Electron spin resonance study of self-trapped holes in CdWO4 scintillatorcrystalsV. V. Laguta, M. Nikl, J. Rosa, B. V. Grinyov, L. L. Nagornaya et al. Citation: J. Appl. Phys. 104, 103525 (2008); doi: 10.1063/1.3028228 View online: http://dx.doi.org/10.1063/1.3028228 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v104/i10 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Electron spin resonance study of self-trapped holes in CdWO4 scintillatorcrystals

V. V. Laguta,1,2,a� M. Nikl,1 J. Rosa,1 B. V. Grinyov,3 L. L. Nagornaya,3 and I. A. Tupitsina3

1Institute of Physics, AS CR, Cukrovarnicka 10, 162 53 Prague, Czech Republic2Institute for Problems of Material Science, NASc of Ukraine, Krjijanovskogo 3, 03680 Kiev, Ukraine3Institute for Scintillation Materials, NASc of Ukraine, 60 Lenin Ave., 61001 Kharkov, Ukraine

�Received 14 May 2008; accepted 8 October 2008; published online 20 November 2008�

The self-trapping of holes at oxygen anions was studied by electron spin resonance in UV irradiatedCdWO4 crystals. Analysis of superhyperfine interaction of the holes with 183W and 111,113Cdisotopes shows that the self-trapped hole is either delocalized in the space between two energeticallyequivalent nearest neighbor oxygen ions or tunnels between them. When the temperature increasesabove 40–50 K the self-trapped holes are thermally liberated and can be retrapped by oxygen ionsperturbed by impurity ions. In case of the Nb5+ or Li+ stabilizing impurities the O− centers arethermally stable up to 160–170 K. The study of kinetic characteristics of the self-trapped holessuggests that holes leave oxygen ions by thermally assisted tunneling mechanism via two slightlydifferent channels. Corresponding ionization probabilities are defined by the Arrhenius law with anaverage thermal ionization energy E=90�5� meV. Calculated pre-exponential factors, about105 s−1, are small, which is consistent with the tunneling mechanism. Thermal stability and kineticcharacteristics of the trapped holes are discussed in light of the scintillation and thermoluminescencecharacteristics of CdWO4. © 2008 American Institute of Physics. �DOI: 10.1063/1.3028228�

I. INTRODUCTION

Cadmium tungstate �CdWO4� single crystal is a widelyused intrinsic scintillating material. Its applications mainlyfocus on the detection of x rays and � rays in the medicalfield.1 Concerning density and light yield, CdWO4 is superiorcompared to other scintillating materials. On the other hand,its practical importance is seriously limited by relativelyslow scintillation response. Scintillation characteristics canbe degraded by the presence of various structural defectsserving as traps or recombination centers for migratingcharge carriers generated by ionizing radiation. Such defectstates can be monitored by thermoluminescence �TL� mea-surements already used in the study of CdWO4 �see, for ex-ample, Refs. 2 and 3�. Trapping centers were assigned todifferent point defects rather intuitively following an analogyof those determined in ZnWO4.2–4 Therefore, an actual localstructure of point defects involved in the charge trapping andmigration processes in CdWO4 is still not well knownmainly due to the lack of supporting information provided byelectron spin resonance �ESR� measurements. Interestingly,such defects have been studied in detail using the ESR tech-nique in the isomorphous ZnWO4 host �see Refs. 5 and 6 andreferences therein�.

A number of previous ESR studies of CdWO4 were de-voted to characterization of transition metal impurities suchas Mn2+, Fe3+, Cu2+, and Mo5+.7–10 Concerning identificationof electron and hole traps, only the Mo5+ and Bi3+ electroncenters, as well as the O− hole bound to Nb5+ impurity, wereconvincingly identified in x-ray or UV irradiated CdWO4

crystals.10,11 The authors of Ref. 10 also mentioned an obser-vation of a self-trapped hole center by ESR assuming the

same defect model as that used in isomorphous ZnWO4

crystal.5 Trapping sites related to the cadmium vacancies�VCd� and OH− molecular ions were studied in Ref. 4. Inparticular, the authors report an observation of h+-VCd-OH−

hole center by ESR. It is worth noticing that there is a sys-tematic research effort in the family of tungstates and mo-lybdates to find a relation between the calculated electronicband structure �especially that within the top of the valenceand bottom of conduction bands� and the presence of specificmaterial defects serving as electron or hole traps.12,13

In the present paper we investigate the hole self-trappingin the UV irradiated CdWO4 scintillator by ESR. In particu-lar, we found that the self-trapped hole is delocalized in thespace between two neighboring oxygen ions connecting twoWO6 octahedrons. Self-trapped hole kinetic characteristicsare also determined and discussed in the context of chargetrapping processes in the scintillation mechanism.

