Scintillator
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A scintillator ( ) is a material that exhibits scintillation, the property of
luminescence Luminescence is a spontaneous emission of radiation from an electronically or vibrationally excited species not in thermal equilibrium with its environment. A luminescent object emits ''cold light'' in contrast to incandescence, where an obje ...
, when excited by
ionizing radiation Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...
. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbed energy in the form of light). Sometimes, the excited state is
metastable In chemistry and physics, metastability is an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball is onl ...
, so the relaxation back down from the excited state to lower states is delayed (necessitating anywhere from a few nanoseconds to hours depending on the material). The process then corresponds to one of two phenomena: delayed
fluorescence Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colore ...
or
phosphorescence Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluor ...
. The correspondence depends on the type of transition and hence the wavelength of the emitted optical
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
.


Principle of operation

A scintillation detector or
scintillation counter A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the Electron excitation, excitation effect of incident radiation on a Scintillation (physics), scintillating material, and detecting the resultant li ...
is obtained when a scintillator is coupled to an electronic light sensor such as a
photomultiplier tube Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible light, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum t ...
(PMT),
photodiode A photodiode is a semiconductor diode sensitive to photon radiation, such as visible light, infrared or ultraviolet radiation, X-rays and gamma rays. It produces an electrical current when it absorbs photons. This can be used for detection and me ...
, or silicon photomultiplier. PMTs absorb the light emitted by the scintillator and re-emit it in the form of electrons via the
photoelectric effect The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physi ...
. The subsequent multiplication of those electrons (sometimes called photo-electrons) results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck the scintillator. Vacuum photodiodes are similar but do not amplify the signal while silicon photodiodes, on the other hand, detect incoming photons by the excitation of charge carriers directly in the silicon. Silicon photomultipliers consist of an array of photodiodes which are reverse-biased with sufficient voltage to operate in avalanche mode, enabling each pixel of the array to be sensitive to single photons.


History

The first device which used a scintillator was built in 1903, by Sir
William Crookes Sir William Crookes (; 17 June 1832 – 4 April 1919) was an English chemist and physicist who attended the Royal College of Chemistry, now part of Imperial College London, and worked on spectroscopy. He was a pioneer of vacuum tubes, inventing ...
and used a ZnS screen. The scintillations produced by the screen were visible if viewed by a microscope in a darkened room; the device was known as a
spinthariscope A spinthariscope () is a device for observing individual Radioactive decay, nuclear disintegrations caused by the interaction of ionizing radiation with a phosphor (see radioluminescence) or scintillator. Invention The spinthariscope was invente ...
. The technique led to a number of important discoveries but was obviously tedious. Scintillators gained additional attention in 1944, when Curran and Baker replaced the naked eye measurement with the newly developed PMT. This was the birth of the modern scintillation detector.


Applications for scintillators

Scintillators are used by the American government as Homeland Security radiation detectors. Scintillators can also be used in
particle detector In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing elementary particle, particles, such as t ...
s, new energy resource exploration, X-ray security, nuclear cameras, computed tomography and gas exploration. Other applications of scintillators include CT scanners and gamma cameras in medical diagnostics, and screens in older style CRT computer monitors and television sets. Scintillators have also been proposed as part of theoretical models for the harnessing of gamma-ray energy through the photovoltaic effect, for example in a nuclear battery. The use of a scintillator in conjunction with a photomultiplier tube finds wide use in hand-held survey meters used for detecting and measuring
radioactive contamination Radioactive contamination, also called radiological pollution, is the deposition of, or presence of Radioactive decay, radioactive substances on surfaces or within solids, liquids, or gases (including the human body), where their presence is uni ...
and monitoring nuclear material. Scintillators generate light in fluorescent tubes, to convert the ultra-violet of the discharge into visible light. Scintillation detectors are also used in the petroleum industry as detectors for Gamma Ray logs.


