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Cryogenic detectors in astronomy

Figure showing detectors used
in astronomy at different wavelengths

The figure above shows the different types of detector used in astronomy across the electromagnetic spectrum. Detector types that are usually or always operated cryogenically are highlighted in yellow, as are the energy/wavelength ranges and types of radiation for which cryogenic instruments are used. CCDs and scintillators are operated at temperatures of 150 K and above; we have not considered this to be a cryogenic temperature range here, though it is common to cool CCDs using liquid nitrogen (which has a boiling point of 77 K).

Further information is given below.

Radio astronomy

Depending on the wavelength, the first stage in detecting a signal in radio astronomy is either a low noise amplifier such as a HEMT (high electron mobility transistor) or heterodyne mixer. In both cases, cryogenic operation reduces noise, and some mixers, such as SIS (superconductor-insulator-superconductor) and HEB (hot electron bolometer) mixers rely on superconductivity and therefore must operate at cryogenic temperatures. In general, operating temperatures vary upwards from 4 K, used for some heterodyne systems operating at wavelengths of around 1 mm.

Sub-mm astronomy

At sub-mm wavelengths (loosely defined as from around 200 μm to a few mm), radio astronomy techniques are used for high spectral resolution measurements. For continuum measurements and low spectral resolution, bolometers are generally employed. These are operated at ultra-low temperatures (300 mK or lower). The main reason is to reduce the heat capacity sufficiently to obtain low enough time constants for the detectors to be useful. Alternative technologies such as STJs (superconducting tunnel junctions) and KIDS (kinetic inductance detectors) rely on superconductivity and thus must also operate at cryogenic temperaturs.

Cryogenic operation would in any case be required at these wavelengths in order to reduce the black-body radiation from the instrument itself, but this would not require such extremely low temperatures. Ideally the telescope itself would also be cooled below room temperature for the same reason; this is possible in space, but not practical on the ground as the entire telescope would have to be inside a vacuum chamber in order to prevent condensation of water and even atmospheric gases on the telescope surface.

Infra-red astronomy

Infrared astronomy covers the wavelength range from around 1 μm to a few hundred μms. As with sub-mm astronomy, cooling of instruments is required to reduce black-body radiation, and in space the telescope itself is also cooled. Photoconductors dominate for detection, and cooling is required to reduce dark noise (electrons promoted into the conduction band by thermal excitation rather than the detection of a photon). The bandgap required depends on the wavelength of photons to be detected; at longer wavelengths, and thus lower photon energies, a small bandgap is required. The dark current depends on temperature and the bandgap size, with smaller bandgaps requiring lower temperatures to effectively supress dark current. Temperatures of a few K are required at 200 μm, while operation at temperatures as high as 100 K is possible at a few μm.

Optical astronomy

CCD (charge coupled device) cameras are used almost exclusively for optical astronomy, having replaced photographic film and other types of detector that have been used historically. While they have many advantages over photographic film, such as excellent linearity and vastly greater sensitivity, they do require cooling for the best sensitivity. As with infra-red detectors, the reason is to reduce thermal excitation of electrons (dark noise). However, since the bandgap is greater than for infra-red detectors, the cooling requirements are more modest, and operating temperatures of 150 K to 200 K are common; this is not generally considered to by a cryogenic temperature range. At optical and shorter wavelengths, there is no need to cool in order to reduce blackbody radiation.

Ultra-violet astronomy

CCDs can be used in the ultra-violet as well as optical wavelengths, operating at similar temperatures. Photoemissive devices such as photomultiplier tubes and microchannel arrays are also used; these operate at room temperature.

X and gamma ray astronomy

CCDs are also used to detect X-rays, but there are many other types of detector used for X and gamma ray astronomy. Operating temperatures are equally variable, covering the range from room temperature (for detectors such as scintillators) to below 100 mK for microcalorimeters (bolometers which detect the heat pulse due to the absorption of a single photon). Due to the near total extinction by the atmosphere at these wavelengths, observations must be carried out from space.

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Page created: Adam Woodcraft
Last edited 2008-4-15     Site map
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