II. SAMPLES AND EXPERIMENTAL DETAILS

The CdWO4 single crystals were grown in air by Czo-chralski technique using Pt crucibles. The charge was pre-pared by high temperature solid phase synthesis of stoichio-metric composition of cadmium and tungsten oxides. Rawmaterials with either 4N purity �sample A� or 5N purity�sample B� were used. Undoped and 0.05% Li-dopedCdWO4 crystals were grown. ESR studies were performed at9.22 GHz with the standard 3 cm wavelength of the ESRspectrometer; measurements were performed in the tempera-ture range of 10–260 K using an Oxford Instruments cry-ostat. A mercury high-pressure arc lamp with optical filterswas used for UV irradiation of the samples.a�Electronic mail: laguta@fzu.cz.

JOURNAL OF APPLIED PHYSICS 104, 103525 �2008�

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III. EXPERIMENTAL RESULTS

A. Electron spin resonance data

Prior to any irradiation, traces of Fe3+ and Mn2+ impurityions were detected in CdWO4 crystals. Broadband UV irra-diation with wavelengths of 300–370 nm at T�40 K in-duces a paramagnetic defect with an ESR spectrum consist-ing of two groups of superhyperfine lines for arbitraryorientation of the magnetic field B. These two groups of linesare related to the existence of two structurally equivalent butmagnetically inequivalent sites in the CdWO4 lattice, whichhas a monoclinic space group P2 /c.14 When the magneticfield rotates in the �010� /b plane or when B is directed alongany of the crystal axes, these two spectra coincide as ex-pected according to the lattice symmetry. Most probably, asimilar spectrum was observed before by Garces et al.10 TheESR spectrum of sample A measured in the �010� plane isshown in Fig. 1�a�. The same spectrum was also measuredfor sample B. It is shown below that the spectrum is associ-ated with a hole trapped by an oxygen ion, i.e., the O− center.The O− center is stable approximately up to 50 K. At highertemperatures thermally liberated holes can be partly re-trapped by oxygen ions adjacent to an impurity ion or adefect as illustrated by spectra �b� and �c� in Fig. 1. Spectrum�b� has already been described and identified as that belong-ing to the O−–Nb hole center.11

In order to determine the spin-Hamiltonian parametersof the O− center, angular dependencies of the central compo-nent of the superhyperfine structure were measured in threecrystallographic planes �Fig. 2�. The resonance fields weredescribed by the spin-Hamiltonian H=�BBgS with the spinS=1 /2 and g-factors and their principal directions presentedin Table I. The g-tensor components are typical for O−

trapped hole centers.5,15

One can see that the g-tensor has predominantly tetrag-onal symmetry with one of the components significantlysmaller than the value ge=2.0023 for the free electron. Such

g-factor values usually correspond to O− p� orbital for ahole localization.16 This is also in good agreement with theelectronic structure of CdWO4. Theoretical calculationsshow17 that the O 2p� states dominate the top of the valenceband. It is worth noting that both the g-tensor componentsand orientation of principal axes corresponding to the self-trapped hole in CdWO4 considerably differ from those inZnWO4. In the latter the g-tensor shows the following com-ponents: 2.0030, 2.0213, and 2.0406.5

The measured g-factors can be reproduced by using ex-pressions previously derived for O2

− molecule in alkalihalides,18 which has the same p� ground state

g1 = ge cos � − gl� �

D��cos � + 1 − sin �� , �1a�

g2 = ge cos � − gl� �

D��cos � − 1 + sin �� , �1b�

g3 = ge + 2gl sin � , �1c�

with sin �=� /2E. Here D is the distance to the highest p�

orbital in the hole representation, E is the splitting of thetwofold degenerate p� orbital, and �=−150 cm−1 is O− spin-orbit coupling constant. Assuming that gl=1, one can deter-mine the splitting of the O− 2p orbitals D=37 300 cm−1 andE=1800 cm−1.

