V. V. Mikhailin

Optical and luminescent properties of anisotropic tungstate crystals

Features of the reflectivity spectra in the fundamental absorption region were analysed for a series of tungstates. The contribution of electronic states of cations to the formation of the bottom of the conduction band and the top of the valence band is demonstrated. Its relation with the dominant mechanism of the energy transfer to the emission centres and the nature of these centres is discussed.

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.

Recombination of Correlated Electron–Hole Pairs With Account of Hot Capture With Emission of Optical Phonons

Electron thermalization and electron-hole recombination in scintillating crystals is simulated with and without account for Coulomb field created by a hole using both analytical estimations and Monte-Carlo approach. The Monte-Carlo simulation is performed both for crystals with one and two branches of longitudinal optical phonons to check the role of additional branches of these phonons. The results of numerical simulation show that the account for Coulomb field at all stages of the thermalization and capture significantly increases the probability of the geminate electron-hole binding in case of high values of optical phonon energies.

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.

Electronic structure and luminescence mechanisms in ZnMoO4 crystals.

Calculations of the band structure, partial densities of states and optical spectra of permittivity, reflectivity and absorption of perfect ZnMoO(4) crystal were performed using the full-potential linear-augmented-plane-wave method. It is shown that the calculated reflectivity spectra reproduce the main features of corresponding experimental spectra in the fundamental absorption region. The bandgap value of ZnMoO(4) is estimated as E(g) = 4.3 eV. Peculiarities of luminescence excitation spectra corrected for near-surface losses and losses on reflectivity are discussed, taking into account the results of the calculations. It is found that the energy structure of the lower part of conduction band is manifested in the excitation spectra of the intrinsic luminescence. The excitation spectra in the region 4.3-8.0 eV are formed by band-to-band electronic transitions mainly within the molybdate groups MoO(4)(2-), whereas electronic states of Zn(2+) cations are not directly involved into the excitation processes. It is shown that the structure of the intrinsic luminescence excitation spectrum depends on the temperature and mechanisms of the structure modification are discussed.

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.

Charge-transfer luminescence and spectroscopic properties of Yb3+ in aluminium and gallium garnets

Luminescence of Yb 3 - from the charge-transfer state with broad emission bands and short radiative lifetimes (few to tens of nanoseconds depending on the host lattice and the temperature) is attractive for the development of fast scintillators capable of discriminating very short events. The most important currently considered application is that in solar neutrino (v e ) real-time spectroscopy, since the v e capture by 1 7 6 Yb is followed by a specific emission signature which can accordingly excite the Yb 3 + fluorescence. Studies on scintillation and luminescence in aluminium garnets containing Yb 3 + have shown that these materials meet some of the required properties for such scintillators. In defining our priorities, the best compromise between host crystal, Yb 3 + concentration, production method, post-growth treatment and performance is to be considered based on the studies of charge-transfer luminescence and quenching mechanisms. The experiments have been extended to a large number of compounds: YAG:Yb-YbAG, YGG:Yb-YbGG, YAP:Yb-YbAP, LaYbO 3 in the form of single crystals and/or powders. In garnets, the temperature-dependent fluorescence intensity and decay time under X-ray and VUV excitations decrease at low temperatures (T<100 K) and demonstrate the important role played by the traps. The thermoluminescence peaks show a strong dependence on the crystal history, composition and impurities introduced intentionally. The fluorescence intensity and decay time are also dependent on Yb 3 + concentration and the presence of Yb 2 + . The results trace the major directions to optimised scintillators in terms of their efficiency and lifetime.

Two-frequency undulator and harmonic generation by an ultrarelativistic electron

The spectrum of electromagnetic radiation of a relativistic electron moving in the magnetic field oscillating in two mutually transversal spatial directions with two different frequencies is analytically investigated. The spectral properties of radiation in the planar biharmonic undulator are also discussed. The technique based on the associated Bessel functions was effectively applied in our calculations. The effect of the magnetic field and the undulator parameters on the radiation of the fundamental as well as low and high harmonics was elucidated. It is demonstrated that the biharmonic undulator can be exploited to regulate the emission of certain selected harmonics and hence contribute to the development of the efficient devices with high extraction and narrow spectrum.

Temperature dependence of the charge transfer and f?f luminescence of Yb3+ in garnets and YAP

For single crystals of YAG–15 mol% Yb, LuAG–15 mol% Yb and YAP–8 mol% Yb the temperature dependence of the charge-transfer (CT) and f–f luminescence of Yb3+ excited in the CT absorption band in the temperature range 7–300 K has been studied. Simulation of the experimental data taking into consideration three processes, photoionization of the CT state and radiative and non-radiative energy transfer to the 4f levels, has demonstrated that for adequate description of the relaxation of the CT state it is essential to get information on the temperature dependence of the direct excitation efficiency of the f–f luminescence as well as to study the role of intrinsic luminescence in the energy transfer to ytterbium ions. Excitation spectra of the CT and IR luminescence in the energy region 4–20 eV at RT and 10 K are presented and discussed.

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.