In condensed matter physics, scintillation—also called radioluminescence—is a process where a material, known as a scintillator, produces ultraviolet or visible light when exposed to high-energy light, such as X-rays or gamma rays, or to fast-moving particles like electrons, alpha particles, neutrons, or ions.

Overview

Scintillation is a type of light emission that occurs when a material absorbs energy and then releases light of a specific type. This process happens in three main steps: first, the absorbed energy is changed into a usable form; second, the energy moves through the material; third, the energy is transferred to the part of the material that produces light. The light that is released usually has less energy than the energy that was originally absorbed, which means scintillation is typically a process that reduces energy levels.

Conversion processes

The first step in scintillation is called conversion. During this step, the energy from incoming radiation is absorbed by the scintillator material. This process creates highly energetic electrons and holes in the material. The way the scintillator absorbs energy depends on the type and energy of the radiation involved. For high-energy photons like X-rays (0.1 keV < E γ < 100 keV) and gamma rays (E γ > 100 keV), three main interactions occur: photoelectric absorption, Compton scattering, and pair production. Pair production only happens when the photon energy is greater than 1022 keV, which is enough to create an electron-positron pair.

These interactions have different absorption rates, which depend on the energy of the incoming radiation, the average atomic number of the material, and the material's density. The absorption of high-energy radiation is described by a formula: I = I₀ * e^(-μd), where I₀ is the initial radiation intensity, d is the material's thickness, and μ is the linear attenuation coefficient. This coefficient is the total of the contributions from each interaction type.

At lower X-ray energies (E γ ≲ 60 keV), the photoelectric effect is the main process. In this process, photons are fully absorbed by bound electrons in the material, usually electrons in the K- or L-shell of atoms. This absorption causes the electrons to be ejected, ionizing the atom. The contribution of the photoelectric effect to the attenuation coefficient is given by a formula involving the material's density (ρ), average atomic number (Z), a constant (n), and the photon energy (E γ). At low X-ray energies, materials with high atomic numbers and densities are better at absorbing radiation.

At higher energies (E γ ≳ 60 keV), Compton scattering becomes the dominant process. This occurs when photons collide with bound electrons, causing inelastic scattering and ionization. The contribution of Compton scattering to the attenuation coefficient depends on the material's density but not on its atomic number.

For gamma rays with energy greater than 1022 keV, pair production begins. This process happens when a photon's energy is converted into an electron-positron pair. These particles then interact with the material to create more electrons and holes. The contribution of pair production to the attenuation coefficient involves the electron's rest mass (mₑ) and the speed of light (c). At high gamma-ray energies, energy absorption depends on both the material's density and atomic number. Unlike the photoelectric effect and Compton scattering, pair production becomes more likely as photon energy increases and becomes the main process above E γ ~ 8 MeV.

The μ oc term includes minor contributions, such as Rayleigh scattering (at low energies) and photonuclear reactions (at very high energies). Rayleigh scattering has little effect, and photonuclear reactions are only important at extremely high energies.

After radiation energy is absorbed and converted into hot electrons and holes, these charge carriers interact with other particles in the scintillator, such as electrons, plasmons, and phonons. This interaction leads to an "avalanche event," where many secondary electron-hole pairs are created until the hot carriers lose enough energy.

The many energetic charge carriers then undergo thermalization, where they lose energy through interactions with phonons (for electrons) and Auger processes (for holes).

The time it takes for energy absorption and thermalization is estimated to be about 1 picosecond, which is much faster than the average decay time in photoluminescence.

Charge transport of excited carriers

The second stage of scintillation involves the movement of electrons and holes that have been slowed down by heat toward areas where light is produced. During this stage, these particles travel through the material. This is an important part of scintillation because many losses in efficiency often happen here. These losses are usually caused by defects in the scintillator crystal, such as impurities, missing atoms, or boundaries between crystal grains. The movement of charges can also slow down the timing of the scintillation process. This stage is not fully understood and depends heavily on the type of material used and how well it conducts electrical charges.

Luminescence

Once electrons and holes reach the luminescence centers, the third and final stage of scintillation occurs: luminescence. During this stage, electrons and holes are captured by the luminescent center and then recombine in a way that releases light. The details of this process depend on the type of material used for scintillation.

For photons like gamma rays, thallium-activated sodium iodide crystals (NaI(Tl)) are often used. For faster responses, though only 5% of the light output is produced, cesium fluoride crystals (CsF) can be used.

In organic molecules, scintillation happens because of π-orbitals. Organic materials form molecular crystals where molecules are loosely held together by forces called Van der Waals forces. The ground state of carbon is 1s² 2s² 2p². When carbon forms compounds, one of the 2s electrons moves to the 2p state, creating a configuration of 1s² 2s¹ 2p³. To explain carbon’s different bonding abilities, scientists say the four valence orbitals (one 2s and three 2p) mix or combine in different ways. For example, in a tetrahedral shape, the s and p orbitals combine to make four hybrid orbitals. In a trigonal shape, one p orbital stays unchanged, and three hybrid orbitals form by mixing the s, p x, and p y orbitals. Orbitals that are symmetrical around the bonding axis and plane are called σ-orbitals, and the bonds are σ-bonds. The remaining p orbital is called a π-orbital. A π-bond forms when two π-orbitals interact, which happens when their nodal planes are in the same plane.

In some organic molecules, π-orbitals interact to create a shared nodal plane. This forms delocalized π-electrons that can be excited by radiation. When these electrons return to their normal state, they release light.

The excited states of π-electron systems can be explained by the perimeter free-electron model (Platt 1949). This model describes polycyclic hydrocarbons made of condensed benzene-like rings, where no carbon atom is part of more than two rings and all carbon atoms are on the outer edges.

The ring can be thought of as a circle with a certain length (l). The electron’s wave function must follow the rules of a plane rotator. The solutions to the Schrödinger wave equation for this system are:

where q is the orbital ring quantum number, representing the number of wave function nodes. Since electrons can spin up or down and move in both directions around the ring, all energy levels except the lowest one are doubly degenerate.

This describes the π-electronic energy levels in organic molecules. When radiation is absorbed, molecules vibrate into an excited singlet state (S₁), and then return to the ground state (S₀) through an allowed transition that emits a photon. This process is called the "fast component" or fluorescence. Triplet states take much longer to decay than singlet states. This "slow component" can result from delayed fluorescence (like thermally activated delayed fluorescence or triplet-triplet annihilation) or phosphorescence (if the decay from a triplet state to the ground state involves a forbidden transition with a change in spin).

The proportion of "fast" and "slow" components depends on how much energy a particle loses as it moves through the material (dE/dx). Because of this, the light output from these components varies for different types of particles. This property allows scintillators to perform pulse shape discrimination: by analyzing the shape of the light signal, scientists can determine which type of particle was detected. The difference in pulse shape is visible in the trailing part of the signal, as it is caused by the decay of the excited states.