The hyperfine splitting of the spectra provides informa-tion about the O− center surroundings. The intense centralcomponent of the hyperfine pattern is mainly associated withthe holes trapped near even W and Cd isotopes with zeronuclear spin where no hyperfine interaction is expected. Thehyperfine splitting can be produced by 183W and 111,113Cdisotopes having a nuclear spin I=1 /2 and the natural abun-dance of 14.3% and about 12.5%, respectively. The high in-tensity of the hyperfine lines �their integrated intensity isequal to intensity of the central component� suggests that thehole interacts with a great number of surrounding nuclei be-cause both 183W and 111,113Cd isotopes have relatively lownatural abundance given above.

The measured hyperfine structure can be satisfactorilyfitted assuming an interaction of the hole with nuclear mag-netic moments of two Cd and two W ions of approximatelythe same magnitude. Measured and calculated spectra aredisplayed in Fig. 1�a�. Corresponding parameters of the fitare listed in Table I. One should note that the hyperfine pat-tern shown in Fig. 1�a� remains practically unchanged when

FIG. 1. �Color online� ESR spectra of various O− hole centers created bylight irradiation in CdWO4: �a� self-trapped hole created by UV irradiationat T�40 K �b� and �c� O−–Nb and O−–Li centers created by subsequentheating to 75–80 K or UV irradiation at these temperatures. Magnetic fieldB is rotated in the ac plane by the angle ��c ,B�=140°. Dots and solid linesare the measured and calculated spectra, respectively.

FIG. 2. Angular dependencies for the ESR spectrum central component ofthe intrinsic O− center in CdWO4 created by UV irradiation at T�40 K.Symbols and solid lines are the measured and calculated spectra,respectively.

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the orientation of the crystal is changed. Therefore, the hy-perfine interaction is nearly isotropic. On the other hand,possible small splitting in the spectrum induced by inequiva-lency of the surrounding nuclei is not resolved due to largelinewidth �around 0.2 mT� of the O− ESR lines.

In CdWO4 structure there are two different energeticallyinequivalent oxygen ion sites.14 Type 1 �solid circles in Fig.3� is bound to one W ion with very short bond length�1.78 Å� and to two Cd ions with longer bond lengths �2.27and 2.41 Å�. Type 2 �empty circles in Fig. 3� is connected totwo W ions with longer bond lengths �1.91 and 2.15 Å� andto one Cd ion with the bond length of 2.20 Å. The type 2oxygen ion is a much more preferable site for a hole local-ization due to weaker binding of the p electron to a W ion.Namely, the W–O bond is responsible for the energy-bandgap of CdWO4 �Ref. 17� and in an isomorphous ZnWO4

crystal the hole is localized at this type of oxygen ion. Thehyperfine structure of the O− spectrum in CdWO4 also sug-gests that the hole is localized at the type 2 oxygen ionbecause it interacts with two nearly equivalent W nuclei.Furthermore, to explain an interaction of the hole withnuclear magnetic moments of two equivalent Cd ions, wehave to assume delocalization of the hole between two clos-est energetically equivalent oxygen ions, O1–O3 or O2–O4,as shown in Fig. 3. These oxygen ions connect two octahe-drons. The distance between these oxygen ions is only2.38 Å, much shorter than the distance between any other

oxygen pair in the octahedron. Since no impurity superhy-perfine interaction was observed and the intensity of thespectrum does not essentially depend on the purity of thecrystal, the O− center can be assigned to the self-trapped holecenter.

