Physics of lead tungstate scintillators This report is a brief review of recent results obtained at the systematic study of the luminescence and photo-thermally stimulated defects creation processes in many undoped and doped crystals, containing different concentrations of various impurity and intrinsic defects, grown by different methods and annealed at different conditions.
V. Babin, V. Kiisk, A. Krasnikov, I. Sildos, A. Stolovits, S. Zazubovich Institute of Physics, University of Tartu, Estonia P. Bohacek, M. Nikl Institute of Physics, Prague, Czech Republic V.V. Laguta Institute for Problems of Material Science, Kiev, Ukraine Institute of Physics, Prague, Czech Republic P. Fabeni, G.P. Pazzi, C. Susini Institute of Applied Physics, Sesto Fiorentino (Firenze), Italy M. Ishii, Y. Usuki Material Research Laboratory, Furukawa Co., Tsukuba, Japan N. Senguttuvan Shonan Institute of Technology, Fujisawa, Japan
Contents 1. Photoluminescence characteristics of crystals 1.1. The blue B emission 1.2. The green G(I) emission 1.3. The green G(II) emission 1.4. The 3.1 ev emission 2. Photo-thermally stimulated decay of the excitonand defect-related states 3. Conclusions 2.1. Characteristics of thermally stimulated luminescence 2.2. Creation of electron and hole centers under irradiation in the exciton region 2.3. Creation of electron and hole centers under irradiation in the defect-related region
Photoluminescence characteristics The B emission In different crystals, E max =2.72-2.78 ev, FWHM=.9-.69 ev, T q =147-197 K Intensity (a.u.) 1 8 6 4 2 2.6 ev 2.6 ev 3.4 ev Intensity (a.u.) 1 1 1 1 2 2 4.1 ev 4.4 ev. 4. 4. 3. 3. 2. 2. Photon energy (ev) Temperature (K) The B emission arises from the (WO 4 ) 2- groups located in the regular crystal regions. These groups can be unperturbed or perturbed by various defects. Thermal quenching of the B emission at T>18 K is caused by the decay of the localized excitons with E a.2 ev. At T<16 K, the B emission intensity decreases due to the STE decay (E a.8 ev) and the electron transfer in close {(WO 4 ) 2- - defect} pairs (E a <.4 ev).
The G(I) emission E max =2.3-2.4 ev, FWHM=.6-.62 ev, T q =182-21 K Intensity (a.u.) 8 7 6 4 3 2 1 3.8-4. ev Intensity (a.u.) 2 1 1 1 1 2 2 Temperature (K) 4. 4. 3. 3. 2. 2. 1. Photon energy (ev) In undoped crystals the G(I) emission arises from the unperturbed and perturbed (WO 4 ) 2- groups located in the crystal regions of leaddeficient structure Pb 7. W 8 O 32 (Korzhik, 1996; Moreau et al., 1999). In Mo 6+ -doped crystals, the G(I) emission arises mainly from the (MoO 4 ) 2- groups, but also from other defects characteristic for the undoped crystal.
PWO 14b 1 % PbO, % WO 3 1 Decay time (μs) 1 1 1 1 1 1 PWO 189h 49% PbO, 1% WO 3 PWO 16 1 1 1 1.1.1 1 1 1 Temperature (K) PWO:Mo
The G(II) emission E max =2. ev, FWHM=.8 ev, I max at 22 K, T q =24 K Intensity (a.u.) 8 6 4 2 Intensity (a.u.) 8 6 4 2 8 12 16 2 24 28 Temperature (K).. 4. 4. 3. 3. 2. 2. 1. Photon energy (ev) The G(II) emission accompanies the recombination of electron and hole centers created as a result of the photo-thermally stimulated decay of various exciton- and defect-related states.
Intensity (a.u.) 1 1 1 1 1.1 1 Intensity (a.u.) 1 1.1 E exc =4.2 ev α 1 E exc =. ev α 1.3 undoped Bridgman 1 1 1 Time (μs) E exc =4.2 ev E exc =3.67 ev α 1.3 1 Time (μs) : 2 ppm Mo Decay kinetics of the G(II) emission was studied under excitation with the pulsed excimer XeCl (4.2 ev), N 2 (3.67 ev) and KrF (. ev) lasers. Decay curve can be approximated by the formula: I(t)~t -α. In log I (log t) coordinates it is a straight line with a slope α. Different decay kinetics were observed under excitation in the localized exciton absorption region and under other excitations.
