Martin Nikl

Scintillation detectors for x-rays

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.

Wide band gap scintillation materials : Progress in the technology and material understanding

Luminescence and scintillation characteristics of four selected material systems, namely CsI:Tl(Na), CeF 3 , PbWO 4 and Ce-doped aluminium perovskites XAIO 3 :Ce (X=Y, Lu, Y-Lu) are reviewed. The progress in their physical understanding and related optimisation of their characteristics and technology are demonstrated. The important role of various defect states in the scintillator performance of these materials is stressed, which has led to the need for a deeper study of the processes of energy transfer and storage to achieve their intrinsic limits and full exploitation in scintillation detectors.

Efficient radioluminescence of the Ce3+-doped Na–Gd phosphate glasses

Photo- and radioluminescence, decay kinetics, and thermoluminescence are reported for a set of Ce3+-doped phosphate glasses. The presence of Gd3+ ions in the glass host matrix at concentrations above 20% enhances the energy transfer towards the Ce3+ emission centers, which results in a remarkable enhancement of the radioluminescence light output.

A study of electron excitations in and single crystals

The excitation spectra of photo- and thermo-luminescence were compared in the VUV - UV spectral region in and scheelite tungstates. Temperature dependences of emission intensities and decay times were measured for in the 80 - 300 K range and approximated by a simple phenomenological model. The energy level structure of the emission centre excited state and related kinetic processes are discussed for both tungstates.

Polaronic centres in single crystals

The autolocalization of free electrons at regular lattice sites, namely at complex anions, has been demonstrated by ESR measurements. It has been shown that the centre is a shallow donor; its energy level is situated meV below the bottom of the conduction band. At temperatures of 50-60 K the captured electrons are detrapped and the formation of other paramagnetic centres occurs. In La-doped , an enhanced cross-section for electron trapping by impurity centres with respect to that for undoped samples is found. In contrast to the case for other tungstates, no intrinsic oxygen hole and/or centres were revealed in . The role of the centres in the processes of transport and recombination of nonequilibrium carriers in the lattice is discussed.

Lead bromide and ternary alkali lead bromide single crystals — growth and emission properties

Abstract Single crystals of PbBr 2 , RbPb 2 Br 5 and CsPbBr 3 (20 mm in diameter and 50 mm long) were grown by the Bridgman method from starting materials purified by bromination with bromine and hydrogen bromide and by zone refining and were tested by measurements of the low-temperature exciton luminescence and decay kinetics. The emission of RbPb 2 Br 5 resembles that of PbBr 2 . Short (10–20 ps) decay times were found in CsPbBr 3 . The emission of all compounds tested is strongly influenced by defects and the exciton character from Frenkel to Wannier type going from PbBr 2 to CsPbBr 3 .

Growth of lead tungstate single crystal scintillators

Abstract In this paper we present information on the growth of first-quality lead tungstate PbWO 4 single crystals and their luminescence and absorption properties. In comparison with so far presented PbWO 4 our single crystals show the prevailing blue emission component in radioluminescence at room temperature and fast scintillation decay in the nanosecond range with a very low level of slow processes on the micro-miliseconds time scale. These facts suggest that especially PbWO 4 single crystals with these properties are suitable as scintillating medium for a crystal calorimeter for the Large Hadron Collider project at CERN.

Scintillator Materials—Achievements, Opportunities, and Puzzles

Participation of shallow and deep traps in the processes of energy transfer and capture is studied by means of time- resolved emission spectroscopy and thermoluminescence in several groups of the Ce3+ and Pr3+-doped complex oxide single crystal scintillators. Tunnelling-driven recombination processes are distinguished in all the groups of examined materials: closely spaced electron and hole traps give rise to the t-1 phosphorescence decays at low temperatures in the Ce-doped aluminum garnets and perovskites, while thermally assisted tunneling process is proposed to explain temperature independent trap depth in glow curve peaks within 50-250 degC in Ce-doped lutetium orthosilicates.

Ce-doped YAG and LuAG Epitaxial Films for Scintillation Detectors

Epitaxial garnet films of undoped and Ce³⁺ -doped yttrium aluminum garnet (YAG) and lutetium aluminum garnet (LuAG) were grown by the isothermal liquid phase epitaxy from two different fluxes: we used a standard PbO-B₂O₃ flux and, as a novelty, BaO-B₂O₃-BaF₂ flux. It is shown that the films obtained from the lead-free BaO flux exhibited superior luminescent properties comparable with those of Czochralski grown single crystals. A detailed comparison of optical, luminescent, and kinetic properties of films grown from these two types of fluxes is made.

A role of Gd3+ in scintillating processes in Tb-doped Na–Gd phosphate glasses

Abstract Sensitization role of Gd 3+ ions in energy transfer processes in radio- and photo-luminescence is described for Tb 3+ -doped and Gd-enriched Na x Gd y phosphate glasses. Limited (Gd–Gd) n energy migration is obtained and the Gd 3+ →Tb 3+ transfer is explained as due to mainly the (super) exchange interaction. These results are discussed and compared with the situation observed in similar glasses or crystals where transfer processes are mainly due to multi-polar interaction (Gd 3+ →Ce 3+ or Ce 3+ →Gd 3+ ) and more efficient migration among Gd 3+ ions that is observed.