Properties of scintillators

There are many desired properties of scintillators, such as high
density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
, fast operation speed, low
cost Cost is the value of money that has been used up to produce something or deliver a service, and hence is not available for use anymore. In business, the cost may be one of acquisition, in which case the amount of money expended to acquire it i ...
, radiation hardness, production capability, and durability of operational parameters. High density reduces the material size of showers for high-energy γ-quanta and electrons. The range of Compton scattered photons for lower energy γ-rays is also decreased via high density materials. This results in high segmentation of the detector and leads to better spatial resolution. Usually high density materials have heavy ions in the lattice (e.g.,
lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...
,
cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...
), significantly increasing the contribution of
photoelectric effect The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physi ...
(~Z4). The increased photo-fraction is important for some applications such as
positron emission tomography Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, r ...
. High stopping power for electromagnetic component of the ionizing radiation needs greater photo-fraction; this allows for a compact detector. High operating speed is needed for good resolution of spectra. Precision of time measurement with a scintillation detector is proportional to . Short decay times are important for the measurement of time intervals and for the operation in fast coincidence circuits. High density and fast response time can allow detection of rare events in particle physics. Particle energy deposited in the material of a scintillator is proportional to the scintillator's response. Charged particles, γ-quanta and ions have different slopes when their response is measured. Thus, scintillators could be used to identify various types of γ-quanta and particles in fluxes of mixed radiation. Another consideration of scintillators is the cost of producing them. Most crystal scintillators require high-purity chemicals and sometimes rare-earth metals that are fairly expensive. Not only are the materials an expenditure, but many crystals require expensive furnaces and almost six months of growth and analyzing time. Currently, other scintillators are being researched for reduced production cost. Several other properties are also desirable in a good detector scintillator: a low gamma output (i.e., a high efficiency for converting the energy of incident radiation into scintillation photons), transparency to its own scintillation light (for good light collection), efficient detection of the radiation being studied, a high
stopping power Stopping power is the supposed ability of a weapon – typically a ranged weapon such as a firearm – to cause a target (human or animal) to be incapacitated or immobilized. Stopping power contrasts with lethality in that it pertains only to a ...
, good linearity over a wide range of energy, a short rise time for fast timing applications (e.g., coincidence measurements), a short decay time to reduce detector dead-time and accommodate high event rates, emission in a spectral range matching the spectral sensitivity of existing PMTs (although wavelength shifters can sometimes be used), an
index of refraction In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
near that of glass (≈1.5) to allow optimum coupling to the PMT window. Ruggedness and good behavior under high temperature may be desirable where resistance to vibration and high temperature is necessary (e.g., oil exploration). The practical choice of a scintillator material is usually a compromise among those properties to best fit a given application. Among the properties listed above, the light output is the most important, as it affects both the efficiency and the resolution of the detector (the efficiency is the ratio of detected particles to the total number of particles impinging upon the detector; the energy resolution is the ratio of the full width at half maximum of a given energy peak to the peak position, usually expressed in %). The light output is a strong function of the type of incident particle or photon and of its energy, which therefore strongly influences the type of scintillation material to be used for a particular application. The presence of quenching effects results in reduced light output (i.e., reduced scintillation efficiency). Quenching refers to all radiationless deexcitation processes in which the excitation is degraded mainly to heat. The overall signal production efficiency of the detector, however, also depends on the
quantum efficiency The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device, or it may refer to the TMR effect of a magnetic tunnel junction. This article deals with the term as a measurement of ...
of the PMT (typically ~30% at peak), and on the efficiency of light transmission and collection (which depends on the type of reflector material covering the scintillator and light guides, the length/shape of the light guides, any light absorption, etc.). The light output is often quantified as a number of scintillation photons produced per keV of deposited energy. Typical numbers are (when the incident particle is an electron): ≈40 photons/keV for , ~10 photons/keV for plastic scintillators, and ~8 photons/keV for
bismuth germanate Bismuth germanium oxide or bismuth germanate is an inorganic chemical compound of bismuth, germanium and oxygen. Most commonly the term refers to the compound with chemical formula (BGO), with the cubic crystal, cubic evlitine crystal structure, ...
(). Scintillation detectors are generally assumed to be linear. This assumption is based on two requirements: (1) that the light output of the scintillator is proportional to the energy of the incident radiation; (2) that the electrical pulse produced by the photomultiplier tube is proportional to the emitted scintillation light. The linearity assumption is usually a good rough approximation, although deviations can occur (especially pronounced for particles heavier than the
proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
at low energies). Resistance and good behavior under high-temperature, high-vibration environments is especially important for applications such as oil exploration ( wireline logging, measurement while drilling). For most scintillators, light output and scintillation decay time depends on the temperature. This dependence can largely be ignored for room-temperature applications since it is usually weak. The dependence on the temperature is also weaker for organic scintillators than it is for inorganic crystals, such as NaI-Tl or BGO. Strong dependence of decay time on the temperature in BGO scintillator is used for remote monitoring of temperature in vacuum environment. The coupled PMTs also exhibit temperature sensitivity, and can be damaged if submitted to mechanical shock. Hence, high temperature rugged PMTs should be used for high-temperature, high-vibration applications. The time evolution of the number of emitted scintillation photons ''N'' in a single scintillation event can often be described by linear superposition of one or two exponential decays. For two decays, we have the form: N = A\exp\left(-\frac\right) + B\exp\left(-\frac\right) where ''τf'' and ''τs'' are the fast (or prompt) and the slow (or delayed) decay constants. Many scintillators are characterized by 2 time components: one fast (or prompt), the other slow (or delayed). While the fast component usually dominates, the relative amplitude ''A'' and ''B'' of the two components depend on the scintillating material. Both of these components can also be a function of the energy loss ''dE''/''dx''. In cases where this energy loss dependence is strong, the overall decay time constant varies with the type of incident particle. Such scintillators enable pulse shape discrimination, i.e., particle identification based on the decay characteristics of the PMT electric pulse. For instance, when BaF2 is used, γ rays typically excite the fast component, while α particles excite the slow component: it is thus possible to identify them based on the decay time of the PMT signal.


