Scintillator

A scintillator is a substance that absorbs high-energy (i.e., ionizing) electromagnetic or charged particle radiation then, in response, fluoresces photons at a characteristic Stokes-shifted (i.e., longer) wavelength, releasing the previously absorbed energy. See also: scintillation (physics).

Scintillators are defined by their

1. light output (i.e., number of emitted photons per unit of absorbed energy),
2. short fluorescence decay times, and
3. optical transparency at wavelengths of their own specific emission energy.

The latter two characteristics distinguish scintillators from phosphors. The lower the decay time of a scintillator—that is, the shorter the duration of its flashes of fluorescence—the less so-called "dead time" the detector will have, resulting in more ionizing events per unit of time that can be detected.

Scintillators are used in several physics research applications to detect electromagnetic waves or particles. In such applications a scintillator converts electromagnetic energy to light of a specific wavelength, which can be detected by devices such as photomultiplier tubes (PMTs).

Types of scintillators

Common scintillators used for radiation detection include inorganic crystals, organic plastics, and liquids; however, many materials scintillate at some level. For example, the scintillation of liquid xenon and neon is used in some ultra-low-background experiments.

Most scintillators for common use are either inorganic crystals or plastics, the most common being thallium-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 sources like tritium. By immersing the sample to be tested in the scintillation fluid, detector absorption problems are negated because of the shortened mean free paths associated with low-energy particles.

Organic crystals

Organic crystals are organic molecules that have an aromatic ring; the ionizing radiation excites the ring into a rotational or vibrational mode. Organic crystal scintillators are characterized by their fast response—on the order of one nanosecond. One of the best-known organic scintillators is anthracene.

Organic liquids

Organic crystal scintillators can be dissolved in a transparent liquids such as mineral oil. The solution will exhibit properties similar to the organic crystal depending on the purity and concentration of the solutions components. See also: liquid scintillation counting.

Organic plastics

Organic crystals can be dissolved in a transparent plastic that becomes solid at ambient temperature (e.g., polystyrene), thus forming plastic scintillators. The solid plastic matrix often has the effect of increasing the relaxation time to 2-3 nanoseconds. The three most common bases for plastic scintillators are polyvinyl toluene, polystyrene, and acrylic. However, acrylic has a low radiation-to-light conversion efficiency on its own as it contains no aromatic structures. Its efficiency can be increased by dissolving naphthalene into the acrylic. Plastics when used on their own typically emit ultraviolet photons. To convert these photons to less-attenuated visible light, a suitable fluorophor can be added to the plastic.

Plastic scintillators are robust and reliable, yet have several disadvantages. Plastic scintillators undergo aging (i.e., gradually losing light yield with time) when exposed to solvents, high temperatures, radiation, or mechanical load. The surface can be damaged by the formation of microscopic cracks, which cause light loss via 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 (i.e., F-Centers), which absorb energy in the ultraviolet range and the blue part of visible light spectrum, thus lowering the optical yield.

Inorganic crystals

Inorganic crystal scintillators are typically composed of alkali halides, such as sodium iodide (NaI). These scintillators are characterized by a high stopping power, which makes them well-suited for detecting high-energy radiation. A disadvantage of inorganic crystal scintillators is that their decay times are longer, on the order of hundreds of nanoseconds.

The following inorganic crystals are commonly used in inorganic scintillators:

• Barium fluoride (BaF₂)
• Bismuth germanate (abbreivated as BGO)
• : Bismuth germanate has a higher stopping power, but a lower optical yield than sodium iodide. It is often used in coincidence detectors for detecting back-to-back gamma rays emitted upon positron annihilation in positron emission tomography machines.
• Cadmium tungstate (CdWO₄)
• : Cadmium tungstate is used in computer tomography and early fluoroscopes.
• Calcium tungstate (CaWO₄)
• Cesium iodide (CsI)
• : Thallium-doped cesium iodide crystals are an alternative to sodium iodide crystals, as they are more durable mechanically and provide superior resistance to moisture.
• Gadolinium oxysulfide (Gd₂O₂S)
• : Gadolinium oxysulfide is often doped with terbium.
• Lanthanum bromide (LaBr₃)
• : Lanthanum bromide is often doped with cerium.
• Lead tungstate (PbWO₄)
• Lutetium iodide (LuI₃)
• Lutetium oxyorthosilicate (Lu₂SiO₅, or abbreviated as LSO)
• : Lutetium oxyorthosilicate is used in positron emission tomography because it exhibits properties similar to bismuth germanate (BGO), but with a higher light yield. Its only disadvantage is the intrinsic background from the beta decay of natural ¹⁷⁶Lu.
• Sodium iodide (NaI)
• : Thallium-doped sodium iodide crystals are used in gamma cameras used for nuclear medicine radioisotope imaging.
• Yttrium aluminium garnet (abbreviated as YAG)
• : Yttrium aluminium garnet is used as a phosphor, but is also suitable for use as a scintillator when in pure single crystal form and doped with cerium.
• Zinc sulfide (ZnS)
• : The first known inorganic scintillator as discovered by Sir William Crookes in 1903 contained zinc sulfide.
• Zinc tungstate (ZnWO₄)

See also

• Gamma spectroscopy
• Liquid scintillation counting
• Scintillation counter

External links

• Crystal Clear Collaboration at CERN
• Gamma Ray and Neutron Spectrometer
• Scintillation crystals and their general characteristics
• Scintillation Properties, from Lawrence Berkeley National Laboratory