The nature of the O− center is further supported by thefollowing experimental observations. When irradiated crystalis heated above approximately 40–50 K the trapped holesstart to get thermally released and related O− ESR signaldisappears. Correspondingly, another ESR spectrum appears�Fig. 1�b��. In accordance with Ref. 11 it is assigned to theO− hole near an accidental Nb5+ impurity ion substituting forW6+. This results in a drastic modification of the hyperfinestructure of the spectrum. Due to the nuclear spin of 93Nb�I=9 /2, natural abundance 100%� the spectrum is split intoten equidistant hyperfine lines. Symmetrically around eachof these ten lines, two doublets with different splitting areseen �see inset of Fig. 1�b��. These doublets are due to hy-perfine interactions with two essentially inequivalent Cdions. The hyperfine structure of O−–Nb center was satisfac-torily fit by taking into account interactions with the abovementioned nuclei. The parameters of the fit are A�93Nb�=17.4�10−4 cm−1, A�111Cd1�=23�10−4 cm−1, A�113Cd1�=24.06�10−4 cm−1, A�111Cd3�=13.5�10−4 cm−1, andA�113Cd3�=14.12�10−4 cm−1. About twice stronger hyper-fine interaction of the hole with one of the two Cd ionsindicates that the hole is predominantly localized only at oneof the two oxygen ions, which are now energetically in-equivalent due to the substitution of one of the W ions nearthe oxygen site �W1 or W2 in Fig. 3� by the Nb impurity.Note that the O−–Nb center is stable up to approximately170 K and is not visible in sample B where the content of theaccidental impurities is low.

Similar transformation of the self-trapped hole centercan be induced by an impurity at the Cd site, e.g., Li+ ion. Inthis case, a hole, released after heating, can be retrapped atan oxygen site in the neighborhood of Li. This is illustratedby spectrum �c� in Fig. 1, when after heating of irradiatedCdWO4 crystal doped by Li the self-trapped hole spectrumtransforms into another type, namely, O−–Li. Intensity ofthis spectrum correlates with the lithium content. The hyper-fine structure of the spectrum could be determined by con-sidering hyperfine interactions with two nearly equivalent Wions �A�183W�=10.5�10−4 cm−1� and only one Cd ion�A�111Cd�=13�10−4 cm−1; A�113Cd�=13.6�10−4 cm−1�because Li substitutes for the second Cd ion. It is interestingto note that the W and Cd related hyperfine interactions showapproximately the same values as in an unperturbed O− cen-

TABLE I. Spectral parameters and thermal stability of the O− self-trapped hole in CdWO4. Orientation of the g-tensor principal axes is given for twostructurally equivalent centers.

g-tensor

Polar and azimuthal angles of axesHyperfine parameters

�10−4 cm−1� Thermal stability

g1 :1.9980�3� 38;38 273;87 �A�111Cd��=11 �45 Kg2 :2.0057�3� 107;107 339;21 �A�113Cd��=11.5 Ea90�5� meVg3 :2.0858�3� 58;122 58;122 �A�183W��=11.2 1 /�0105 1 /s

1Hyperfine �HF� parameters are given along the g2 principal direction.

FIG. 3. Fragment of CdWO4 crystal structure �projection onto the �001�plane� with the model of the self-trapped hole center. The self-trapped holeis shared between two closest oxygen ions O1–O3 or O2–O4 forming twoenergetically equivalent but magnetically different hole centers. The holeinteracts with nuclear magnetic moments of two closest W ions �W1–W2�and two Cd ions �Cd1–Cd3 or Cd2–Cd4�.

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ter. Therefore, such a hole seems still delocalized betweentwo closest oxygen ions. Although no interaction with the7Li isotope was resolved due to its small strength in com-parison with the linewidth �for example, in similar O−–Licenter in ZnWO4 the 7Li hyperfine constant is only�0.8–1.6��10−4 cm−1�, there is no doubt that we are dealingwith the Li-perturbed O− center.

B. Thermal stability of hole centers

An important characteristic feature of any charge trap inthe scintillating material is its thermal stability becausecharge carriers thermally released from a trap can contributeto the delayed radiative recombination processes. The trapsare well visualized by TL measurements. The glow curve ofCdWO4 consists of a principal broad peak at 60–80 K andseveral smaller peaks at higher temperatures.2,3 It is evidentthat, namely, self-trapped holes described above could con-tribute to the TL peak at 60–80 K. To obtain a quantitativecorrelation between ESR centers and TL traps, we studiedthe thermal stability of the O− centers using the method ofisochronal annealing. After irradiation at low temperature�usually 20–30 K�, the sample was heated to a certain tem-perature Tan, held at that temperature for three minutes, andthen quickly cooled �with a rate of about 4 K/s� to 35–40 K,where the ESR spectrum was measured. The 3 min intervalwas found as the best compromise to ensure good thermali-zation of the sample and sufficient reproducibility of mea-sured ESR intensities. The signal amplitudes are depicted inFig. 4. It can be seen that the intensity of the self-trappedhole spectrum sharply decreases at a temperature interval of60–80 K, where the TL glow peak is observed. The spectrumcompletely disappears after heating to a temperature of�85 K. As mentioned above, different ESR spectra appearindicating partial recapture of the holes at different latticesites, perturbed by defects. This process illustrates an ex-ample in Fig. 4 displaying the O−–Li center, thermally stableup to 150–160 K.