Types of scintillators


Organic crystals

Organic scintillators are
aromatic hydrocarbon Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were ...
compounds which contain
benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
ring structures interlinked in various ways. Their luminescence typically decays within a few nanoseconds. Some organic scintillators are pure crystals. The most common types are
anthracene Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes, as a scintil ...
(, decay time ≈30 ns),
stilbene Stilbene may refer to one of the two stereoisomers of 1,2-diphenylethene: * (''E'')-Stilbene (''trans'' isomer) * (''Z'')-Stilbene (''cis'' isomer) See also * Stilbenoid Stilbenoids are hydroxylated derivatives of stilbene. They have a C6–C ...
(, 4.5 ns decay time), and
naphthalene Naphthalene is an organic compound with formula . It is the simplest polycyclic aromatic hydrocarbon, and is a white Crystal, crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 Parts-per notation ...
(, few ns decay time). They are very durable, but their response is
anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...
(which spoils energy resolution when the source is not collimated), and they cannot be easily machined, nor can they be grown in large sizes; hence they are not very often used. Anthracene has the highest light output of all organic scintillators and is therefore chosen as a reference: the light outputs of other scintillators are sometimes expressed as a percentage of anthracene light output.


Organic liquids

These are liquid solutions of one or more organic scintillators in an
organic solvent A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...
. The typical solutes are fluors such as ''p''-terphenyl (), PBD (), butyl PBD (), PPO (), and wavelength shifter such as POPOP (). The most widely used solvents are
toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water Water is an inorganic compound with the c ...
, xylene,
benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
, phenylcyclohexane, triethylbenzene, and decalin. Liquid scintillators are easily loaded with other additives such as wavelength shifters to match the spectral sensitivity range of a particular PMT, or 10B to increase the neutron detection efficiency of the
scintillation counter A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the Electron excitation, excitation effect of incident radiation on a Scintillation (physics), scintillating material, and detecting the resultant li ...
itself (since 10B has a high interaction cross section with
thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium wit ...
s). Newer approaches combine several solvents or load different metals to achieve identification of incident particles. For many liquids, dissolved
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
can act as a quenching agent and lead to reduced light output, hence the necessity to seal the solution in an oxygen-free, airtight enclosure.


Plastic scintillators

The term "plastic scintillator" typically refers to a scintillating material in which the primary fluorescent emitter, called a fluor, is suspended in the base, a solid polymer matrix. While this combination is typically accomplished through the dissolution of the fluor prior to bulk polymerization, the fluor is sometimes associated with the polymer directly, either covalently or through coordination, as is the case with many Li6 plastic scintillators.
Polyethylene naphthalate Polyethylene naphthalate (poly(ethylene 2,6-naphthalate) or PEN) is a polyester derived from naphthalene-2,6-dicarboxylic acid and ethylene glycol. As such it is related to poly(ethylene terephthalate), but with superior barrier properties. Prod ...
has been found to exhibit scintillation by itself without any additives and is expected to replace existing plastic scintillators due to higher performance and lower price. The advantages of plastic scintillators include fairly high light output and a relatively quick signal, with a decay time of 2–4 nanoseconds, but perhaps the biggest advantage of plastic scintillators is their ability to be shaped, through the use of molds or other means, into almost any desired form with what is often a high degree of durability. Plastic scintillators are known to show light output saturation when the energy density is large ( Birks' Law).


Bases

The most common bases used in plastic scintillators are the aromatic plastics, polymers with aromatic rings as pendant groups along the polymer backbone, amongst which polyvinyltoluene (PVT) and
polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It i ...
(PS) are the most prominent. While the base does fluoresce in the presence of ionizing radiation, its low yield and negligible transparency to its own emission make the use of fluors necessary in the construction of a practical scintillator. Aside from the aromatic plastics, the most common base is polymethylmethacrylate (PMMA), which carries two advantages over many other bases: high ultraviolet and visible light transparency and mechanical properties and higher durability with respect to brittleness. The lack of fluorescence associated with PMMA is often compensated through the addition of an aromatic co-solvent, usually naphthalene. A plastic scintillator based on PMMA in this way boasts transparency to its own radiation, helping to ensure uniform collection of light. Other common bases include polyvinyl xylene (PVX) polymethyl, 2,4-dimethyl, 2,4,5-trimethyl styrenes, polyvinyl diphenyl, polyvinyl naphthalene, polyvinyl tetrahydronaphthalene, and copolymers of these and other bases.


Fluors

Also known as luminophors, these compounds absorb the scintillation of the base and then emit at larger wavelength, effectively converting the ultraviolet radiation of the base into the more easily transferred visible light. Further increasing the attenuation length can be accomplished through the addition of a second fluor, referred to as a spectrum shifter or converter, often resulting in the emission of blue or green light. Common fluors include polyphenyl hydrocarbons, oxazole and oxadiazole aryls, especially, n-terphenyl (PPP), 2,5-diphenyloxazole (PPO), 1,4-di-(5-phenyl-2-oxazolyl)-benzene (POPOP), 2-phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD), and 2-(4’-tert-butylphenyl)-5-(4’’-biphenylyl)-1,3,4-oxadiazole (B-PBD).


