Doped PbWO4 Crystals Research Articles

Fiber and crystals dual readout calorimeters

The RD52 (DREAM) collaboration is performing R&D on dual readout calorimetry techniques with the aim of improving hadronic energy resolution for future high energy physics experiments. The simultaneous detection of Cherenkov and scintillation light enables us to measure the electromagnetic fraction of hadron shower event-by-event. As a result, we could eliminate the main fluctuation which prevented from achieving precision energy measurement for hadrons. We have tested the performance of the lead and copper fiber prototypes calorimeters with various energies of electromagnetic particles and hadrons. During the beam test, we investigated the energy resolutions for electrons and pions as well as the identification of those particles in a longitudinally unsegmented calorimeter. Measurements were also performed on pure and doped PbWO4 crystals, as well as BGO and BSO, with the aim of realizing a crystal based dual readout detector. We will describe our results, focusing on the more promising properties of homogeneous media for the technique. Guidelines for additional developments on crystals will be also given. Finally we discuss the construction techniques that we have used to assemble our prototypes and give an overview of the ones that could be industrialized for the construction of a full hermetic calorimeter.

Thermal annealing behavior of fluorescence of Pr3+‐doped lead tungstate single crystal

AbstractThe effects of thermal annealing treatment on the spectroscopic properties of Pr3+‐doped PbWO4 crystals were discussed in detail. The polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curves of main emission manifolds of the as‐grown and annealed Pr3+‐doped PbWO4 crystals were measured at room temperature and analyzed. Thermal annealing treatment can diminish most defect centers related to oxygen vacancies and thermal stress. The results demonstrate that thermal annealing treatment can exert a distinct influence on the spectroscopic properties of Pr3+: PbWO4 crystals as promising phosphors and laser materials.

Multisite Structure of PbWO4:Eu3+ Crystals Investigated by Site-Selective Laser-Excitation Spectroscopy

Defect structures of Eu(3+)-doped PbWO(4) crystals are investigated by site-selective laser-excitation spectroscopy. The excitation spectrum of the (7)F(0) --> (5)D(0) transition shows a strong dependence on Eu(3+)-concentration in PbWO(4) crystals. Only one substitution site for Eu(3+) is identified in the PbWO(4):Eu(3+) (0.01 at.%) crystal, while at least six inequivalent sites are observed in the PbWO(4):Eu(3+) (0.05 and 0.5 at.%) crystals. The charge compensation mechanisms for the observed sites are discussed in relation with annealing effects and temperature-dependent decay curves. The dominant site for Eu(3+) in the PbWO(4) lattice seems to be due to a formation of Eu(Pb)(3+)-V(Pb)'' dipole by charge compensation. The minor sites for Eu(3+) are attributable to the Eu(Pb)(3+)-O(i)'' dipole, a site perturbed by Eu(Pb)(3+*)-V(Pb)'' dipole, and a clustering of Eu(3+) in the PbWO(4) lattice. The energy transfer and site distribution of Eu(3+) are discussed in PbWO(4):Eu(3+) (0.01, 0.05, and 0.5 at.%).

Localized excitons and defects in PbWO4 single crystals: a luminescence and photo‐thermally stimulated disintegration study

AbstractLuminescence from the exciton‐ and defect‐related states and photo‐thermally stimulated disintegration of these states under selective UV irradiation were studied in the PbWO4:Mo, Ce crystal and compared with the characteristics of other undoped and doped PbWO4 crystals of different origin. For the first time, various localized exciton states were identified and their decay into stable defects was found. Different positions, halfwidths and temperature dependences were observed for the blue emission spectrum in different PbWO4 crystals and explained by the presence of strongly overlapping emission bands of the WO42–‐type self‐trapped and localized excitons. Binding energies of the self‐trapped and localized exciton states were calculated. The dominant thermally stimulated luminescence peaks, created in the PbWO4:Mo, Ce crystal by the UV radiation, are located at 110, 202 and 247 K. The related trap depths were calculated using the initial rise method. It was concluded that under irradiation in the exciton absorption region, defects are mainly created due to the disintegration of the localized exciton states, which occurs without release of free charge carriers. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Defects creation under UV irradiation of PbWO4 crystals

A systematic study of photothermally stimulated defects creation processes is carried out by the thermally stimulated luminescence (TSL) method for a large number of undoped and doped PbWO4 crystals under irradiation at 80-180 K in the 3.4-4.3 eV energy range. The activation energy Ea for the regular exciton state disintegration is found to be approximately 0.1 eV. For defect-related states disintegration, Ea varies in the crystals studied from 0.03 to 0.36 eV. The origin of the defect-related states is discussed. The conclusion is made that not only a release of charge carriers but also charge transfer processes take place under UV irradiation of PbWO4 crystals.

PbO compensation in the growth of PbWO4: Y3+ crystals

The light yield of Y3+-doped PbWO4 crystals increases after low-dose-rate irradiation with γ rays, and the radiation hardness is sensitive to annealing temperature. In the PWO growth procedure, an excess of PbO in the starting materials is a convenient method of compensating for PbO volatility. The relationship between the excess of PbO and the abnormal radiation behaviour has been investigated. The mechanism of the normal excess of PbO in the growth of PWO: Y3+ is discussed.

