V. Nagirnyi

Excitonic and recombination processes in CaWO4 and CdWO4 scintillators under synchrotron irradiation

Excitation spectra of intrinsic (IL) and extrinsic luminescence (EL) from CaWO 4 and CdWO 4 crystals, measured using synchrotron radiation (4-30 eV) at 8 K, have been analysed. In CaWO 4 the IL (2.9 eV) is efficiently excited by photons of 5.5-7.0 eV at the creation of molecular excitons. The EL is efficiently excited in interband transitions (hv>6.8 eV). The energy threshold for the multiplication of electronic excitations is E t 15 eV in CaWO 4 and E t 10 eV CdWO 4 . In CdWO 4 , a fraction of the molecular excitons undergoes autoionization due to an overlap with the continuum of the interband transitions. Fast emission (<2 ns) is found at 80 K under the pulse excitation by electrons with an energy of 300 keV. The continuous temperature-independent emission of CaWO 4 crystals in the region of 2-5 eV is interpreted as an intraband luminescence. The emission at 5.5 eV is ascribed to a metastable molecular excitons.

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.

Dependence of luminescence processes and transmission in vacuum-ultraviolet region on surface condition in CaF2 single crystals

Abstract We investigated the effect of the surface condition on luminescence processes in a surface layer of single CaF2 crystals under the excitation by vacuum-ultraviolet (VUV) light (hν≤13.5 eV). It has been shown that the intensity of the excitonic emission at 4.4 eV in polished crystals essentially decreases with respect to that in freshly cleaved crystals. The decrease at the interband excitation is larger than that at the direct optical creation of excitons. A new luminescence band peaking at 2.55 eV has been discovered and ascribed to the oxygen-related centres situated in a surface layer of CaF2. The main excitation band of this band is in strict correlation with the excitonic absorption band at 11.2 eV. It is almost absent in polished crystals. The ageing of the cleaved surface of CaF2 crystals causes decrease in the intensities of the 4.4 and 2.55 eV emissions with different rates. The etching of a polished surface restores both emission bands. The correlation has been established between the evolution of these luminescence phenomena and the changes in VUV transmission spectra at 7.5–10.2 eV near the fundamental absorption edge of CaF2, induced by the cleavage, polishing or etching of the sample.

Excitonic processes in pure and doped

The optical properties of nominally pure crystals as well as of those of pure and and -doped powders are compared. The emission band peaking at 3.9 eV and the excitation bands in the region 10-10.6 eV observed for and powders are ascribed to the radiative decay of a self-trapped exciton perturbed by an anion vacancy or impurity-vacancy dipole. A -perturbed exciton has been found in .

Creation of groups of spatially correlated defects in a KBr crystal at 8 K

The creation spectrum of triplets of spatially correlated defects (F centre + self-trapped hole, centre + interstitial halogen ion, I centre) was measured for the first time in a KBr crystal at 8 K, using synchrotron radiation of 6-25 eV and highly sensitive luminescent methods. The spectrum of spatially correlated F and H centre creation by synchrotron radiation at 8 K was measured as well. The efficiency of triplet creation is especially high at crystal irradiation by photons under the conditions of multiplication of electronic excitations when the absorption of one photon leads, respectively, to the formation of an electron-hole (e-h) pair and a secondary exciton (15.0-16.5 eV), or two e-h pairs (16.7-19.0 eV), or to the formation of a cation exciton (19.5-20.5 eV). It is shown that the formation of an triplet is caused by the recharging of a primary F-H pair by an e-h pair or by a free exciton.

Exciton–Exciton Interaction in CdWO

Using time-resolved luminescence spectroscopy, excitation density effects were investigated under conditions of resonant creation of excitons by femtosecond laser pulses in CdWO4 scintillator at room temperature. It was revealed that the decay kinetics of intrinsic emission becomes accelerated and deviates from the exponential law in the initial stage due to the Förster dipole-dipole interaction of self-trapped excitons. It is shown that these processes originate from the relaxed exciton states even at excitation power up to ~1012 W/cm2. The quantitative analysis of the experimental data is performed, and the dipole-dipole interaction parameters discussed. The results obtained are shown to be of general importance for understanding the problem of nonproportional response of scintillators, whose operation is based on intrinsic luminescence.

_{4}

Abstract Intrinsic (WO 4 ) 3− centers can be created in lead tungstate by irradiation with light having energies well below the bandgap, i.e. the threshold for excitation of the blue tungstate luminescence. Detailed photo-ESR studies show that this happens by ionization of defect states in the bandgap which have been prefilled by suitable radiation. Depending on the energy depth of the respective defects a wavelength between near-infrared and near-UV is necessary for the ionization process. A simultaneous production of different electron centers is observed via participation of the conduction band. This allows an easy determination of the trap level energies.

Under Resonant Excitation by Intense Femtosecond Laser Pulses

Abstract The excitation spectra for various impurity emissions have been measured in MgO:Al, MgO:OH, MgO:Cr crystals and MgO:F, MgO:LiF ceramics at 8 K using synchrotron radiation of 5–32 eV. Processes of radiative recombination at the impurity centres of various structure differ significantly for two stages of multiplication of electron–hole (e–h) pairs with threshold photon energies at 20 and 25 eV. The multiplication processes have been analysed taking into account the peculiarities of the electronic structure of MgO.