Lecoq
3.1 Basic Detector Principles and Scintillator Requirements
3.1.1 Interaction of Ionizing Radiation with Scintillator
A scintillator is a type of radiation detector that not only absorbs ionizing radiation but also converts a portion of the energy it absorbs into light Charged particles interact with the scintillator through Coulomb interactions, leading to atomic excitation or ionization In contrast, neutral particles initially interact with the nucleus, producing recoil protons or spallation fragments that subsequently transfer their energy to the scintillator in a manner similar to charged particles.
The energy loss rate (−dE/dx) for charged particles is significantly influenced by their energy levels and is accurately represented by the Bethe-Bloch formula, particularly for particles in the MeV-GeV range This formula incorporates atomic shell corrections for lower energy particles and accounts for radiative loss corrections at higher energy levels In heavy materials commonly utilized as scintillators, with densities ranging from 6 to 8 g/cm³, the energy loss is generally on the order of specific values.
10 MeV/cm for a minimum ionizing particle but it can be a factor up to 100 more at very low or very high energy (radiative losses).
CERN, Geneva, Switzerland e-mail: Paul.Lecoq@cern.ch © The Author(s) 2020
C W Fabjan, H Schopper (eds.), Particle Physics Reference Library, https://doi.org/10.1007/978-3-030-35318-6_3
In the case of X- orγ- rays, the three fundamental mechanisms of electromag- netic interaction are [3]:
At low energy levels, particularly up to a few hundred keV for heavy materials, photoelectric absorption is the dominant process In this interaction, a photon transfers its energy to an electron from a deep electron shell of an absorber atom, ejecting the photoelectron with kinetic energy equal to the incident photon energy minus the electron's binding energy This event triggers a rapid reorganization of the electron cloud to fill the vacancy, leading to the emission of characteristic X-rays or Auger electrons The probability of the photoelectric effect is highest when the photon energy is similar to the kinetic energy of the electron in its shell, resulting in distinct peaks in the cross-section curve that correspond to resonances of various electron shells Overall, the cross-section exhibits a rapid decrease with increasing energy and shows a strong dependence on the atomic number (Z) of the absorber, highlighting the preference for high-Z materials in X-ray and gamma-ray detection and shielding, as indicated by the relationship σ_ph ∝ Z^5.
Fig 3.1 Energy dependence of photon total cross sections in Lead (from Particle Data Group)
3 Scintillation Detectors for Charged Particles and Photons 47
At energies above a few hundred keV, Compton scattering becomes predominant.
In this scenario, the incident photon transfers a portion of its initial energy \( E_\gamma \) to an electron within the atomic shells, resulting in scattering at an angle \( \theta \) from its original trajectory The recoiling electron is swiftly absorbed by the scintillator, releasing energy as described by the relevant formula.
E e=E γ −E γ −E ebinding (3.2) whereE γ is the energy of the scattered photon given by (withm 0the rest mass of the electron):
The energy emitted in the scintillator due to the recoil electron varies continuously between zero and a maximum value of Eγ - 256 keV, where Eγ represents the gamma energy, significantly exceeding the electron's rest mass energy.
The probability of Compton scattering is related to the electron density in the medium and increases linearly with the atomic number of the absorber, favouring therefore highZmaterials.
Above a threshold of 1.02 MeV, which is twice the rest mass of the electron, e+ e− pair production occurs primarily in the electric field of atomic nuclei and, to a lesser extent, in the electron cloud This process, akin to photo-absorption and Compton scattering, exhibits a higher likelihood in high-Z materials, with the cross section approximated by the formula σ pair ∝ Z² ln.
Below the threshold of electron-positron pair production electrons will continue to loose energy mainly through Coulomb scattering.