Under photoexcitation in the localized exciton region, the monomolecular tunneling recombination process takes place, which occurs between genetic pairs of the electron and hole centers optically created without release of free charge carriers. Under photoexcitation in the band-to-band and defectrelated regions, the bimolecular tunneling recombination process takes place, which occurs between chaotic (stochastic) pairs of the electron and hole centers created at the trapping of optically released free charge carriers.
The 3.1 ev emission Intensity (a.u.) 4 3 2 1 x1 Intensity (a.u.) 8 6 4 2 3.1 ev G(I) 8 12 16 2 24 28 Temperature (K).. 4. 4. 3. 3. 2. 2. 1. Photon energy (ev) Both the 3.1 ev and G(I) emission bands in :Ce 3+ can arise from the same localized exciton (ex o Ce 3+ ). T q =13-14 K, E q ~.1-.17 ev. Two types of red emissions were also detected in crystals.
Photo-thermally stimulated decay of the exciton- and defect-related states Defects creation processes were studied under selective ( nm) irradiation with E irr =3.4-. ev at =8-3 K. Defects creation reveals itself in the appearance of TSL peaks. Dependences of each TSL peak intensity (I TSL ) on E irr, and t irr were measured. Characteristics of thermally stimulated luminescence The TSL peaks located at K, around 1 K and at 19-2 K were ascribed to the thermal destruction of electron (WO 4 ) 3-, (WO 4 ) 3- -A 3+ and {Pb + -WO 3 } centers, respectively. The peak at 2 K was ascribed to (MoO 4 ) 3- centers, and the peaks at T>3 K, to (WO 3 ) - -A Pb or (WO 3 ) - -V Pb centers. In some papers, the intense peak at 22-23 K was also connected with (MoO 4 ) 3- centers. The origin and structure of electron centers was established by the ESR studies.
The systematic TSL study of more than thirty different crystals allowed us to conclude that both in the undoped and in the Mo-doped crystals, the G(II) emission is observed in the TSL spectrum at 1-3 K. At higher temperatures, thermally stimulated processes are accompanied with the red emission. Total TSL intensity and the intensity ratio of various peaks at the TSL glow curve are different in different crystals. They are determined mainly by the concentration and type of oxygen (V O ) and lead (V Pb ) vacancies, which strongly depend on the crystal growth and annealing conditions and on the concentration of A 3+ ions.
1 a :Mo, Ce =12 K 1 TSL intensity (a.u.) 1 2 1 1 b c x 12 16 2 24 28 32 36 x1 Temperature (K) :Mo, La =2K Bridgman =12 K annealed as-grown
In the same crystal, the shape of the TSL glow curve depends strongly on the irradiation energy E irr and irradiation duration t irr. TSL intensity (a.u.) 1 1 1 4.8 ev x1 min 4.12 ev :2 3.8 ev 2 min Bridgman =133 K 12 16 2 24 28 32 36 Temperature (K) Bridgman =133 K
1 1 1 11 K x2 :Mo, Ce =12 K :Mo, Ce =8 K 2 K TSL intensity (a.u.) 1 2 1 1 3 2 2 :Mo, La =2K Bridgman =12 K 2 K 2 K 23 K 1 1. 4.8 4.6 4.4 4.2 4. 3.8 3.6 3.4 E irr (ev) 2 K 23 K The TSL peaks creation spectra (dependences of I TSL on irradiation energy E irr ) are different for different peaks.
12 a 8 23 K 4 6 b 2 K 2 K Bridgman =12 K E irr =4.7 ev TSL intensity (a.u.) 4 2 2 16 12 8 4 16 12 8 4 c d 2 K 23 K 23 K 226 K :Mo, La =2 K E irr =4.1 ev :Mo, La =12 K E irr =3.7 ev 1 2 3 4 6 t irr (min) :Mo, La =2 K E irr =3.8 ev The dependences of the TSL intensity on the irradiation duration t irr are also different for different peaks.
The 22-23 K peak consists of at least three components arising from different centers. Its complex structure appears in the peak position dependence on E irr, and t irr. 23 228 :Mo, La =12 K TSL peak position 226 224 222 22. 4.8 4.6 4.4 4.2 4. 3.8 3.6 E irr (ev)
The values of trap depths E t corresponding to each component of the 22-23 K peak are also different and vary from.32 ev to.9 ev.
