PWO Crystals Research Articles

The response of lead-tungstate scintillators (PWO) to photons with energies in the range 13 MeV–64 MeV

The response of a matrix of 25 lead tungstate (PWO) scintillator detectors, operated at −25°C, to photons in the range 13MeV–64MeV has been measured at the tagged-photon facility at MAX-lab, Lund. The tapered PWO crystals, each with a length of 200mm and a cross-section of 24.4×24.4mm2 in the front end, read out by 19mm photomultiplier tubes, were arranged in a 5×5 matrix. The response was measured for photons directed towards the centre of the central crystal as well as for photons directed towards the corner of the central crystal, where four crystals meet. The obtained energy resolution surpasses what has been published so far and is close to the limit given by Poisson statistics and escaped energy. For photons directed towards the centre(corner) of the central crystal the relative energy resolution, defined as (FWHM/2.35)/Eγ, decreases from 7.3%(11.0%) at Eγ=13MeV to 3.3%(3.6%) at Eγ=64MeV. The reconstructed point of impact of a photon in this energy range is determined with an uncertainty (one standard deviation) of 7.3±0.1mm.

Bridgman crystal growth and spectral properties of Er doped PbWO4 as stimulated Raman crystals

Several Er doped (0.2at%, 0.5at%, and 1.0at%) PbWO4 single crystals were grown by Bridgman method with the size of Φ20×100mm. The absorption spectra of Er:PWO samples were measured to investigate the spectral properties of Er3+ ions according to Judd-Ofelt theory. The phenomenological intensity parameters Ωλ were fitted to calculate the spectral parameters of several radiative transitions of Er3+ ions, and the possibility of 4I13/2→4I15/2 transition at about 1.5μm of Er3+ ions in PWO crystal was also discussed. The energy transfer from PWO matrix to Er3+ ions was investigated based on the photoluminescence spectra, and the frequency shifting effect of Er3+ ions on PWO crystal was discussed according to the spontaneous Raman spectra. The 1.5μm fluorescence spectra of different Er:PWO samples were measured under the excitation at 980nm.

A light-emitting diode monitoring system of the PHOS photon spectrometer in the ALICE experiment on the large hadron collider

A light-emitting diode (LED) monitoring system of the PHOS photon spectrometer in the ALICE experiment on the CERN large hadron collider is described. The spectrometer includes three modules in the form of 64 × 56 matrices consisting of plumbum tungstate (PWO) crystals. As test light signal sources, Kingbright L934SCC superbright green light-emitting diodes with an individual instrumental regulation of light flash intensities in each channel of the spectrometer are used. The system ensures adjustment and monitoring of the electronics at the stage of preparing for the physical trigger of the PHOS modules and is intended to perform special test measurements with the spectrometer without using high-energy particles. In addition, in the course of the experiment, it allows one to promptly monitor each channel of the spectrometer and keep track of temperature variations of the light yield of the PWO crystals. In this case, the long-term relative channel monitoring stability is ensured at a 1.2 × 10−3 level.

Application of X‐ray fluorescence spectrometry on quality control of scintillation crystals

AbstractThe major components PbO and WO3 in PWO crystals, Bi2O3 and GeO2 in BGO crystals, as well as Gd, La, Nb, Mg, Mo, Bi, Sb and Y doped in PWO crystals and Eu doped in BGO crystals were successfully determined with X‐ray fluorescence spectroscopy (XRF) using fusion techniques. Calibration standards were synthesized with high‐purity oxides and standard solutions. The analysis results can meet the general requirements of the quality control of the crystal growth and research purpose. The relative standard deviations for Bi2O3, GeO2, PbO and WO3 are 0.21%, 0.18%, 0.25% and 0.22%, respectively (k = 8). The detection limits for dopants in PWO are below 5 µg/g for Gd, La, Nb, Mo, Sb and Y and below 20 µg/g for Mg and Bi. The detection limits for Eu doped in BGO are 8 µg/g. The testing results of XRF were compared with those of inductively coupled plasma optical emission spectroscopy. It was found that the relative differences of the testing results between the two methods are less than 10% for most dopants in PWO crystals. Copyright © 2011 John Wiley & Sons, Ltd.

