end of this section, you will be able to:
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It is well known that ionizing radiation affects us but does not trigger nerve impulses. Newspapers carry stories about unsuspecting victims of radiation poisoning who fall ill with radiation sickness, such as burns and blood count changes, but who never felt the radiation directly. This makes the detection of radiation by instruments more than an important research tool. This section is a brief overview of radiation detection and some of its applications.
The first direct detection of radiation was Becquerel's fogged photographic plate. Photographic film is still the most common detector of ionizing radiation, being used routinely in medical and dental x rays. Nuclear radiation is also captured on film, such as seen in Figure 1. The mechanism for film exposure by ionizing radiation is similar to that by photons. A quantum of energy interacts with the emulsion and alters it chemically, thus exposing the film. The quantum come from an α-particle, β-particle, or photon, provided it has more than the few eV of energy needed to induce the chemical change (as does all ionizing radiation). The process is not 100% efficient, since not all incident radiation interacts and not all interactions produce the chemical change. The amount of film darkening is related to exposure, but the darkening also depends on the type of radiation, so that absorbers and other devices must be used to obtain energy, charge, and particle-identification information.
Another very common radiation detector is the Geiger tube. The clicking and buzzing sound we hear in dramatizations and documentaries, as well as in our own physics labs, is usually an audio output of events detected by a Geiger counter. These relatively inexpensive radiation detectors are based on the simple and sturdy Geiger tube, shown schematically in Figure 2b. A conducting cylinder with a wire along its axis is filled with an insulating gas so that a voltage applied between the cylinder and wire produces almost no current. Ionizing radiation passing through the tube produces free ion pairs that are attracted to the wire and cylinder, forming a current that is detected as a count. The word count implies that there is no information on energy, charge, or type of radiation with a simple Geiger counter. They do not detect every particle, since some radiation can pass through without producing enough ionization to be detected. However, Geiger counters are very useful in producing a prompt output that reveals the existence and relative intensity of ionizing radiation.
Another radiation detection method records light produced when radiation interacts with materials. The energy of the radiation is sufficient to excite atoms in a material that may fluoresce, such as the phosphor used by Rutherford's group. Materials called scintillators use a more complex collaborative process to convert radiation energy into light. Scintillators may be liquid or solid, and they can be very efficient. Their light output can provide information about the energy, charge, and type of radiation. Scintillator light flashes are very brief in duration, enabling the detection of a huge number of particles in short periods of time. Scintillator detectors are used in a variety of research and diagnostic applications. Among these are the detection by satellite-mounted equipment of the radiation from distant galaxies, the analysis of radiation from a person indicating body burdens, and the detection of exotic particles in accelerator laboratories.
Light from a scintillator is converted into electrical signals by devices such as the photomultiplier tube shown schematically in Figure 3. These tubes are based on the photoelectric effect, which is multiplied in stages into a cascade of electrons, hence the name photomultiplier. Light entering the photomultiplier strikes a metal plate, ejecting an electron that is attracted by a positive potential difference to the next plate, giving it enough energy to eject two or more electrons, and so on. The final output current can be made proportional to the energy of the light entering the tube, which is in turn proportional to the energy deposited in the scintillator. Very sophisticated information can be obtained with scintillators, including energy, charge, particle identification, direction of motion, and so on.
Solid-state radiation detectors convert ionization produced in a semiconductor (like those found in computer chips) directly into an electrical signal. Semiconductors can be constructed that do not conduct current in one particular direction. When a voltage is applied in that direction, current flows only when ionization is produced by radiation, similar to what happens in a Geiger tube. Further, the amount of current in a solid-state detector is closely related to the energy deposited and, since the detector is solid, it can have a high efficiency (since ionizing radiation is stopped in a shorter distance in solids fewer particles escape detection). As with scintillators, very sophisticated information can be obtained from solid-state detectors.
Learn about different types of radiometric dating, such as carbon dating. Understand how decay and half life work to enable radiometric dating to work. Play a game that tests your ability to match the percentage of the dating element that remains to the age of the object.
Geiger tube: a very common radiation detector that usually gives an audio output
photomultiplier: a device that converts light into electrical signals
radiation detector: a device that is used to detect and track the radiation from a radioactive reaction
scintillators: a radiation detection method that records light produced when radiation interacts with materials
solid-state radiation detectors: semiconductors fabricated to directly convert incident radiation into electrical current
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Radiation instrumentation yields a signal when radiation interacts with a sensitive material. Radiation consists of energetic uncharged or charged particles generated from radioactive decay, manmade sources, or cosmic ray interactions in the atmosphere. The sensitive material may be solid, liquid, or gas. The signal is generated as the charge produced by the radiation interaction, either directly or indirectly, is collected. The sensors may also record location, time and/or energy of the radiation interaction.
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Capabilities of radiation instrumentation systems
A sensor may be designed to detect radiation, which means that the signal from a radiation source is significantly higher than that of the background radiation and/or instrument noise. Some sensing systems are capable of localizing a radiation source. Furthermore, some radiation instruments are capable of imaging radiation sources. Additionally, some sensors are able to identify the type a particular radiation source. Moreover, some sensing systems are able to characterize, or determine more detailed characteristics, of a source and/or the environment surrounding the source.
