Radiation Detector


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radiation detector

[‚rād·ē′ā·shən di‚tek·tər]
(nucleonics)

Radiation Detector

 

a device for converting electromagnetic-radiation signals into signals of a different physical nature for the purpose of detecting them and using or studying the information they carry. The radiation may range from a wavelength λ = 10-9 cm for X rays to a wavelength λ = 10-1 cm for radio waves (see for a discussion of detectors of electromagnetic radiation with shorter wavelengths). Radiation detectors are frequently one of the main components of automatic devices and control systems. They play an important part in scientific research, for example, in spectroscopy, quantum electronics, and astronomy.

The conversion of signals in such detectors is accomplished in the course of interaction of the electromagnetic-radiation field with some substance. The field changes the energy states of the electrons, atoms, or molecules of the substance, and these changes are recorded.

Various types of radiation detectors exist in which substances are used in different states of aggregation. Thus, for example, the radiation may ionize a gas and cause an electrical discharge in it; in this case, a current or voltage pulse is recorded, and the detector is called a photon counter. It is possible to record the increase in the volume of a gas heated by absorbed radiation; this is the principle of operation of optic-acoustic or pneumatic detectors, which can operate throughout the entire spectral range indicated above but are more frequently used in the far infrared (IR) region at wavelengths from 50 to 1,000 microns (μ).

Radiation detectors consisting of a solid that is sensitive to radiation constitute the most extensive group. They include bolometers, in which the resistance to electric current changes as a result of the absorption of radiation; thermocouples, which produce an electromotive force in response to heating by radiation; and pyroelectric radiation detectors fabricated from ferroelectric crystals. In this last type of detector, a static electric charge appears on the surface of the crystal as a result of the interaction with radiation. All of these types are classified as thermal radiation detectors, since it is the heating of the substance by radiation that plays the main role in the energy-conversion mechanism. They are used throughout the entire spectral region considered here.

In photoelectric detectors, the radiation acts directly on the electrons of the substance; the principal phenomena involved are the inner and outer photoelectric effects. Photocells and photo-multiplier tubes, which use the outer photoelectric effect or photoemission effect, are mainly used for wavelengths λ < 1–2μ; photoresistive cells, photodiodes, and other radiation detectors using the inner photoelectric effect are sensitive to radiation with wavelengths equal to submillimeter wavelengths in the radio range. At the shorter wavelengths in the spectral region considered here, photomultiplier tubes and semiconductor avalanche photodiodes can operate as photon counters.

There are also photon counters in which the effect of the ionization of a liquid or a solid by radiation is used. In detecting radiation in the far IR and submillimeter ranges, detectors are used in which the photons do not change the concentration of conduction electrons in a solid but either change their mobility or cause a pressure on the electrons by transferring momentum to them—the effect of entrainment of electrons by photons. Photoelectric detectors for the 5–1,000μ range require cooling to a temperature of 4°-77°K; their working temperature must decrease as the wavelength of the recorded radiation increases. At low operating temperatures, the phenomenon of superconductivity and related effects are used to detect radiation, as in detectors based on the Josephson effect and superconducting bolometers.

In addition to single-element detectors, there are also multielement radiation detectors with individual detecting elements that are discretely or continuously distributed over a surface. They are used to obtain a two-dimensional image of the radiating object. Photographic plates and films are classical examples of this type. This group also includes image converters, which operate at wavelengths λ < 1.2μ, television camera tubes, and luminescent converters with thermal quenching for the entire spectrum under consideration and “flash” operation for radiation with a wavelength λ ~ 2 μ. Also included are multiple-element semiconductor bolometers, photoresistive cells made of lead sulfide (for wavelengths up to λ ~ 3.5 μ) and indium antimonide (for wavelengths up to λ ~ 5.5 μ), and evaporographs, in which an oil film heated by the radiation evaporates.

An important parameter of any radiation detector is the ratio of the useful signal and the interference level. During the conversion process, the detector must not substantially lower this ratio. The capacity of detectors to record signals of small duration is characterized by their time constant. Such characteristics as the conversion coefficient and threshold sensitivity—the magnitude of the minimum detectable signal—are important for practical purposes. The sensitivity of the best counters and photomultiplier tubes is such that individual photons of the incident radiation can be recorded. Radiation detectors for the IR range are less sensitive.

The quantity D* is the reciprocal of the threshold sensitivity of the radiation detector referred to a unit band of the working frequencies and unit area of the detecting surface. For thermal detectors it reaches 109; for photoelectric detectors it reaches 1012 for wavelengths λ ~3 μ and 1010-1011 for wavelengths λ ~ 1,000μ. The value of the time constant of image converters extends up to 10-12 sec. It reaches 10–9 sec for special photocells and 10–7 for detectors using the inner photoelectric effect; in certain cases, such as with doped photoresistive cells, it may reach 10–10 sec. The time constant of thermal radiation detectors may reach 10–9 sec, but more frequently (for high values of D*) it is in the range of 10-2—10–3 sec.

M. N. MARKOV

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