Semiconductor detector 802913 199395931 2008-03-19T18:18:53Z 129.143.4.66 {{Cleanup|date=June 2007}} <!--|the section splitting does not make much sense--> A '''semiconductor detector''' is a device that uses a semiconductor (usually [[silicon]] or [[germanium]]) to detect traversing charged particles or the absorption of photons. In the field of particle physics, these detectors are usually known as ''silicon detectors.'' When their sensitive structures are based on a single [[diode]], they are called '''semiconductor diode detectors'''. When they contain many diodes with different functions, the more general term semiconductor detector is used. Semiconductor detectors have found broad application during recent decades, in particular for [[gamma ray|gamma]] and [[X-ray]] [[spectrometry]] and as [[particle detector]]s. == Semiconductor radiation detector == In these detectors, [[radiation]] is measured by means of the number of [[charge carrier]]s set free in the detector, which is arranged between two [[electrode]]s. Ionizing radiation produces free [[electron]]s and [[Electron hole|holes]]. The number of electron-hole pairs is proportional to the [[energy]] transmitted by the radiation to the semiconductor. As a result, a number of electrons are transferred from the [[valence band]] to the [[conduction band]], and an equal number of holes are created in the valence band. Under the influence of an [[electric field]], electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer [[electrical network|circuit]]. The holes travel into the opposite direction and can also be measured. As the amount of energy required to create an electron-hole pair is known, and is independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the energy of the incident radiation to be found.<ref>{{cite book |last=Knoll |first=G.F. |title=Radiation Detection and Measurement, 3rd edition |publisher=Wiley |date=1999 |id=ISBN 978-0471073383}} p365</ref> The energy required for production of electron-hole-pairs is very low compared to the energy required for production of paired ions in a gas detector. Consequently, in semiconductor detectors the [[Statistical variability|statistical variation]] of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good, and is dependent upon [[rise time]].<ref>Knoll, p119</ref> Compared with [[gaseous ionization detectors]], the [[density]] of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions. == Semiconductor particle detectors == ===Silicon detector=== Most silicon [[Elementary particle|particle]] detectors work, in principle, by [[Doping (semiconductor)|doping]] narrow (usually around 100 micrometers wide) strips of [[silicon]] to make them into [[diode]]s, which are then reverse biased. As charged particles pass through these strips, they cause small ionization currents which can be detected and measured. Arranging thousands of these detectors around a collision point in a [[particle accelerator]] can give an accurate picture of what paths particles take. Silicon detectors have a much higher resolution in tracking charged particles than older technologies such as [[cloud chamber]]s or [[wire chamber]]s. The drawback is that silicon detectors are much more expensive than these older technologies and require sophisticated cooling to reduce leakage currents (noise source) as well as suffer degradation over time from [[radiation]]. ===Diamond detector=== [[Diamond]] detectors have many similarities with silicon detectors, but are expected to offer significant advantages, in particular a high radiation hardness and very low drift currents. At present they are much more expensive and more difficult to manufacture. ===Germanium detector=== [[Image:HPGe detector.jpg|thumb|right|High Purity Germanium detector (disconnected from liquid Nitrogen dewar)]] [[Germanium]] detectors are mostly used for spectroscopy in [[nuclear physics]]. While silicon detectors cannot be thicker than a few millimeters, germanium can have a depleted, sensitive thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to few MeV. These detectors are also called '''High-Purity Germanium''' detectors ('''HPGe''') or Hyperpure Germanium detectors. Before current purification techniques were refined, Germanium [[single crystal|crystals]] could not be produced with purity sufficient to enable their use as spectroscopy detectors. Impurities in the crystals trapped electrons and holes, ruining the performance of the detectors. Therefore, Germanium crystals were doped with [[Lithium]] ions ('''Ge(Li)'''), in order to produce an [[intrinsic]] region in which the electrons and holes would be able to reach the contacts and produce a signal. When Germanium detectors were first developed, only very small crystals were available. Low efficiency was the result, and Germanium detector efficiency is still often quoted in relative terms, as discussed above. Crystal growth techniques have improved, allowing detectors to be manufactured that are as large as or larger than commonly available NaI crystals, although such detectors cost more than €100,000. Present-day HPGe detectors commonly still use lithium diffusion to make an [[N-type semiconductor|n<sup>+</sup>]] [[ohmic contact]], and boron implantation to make a [[P-type semiconductor|p<sup>+</sup>]] contact. Coaxial detectors with a central n<sup>+</sup> contact are referred to as n-type detectors, while p-type detectors have a p<sup>+</sup> central contact. The thickness of these contacts represents a '''dead layer''' around the surface of the crystal within which energy depositions do not result in detector signals. Typical dead layer thickness are several hundred micrometers for an Li diffusion layer, and a few tenths of a micrometer for a B implantation layer. The major drawback of Germanium detectors is that they must be cooled to [[Nitrogen|liquid nitrogen]] temperatures to produce spectroscopic data. At higher temperatures, the electrons can easily cross the [[Band gap]] in the crystal and reach the conduction band, where they are free to respond to the electric field. The system therefore produces too much electrical noise to be useful as a spectrometer. Cooling to liquid nitrogen temperatures, 77.36 K, reduces thermal excitations of valence electrons so that only a gamma ray interaction can give an electron the energy necessary to cross the band gap and reach the conduction band. Cooling with liquid nitrogen is inconvenient, as the detector requires hours to cool down to operating temperature before it can be used, and cannot be allowed to warm up during use. Ge(Li) crystals could never be allowed to warm up, as the Lithium would drift out of the crystal, ruining the detector. HPGe detectors can be allowed to warm up to room temperature when not in use. Appropriate care must be taken when working with liquid nitrogen; the major hazards are [[Cold burn]] and oxygen depletion as the liquid nitrogen boils, producing a significant volume of nitrogen gas. Commercial systems are now available that use advanced refrigeration techniques to eliminate the need for liquid nitrogen cooling. ==See also== * [[Particle detector]] * [[Lazarus effect]] * [[X-ray spectroscopy]] * [[Synthetic diamond#Applications|Synthetic diamonds]] ==References== {{reflist}} [[Category:Particle detectors]] [[Category:Spectrometers]] [[Category:X-ray instrumentation]] [[Category:Medical imaging]] [[Category:Experimental particle physics]] [[de:Halbleiterdetektor]] [[fr:Détecteur à semiconducteur]] [[hu:Félvezető detektor]] [[it:Rivelatore a semiconduttore]]