Scintillator 454323 218495913 2008-06-10T22:16:53Z 129.130.121.32 /* Inorganic crystals */ [[Image:Plastic scintillator.jpg|thumb|right|Extruded plastic scintillator material fluorescing under UV inspection lamp at [[Fermilab]] for the [[MINERνA]] project.]] A '''scintillator''' is a substance that absorbs high energy ([[ionizing]]) [[electromagnetic radiation|electromagnetic]] or charged [[particle radiation]] then, in response, [[fluorescent|fluoresces]] photons at a characteristic [[Stokes shift|Stokes-shifted]] (longer) wavelength, releasing the previously absorbed energy. See also ''[[Scintillation (physics)]]''. Scintillators are defined by their light output (number of emitted photons per unit absorbed energy), short fluorescence decay times, and optical transparency at wavelengths of their own specific emission energy. The latter two characteristics set them apart from [[phosphor]]s. The lower the decay time of a scintillator, that is, the shorter the duration of its flashes of fluorescence are, the less so-called "dead time" the detector will have and the more ionizing events per unit of time it will be able to detect. Scintillators are used in many physics research applications to detect [[electromagnetic wave]]s or particles. There, a scintillator converts the energy to light of a wavelength which can be detected by inexpensive or easy to handle detectors such as [[photomultiplier]] tubes (PMTs). ==Types of Scintillators== [[Image:Lead Tungstate Crystal Preparation.jpg|thumb|right|Preparing single crystal lead tungstate scintillators for the ECAL on the [[Compact Muon Solenoid]].]] Common scintillators used for radiation detection include inorganic crystals, organic plastics and liquids. However, many materials scintillate at some level; scintillation of liquid [[xenon]] and [[neon]] plays a role in some ultra-low-background experiments. Most scintillators for common use are either inorganic crystals or plastics, the most common being [[thallium]]-[[doping (semiconductors)|doped]] [[sodium iodide]] crystals, which have a high radiation-to-light conversion efficiency. However, organic liquid scintillating fluids are well-suited for detecting very low energy particle radiation such as beta radiation from [[tritium]] by simply immersing the sample to be tested in the scintillation fluid, thereby negating detector absorption problems due to the very short [[mean free path]]s associated with low energy particles. ===Organic liquids=== The organic crystal scintillator can be dissolved in a transparent liquid, for example in mineral oil, maintaining properties similar to the organic crystal, depending on purity and concentration. For the specific use of this form of scintillator, see [[Liquid scintillation counting]]. ===Organic crystals=== These are organic molecules which have an [[aromatic ring]]; the [[ion]]ising radiation excites it to a rotational or vibrational mode. They are characterized by a fast response, on the order of one nanosecond. When pure, they form crystals, which are difficult to shape. One of the best known organic scintillators is [[anthracene]]. ===Organic plastics=== The organic crystals can be also be dissolved in a transparent plastic that becomes solid at ambient temperature, like [[polystyrene]], these mixtures are called plastic scintillators. The plastic can be easily shaped and tooled. The solid plastic matrix has often the effect of increasing the relaxation time to 2-3 nanoseconds. The three most common bases for plastic scintillators are [[polyvinyl toluene]], [[polystyrene]], or [[Acrylic glass|acrylic]]. However acrylic, as it contains no [[aromatic]] structures, has very low scintillation efficiency of its own; it gains acceptable efficiency if e.g. [[naphthalene]] is dissolved in it in amount of 5-20 weight %. The plastics when used on their own typically emit [[ultraviolet]] photons; to convert them to less attenuated visible light, a suitable [[fluorophor]] is added in amount of about 1 wt.