Detector (disambiguation)

A detector is a device that detects or measures some phenomenon or stimulus, and produces some signal in response. More generally, the term may be applied to biological organs that perform a similar function. The eye for example converts the energy of those photons reaching the retina into electrical signals which are further processed by the brain.

Most optical detectors are quantum devices in which an individual photon produces a discrete effect.

• In a photographic plate, a silver halide molecule is split into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly.
• In electronic detectors, the photon excites an electron:
  • into the conduction band (in semiconductor detectors)
  • into vacuum (in photomultiplier tubes)
  • in a superconductor, generating a transient heat pulse that breaks a large number of Cooper pairs (see Josephson effect).

There are optical detectors that are effectively thermometers, responding purely to the heating effect of the incoming radiation, such as pyroelectric detectors, Golay cells, thermocouples and thermistors, but the latter two are much less sensitive. Cryogenic detectors are sufficiently sensitive to measure the energy of single x-ray, visible and near infra-red photons (Enss 2005).

In astronomy, the detecting devices generally used to record images are charge-coupled devices (CCD, a special semiconductor detector), although before the 1990s photographic plates were the most common. Glass-backed plates were used rather than film, because they do not shrink or deform in going between wet and dry condition, or under other disturbances. Unfortunately, Kodak discontinued producing several kinds of plates between 1980 and 2000, terminating the production of important sky surveys. See, for example, T. M. Girard et al, Astronomical Journal, 127, 3060 (May, 2004)[1]. The next generation of astronomical instruments (see for example Astro-E2) will include Cryogenic detectors. In experimental particle physics, a particle detector is a device used to track and identify elementary particles.

In an AM radio receiver, a "detector" is a circuit or device that recovers the signal of interest from the modulated wave. This device is usually based on a diode, which converts the (usually pre-amplified) incoming high-frequency (many kHz) AC signal into a "DC" (low frequency AC) signal. The latter has noise remaining, but its envelope is the required audio frequency output. In a FM radio, a circuit called a discriminator is used to convert frequency variations to amplitude variations.

References

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