To get a deeper insight into the kinetics of the self-trapped holes and their contribution to TL processes, we de-termined the thermal ionization energy and frequency factor

from the ESR time decay data. These parameters can usuallybe obtained from the ionization probability expressed in theform of the Arrhenius law,19

P�T� = NcSv exp�− E/kT� . �2�

Here Nc is an effective density of states in the valence band,v and S are the hole thermal velocity and cross section of itscapture, respectively, and E stands for thermal ionization en-ergy. The value of P as a function of temperature can beobtained from measurements of the time decay ��T�=1 / P�T� of O− concentration after the radiation is turned off.As thermally released holes are either recaptured by deepertraps or recombined with electrons at recombination centers�the first-order kinetic process� their concentration time de-cay is expected to be a simple exponential function of n=n0 exp�−t /��T��. However, the measured concentrationsn�t� only approximately follow the single exponential decay.Much better fit was obtained using the two-exponential de-cay function

n = n0�1� exp�− t/��1�� + n0

�2� exp�− t/��2�� . �3�

The nonsingle exponential decay of the ESR signal is wellseen in Fig. 5, where the concentrations n�t� are presented inthe logarithmic scale. Solid lines represent the two-exponential fits. The two-exponential character of the con-centration decay suggests that the hole might escape the trapvia two slightly different channels. Corresponding character-istic values �1 /�1 and 1 /�2� as a function of reciprocal tem-perature are shown in the logarithmic scale in the inset ofFig. 5. The slopes of the lines provide the thermal ionizationenergies E1=0.093 eV and E2=0.085 eV representing twochannels. The frequency factors are rather small: 1 /�01=7.6�104 s−1 and 1 /�02=1.2�105 s−1. The two-channelmechanism of the O− center destruction can be understoodwhen taking into account that the hole is delocalized in the

FIG. 4. �Color online� Temperature dependence of the O− ESR intensities inCdWO4 using the isochronal annealing. The dashed line is the ESR intensityof the O−–Li center calculated with the same trap parameters E and 1 /�0 asthose determined by the TL analysis of Ref. 2.

FIG. 5. �Color online� Time decay of the O− ESR intensity measured atvarious temperatures. The inset shows the temperature dependence of thethermal ionization probability of the O− center. Solid lines in both graphs arenumerical fits of the data according to Eqs. �2� and �3�.

103525-4 Laguta et al. J. Appl. Phys. 104, 103525 �2008�

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space between two oxygen ions. Therefore it can be excitedfrom two slightly different lattice sites, i.e., with differenttransition probabilities, toward a recombination �trapping�center. We cannot exclude a more complicated kinetic pro-cess for holes than that proposed here. However, as the dif-ference between two sets of thermal-escape parameters de-rived by us is not too large, the self-trapped hole can becharacterized by just one set of average values: Ea

=90�5 meV and 1 /�0105 s−1 �Table I�.It should be noted that the frequency factor 2

�1012 s−1 reported for the impurity-related TL peak inCdWO4:Li �Ref. 2� exhibits usually expected order of mag-nitude. Temperature dependence of the O−–Li ESR intensity�Fig. 4� was satisfactorily fit using this value of frequencyfactor and the trap depth E=0.48 eV; both values were de-termined from the TL data. Therefore extremely low fre-quency factor values 1 /�01 and 1 /�02 reported above canindicate the tunneling of self-trapped holes toward either re-combination or deeper trapping sites. Thermally assisted tun-neling where the hole tunnels from its excited state towardan electron center also cannot be excluded. It is still a ther-mally activated process but with substantially reduced prob-ability and thus correspondingly lower frequency factor �see,for example, Ref. 20�. Similar thermally assisted tunnelingprocess was recently found for trapped holes in ZnO:Li.21

Moreover, at low temperatures the self-trapped hole can tun-nel with high probability through the barrier between twoclosest oxygen sites. This feature naturally explains its hy-perfine interaction with two equivalent Cd ions. Such hy-pothesis is further supported by earlier measurements of ther-mostimulated conductivity �TSC�.22 In particular, besideusual TSC peaks, the peak of the negative TSC at T80–90 K was observed in x-ray irradiated CdWO4 cut inthe �010� plane. The peak was explained as due to tunnelingof captured carriers along the crystallographic axes. Unfor-tunately, no model of the tunneling mechanism was pro-vided.