%. Plastic scintillators are robust and reliable, but also quirky. They undergo aging, gradually losing light yield with time, with solvents, high temperatures, radiation, or mechanical load accelerating the process. The surface can be damaged by formation of microcracks which cause light loss by reflection. Plastic scintillators are also sensitive to airborne oxygen which lowers their yield; this is known as ''atmospheric quenching''. Some plastics change their yield slightly when subjected to magnetic fields. Radiation damage leads to formation of color centers ([[F-Centers]]) which absorb in ultraviolet and blue part of spectrum, lowering the optical yield. [http://pdg.lbl.gov/2002/pardetrpp.pdf] Some polymers can scintillate on their own. A commonly used polymer scintillator is [[polyvinyl toluene]] (PVT). ===Inorganic crystals=== Are usually composed of [[alkali]] [[halide]]s, like [[sodium iodide|NaI]]. They are characterized by a high stopping power, which makes them most appropriate to detect high energy radiation. But they have longer decay times, in the order of hundreds of [[nanoseconds]]. * ZnS was the first known inorganic scintillator as discovered by Sir [[William Crookes]] in 1903. * NaI(Tl) ([[thallium]] [[doping (semiconductors)|doped]] [[sodium iodide]]) crystals *: are used in [[gamma camera]]s used for [[nuclear medicine]] radioisotope imaging. NaI was discovered by [[Robert Hofstadter]] in the 1940's. * CsI(Tl) (thallium doped [[caesium iodide]]) crystals are an alternative to NaI(Tl). They are more mechanically durable and have better resistance to moisture. * BaF<sub>2</sub> ([[Barium fluoride]]) * BGO ([[bismuth germanate]] - Bi<sub>4</sub>Ge<sub>3</sub>O<sub>12</sub>) has a higher stopping power, but lower yield than NaI(Tl) *: It is often used in [[coincidence detector]]s for detecting back-to-back gamma rays emitted upon [[positron]] [[annihilation]] in [[positron emission tomography]] machines. * Cerium-doped [[yttrium aluminium garnet]] (Ce:YAG), the yellowish-white coating on the chip in some "white" [[light-emitting diode]]s (LEDs). This is used as a [[phosphor]] but is also suitable for use as a scintillator when in pure single crystal form. This converts part of the visible blue light emitted by the LED chip to visible yellow light. The blue and yellow light together create the subjective impression of white light. * LaBr<sub>3</sub>(Ce) ([[cerium]]-doped [[lanthanum bromide]]) * LuI<sub>3</sub> ([[lutetium iodide]]) * Gd<sub>2</sub>O<sub>2</sub>S ([[terbium]]-doped [[gadolinium oxysulfide]], GOS) * CaWO<sub>4</sub> ([[calcium tungstate]]) * CdWO<sub>4</sub> ([[cadmium tungstate]]), used in [[computer tomography]] and early [[fluoroscope]]s * PbWO<sub>4</sub> ([[lead tungstate]]) * ZnWO<sub>4</sub> ([[zinc tungstate]]) * Lu<sub>2</sub>SiO<sub>5</sub> ([[lutetium oxyorthosilicate]]), also known as LSO. *: It is used in [[Positron Emission Tomography]], because it exhibits properties similar to BGO, but higher light yield. Its only drawback is the intrinsic background from [[beta decay|β<sup>-</sup> decay]] of natural [[Lutetium|<sup>176</sup>Lu]]. == See also == *[[Scintillation Counter]] *[[Liquid scintillation counting]] *[[Gamma spectroscopy]] ==External links== * [http://crystalclear.web.cern.ch/crystalclear/ Crystal Clear Collaboration] at [[CERN]] * [http://www.mt-berlin.com/frames_cryst/descriptions/scintillators_gen%20.htm Scintillation crystals and their general characteristics] * [http://scintillator.lbl.gov/ Scintillation Properties], from [[Lawrence Berkeley National Laboratory]] * [http://btc.montana.edu/messenger/instruments/grns.htm Gamma Ray and Neutron Spectrometer] [[Category:Photochemistry]] [[Category:Particle detectors]] [[Category:Phosphors and scintillators| ]] [[de:Szintillator]] [[fa:آشکارساز سوسوزن]] [[fr:Scintillateur]] [[it:Scintillatore]] [[hu:Szcintillátor]] [[ru:Сцинтилляторы]] [[tr:Sintilatör]]