Specific Heat and Thermal Diffusivity and Conductivity of PbWO4Single Crystal Related to Its Crystal Orientation at (300 to 673) K

Lead tungstate (PbWO4, PWO) single crystal has been chosen as a new generation scintillation material. The specific heat and thermal diffusivity of PWO single crystal grown by the vertical Bridgman technique is investigated for the first time by differential scanning calorimetry (DSC) and a laser flash technique at (300 to 673) K, respectively. The thermal conductivity is calculated, and it exhibits anisotropy with the values along the [100] axis (Kx,y) higher than that along the [001] axis (Kz), which is 2.40 W·m−1·K−1 and 2.00 W·m−1·K−1 at room temperature, respectively. Because of the heavier elements of the Pb atom, the PWO crystal has a higher degree of anharmonicity of lattice vibrations and thus has a smaller thermal conductivity than another scintillation material bismuth germanate (BGO).

High-quality PWO crystals for the PANDA-EMC

The paper provides a status report on the crystal production and quality control of a major part of the PbWO4 crystals for the PANDA-EMC. The results confirm the excellent performance of the new generation of PWO-II. The mechanism of stimulated recovery provides an additional tool to recover radiation damage at room and low temperatures by applying an external infrared light source. Even on-line recovery can be considered if the photo sensor is insensitive in that particular wavelength region.

Luminescence characteristics of lead tungstate (PbWO4) scintillation crystal doped with fluorine anions

Lead tungstate (PbWO4, shorten as PWO) crystal has attracted much interest as a very dense, fast and radiation hard scintillator. However, its application is strongly hampered by its poor light output. At present, great efforts have been devoted to improve the scintillating efficiency of PWO crystals. In this paper, fluorine anions were used as dopant to improve the light output of PWO crystals. The crystals were grown by modified Bridgman method. The charge was obtained from PbO and WO3 powder with purity of 99.99% and 99.999% respectively. The transmittance spectra measured with a spectrophotometer Shimadzu UV-2501PC reveal that the transmission of PWO:F is much higher than that of undoped PWO, especially in the short wavelength region (330—500 nm). The most significant characteristic of the F-doped PWO is that two emission components can be identified in their photoluminescence spectra, a fast component related to the blue emission (419 nm) and a slow component related to the green emission (553 nm). The measured light yield of PWO:F, based on the pulse height spectra stimulated by 137Cs, is as high as 2 to 3 times of the undoped PWO and meanwhile will be increased with the integration of time gate. This means that the significant contribution to the light yield of PWO may come from the green luminescence. The decay constants excited by 22Na gamma ray can be fitted into three components, they are τ1=2.68[57.5%] , τ2=47.6[31.0%] and τ3=183[11.5%]. 88.5% of total scintillating light decays within 50 ns. However, the distribution of light yield along the crystal axis is not uniform, i.e. higher at the tail end than that in the seed end. It is suggested that the blue emission is ascribed to the regular lattice centers, namely [WO4]2- and the green one to a defect [WO3+F] cluster. The significant defects in PWO lattice induced by F-anion should be responsible for the increase of the light yield.

Dual-readout Calorimetry with Scintillating Crystals

The possibility of evaluating the amount of the energy released by the electromagnetic part of a hadronic shower (f) would allow to account for one of the main sources of the fluctuation of the hadronic calorimeter response. The dual-readout method allows an event-by-event measurement of f, as it was originally demonstrated with the DREAM sampling hadronic calorimeter. This approach can be extended to homogeneous detectors like crystals if Cherenkov and scintillation light can be separated by exploiting their main properties. In this paper we present several methods developed for distinguishing the two components in PWO and BGO crystal based calorimeters and the results obtained.