Inorganic crystals

Inorganic scintillators are usually crystals grown in high temperature furnaces, for example,
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fl ...
s, often with a small amount of activator impurity. The most widely used is (
thallium Thallium is a chemical element; it has Symbol (chemistry), symbol Tl and atomic number 81. It is a silvery-white post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Che ...
-doped
sodium iodide Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions ...
); its scintillation light is blue. Other inorganic alkali halide crystals are: , , (pure), , , . Some non-alkali crystals include: BGO, , , , , , (), , , GAGG:Ce. (For more examples, see also phosphors). Newly developed products include , lanthanum chloride doped with cerium, as well as a cerium-doped lanthanum bromide, . They are both very
hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal or room temperature. If water mol ...
(i.e., damaged when exposed to moisture in the air) but offer excellent light output and energy resolution (63 photons/keV γ for versus 38 photons/keV γ for ), a fast response (16 ns for versus 230 ns for ), excellent linearity, and a very stable light output over a wide range of temperatures. In addition LaBr3(Ce) offers a higher stopping power for γ rays (density of 5.08 g/cm3 versus 3.67 g/cm3 for ). LYSO () has an even higher density (7.1 g/cm3, comparable to ), is non-hygroscopic, and has a higher light output than (32 photons/keV γ), in addition to being rather fast (41 ns decay time versus 300 ns for ). A disadvantage of some inorganic crystals, e.g., NaI, is their hygroscopicity, a property which requires them to be housed in an airtight container to protect them from moisture. and BaF2 are only slightly hygroscopic and do not usually need protection. CsF, , , are hygroscopic, while , , , and are not. Inorganic crystals can be cut to small sizes and arranged in an array configuration so as to provide position sensitivity. Such arrays are often used in medical physics or security applications to detect X-rays or γ rays: high-''Z'', high density materials (e.g. LYSO, BGO) are typically preferred for this type of applications. Scintillation in inorganic crystals is typically slower than in organic ones, ranging typically from 1.48 ns for to 9000 ns for . Exceptions are (~5 ns), fast (0.7 ns; the slow component is at 630 ns), as well as the newer products (, 28 ns; , 16 ns; , 41 ns). For the imaging application, one of the advantage of inorganic crystals is very high light yield. Some high light yield scintillators above 100,000 photons/MeV at 662 keV are very recently reported for , , and . Many semiconductor scintillator phosphors are known, such as ZnS(Ag) (mentioned in the history section), CdS(Ag), ZnO(Zn), ZnO(Ga), CdS(In), ZnSe(O), and ZnTe(O), but none of these are available as single crystals. CdS(Te) and ZnSe(Te) have been commercially available in single crystal form, but their luminosity is partially quenched at room temperature. GaAs(Si,B) is a recently discovered cryogenic semiconductor scintillator with high light output in the infra-red and apparently no afterglow. In combination with ultra-low noise cryogenic photodetectors it is the target in experiments to detect rare, low-energy electronic excitations from interacting dark matter.


Gaseous scintillators

Gaseous scintillators consist of
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
and the
noble gas The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...
es
helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
,
argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
,
krypton Krypton (from 'the hidden one') is a chemical element; it has symbol (chemistry), symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace element, trace amounts in the Earth's atmosphere, atmosphere and is of ...
, and
xenon Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
, with helium and xenon receiving the most attention. The scintillation process is due to the de-excitation of single atoms excited by the passage of an incoming particle. This de-excitation is very rapid (~1 ns), so the detector response is quite fast. Coating the walls of the container with a wavelength shifter is generally necessary as those gases typically emit in the
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
and PMTs respond better to the visible blue-green region. In nuclear physics, gaseous detectors have been used to detect fission fragments or heavy
charged particle In physics, a charged particle is a particle with an electric charge. For example, some elementary particles, like the electron or quarks are charged. Some composite particles like protons are charged particles. An ion, such as a molecule or atom ...
s.


Glasses

The most common
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
scintillators are
cerium Cerium is a chemical element; it has Chemical symbol, symbol Ce and atomic number 58. It is a hardness, soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it ...
-activated lithium or boron silicates. Since both lithium and boron have large neutron cross-sections, glass detectors are particularly well suited to the detection of thermal (slow) neutrons. Lithium is more widely used than boron since it has a greater energy release on capturing a neutron and therefore greater light output. Glass scintillators are however sensitive to electrons and γ rays as well (pulse height discrimination can be used for particle identification). Being very robust, they are also well-suited to harsh environmental conditions. Their response time is ≈10 ns, their light output is however low, typically ≈30% of that of anthracene.