Applications
Radiation instrumentation is used in a host of applications, ranging from big science to industry. Considering big science, radiation instruments were the enabler of the recent effort at CERN to discover the Higgs Boson' the particle thought to give mass to matter. In this case, the radiation instruments were very large and complex. Additionally, radiation instruments are used on space stations, to sense emissions from distant galaxies, to explore particular planets (e.g., Mars rover), or to sense radiation dose in space for our protection. Radiation instruments used at neutron science centers enable a host of new materials discovery and development. Radiation instruments are also used in medical imaging systems. For example, the instruments used in Position Emission Tomography systems enable doctors to determine whether or not cancer has spread throughout the body. In radiation therapy, radiation instruments are used to verify that that radiation dose is appropriately delivered to patients so that the cancer is killed and normal tissues are spared. Radiation instruments are also used in security at airports and border crossings to look for contraband. Furthermore, they are used in nuclear facilities and for nuclear weapon treaty verification purposes. Radiation instruments find a place in all areas of nuclear and plasma science and engineering. In nuclear reactors, radiation instruments are used to monitor reactor power. In nuclear fusion research, radiation instruments are used to help understand how well the reaction is being sustained. Radiation instruments are also used for a host of industrial processes, including drilling for oil, as well as steel and cast iron manufacturing.
UTK's Rad IDEAS group
Our group mainly designs instrumentation for sensing of uncharged particles, including gamma rays and neutrons' both slow and fast. We have also designed detectors for alpha sensing and cosmic ray muon sensing.
Many of our projects are concerned with the neutron and gamma emissions that emanate from special nuclear materials, including uranium and plutonium. These particles are emitted as a result of spontaneous nuclear decay, the result of fission chain reactions, and/or due to stimulation from energetic neutron or photon sources. The intensity of these radiations decreases as one over the distance between the source and the sensor squared. Thus, the source radiation intensity is often comparable to that of the radiation background when the detector is located at a standoff distance some tens of meters away from the source. Furthermore, shielding decreases radiation intensity in an exponential manner as thickness of the material increases.
Concerning detector types, our focus is on solid radiation scintillators, or materials that light up when radiation strikes them, especially those used in radiation imaging systems. We also do some work with semiconductor and gaseous detectors. Many of the detector systems that we work with are position-sensitive, meaning that the interaction location of the radiation interaction is measured.
Our group is involved with pioneering work all the way from new radiation detection materials discovery to the design of new methods and algorithms used to interpret the data generated by proof-of-concept systems.
Our main application space is nuclear non-proliferation technology, or technology aimed at stopping the spread of nuclear weapons and nuclear materials, but our interest in the science itself continues to push us into new application areas.
Radiation instrument functions, capabilities and performance
When radiation interacts in an instrument, those interactions are recorded in some form. Some minimum set of recorded events is necessary to make any decision using the instrument, e.g., confirm that a radiation source is detected. Sometimes the radiation instrument simply counts radiation. In other cases, a particular property of the radiation, e.g., its energy, is binned. The resulting histogram is called a spectrum, e.g., an energy spectrum. An instrument that examines any particular radiation property is said to be spectroscopic, or to perform spectroscopy. Thus, two functions of radiation instruments are radiation counting and radiation spectroscopy.
The performance of a radiation instrument is measured according to its intended capabilities, which may include one or more of the following: detect, localize, image, identify, characterize. One property which is often important is detector efficiency. Larger sensors tend to be more efficient, meaning that the source radiation is more likely to interact in the detector. Certain material properties can also increase efficiency. For example, denser materials are more efficient for detecting gamma rays compared with lighter materials. The opposite tends to be true for fast neutrons.
A second performance property that may be of interest, depending upon the intended capability of a radiation instrument, is called resolution. Resolution is the ability to resolve a particular property of the interacting radiation, which could include its energy (energy resolution), interaction location in the instrument (position or spatial resolution), time of interaction in the instrument (timing resolution), or the related angle from which the source radiation originated (angular resolution). For example, in a high energy resolution instrument, two distinct, closely spaced energies are more readily resolved than in a moderate or low energy resolution instrument.
Imaging radiation may be performed in a variety of ways. An image formed by measuring some fraction of radiation, e.g., x-rays, that pass through an imaged object is called transmission radiography. A more detailed image of the object may be reconstructed if there is more than one view of the object taken from one than one angle of observation. A common example of this is x-ray computed tomography, also called a CT scan. If one desires to image a source of radiation rather than an object, the source itself can be imaged through recording some fraction of it that passes through a pinhole collimator onto a position-sensitive radiation detector. As the source moves, the projection of the source onto the detector moves as well. A common example of this is single-photon emission computed tomography, or SPECT. Although we are mainly familiar with imaging x-rays, gamma rays or neutron radiations may be imaged via very similar means.
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