A. Kotlov

Energy transfer in ZnWO4 and CdWO4 scintillators

A systematic spectroscopic study of oriented single ZnWO4 crystals was performed using UV and synchrotron excitation. Emission and phosphorescence spectra in the energy region 1-4 eV as well as reflection spectra and excitation spectra of various emissions and phosphorescence in the energy region 3.5-32eV were studied at 4.2-300K using synchrotron radiation. The results are compared to those available for CdWO4. The peculiarities of the band structure and those of excitonic and electron-hole processes in the systems investigated are discussed.

Separation of excitonic and electron-hole processes in metal tungstates

Absorption, reflection and emission polarization spectra of CdWO4 crystals have been studied in the region 3.5-30 eV in order to distinguish the excitonic and electron-hole processes in the vicinity of the band gap. The following parameters have been defined from the Urbach tail study at 6-300 K: E-0 = 5 eV, sigma(0) = 0.31, homega = 70 meV (565 cm(-1)). Excitonic processes have been shown to dominate at excitation of tungstate crystals in the lower part of the conduction band. Excitons are formed due to the transitions into the tungstate W5d states hybridized with O6p and possess a very strong tendency for self-trapping. Free electrons and holes can be created at the energies 1-2 eV (depending on the crystal) higher than the bottom of the conduction band due to the transitions into cationic states

Investigation of Cu-doped Li2B4O7 single crystals by electron paramagnetic resonance and time-resolved optical spectroscopy

A low-temperature study of the thermoluminescent dosimeter material, lithium tetraborate (Li2B4O7) doped by Cu, has been carried out by the methods of electron paramagnetic resonance (EPR) and time-resolved polarization spectroscopy using 4–20 eV synchrotron radiation and 1 µs Xe flash lamp pulses in the region 3–6 eV. The observed EPR spectra of an unpaired hole with strong d-character and characteristic hyperfine splittings can be ascribed to Cu2+ substituted at a Li lattice site and displaced due to relaxation. The results on the Cu+-related luminescence strongly support the conclusion about a low-symmetry position of copper impurity ions in the lithium tetraborate lattice. The temperature dependence of the decay kinetics of the Cu+-related 3.35 eV emission indicates a triplet nature for the relaxed excited state of the Cu+ centres. An off-centre position of the Cu+ ion in the relaxed excited state is suggested.

Different incorporation of Cu+ and Cu2+ in lithium tetraborate single crystals

The incorporation site of Cu in the scintillator and thermoluminescent dosimeter material lithium tetraborate Li2B4O7:Cu has been shown to depend on the charge state of the dopant. As confirmed by a refined analysis, the EPR spectra of an unpaired hole can be ascribed to Cu2+ situated near the Li lattice site with C1 symmetry, in contrast to the higher (C2) overall symmetry reported earlier for Cu+ in its relaxed excited state, the latter suggesting interstitial incorporation for the monovalent dopant. Consequences for charge transfer processes involving copper ions are discussed.

Iron-related luminescence centers in ZnWO4 : Fe

A systematic spectroscopic study of single ZnWO 4 :Fe crystals with different iron concentrations has been performed under excitation by ultraviolet light, by synchrotron radiation or under photostimulation by near-infrared light. The luminescence of Fe 3+ -related centres has been studied. It is shown that iron centres of different types efficiently promote the formation of crystal defects at low temperatures. Electrons and holes can be trapped near Fe 2+ or Fe 3+ ions, which is further revealed in phosphorescence, thermostimulated or photostimulated luminescence. At room temperature the main effect of iron impurity is to reduce the light yield of a ZnWO 4 scintillator.

Relaxation of electronic excitations in wide-gap crystals studied by femtosecond interferometry technique

Time-resolved interferometry with a 100-fs temporal resolution was applied for the first time to studying the relaxation of electronic excitations in complex oxides, namely, tungstates CDWO4 with a crystal lattice of the wolframite-type and CaWO4 with a scheelite-type lattice. Two stages of charge carrier relaxation, namely, very fast carrier trapping in 200 fs resulting in self-trapped exciton formation and a relatively slow picosecond relaxation process probably due to configurational relaxation within the oxyanion molecule and modification of the surrounding lattice, are revealed in tungstate crystals. Corresponding models of self-trapped exciton creation in tungstate crystals are discussed.

Time-resolved spectroscopy in ZnWO4 and ZnWO4:Fe

Time-resolved luminescence and absorption of ZnWO4 and ZnWO4:Fe have been studied. The fast decaying luminescence at ∼1.7eV is attributed to either Fe2☎ or a Fe3☎ related center. The two observed stages in luminescence decay kinetics under ionising radiation are suggested to be due to two types of self-trapped excitons.