Solution-based perovskite scintillators

Scintillation properties of organic-inorganic methylamonium (MA) lead halide
perovskites A perovskite is a crystalline material of formula ABX3 with a crystal structure similar to that of Perovskite, the mineral perovskite, this latter consisting of calcium titanium oxide (CaTiO3). The mineral was first discovered in the Ural Moun ...
under proton irradiation were first reported by Shibuya et al. in 2002 and the first γ-ray pulse height spectrum, although still with poor energy resolution, was reported on () by van Eijk et al. in 2008 . Birowosuto at al. studied the scintillation properties of 3-D and 2-D layered perovskites under X-ray excitation. MAPbBr3 () emits at 550 nm and MAPbI3 () at 750 nm which is attributed to exciton emission near the band gap of the compounds. In this first generation of Pb-halide perovskites the emission is strongly quenched at room temperature and less than 1 000 ph/MeV survive. At 10 K however intense emission is observed and write about yields up to 200 000 ph/MeV. The quenching is attributed to the small e-h binding energy in the exciton that decreases for Cl to Br to I . Interestingly one may replace the organic MA group with Cs+ to obtain full inorganic CsPbX3 halide perovskites. Depending on the Cl, Br, I content the triplet X-ray excited exciton emission can be tuned from 430 nm to 700 nm . One may also dilute Cs with Rb to obtain similar tuning. Above very recent developments demonstrate that the organic-inorganic and all inorganic Pb-halide perovskites have various interesting scintillation properties. However, the recent two-dimensional perovskite single crystals with light yields between 10 000 and 40 000 ph/MeV and decay times below 10 ns at room temperature will be more favorable as they may have much larger Stokes shift up to 200 nm in comparison with CsPbBr3 quantum dot scintillators and this is essential to prevent self reabsorption for scintillators. More recently, a new material class first reported by Professo
Biwu Ma's
research group, called 0D organic metal halide hybrid (OMHH), an extension of the perovskite materials. This class of materials exhibits strong exciton binding of hundreds of meV, resulting in their high photoluminescent quantum efficiency of almost unity. Their large stoke shift and reabsorption-free properties make them desirable. Their potential applications for scintillators have been reported by the same group, and others. In 2020,(C38H34P2)MnBr4 was reported to have a light yield up to 80 000 Photon/MeV despite its low Z compared to traditional all inorganic. Impressive light yields from other 0D OMHH have been reported. There is a great potential to realize new generation scintillators from this material class. However, they are limited by their relatively long response time in microseconds, which is an area of intense research.


Physics of scintillation


Organic scintillators

Transitions made by the free
valence electron In chemistry and physics, valence electrons are electrons in the outermost shell of an atom, and that can participate in the formation of a chemical bond if the outermost shell is not closed. In a single covalent bond, a shared pair forms with b ...
s of the
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s are responsible for the production of scintillation light in organic crystals. These electrons are associated with the whole molecule rather than any particular atom and occupy the so-called -
molecular orbital In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding ...
s. The
ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state ...
S0 is a
singlet state In quantum mechanics, a singlet state usually refers to a system in which all electrons are paired. The term 'singlet' originally meant a linked set of particles whose net angular momentum is zero, that is, whose overall spin quantum number s=0. A ...
above which are the excited singlet states (S*, the lowest
triplet state In quantum mechanics, a triplet state, or spin triplet, is the quantum state of an object such as an electron, atom, or molecule, having a quantum spin ''S'' = 1. It has three allowed values of the spin's projection along a given axis ''m''S = ...
(T0), and its excited levels (T*, A
fine structure In atomic physics, the fine structure describes the splitting of the spectral lines of atoms due to electron spin and relativistic corrections to the non-relativistic Schrödinger equation. It was first measured precisely for the hydrogen atom ...
corresponding to
molecular vibration A molecular vibration is a Periodic function, periodic motion of the atoms of a molecule relative to each other, such that the center of mass of the molecule remains unchanged. The infrared spectroscopy correlation table, typical vibrational fre ...
al modes is associated with each of those electron levels. The energy spacing between electron levels is ≈1 eV; the spacing between the vibrational levels is about 1/10 of that for electron levels. An incoming particle can excite either an electron level or a vibrational level. The singlet excitations immediately decay (< 10 ps) to the S* state without the emission of radiation (internal degradation). The S* state then decays to the ground state S0 (typically to one of the vibrational levels above S0) by emitting a scintillation
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
. This is the prompt component or
fluorescence Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colore ...
. The transparency of the scintillator to the emitted photon is due to the fact that the energy of the photon is less than that required for an S0 → S* transition (the transition is usually being to a vibrational level above S0). When one of the triplet states gets excited, it immediately decays to the T0 state with no emission of radiation (internal degradation). Since the T0 → S0 transition is very improbable, the T0 state instead decays by interacting with another T0 molecule: T_0 + T_0 \rightarrow S^ + S_0 + \text and leaves one of the molecules in the S* state, which then decays to S0 with the release of a scintillation photon. Since the T0-T0 interaction takes time, the scintillation light is delayed: this is the slow or delayed component (corresponding to delayed fluorescence). Sometimes, a direct T0 → S0 transition occurs (also delayed), and corresponds to the phenomenon of
phosphorescence Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluor ...
. Note that the observational difference between delayed-fluorescence and phosphorescence is the difference in the
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s of the emitted optical photon in an S* → S0 transition versus a T0 → S0 transition. Organic scintillators can be dissolved in an
organic solvent A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...
to form either a liquid or plastic scintillator. The scintillation process is the same as described for organic crystals (above); what differs is the mechanism of energy absorption: energy is first absorbed by the solvent, then passed onto the scintillation
solute In chemistry, a solution is defined by IUPAC as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are ...
(the details of the transfer are not clearly understood).


