A scintillator is material which exhibits scintillation the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming ionizing radiation, absorb its energy and scintillate, i.e. reemit the absorbed energy in the form of a small flash of light in the visible range. If the reemission occurs promptly, i.e. within the ~10−8s required for an atomic transition, the process is called fluorescence. Sometimes, the excited state is metastable, so the relaxation back out of the excited state is delayed the process then corresponds to either one of two phenomena, depending on the type of transition and hence the wavelength of the emitted optical photon: delayed fluorescence or phosphorescence.
Some properties desirable in a good scintillator detector are: 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, efficient detection of the radiation being studied, a high stopping power, good linearity over a wide range of energy, a short rise time for fast timing applications, 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 photomultiplier tube, an index of refraction near that of glass (≈1.5) to allow optimum coupling to the photomultiplier tube window. Ruggedness and good behavior under high temperature may be desirable where resistance to vibration and high temperature is necessary. The practical choice of a scintillator material is usually a compromise between those properties to best fit a given application. the light output is the most important, as it affects both the efficiency and the resolution of the detector . 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 radiation less 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 of the Photomultiplier tube, 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.
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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.
The time evolution of the number of emitted scintillation photons N in a single scintillation event can often be described by the linear superposition of one or two exponential decays. For two decays, we have the form:
Where τf and τs are the fast and the slow decay constants.
TYPES OF SCINTILLATORS:-
1) Organic crystals
Organic scintillators are aromatic hydrocarbon compounds containing linked or condensed benzene ring structures. They typically have a very rapid decay time. Some organic scintillates are pure crystals. The most common types are anthracene (C14H10, decay time ≈30 ns), stilbene (C14H12, few ns decay time), and naphthalene (C10H8, few ns decay time). They are very durable, but their response is anisotropic, 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 percent of anthracene light.
2) Organic Liquids
These are liquid solutions of one or more organic scintillators in an organic solvent. Thse typical solutes are fluors such as p-terphenyl (C18H14), PBD (C20H14N2O), butyl PBD (C24H22N2O), PPO(C15H11NO), and wavelength shifter such as POPOP (C24H16N2O). The most widely used solvents are toluene, xylene, benzene, 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 Photo multiplier tube to increase the neutron detection efficiency of the scintillation counter itself. For many liquids, dissolved oxygen can act as a quenching agent and lead to reduced light output, hence the necessity to seal the solution in an oxygen-free, air-tight enclosure.
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Plastic scintillators are solutions of organic scintillators in a solvent which is subsequently polymerized to form a solid. Some of the common solutes are p-Terphenyl The most widely used plastic solvents are polyvinyl toluene and polystyrene.
Plastics scintillators give a fast signal and a high light output. The number of emitted scintillation photons is best described by the convolution of an exponential decay and a Gaussian (rather than the exponential decay alone):
Where the a function f is a Gaussian.
HOW SCINTILLATOR DETECTOR OBTAINED:-
A scintillation detector or scintillation counter is obtained when a scintillator is coupled to an electronic light sensor such as a photomultiplier tube or a photodiode. Photomultiplier tube absorb the light emitted by the scintillator and reemit it in the form of electrons via the photoelectric effect. The subsequent multiplication of those 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 accomplish the same thing directly in the silicon.
A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic , ororganic liquid that fluoresces when struck by ionizing radiation. A sensitive photo multiplie rtube measures the light from the crystal. The Photo multiplie tube is attached to an electronic amplifier and other electronic equipment to count and possibly quantify the amplitude of the signals produced by the photomultiplier.
WORKING OF SCINTILLATOR COUNTER:-
When a charged particle strikes the scintillator, a flash of light is produced, which may or may not be in the visible region of the spectrum. Each charged particle produces a flash. If a flash is produced in a visible region, it can be observed through a microscope and counted - an impractical method. The association of a scintillator and photomultiplier with the counter circuits forms the basis of the scintillation counter apparatus. When a charged particle passes through the phosphor, some of the phosphor's atoms get excited and emit photons. The intensity of the light flash depends on the energy of the charged particles. Cesium iodide (CsI) in crystalline form is used as the scintillator for the detection of protons and alpha particles; sodium iodide (NaI) containing a small amount of thallium is used as a scintillator for the detection of gamma waves.
The scintillation counter has a layer of phosphor cemented in one of the ends of the photomultiplier. Its inner surface is coated with a photo-emitter with less work potential. This photoelectric emitter is called as photocathode and is connected to the negative terminal of a high tension battery. A number of electrodes called dynodes are arranged in the tube at increasing positive potential.
When a charged particle strikes the phosphor, a photon is emitted. This photon strikes the photocathode in the photomultiplier, releasing an electron. This electron accelerates towards the first dynode and hits it. Multiple secondary electrons are emitted, which accelerate towards the second dynode. More electrons are emitted and the chain continues, multiplying the effect of the first charged particle. By the time the electrons reach the last dynode, enough have been released to send a voltage pulse across the external resistors. This voltage pulse is amplified and recorded by the electronic counter.
APPLICATION OF SCINTILLATOR COUNTER:-
Scintillation counters can be used in a variety of applications.
National and homeland security
Several products have been introduced in the market utilising scintillation counters for detection of potentially dangerous gamma-emitting materials during transport. These include scintillation counters designed for freight terminals, border security, ports, weigh bridge applications, scrap metal yards and contamination monitoring of nuclear waste. There are variants of scintillation counters mounted on pick-up trucks and helicopters for rapid response in case of a security situation due to dirty bombs or radioactive waste, Hand-held units are also commonly used.
SCINTILLATOR COUNTER AS SPECTROMETER:-
Scintillators often convert a single photon of high energy radiation into high number of lower-energy photons, where the number of photons per megaelectronvolt of input energy is fairly constant. By measuring the intensity of the flash, it is therefore possible to discern the original photon's energy.
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The spectrometer consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier. The pulses are counted and sorted by their height, producing a x-y plot of scintillator flash brightness vs number of the flashes, which approximates the energy spectrum of the incident radiation, with some additional artifacts. A monochromatic gamma radiation produces a photopeak at its energy. The detector also shows response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously, appearing as sum peaks with energies up to the value of two or more photopeaks added.