Room‐temperature 1.53 µm emission in Er3+‐doped PbWO4 single crystals

AbstractIn this work, room temperature 1.53 µm emissions were investigated for the first time in Er3+‐doped PbWO4 crystals. J–O calculations demonstrated that the high rates of nonradiative relaxation from the excited states to the level 4I13/2 ensure its effective population. Consequently there is a possibility of generation of stimulated emission based on the 4I13/2 → 4I15/2 transition. Fluorescence intensity and lifetime of 1.53 µm emission (4I13/2 → 4I15/2) with a wider full wave at half maximum show a dependence on crystal orientation, which become stronger with increasing Er3+ doping. Annealing exerts a distinct influence on the 1.53 µm emission and its fluorescence lifetime in PbWO4:Er3+ crystals, which might be caused by a reduction of defects in the lattice and to make Er3+ ions optically active. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Decay kinetics of the green emission in tungstates and molybdates

Luminescence characteristics of a number of undoped and variously doped PbWO4 crystals were studied at 0.4–400K by the time-resolved spectroscopy and compared with those of ZnWO4,CdWO4 and PbMoO4 crystals. Two types of green emission centres are detected in PbWO4 crystals. The centres of the first type are responsible for the low-temperature 2.3–2.4eV emission observed under excitation around 3.90–3.95eV. The structure and parameters of their relaxed excited states were determined. It was concluded that the origin of defects responsible for the green emission of the first type could vary for different crystals. The centres of the second type with the emission around 2.5eV appear in crystals containing oxygen vacancies after the thermal destruction of Pb+-WO3 centres at T>180K. Decomposition of the exciton and various defect-related states was also studied, and activation energies of this process were calculated.

Red emission of PbWO4 crystals

Luminescence characteristics of a large number of undoped and doped PbWO4 crystals, grown by the Czochralski or Bridgman method, as-grown or annealed in the nitrogen atmosphere or in air, were studied in the 4.2–300K temperature range. Two types of red emission centres were found. The centres with the emission band, peaking at 4.2K at 1.57eV, were observed in most of the crystals studied. The centres with the emission band, peaking at 4.2K at 1.48eV, were observed only in the PbWO4:Mo6+, Y3+ crystal. It is suggested that incompletely compensated lead vacancies are responsible for the appearance of the red emission.

Characteristics of the optical absorption edge and defect structures ofLa3+-dopedPbWO4 crystals

The UV optical absorption edge of purePbWO4 (PWO) anda series of La3+-doped PbWO4 crystals was measured at room temperature. Parameters, including the energy gapEg and the energyof phonon Ep involved in the indirect transition, were calculated, and the dependence of the transition onimpurities instead of phonons in heavily doped PWO:La crystals is presented. The shift ofthe absorption edge to higher energy observed in the case of doping at low concentration(<400 ppm) and the red-shift of the cut-off edge found for doping at higher concentration(>1600 ppm)are explained by the combined effects of ion polarization and localized defect energy level. FT-IR spectraof the WO4 tetrahedron confirmed the coexistence of interstitial oxygen and Pb vacancies to keep thecharge balance in heavily doped PWO:La crystals.

Luminescence enhancement of Er3+ ions and energy transfer in PbWO4 single crystals

A series of Er3+-doped PbWO4 crystals have been grown by the Czochralski method with good optical quality. Optical absorbance, x-ray excited luminescence, photoluminescence and excitation spectra have been investigated in these Er3+-doped PbWO4 crystals. Efficient luminescence from Er3+ ions doped in PbWO4 is reported. With increasing luminescence of rare earth Er3+ ions in PbWO4 : Er3+, the intrinsic luminescence of the PbWO4 host is seen to decrease intensively. It was demonstrated that excitation into the PbWO4 host is followed by an efficient energy transfer towards Er3+ ions. The excitation bands of Er3+ ions overlap with the luminescence spectra of the PbWO4 host, which supports the hypothesis of resonant energy transfer from host to Er3+ ions. Annealing at higher temperature results in further enhancement of the luminescence of Er3+ ions in the PbWO4 host.

Decay kinetics of the intrinsic luminescence in undoped and doped PbWO4 crystals excited by femtosecond laser

Decay kinetics of the luminescence in undoped and La3+- or Cd2+-doped PbWO4 (PWO) crystals has been studied under the excitation with the fourth harmonic light (300nm) of a femtosecond laser at room temperature. The measurements are carried out by making use of a combination of a monochromator and a streak scope. The intrinsic “blue” luminescence at 440nm in undoped PWO exhibits two fast decay components (1.5 and 13ns). In addition, there appears a rise component of about 60ps. The luminescence and decay characteristics of the doped PWO are essentially the same as those of the undoped one, i.e., there is no appearance of the extrinsic “green” luminescence in the later stage by doping of La3+(1350atppm) or Cd2+(1at%) ions. It is found that the blue luminescence is enhanced in intensity by several 10% in these doped crystals. The influence of dopant ions on the decay kinetics and the luminescence intensity will be discussed.

The study of time-resolved absorption and luminescence in PbWO4 crystals

The transient absorption and luminescence under pulsed electron beam excitation were studied for undoped and La3+ doped PbWO4 crystals. The absorption band at ∼1.0 eV is suggested to be due to self-trapped electrons and the absorption band at 3.5 eV may be due to self-trapped holes. The formation of luminescence centers via electron–hole recombination is affected by La3+ in doped crystals. The large fraction of electrons and holes undergoes thermostimulated recombination within geminate pairs and the spatial separation of geminate pairs components is important in recombination process. The intrinsic (blue) luminescence arises in this recombination. The mechanism of La3+ influence on recombination is discussed.