In ordered materials like crystals, energy degradation leads to interactions between high-energy electrons and lattice atoms, resulting in the excitation of electrons from valence or core bands to the conduction band, forming electron-hole pairs When the energy of the electrons surpasses the ionization threshold, free carriers are generated, moving randomly until they are either trapped by defects or recombine at luminescent centers If the threshold is not met, the electron and hole dissipate energy by coupling with lattice vibrations, eventually reaching the top of the valence band and the bottom of the conduction band Additionally, they may bind to form excitons, which possess energy slightly lower than the bandgap between the valence and conduction bands, maximizing the probability of interactions at this stage.
48 P Lecoq for their relaxation on luminescent centres through an energy or a charge transfer mechanism.
A scintillator material must include luminescent centers, which can be either extrinsic, such as doping ions, or intrinsic, like molecular systems within the lattice or lattice defects These centers must have a radiative transition between an excited state and a lower energy state, with energy levels that are lower than the forbidden energy bandgap This requirement is crucial to prevent the re-absorption of emitted light or the photo-ionization of the luminescent centers.
A scintillator acts as a wavelength shifter by transforming the energy of incoming particles or high-energy photons, such as UV, X-rays, or gamma rays, into multiple lower-energy photons with longer wavelengths in the visible or near-visible spectrum These emitted photons can be detected using devices like photomultipliers, photodiodes, or avalanche photodiodes.
Scintillators are among the most popular ionizing radiation detectors.
Scintillators are classified into two primary categories: inorganic and organic In inorganic scintillators, typically ionic crystals, scintillation occurs when thermalized electrons and holes are displaced to the conduction band and valence band, respectively, due to scattering from fast charge carriers In contrast, organic scintillators produce scintillation through transitions between excited molecular levels and their electronic ground states While inorganic scintillators tend to be brighter, they also exhibit slower decay times compared to their organic counterparts.
There is no single "ideal" scintillator material; the selection depends on the specific application and involves balancing various physical, chemical, and optical characteristics, including density, scintillation efficiency, and radiation resistance Additionally, production and processing costs are crucial factors, especially given the substantial quantities needed for certain applications.
Physico-chemical properties are related to the material composition, structure and density, as well as to its chemical stability when exposed to different environmental conditions: air, humidity, ionizing radiation.
To optimize detector volume and cost, it is crucial to enhance density and compactness by utilizing high stopping power materials This approach effectively minimizes the shower size for high-energy gamma rays and electrons, as well as the range of Compton scattered photons from lower energy gamma rays Additionally, dense materials limit the lateral spread of the shower, a key factor for most High Energy Physics detectors.
3 Scintillation Detectors for Charged Particles and Photons 49
Fig 3.2 Density for various binary compounds as a function of the binding anion (courtesy P. Derenbos, from ref [5])
Crystals with densities exceeding 8 g/cm³, like Lead Tungstate (PWO: 8.28 g/cm³) and Lutetium Aluminium Perovskite (LuAP: 8.34 g/cm³), are currently available Researchers are also identifying and studying materials with even higher densities, around 10 g/cm³, including Lutetium Oxide (Lu₂O₃) and Lutetium Hafnate.
Lutetium compounds such as Lutetium Tantalate (Lu3TaO7), Lutetium Lead Tantalate (LuPb2TaO6), and Thorium Oxide (ThO2) are notable scintillators characterized by their wide bandgap and high density The choice of high atomic number cations and anions with small ionic radii enhances ionic density within the crystal lattice Generally, oxides exhibit greater density than iodides due to the smaller ionic radius of oxygen compared to iodine, despite oxygen's lighter weight Additionally, the oxidation potential of anions plays a crucial role in minimizing the number of lighter anions required to balance the charge of heavier cations Consequently, oxygen serves as a superior ligand over fluorine, owing to its higher oxidation states (2 or 3 versus 1) This relationship is further illustrated in various binary compounds, highlighting the impact of anion type on density and stability.
HighZ materials are favored for low and medium energy spectroscopy due to the significant relationship between the photoelectric cross-section and atomic number Z Additionally, high density is essential at elevated energies to minimize the radiation length X₀, which represents the average distance over which an electron loses 1/e of its energy, and is determined by the material's density ρ, atomic mass A, and atomic number Z.