The studies of Bridgman crystals allowed to ascribe the 22-23 K peak and the 2 K peak to the electron centres connected with the oxygen vacancies of the type of WO 2, WO and with {Pb + -WO 3 } centers, respectively. TSL intensity (a.u.) 2 1 1 Bridgman =12 K annealed as-grown 12 16 2 24 28 32 36 x1 Temperature (K)
ESR studies indicate that also hole centres are thermally destroyed in this temperature range: the decrease in the concentration of electron (CrO 4 ) 3- and (MoO 4 ) 3- centers, which are stable up to T>3 K and 24 K, respectively, was observed at about 21 K due to their recombination with thermally released holes. ESR intensity (arb.units) 6 4 3 2 1 {Pb + -WO 3 } 3- MoO 4 3- CrO 4 1 1 2 2 3 T ann (K)
Electrons are trapped at various V O -related defects and at (MoO 4 ) 2- groups, but the holes responsible for the G(II) emission, at V Pb -related defects. Co-doping with A 3+ ions, resulting in the reduction of the number of the isolated oxygen and lead vacancies, leads to the suppression of TSL just due to the suppression of the G(II) emission.
1 1 1 11 K x2 1 :Mo, Ce =8 K Creation of electron and hole centers under irradiation in the exciton region 1 :Mo, Ce =12 K 2 K TSL intensity (a.u.) 1 2 1 1 3 2 2 :Mo, La =2K Bridgman =12 K 2 K 2 K 23 K 1 1. 4.8 4.6 4.4 4.2 4. 3.8 3.6 3.4 E irr (ev) 2 K 23 K Creation spectra of TSL peaks are narrow bands (FWHM.1 ev) whose maxima positions are temperaturedependent and different for different TSL peaks.
Strongly selective creation of TSL peaks under irradiation in the exciton absorption region indicates that they are created at the decay of different localized exciton states and that this process does not result in the release of free electrons. Narrow bands in the creation spectra of different peaks correspond to various localized excitons. We assume that the excitons of the type of (WO 4 ) 2- localized near/at A 3+ ions (ex A 3+ ), (MoO 4 ) 2- groups (ex MoO 4 2- ), and oxygen-deficient complexes of the type of WO 3 (ex WO 3 ), WO 2 (ex WO 2 ) and WO (ex WO) can exist in crystals. An electron and a hole are released at the photo-thermally stimulated decay of a localized exciton. The released electron can be immediately trapped at (WO 4 ) 2- or (MoO 4 ) 2- groups, at Pb 2+ ions located close to WO 3, around WO 2 and WO complexes, etc.
The following exciton decay processes are assumed to take place: ex A 3+ {(WO 4 ) 3- -A 3+ }; ex (MoO 4 ) 2- (MoO 4 ) 3- ; ex WO 3 {Pb+ -WO 3 }; ex WO 2 {e - or 2e - at/near WO 2 }; ex WO {e - or 2e - at/near WO}. Activation energy E a, calculated for a TSL peak creation from the slope of the lni TSL ( ) dependence, corresponds to the activation energy of the corresponding exciton decay.
The E a values are close to E q values obtained for thermal quenching of the 3.1 ev, B and G(I) emissions. It means that the luminescence quenching is caused by the decay of the corresponding exciton state. At the localized exciton decay, mobile holes are most probably released which are further trapped at the V Pb -related centers. Recently, the photo-thermally stimulated conductivity was observed by C. Itoh under selective excitation around 4.1 ev. The creation spectrum and E a.2 ev obtained for this process coincide with those obtained for the TSL peak creation. It means that the photoconductivity appears due to the decay of the localized excitons. As free electrons are not released in this process, it means that the hole photoconductivity was observed.
1 1 1 11 K x2 1 :Mo, Ce =8 K Creation of electron and hole centers under irradiation in the defect-related region 1 :Mo, Ce =12 K 2 K TSL intensity (a.u.) 1 2 1 1 3 2 2 :Mo, La =2K Bridgman =12 K 2 K 2 K 23 K 1 1. 4.8 4.6 4.4 4.2 4. 3.8 3.6 3.4 E irr (ev) 2 K 23 K Under irradiation at >13 K in the defectrelated absorption region (at 3.7-3.8 ev), various electron centers are created. They are detected by the ESR and the TSL methods.
