The 5d level positions of the trivalent lanthanides in inorganic compounds

Recent research in the field of phosphor and scintillator materials and related detectors is reviewed. After a historical introduction the fundamental issues are explained regarding the interaction of x-ray radiation with a solid state. Crucial parameters and characteristics important for the performance of these materials in applications, including the employed measurement methods, are described. Extended description of the materials currently in use or under intense study is given. Scintillation detector configurations are further briefly overviewed and selected applications are mentioned in more detail to provide an illustration.

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Scintillation detectors for x-rays

The light-radiating semiconductor component has a radiation-emitting semiconductor body and a luminescence conversion element. The semiconductor body emits radiation in the ultraviolet, blue and/or green spectral region and the luminescence conversion element converts a portion of the radiation into radiation of a longer wavelength. This makes it possible to produce light-emitting diodes which radiate polychromatic light, in particular white light, with only a single light-emitting semiconductor body. A particularly preferred luminescence conversion dye is YAG:Ce.

Light-radiating semiconductor component with a luminescence conversion element

The 4fn↔4fn − 15d transitions of the trivalent lanthanides in halogenides and chalcogenides

In this review, the major achievements and research and development (R&D) trends from the last decade in the field of single crystal scintillator materials are described. Two material families are included, namely, those of halide and oxide compounds. In most cases, the host crystals are doped with Ce3+, Pr3+ or Eu2+ rare earth ions. Their spin- and parity-allowed 5d–4f transitions enable a rapid scintillation response, on the order of tens to hundreds of nanoseconds. Technological recipes, extended characterization by means of optical and magnetic spectroscopies, and theoretical studies are described. The latter provide further support to experimental results and provide a better understanding of the host electronic band structure, energy levels of specific defects, and the emission centers themselves. Applications in medical imaging and dosimetry, security measures, high-energy physics and the high-tech industry, in which X(γ)-rays or particle beams are used and monitored, are recognized as the main driving factor for R&D activities in this field.

Recent R&D Trends in Inorganic Single-Crystal Scintillator Materials for Radiation Detection

Gamma-ray and UV-excitation of cerium-doped rare-earth oxyorthosilicates (RE = Y, Gd, and Lu) has been used to investigate the emission mechanism for this family of scintillators. The data clearly indicate the presence of two different luminescence centers, which are attributed to cerium substitution in the two different rare-earth crystallographic sites. While this model explains much of the present and previous UV data, the gamma-ray induced emission from GSO requires a better understanding of the energy transfer between Gd and Ce.

UV and gamma-ray excited luminescence of cerium-doped rare-earth oxyorthosilicates

The crystal structure of Lu/sub 2/(SiO/sub 4/)O:Ce ("LSO") has two trivalent cation sites which may be occupied by Ce/sup 3+/ to form luminescence centers. Previous investigations revealed the existence of two distinct sets of Ce/sup 3+/ excitation and emission spectra and suggested that the differences in the spectra are due to differences in the crystal fields at the two lattice sites that shift the 5d levels of Ce/sup 3+/. In the present report, we reexamine this issue and present new evidence which suggests a different interpretation. In particular, spectra measured at 13 K suggest that both lattice sites may give rise to indistinguishable excitation and emission spectra while the second set of observed spectra may arise from Ce/sup 3+/ located in interstitial sites. The evidence for the interstitial sites is a disappearance of the doublet structure in the emission spectra and a large variation in the population of the sites as a function of total cerium concentration. In addition, the results indicate that the scintillation properties of LSO are influenced by the relative population of Ce in interstitial sites compared to lattice sites as well as by differences in the transfer efficiencies to different sites.

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The role of cerium sites in the scintillation mechanism of LSO

We have found that irradiation of a variety of thin film-substrate combinations by heavy ion beams at energies of mega-electronvolts per atomic mass unit will produce a remarkable enhancement in the adherence of the film. For example, gold films can be firmly attached to soft materials such as Teflon using a 1 MeV beam of protons (10^(14) cm^(−2)) or helium ions (10^(13) cm^(−2)) and to harder materials such as silicon (10^(15) cm^(−2)), quartz (2 × 10^(15) cm^(−2)) and tungsten (2 × 10^(14) cm^(−2)) with 0.5 MeV a.m.u.^(−1) beams of fluorine or chlorine ions. In the case of metal films on semiconductors a low resistance contact results. The mixed layer at the interface is observed to be quite thin (approximately 50 A or less); for silver on silicon electron diffraction and imaging studies of the interface region reveal the presence of crystalline silver compounds.

Enhanced adhesion from high energy ion irradiation

We numerically integrate the one-dimensional, cylindrically symmetric Ginzburg-Landau equations to calculate the spatial variation of the order parameter and supercurrents for a vortex trapped by a cylindrical defect. We use the resulting field distributions to estimate the pinning energy, and make use of the vortex/two-dimensional boson analogy to calculate the depinning temperature. The microscopic behavior oi the fields depends on the size, and the conductivity of the cylindrical defect appears to be important for the pinning.

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T. A. Tombrello

Papers

UV and gamma-ray excited luminescence of cerium-doped rare-earth oxyorthosilicates

The role of cerium sites in the scintillation mechanism of LSO

Enhanced adhesion from high energy ion irradiation

Enhanced adhesion from high energy ion irradiation

Vortex pinning by cylindrical defects in type-II superconductors: Numerical solutions to the Ginzburg-Landau equations