Inorganic scintillators

The scintillation process in inorganic materials is due to the
electronic band structure In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called ''band gaps'' or '' ...
found in
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s and is not molecular in nature as is the case with organic scintillators. An incoming particle can excite an electron from the
valence band In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...
to either the
conduction band In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...
or the
exciton An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb's law, Coulomb force resulting from their opposite charges. It is an electrically neutral quasiparticle regarded as ...
band (located just below the conduction band and separated from the valence band by an energy gap; se
picture
. This leaves an associated
hole A hole is an opening in or through a particular medium, usually a solid Body (physics), body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in m ...
behind, in the valence band. Impurities create electronic levels in the forbidden gap. The excitons are loosely bound electron-hole pairs which wander through the
crystal lattice In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystal, crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that ...
until they are captured as a whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light (fast component). The activator impurities are typically chosen so that the emitted light is in the visible range or near-UV where
photomultiplier A photomultiplier is a device that converts incident photons into an electrical signal. Kinds of photomultiplier include: * Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sh ...
s are effective. The holes associated with electrons in the conduction band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons. The delayed de-excitation of those metastable impurity states again results in scintillation light (slow component). BGO ( bismuth germanium oxide) is a pure inorganic scintillator without any activator impurity. There, the scintillation process is due to an optical transition of the ion, a major constituent of the crystal. In tungstate scintillators and the emission is due to radiative decay of self-trapped excitons. The scintillation process in GaAs doped with silicon and boron impurities is different from conventional scintillators in that the silicon ''n''-type doping provides a built-in population of delocalized electrons at the bottom of the conduction band. Some of the boron impurity atoms reside on arsenic sites and serve as acceptors. A scintillation photon is produced whenever an acceptor atom such as boron captures an ionization hole from the valence band and that hole recombines radiatively with one of the delocalized electrons. Unlike many other semiconductors, the delocalized electrons provided by the silicon are not “frozen out” at cryogenic temperatures. Above the Mott transition concentration of free carriers per cm3, the “metallic” state is maintained at cryogenic temperatures because mutual repulsion drives any additional electrons into the next higher available energy level, which is in the conduction band. The spectrum of photons from this process is centered at 930 nm (1.33 eV) and there are three other emission bands centered at 860, 1070, and 1335 nm from other minor processes. Each of these emission bands has a different luminosity and decay time. The high scintillation luminosity is surprising because (1) with a refractive index of about 3.5, escape is inhibited by total internal reflection and (2) experiments at 90K report narrow-beam infrared absorption coefficients of several per cm. Recent Monte Carlo and Feynman path integral calculations have shown that the high luminosity could be explained if most of the narrow beam absorption is actually a ''novel'' optical scattering from the conduction electrons with a cross section of about 5 x 10−18 cm2 that allows scintillation photons to escape total internal reflection. This cross section is about 107 times larger than Thomson scattering but comparable to the optical cross section of the conduction electrons in a metal mirror.


Gases

In gases, the scintillation process is due to the de-excitation of single atoms excited by the passage of an incoming particle (a very rapid process: ≈1 ns).


Response to various radiations


Heavy ions

Scintillation counters are usually not ideal for the detection of heavy ions for three reasons: # The very high ionizing power of heavy ions induces quenching effects which result in a reduced light output (e.g. for equal energies, a
proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
will produce 1/4 to 1/2 the light of an
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
, while
alphas ''Alphas'' is an American superhero drama television series created by Zak Penn and Michael Karnow. It follows a group of people with superhuman abilities, known as "Alphas", as they work to prevent crimes committed by other Alphas. The serie ...
will produce only about 1/10 the light); # The high
stopping power Stopping power is the supposed ability of a weapon – typically a ranged weapon such as a firearm – to cause a target (human or animal) to be incapacitated or immobilized. Stopping power contrasts with lethality in that it pertains only to a ...
of the particles also results in a reduction of the fast component relative to the slow component, increasing detector dead-time; # Strong non-linearities are observed in the detector response especially at lower energies. The reduction in light output is stronger for organics than for inorganic crystals. Therefore, where needed, inorganic crystals, e.g. , (typically used in thin sheets as α-particle monitors), , should be preferred to organic materials. Typical applications are α- survey instruments,
dosimetry Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingest ...
instruments, and heavy ion ''dE''/''dx'' detectors. Gaseous scintillators have also been used in
nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies th ...
experiments.


Electrons

The detection efficiency for
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s is essentially 100% for most scintillators. But because electrons can make large angle
scattering In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...
s (sometimes
backscatter In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, ...
ings), they can exit the detector without depositing their full energy in it. The back-scattering is a rapidly increasing function of the atomic number ''Z'' of the scintillator material. Organic scintillators, having a lower ''Z'' than inorganic crystals, are therefore best suited for the detection of low-energy (< 10 MeV)
beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus, known as beta decay. There are two forms of beta decay, β− decay and ...
s. The situation is different for high energy electrons: since they mostly lose their energy by
bremsstrahlung In particle physics, bremsstrahlung (; ; ) is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic ...
at the higher energies, a higher-''Z'' material is better suited for the detection of the bremsstrahlung photon and the production of the electromagnetic shower which it can induce.