The data presented in this paper show that, despite somesimilarity observed for the hole self-trapping phenomenon inCdWO4 and ZnWO4, there is an essential difference: inZnWO4 the self-trapped hole is localized exclusively at oneof two energetically equivalent oxygen ions. Thermally acti-vated jumps between the two lattice sites occur above 25 K.5

In contrast, in CdWO4 the self-trapped hole is at any tem-perature much more delocalized in the space between twoenergetically equivalent oxygen ions.

It is interesting to consider obtained results in light oftime evolution of charge trapping in the scintillation mecha-nism of CdWO4. It is well known that the decay time ofscintillation response is about 12–15 �s. The value coin-cides with photoluminescence decay characteristics. Also theshape of emission spectrum with the maximum at about 480nm is the same under both UV and gamma excitations. Theemission is assigned to the radiative transition within theexcited WO6 complex anion, even if in the process of lumi-nescent center excitation the Cd2+ states might be involved.13

Under any excitation that creates free holes in the valenceband and free electrons in the conduction band, the holes willimmediately get self-trapped. At room temperature, consid-

ering Eq. �2� and the values of frequency factors and energybarriers obtained in Sec. IIIB, the self-trapped holes will getthermally released in the time scale on the order of hundredsof microseconds. Taking into account the decay time of scin-tillation response �12–15 �s� it is evident that self-trappedholes are immobile during the scintillation response. Hence,the scintillation mechanism of CdWO4 consists in radiativerecombination of electrons migrating in the conduction bandwith self-trapped holes without any additional delay.

To summarize, we have shown that the holes createdafter UV irradiation of CdWO4 scintillator crystals at T�30 K are self-trapped at regular oxygen ions forming O−

paramagnetic centers. Superhyperfine interaction of the self-trapped holes with 183W and 111,113Cd isotopes suggests thatthe hole is delocalized �or quickly tunnels� between twoneighboring oxygen ions that connect two WO6 octahedrons.The self-trapped hole center is stable up to approximately 50K. At higher temperatures the thermally released holes canbe either retrapped at oxygen ions adjacent to aliovalent im-purities such as Li+ and Nb5+ substituting for Cd2+ and W6+,respectively, or radiatively recombine with electrons storedat deeper traps giving contribution to the TL glow peakswithin 60–80 K.

Kinetic characteristics of the self-trapped holes indicatethat the holes escape from oxygen trapping sites via twoslightly different channels. Corresponding ionization prob-abilities are described by the Arrhenius law with the thermalionization energies E1=93 meV and E2=85 meV and thefrequency factors 1 /�01=7.6�104 s−1 and 1 /�02=1.2�105 s−1. Extremely low frequency factors suggest ther-mally assisted tunneling of the self-trapped holes to eitherrecombination or deeper trapping sites. Self-trapped holessurvive at room temperature up to several hundreds of mi-croseconds. This fact implies that during scintillation re-sponse of CdWO4, the holes are immobile and can serve ascenters of radiative recombination with electrons migratingin the conduction band.

The O−–Nb and O−–Li centers are stable up to 160–170K and their thermal destruction may contribute to the TLpeaks observed at 160–180 K.2 Temperature decay of theO−–Li center concentration was satisfactorily fit using thesame values of the trap depth �0.48 eV� and frequency factor�2�1012 s−1� as those determined from the TL dataanalysis.2

ACKNOWLEDGMENTS

Financial support from the Czech Project No. GA AVIAA100100810 and the Institutional Research Plan No.AVOZ10100521 is gratefully acknowledged. Thanks are dueto Y. Usuki for providing a CdWO4 sample for further com-parison and E. Mihokova for language corrections.

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