A.N. Belsky

Luminescence properties and scintillation mechanisms of cerium- and praseodymium-doped lutetium orthoaluminate

Absorption, reflection as well as luminescence emission, excitation, and decay curves for single crystals of and grown by the Bridgman technique have been measured at various temperatures. The fluorescence spectra photo-excited over a wide energy domain ranging from the UV to the x-ray region, and the kinetics are typical of the cerium and praseodymium ions. These experimental results show that the exciton transfer to the dopant occurs at around 8 eV, and the energy transfer via sequential hole and electron trapping is dominant at higher energy. This process must be considered as the main scintillation mechanism in this crystal. The high efficiency of this mechanism is explained by the small energy difference between the 4f level of the dopant and the top of the valence band, estimated from XPS measurements.

EXTENSIVE STUDIES ON CEF3 CRYSTALS, A GOOD CANDIDATE FOR ELECTROMAGNETIC CALORIMETRY AT FUTURE ACCELERATORS

Abstract In the framework of its search for new heavy, fast and radiation hard scintillators for calorimetry at future colliders, the Crystal Clear Collaboration performed a systematic investigation of the properties and of the scintillation and radiation damage mechanisms of CeF 3 monocrystals. Many samples of various dimensions up to 3 × 3 × 28 cm 3 were produced by industry and characterised in the laboratories by different methods such as: optical transmission, light yield and decay time measurements, excitation and emission spectra, gamma and neutron irradiations. The results of these measurements are discussed. The measured light yield is compared to the theoretical expectations. Tests in high energy electron beams on a crystal matrix were also performed. The suitability of CeF 3 for calorimetry at high rate machines is confirmed. Production and economical considerations are discussed.

Time-resolved luminescence of CeF3 crystals excited by X-ray synchrotron radiation

Abstract Under X-ray synchrotron radiation (SR) of storage ring VEPP-3 excitation time-resolved luminescence spectra and decay curves for two different CeF 3 crystals have been measured. The temperature dependence of the decay kinetics of emission at 300 nm of new pure CeF 3 crystal has been studied. The maximum of the intensity of this emission was observed at 390 K under SR X-ray excitation. The results are compared with data obtained under UV SR excitation and are discussed in terms of two models of luminescence quenching.

Fast luminescence of undoped PbWO4 crystal

Abstract The results of an investigation of PbWO4 luminescence properties obtained by time-resolved techniques under VUV, X-ray synchrotron radiation and γ-radiation excitation are presented. The fast luminescence in the 430 nm band with fwhm of 0.5 eV is characterized by a decay time of about 4–5 ns under X-ray excitation. A slow luminescence with decay time of more than 1 μs can be attributed to a green emission band. An intermediate decay time was also detected. This time changes from 20 ns (for 360 nm emission) to 50 ns (for 550 nm). The PbWO4 fast band excitation spectrum and reflectivity were measured in the fundamental absorption region using VUV synchrotron radiation (4–150 eV). A comparison between these two spectra allowed us to suppose the excitonic type of the excitation mechanism of the fast emission band. The role of Pb2+ ions in the PbWO4 luminescence is discussed.

Fast fluorescence and scintillation properties of cerium and praseodymium doped lutetium orthoborates

Abstract Both calcite and vaterite polyerystalline phases of LuBO3 doped with cerium and praseodymium have been prepared and their fluorescence properties investigated in the visible, near UV and VUV regions. The systems exhibit a strong and rapid near UV-blue fluorescence typical of the 5d → 4f parity-allowed transitions of dopants. It is shown that the excitation transfers from excitions and electron-hole pairs to dopants are very efficient. The light yield can be three times more efficient than that of the well-known Bi4Ge3O12 scintillator for the best samples.

Luminescence quenching as a probe for the local density of electronic excitations in insulators

Abstract Luminineence quenching which originates from the interaction of closely spaced electronic excitations (EEs: electrons, core- and valence-band holes, excitons, etc.) is considered. ‘Reactions’ between them at a distance shorter than that of dipole-dipole interaction are discussed. Creation of regions with high local density of EEs by VUV and XUV photons is demonstrated. Features of this type of quenching specific for different regions of excitation and different crystals are illustrated by experimental examples.

Influence of stoichiometry on the optical properties of lead tungstate crystals

Abstract A series of PbWO 4 crystals, grown consistently from one melt, were studied under excitation by VUV and X-ray synchrotron radiation. We observed a systematic change in the spectroscopic data (emission spectra, luminescence decay, thermoluminescence, radiation hardness) from the first grown to the last. In the reflectivity spectrum, the intensity in the 4–6 eV absorption region, which is assigned to the Pb 2+ 6s–6p transitions, decreases relative to the intensity in the 7–10 eV region which is due to (WO 4 ) 2− absorption. We connect these changes with the variation in stoichiometry of the melt during crystal growth.

Crossluminescence in ionic crystals

Abstract Crossluminescence (CL) of ionic crystals is regarded with the account for the many-body effects. The crossluminescence yield in the pre-threshold region is considered on the basis of Fano interaction. Relaxation of the doubly-ionised 5p state of the cation ion as manifested in the CL and STE excitation spectra in the region of 4d core-hole creation is cliscussed. The effects of CL quenching are briefly reviewed.

Elaboration and spectroscopic properties of new dense cerium-doped lutetium based scintillator materials

Abstract Two new systems, namely LaLuO 3 and Lu 4 Al 2 O 9 doped with cerium ions, have been investigated. The materials have been elaborated in the form of polycrystalline powders by melting the component oxides and by solid state reaction techniques and in the form of single crystal fibers grown by the laser heated pedestal growth (LHPG) method. The optical spectroscopy was carried out in the UV and VUV regions using laser and synchroton radiation and their fluorescence and scintillation mechanisms were analyzed.

Luminescence properties of the RbCaF3 crystal at X-ray excitation

Abstract The spectra and decay time of RbCaF3 emission at monochromatic X-ray excitation have been measured. It is shown that the short wavelength fast (2.8 ns) intrinsic luminescence of the crystal consists of two bands (245 and 285 nm), which confirms the model of radiative core-to-valence transitions for perovskite structure crystals. A stepwise decrease of the light yield near the K-edge adsorption of Rb (15.2 keV) is observed.