What Is a Scintillator?
A scintillator is a general term for materials that emit light when exposed to charged particles or radiation.
They are classified into inorganic and organic scintillators. Inorganic scintillators, suitable for detecting gamma rays and X-rays, are made from crystals with high atomic numbers. They emit a large amount of light and have excellent energy resolution but respond slowly. Organic scintillators, including plastic and liquid types, are inexpensive, lightweight, and respond quickly, making them ideal for detecting alpha and beta rays.
Scintillators are used in various fields as part of scintillation detectors.
Uses of Scintillators
Scintillators convert radiation into light, ranging from ultraviolet to visible. This light is then transformed into electrical signals by photomultiplier tubes or opto-semiconductors, which are processed to generate images and other radiation-related information.
Applications include X-ray computed tomography (X-ray CT), positron emission tomography (PET), airport baggage screening, food inspection, non-destructive testing of electronic components, exploration of oil and mineral resources, nuclear reactor radiation monitoring, and research in particle, nuclear, and space physics.
Principle of Scintillators
The emission principle of scintillators varies between inorganic and organic types.
1. Inorganic Scintillators
In inorganic scintillators, charged particles or radiation excite electrons from the valence to the conduction band. The recombination of these electrons with holes in the valence band produces scintillation light, corresponding to the energy difference. Impurities in the crystal lattice create new energy levels, allowing the emission of visible light from these modified structures.
2. Organic Scintillators
Organic scintillators emit light through the excitation of individual molecules. Radiation excites electrons to higher energy states, predominantly to the first excited state, resulting in fluorescence. Some electrons may also transition to higher excited states before returning to the first excited state and eventually to the ground state, emitting phosphorescence or delayed fluorescence.
Structure of Scintillators
The structure of scintillators differs between inorganic and organic types.
1. Inorganic Scintillators
Examples include NaI:Tl, LSO:Ce, lead tungstate (PbWO4), gadolinium silicate (GSO: Gd2SiO5 with Ce), and bismuth germanate (BGO: Bi4Ge3O12). NaI:Tl, a commonly used scintillator, must be sealed to prevent degradation from moisture absorption.
LSO:Ce, an oxide-based scintillator, emits light via transitions in Ce3+ and offers faster response times compared to NaI:Tl.
2. Organic Scintillators
Organic scintillators include crystal, liquid, and plastic types. Liquid scintillators, such as naphthalene, are durable against strong irradiation. Organic molecules like anthracene and stilbene, with their pi-electronic structures, offer multiple excited states but are less commonly used due to their anisotropic response and processing challenges. Plastic scintillators, created by dissolving organic emitters in plastic, are user-friendly and suitable for detecting alpha and beta rays, though less effective for gamma rays.