It means that free electrons are optically released into the conduction band. They can be trapped at (MoO 4 ) 2- groups, which are known as effective electron traps, and also at some other defects (e.g., {Pb 2+ -WO 3 }, WO 2, WO, WO 3 -A Pb, etc). In case the number of other electron traps is negligible, like in A 3+ - containing crystals, the released electrons become trapped mainly at (MoO 4 ) 2-. This explains much larger number of (MoO 4 ) 3- centers in :Mo, A 3+ as compared with :Mo. In case the number of hole traps is also negligible, the fast electron-hole recombination takes place at (MoO 4 ) 2-. Owing to that, in :Mo,A 3+ the intensity ratio of the fast Mo-related G(I) emission to the slow G(II) emission is much larger than in :Mo, which explains good scintillation characteristics of the A 3+ containing :Mo crystals.
In :Mo with large number of various defects responsible for the G(I) emission, not only (MoO 4 ) 2- groups but also some other centers can be ionized by photons of the 3.7-3.8 ev energy. Indeed, strongly different E a values are obtained in different :Mo crystals for the creation of the same TSL peaks in the same energy range (inspite of the fact that (MoO 4 ) 3- centers of only one type exist). Even in the same crystal, E a for different TSL peaks creation with the same E irr are strongly different. ln I TSL 1 a 8 6 4 2 6 4 E a ~.38 ev E a ~.17 ev E a ~.17 ev b 2.4..6.7.8.9 1/, K -1
TSL intensity (a.u.) 2 16 12 8 4 16 12 8 4 2 K 23 K 23 K :Mo, La =12 K E irr =3.7 ev 1 2 3 4 6 t irr (min) :Mo, La =2 K E irr =3.8 ev The dependences I TSL (t irr ) can also be different for different TSL peaks creation with the same E irr. The superlinear dependence obtained for the 22-23 K peak may mean that the corresponding electron centers are created due to the two-photon process, i.e., the ionization of (MoO 4 ) 3- created by the first photon (as a result of a hole release), takes place by the second photon. The same dependence observed for the 2 K and ~23 K peaks creation may mean that the electron and hole centers are created by one photon.
Conclusions The complex B and G(I) emissions are of the exciton-like origin and arise from the (WO 4 ) 2- groups located in the regular and in the lead-deficient crystal regions, respectively, and perturbed by various defects. The 3.1 ev band and the weak G(I) band are assumed to arise from two configurations of an exciton localized near an A 3+ ion. The slow G(II) emission accompanies photo- and thermally stimulated tunneling recombination processes in different (genetic or stochastic) optically created pairs of the V - and (MoO 4 ) 3- -related electron centers and the V Pb -related hole centers. Thermal quenching of the emissions is caused by the decay of the corresponding localized excitons. Thermal quenching of the G(II) emission is caused by thermal destruction of the corresponding hole centers.
Various localized exciton states (the excitons of the type of (WO 4 ) 2- localized near A 3+ ions, oxygen vacancies of the type of WO 3, WO 2 or WO, and (MoO 4 ) 2- groups) are identified. Their photo-thermally stimulated decay is assumed to result in the following processes: ex A 3+ (WO 4 ) 3- -A 3+ ; ex WO 3 {Pb + -WO 3 }; ex (MoO 4 ) 2- (MoO 4 ) 3- ; ex WO 2 {e - or 2e - at/near WO 2 }; ex WO {e - or 2e - at/near WO}. The creation of electron centers in these processes takes place without release of the free electrons. The optically released mobile holes can be responsible for the photoconductivity observed under irradiation in the localized exciton region at T>1 K.
At the photo-thermally stimulated decay of the self-trapped excitons and defect-related states, free charge carriers are released. Lead and oxygen vacancies play an important role in the trapping of holes and electrons, respectively, and in the optically and thermally stimulated recombination processes. Co-doping with stable A 3+ ions reduces the number of the isolated vacancies. This leads to both a strong suppression of the slow (μs-ms) tunnelling recombination G(II) luminescence and the enhancement of the fast (2-4 ns at RT) B and G(I) emissions. As a result, considerable improvement of scintillation characteristics of crystals takes place. The study of dependences of the number of various defects, created under selective UV irradiation, on E irr, and t irr, carried out at the same samples and in comparable conditions by both the ESR and TSL methods, is an excellent method for investigation of the origin of the defects and the mechanisms of their creation.