Study on the doping mechanism and electronic structure for Nb 5+ doping PbWO 4 crystals

The existing forms of the impurity Nb 5+ in the Nb 5+:PbWO 4 (PWO) crystals are simulated by computer technology. The possible various kinds of defects in the Nb 5+:PWO crystals are also simulated. By analyzing the calculation results of defect formation energies and binding energies, the optimal substitution positions of the Nb 5+ ions and the charge compensating mechanism [ Nb w - + V O 2 + + Nb w - ] in the Nb 5+:PWO crystals were obtained. The electronic structures for Nb 5+:PWO were calculated with density functional theory code CASTEP. It shows that the doping of Nb 5+ ions could suppress the 350 nm absorption band.

PANDA electromagnetic calorimeters

PANDA is a challenging experimental setup to be implemented at the high-energy storage ring (HESR) at the international facility FAIR, GSI (Germany). PANDA physics program relies heavily on the capability to measure photons with excellent energy, position and timing resolution. For this purpose PANDA proposed to employ electromagnetic calorimeters using two different technologies: compact crystal calorimeter cooled to - 25 ∘ C around target and lead–scintillator sandwich calorimeter with optical fibers light collection (so-called shashlyk calorimeter) in the forward region. Institute for High Energy Physics (IHEP) PANDA group reports on two types of measurements performed at IHEP, Protvino: radiation hardness of the PWO crystals at - 25 ∘ C and testbeam studies of the energy and position resolution of the shashlyk calorimeter prototype in the energy range up to 19 GeV.

Transformations of Absorption and Emission Centers in ${\hbox {PbWO}} _{4}$

Non-stoichiometric PWO crystals were studied at annealing experiments in various atmospheres. It was found that a substantial part of absorption centers in the PWO platelets was brought into absorption activity due to mechanical processing of their surfaces. The green emission of the type G(II) is conditioned by presence of mechanical tension at the emission centers. A new process found at annealing is the transformation of the 330/355 centers into the 340/430 centers caused by separation of lead vacancies from the former ones.

Radiation Hardness and Recovery Processes of PWO Crystals at $-25\, ^{\circ}$C

For the first time, full size lead tungstate crystals of different suppliers, quality and dopant concentration have been irradiated with gamma-rays at low temperatures down to -25degC at IHEP Protvino. In contrast to the behavior at room temperature, increased damage and extremely slow recovery processes have been observed. These first results are discussed in the light of several very different interpretations. Further more sensitive measurements are under preparation. The outcome will have a strong impact on the presently assembled ALICE-PHOS detector and the design of the EM calorimeter of PANDA at the future FAIR facility.

Overview of the 63000 PWO Barrel Crystals for CMS_ECAL Production

In March 2007, the PWO crystal production for the barrel part of the CMS electromagnetic calorimeter has been completed. Since September 1998, 63000 crystals (61000 in Russia, 2000 in China) have been produced, received and tested in two regional centers (CERN and INFN Rome). This paper presents an overview of the procedures used from the R&D phase up to the final large scale reception and quality control. The crystals characteristics and the lessons learned from the production of this unprecedented amount of crystals in a HEP experiment are also presented.

First study of radiation hardness of lead tungstate crystals at low temperatures

The electromagnetic calorimeter of PANDA at the FAIR facility will rely on the operation of lead tungstate (PbWO 4, PWO) scintillating crystals at temperatures near - 25 ∘ C to provide sufficient resolution for photons in the energy range from 8 GeV down to 10 MeV. The radiation hardness of PWO crystals was studied at the IHEP (Protvino) irradiation facility in the temperature range from + 20 ∘ C (room temperature) down to - 23 ∘ C . These studies have indicated significantly different behavior in the time evolution of the damaging processes well below room temperature. Different signal loss levels at the same dose rate but at different temperatures were observed. The effect of a deep suppression of the crystal recovery process at temperatures below 0 ∘ C has been seen.

First-principles study on the effect of the interstitial oxygen ion on the spectral properties of PWO crystal and the origin of the green luminescence band

The electronic structures and absorption spectra of PWO crystals containing interstitial oxygen ions have been calculated using density functional theory code CASTEP with lattice structure optimized. It is shown by calculation that: (1) the interstitial oxygen ion in the perfect PWO crystal doesn’t bring any obvious absorption in the visible region; (2) the green emission of lead tungstate origin is closely related to the interstitial oxygen ion, and probably originates from the center of “ WO 4+O i ”.