Gamma rays

High-''Z'' materials, e.g. inorganic crystals, are best suited for the detection of
gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
s. The three basic ways that a gamma ray interacts with matter are: the
photoelectric effect The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physi ...
,
Compton scattering Compton scattering (or the Compton effect) is the quantum theory of high frequency photons scattering following an interaction with a charged particle, usually an electron. Specifically, when the photon hits electrons, it releases loosely bound e ...
, and
pair production Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers ...
. The photon is completely absorbed in photoelectric effect and pair production, while only partial energy is deposited in any given Compton scattering. The cross section for the photoelectric process is proportional to ''Z''5, that for pair production proportional to ''Z''2, whereas Compton scattering goes roughly as ''Z''. A high-''Z'' material therefore favors the former two processes, enabling the detection of the full energy of the gamma ray. If the gamma rays are at higher energies (>5 MeV), pair production dominates.


Neutrons

Since the
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
is not charged it does not interact via the
Coulomb force Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that calculates the amount of force between two electrically charged particles at rest. This electric force is conventionally called the ''electrostatic ...
and therefore does not ionize the scintillation material. It must first transfer some or all of its energy via the strong force to a charged
atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford at the Department_of_Physics_and_Astronomy,_University_of_Manchester , University of Manchester ...
. The positively charged nucleus then produces
ionization Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...
.
Fast neutrons The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...
(generally >0.5 MeV ) primarily rely on the
recoil Recoil (often called knockback, kickback or simply kick) is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, for according to Newton's third law the force requ ...
proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
in (n,p) reactions; materials rich in
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
, e.g. plastic scintillators, are therefore best suited for their detection.
Slow neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...
s rely on
nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a t ...
s such as the (n,γ) or (n,α) reactions, to produce ionization. Their
mean free path In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
is therefore quite large unless the scintillator material contains nuclides having a high cross section for these nuclear reactions such as 6Li or 10B. Materials such as LiI(Eu) or
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
s are therefore particularly well-suited for the detection of slow (thermal) neutrons.