First‐principles study of PbWO4 crystal with interstitial oxygen atoms

AbstractThe positions of the interstitial oxygen atoms in PbWO4 (PWO) crystal are simulated by computer technologies. It was found that the formation energy of isolated point defects is low when interstitial oxygen atoms exist around the W6+ ion. Based on the computed results, the total density of states and the partial densities of states for perfect PWO crystals and PWO crystals containing interstitial oxygen atoms were calculated using the DV‐Xα program. By analyzing the calculated results it can be concluded that an interstitial oxygen atom could combine with one or two formal lattice oxygen ions forming oxygen molecular ions, which causes the 350 nm absorption band. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Computer simulation of Nb-doping PBWO 4 crystal

The existing forms of the impurity Nb 5+ ions in the Nb:PWO crystals are simulated by the computer simulation technology. The various kinds of defects and substitution reactions in the Nb:PWO crystals are also simulated. By analyzing the calculated results of defect formation energies and solution energies, the optimal positions of the Nb 5+ ions and the charge compensating mechanism [NbO 3+V O] +–[NbO 4] − in the Nb:PWO crystals are obtained; this charge compensating mechanism change the compensating form of oxygen vacancies which relates to 350 nm absorption band, then the 350 nm absorption band is depressed.

The interstitial oxygen atoms and its effects for optical properties of PWO crystals

The positions of the interstitial oxygen atoms in PWO crystals are numerically simulated by. It was found that the formation energy of isolated point defect is small when oxygen atom exists around the (WO4)2-. Based on the computed results, the total density of states and partial density of states of perfect PWO crystals and PWO crystals with interstitial oxygen atoms were calculated using DV-X α program. It is concluded that interstitial oxygen atoms will combine with oxygen ions of (WO4)2-, forming oxygenic molecule ions which are related to the 350 nm absorption band.

First-Principles Study on the Electronic Structures forY3+:PbWO4 Crystals

The possible defect models of Y3+:PbWO4 crystals arediscussed by defect chemistry and the most possible substitutingpositions of the impurity Y3+ ions are studied by using thegeneral utility lattice program (GULP). The calculated results indicatethat in the lightly doped Y3+:PWO crystal, the main compensatingmechanism is [2YPb++VPb2−], and in theheavily doped Y3+:PWO crystal, it will bring interstitial oxygenions to compensate the positive electricity caused byYPb+, forming defect clusters of[2YPb++Oi2−] in the crystal. The electronicstructures of Y3+:PWO with different defect models arecalculated using the DV-Xα method. It can be concluded from theelectronic structures that, for lightly doped cases, the energy gap ofthe crystal would be broadened and the 420 nm absorption band will berestricted; for heavily doped cases, because of the existence ofinterstitial oxygen ions, it can bring a new absorption band and reducethe radiation hardness of the crystal.

First-principles study on the La3+ doping PbWO4 crystal for different doping concentrations

Abstract The defect models of La3+: PbWO4 (PWO) for different doping concentrations are discussed by defect chemistry and the optimal substituting positions of the impurity La3+ ions are obtained using the General Utility Lattice Program (GULP). The calculated results indicate that, in the lightly doped La3+: PWO crystal, the main compensating mechanism is [2La+Pb + V2−Pb], and in the heavily doped La3+: PWO crystal, it will bring interstitial oxygen ions to compensate the positive electricity caused by La+Pb, forming defect cluster [2La+Pb + O2−i] in the crystal. The electronic structures of the La3+: PWO with the optimal defect models are calculated using the DV-Xα method. It can be concluded from the electronic structures that, for lightly doped case, the energy gap of the crystal would be broaden and the 420 nm absorption band will be restricted; for heavily doped case, because of the existence of interstitial oxygen ions, it can bring new absorption bands and reduce the radiation hardness of the crystal.