List of inorganic scintillators

The following is a list of commonly used inorganic crystals: * or barium fluoride: contains a very fast and a slow component. The fast scintillation light is emitted in the UV band (220 nm) and has a 0.7 ns decay time (smallest decay time for any scintillator), while the slow scintillation light is emitted at longer wavelengths (310 nm) and has a 630 ns decay time. It is used for fast timing applications, as well as applications for which pulse shape discrimination is needed. The light yield of is about 12 photons/keV. is not hygroscopic. * or
bismuth germanate Bismuth germanium oxide or bismuth germanate is an inorganic chemical compound of bismuth, germanium and oxygen. Most commonly the term refers to the compound with chemical formula (BGO), with the cubic crystal, cubic evlitine crystal structure, ...
: bismuth germanate has a higher stopping power, but a lower optical yield than . It is often used in coincidence detectors for detecting back-to-back
gamma rays A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
emitted upon
positron The positron or antielectron is the particle with an electric charge of +1''elementary charge, e'', a Spin (physics), spin of 1/2 (the same as the electron), and the same Electron rest mass, mass as an electron. It is the antiparticle (antimatt ...
annihilation In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy a ...
in
positron emission tomography Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, r ...
machines. * or cadmium tungstate: a high density, high atomic number scintillator with a very long decay time (14 μs), and relatively high light output (about 1/3 of that of ). is routinely used for X-ray detection (CT scanners). Having very little 228Th and 226Ra contamination, it is also suitable for low activity counting applications. * or
calcium fluoride Calcium fluoride is the inorganic compound of the elements calcium and fluorine with the formula CaF2. It is a white solid that is practically insoluble in water. It occurs as the mineral fluorite (also called fluorspar), which is often deeply col ...
doped with
europium Europium is a chemical element; it has symbol Eu and atomic number 63. It is a silvery-white metal of the lanthanide series that reacts readily with air to form a dark oxide coating. Europium is the most chemically reactive, least dense, and soft ...
: The material is not hygroscopic, has a 940 ns decay time, and is relatively low-''Z''. The latter property makes it ideal for detection of low energy β particles because of low backscattering, but not very suitable for γ detection. Thin layers of have also been used with a thicker slab of to make phoswiches capable of discriminating between α, β, and γ particles. * or
calcium tungstate Scheelite is a calcium tungstate mineral with the chemical formula calcium, Catungsten, Woxygen, O4. It is an important ore of tungsten (wolfram). Scheelite is originally named after Swedish chemist Carl Wilhelm Scheele (1742–1786). Well-form ...
: exhibits long decay time 9 μs and short wavelength emission with maximum at 420 nm matching sensitivity curve of bialkali PMT. The light yield and energy resolution of the scintillator (6.6% for 137Cs) is comparable with that of . * : undoped cesium iodide emits predominantly at 315 nm, is only slightly hygroscopic, and has a very short decay time (16 ns), making it suitable for fast timing applications. The light output is quite low at room temperature, however, it significantly increases with cooling. * or cesium iodide doped with sodium: the crystal is less bright than , but comparable in light output to . The wavelength of maximum emission is at 420 nm, well matched to the photocathode sensitivity of bialkali PMTs. It has a slightly shorter decay time than (630 ns versus 1000 ns for ). is hygroscopic and needs an airtight enclosure for protection against moisture. * or cesium iodide doped with
thallium Thallium is a chemical element; it has Symbol (chemistry), symbol Tl and atomic number 81. It is a silvery-white post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Che ...
: these crystals are one of the brightest scintillators. Its maximum wavelength of light emission is in the green region at 550 nm. is only slightly hygroscopic and does not usually require an airtight enclosure. * GaAs or gallium arsenide (suitably doped with silicon and boron impurities) is a cryogenic ''n''-type semiconductor scintillator with a low cryogenic bandgap (1.52 eV) and high light output (100 photons/keV) in the infra-red (930 nm). The absence of thermally stimulated luminescence is evidence for the absence of afterglow, which makes it attractive for detecting rare, low energy electronic excitations from interacting dark matter. Large (5 kg) high-quality crystals are commercially grown for electronic applications. * or gadolinium oxysulfide has a high stopping power due to its relatively high density (7.32 g/cm3) and the high atomic number of
gadolinium Gadolinium is a chemical element; it has Symbol (chemistry), symbol Gd and atomic number 64. It is a silvery-white metal when oxidation is removed. Gadolinium is a malleable and ductile rare-earth element. It reacts with atmospheric oxygen or moi ...
. The light output is also good, making it useful as a scintillator for x-ray imaging applications. * (or lanthanum bromide doped with cerium): a better (novel) alternative to ; denser, more efficient, much faster (having a decay time about ~20ns), offers superior energy resolution due to its very high light output. Moreover, the light output is very stable and quite high over a very wide range of temperatures, making it particularly attractive for high temperature applications. Depending on the application, the intrinsic activity of 138La can be a disadvantage. is very hygroscopic. * (or lanthanum chloride doped with
cerium Cerium is a chemical element; it has Chemical symbol, symbol Ce and atomic number 58. It is a hardness, soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it ...
): very fast, high light output. is a cheaper alternative to . It is also quite hygroscopic. * or lead tungstate: due to its high-''Z'', is suitable for applications where a high stopping power is required (e.g. γ ray detection). * or lutetium iodide. * or lutetium oxyorthosilicate (): used in
positron emission tomography Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, r ...
because it exhibits properties similar to bismuth germanate (), but with a higher light yield. Its only disadvantage is the intrinsic background from the
beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...
of natural 176Lu. * (): comparable in density to , but much faster and with much higher light output; excellent for medical imaging applications. is non-hygroscopic. * or
sodium iodide Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions ...
doped with
thallium Thallium is a chemical element; it has Symbol (chemistry), symbol Tl and atomic number 81. It is a silvery-white post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Che ...
: is by far the most widely used scintillator material. It is available in single crystal form or the more rugged polycrystalline form (used in high vibration environments, e.g. wireline logging in the oil industry). Other applications include nuclear medicine, basic research, environmental monitoring, and aerial surveys. is very hygroscopic and needs to be housed in an airtight enclosure. * or yttrium aluminum garnet: is non-hygroscopic. The wavelength of maximum emission is at 550 nm, well-matched to red-resistive PMTs or photo-diodes. It is relatively fast (70 ns decay time). Its light output is about 1/3 of that of . The material exhibits some properties that make it particularly attractive for electron microscopy applications (e.g. high electron conversion efficiency, good resolution, mechanical ruggedness and long lifetime). * or
zinc sulfide Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various i ...
: is one of the older inorganic scintillators (the first experiment making use of a scintillator by Sir
William Crookes Sir William Crookes (; 17 June 1832 – 4 April 1919) was an English chemist and physicist who attended the Royal College of Chemistry, now part of Imperial College London, and worked on spectroscopy. He was a pioneer of vacuum tubes, inventing ...
(1903) involved a ZnS screen). It is only available as a polycrystalline powder, however. Its use is therefore limited to thin screens used primarily for α particle detection. * or
zinc tungstate Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic table ...
is similar to scintillator exhibiting long decay constant 25 μs and slightly lower light yield.


See also

*
Gamma spectroscopy Gamma-ray spectroscopy is the ''qualitative'' study of the energy spectra of gamma-ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics. Gamma-ray spectrometry, on the other hand, is the method used to acqu ...
*
Liquid scintillation counting Liquid scintillation counting is the measurement of radioactive activity of a sample material which uses the technique of mixing the active material with a liquid scintillator (e.g. zinc sulfide), and counting the resultant photon emissions. The pu ...
*
Scintillation counter A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the Electron excitation, excitation effect of incident radiation on a Scintillation (physics), scintillating material, and detecting the resultant li ...
* Scintillating bolometer * Neutron detection * Total absorption spectroscopy


Notes


References


Sources

* * * * * * * * * * * * * * * * * * * * * *


External links


Crystal Clear Collaboration
at
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in Meyrin, western suburb of Gene ...
*
Scintillation crystals and their general characteristics
* {{refend Photochemistry Particle detectors Ionising radiation detectors