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Physics of halide scintillators
Radiation Measurements 33 (2001) 699–704
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Physics of halide scintillators Svetlana Zazubovich ∗ Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia Received 20 August 2000; received in revised form 2 February 2001; accepted 11 February 2001
Abstract The paper reviews the results obtained within the last 5 – 6 years in the .eld of the physics of some halide crystals which have led to a better understanding of their characteristics. The luminescence and scintillation characteristics of caesium iodides, widely used in the existing detectors, as well as the properties of some other selected materials are discussed in view of their possible applications. For all the systems concerned, the emission important for their practical applications arises from the radiative decay of excitons of various types. The following features are considered in more detail: (1) the excited states structure, the mechanism of excitation of the localized exciton emission in the impurity-induced absorption bands, the processes of defects creation, and the scintillation mechanism in CsI : Tl and CsI : Na scintillators; (2) the luminescence of on-centre excitons in CsI and RbI crystals; (3) the luminescence of free cation excitons in the CsPbX3 -type aggregates thermally created in CsX : Pb single crystals (X = Cl, Br); (4) the model of the excited states responsible for the cation exciton luminescence c 2001 Elsevier Science Ltd. All rights reserved. of lead halides. Keywords: Luminescence; Exciton; Scintillator; Halides
1. Introduction The progress in high energy physics and nuclear physics has stimulated scintillation materials science in two directions. First, there is the search and development of new perspective materials for scintillators. Second, there is a further improvement of the traditional scintillators, including alkali halides. The scintillators created on the basis of alkali iodides (CsI, CsI : Tl, CsI : Na, NaI : Tl) .nd wide applications in the areas of high energy physics, nuclear physics, medicine, industry, security and environmental control devices, geology, astrophysics, etc. Their bene.t is a very high light output, high conversion e=ciency of the ionizing radiation energy into visible light convenient for registration by the modern sensitive silicon detectors, high spatial and energy resolution. The crystals on the basis of alkali halides are often preferred to other materials due to their
∗ Tel.: +372-7-428946; fax: +372-7-383033. E-mail address: [email protected] (S. Zazubovich).
low price and the presence of the developed technologies and equipment for the growth of large single crystals of good quality. The attention paid to these materials has, in particular, increased during the last years due to the creation of high-resolution gamma detectors suitable for precise gamma cameras, positron emission tomography, single photon emission mammography and astrophysics applications. CsI : Tl scintillators are used now in several famous investigation centres (National Laboratory for High Energy Physics, Japan; Stanford Linear Accelerator Centre, USA; Fermi Lab, etc) to construct precision electromagnetic calorimeters for the next generation of high energy physics experiments. Pure CsI is used in the fast selective scintillators for nuclear physics. Besides their practical importance, these systems are also of great scienti.c interest since for these relatively simple crystals a detailed study of the mechanisms of physical phenomena, and elaboration of the theoretical models is possible. This is necessary for an understanding and prediction of the properties of more complicated materials which are or can be applied in various devices.
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 0 8 6 - 5
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S. Zazubovich / Radiation Measurements 33 (2001) 699–704
The characteristics of various halide scintillators were recently reviewed by Rodnyi (1997) and Nikl (2000). In the present paper, we consider only some selected systems, namely localized excitons in doped CsI crystals, on-centre self-trapped excitons in CsI and RbI, free excitons in the CsPbX3 aggregates dispersed in a CsX single crystal matrix, and cation excitons in lead halides. 2. Luminescence and defects creation in CsI : Tl and CsI : Na Despite active spectroscopic studies and long-time practical applications, many features in these systems remained unclear up to recent times. For example, the origin of the visible emission of the CsI : Tl scintillator was established only 6 years ago. On the basis of the low-temperature (down to 0:35 K) photoluminescence decay kinetics and polarization studies (Nagirnyi et al., 1994) and from ODMR experiments (Spaeth et al., 1994) it has been concluded that this emission is not caused by the electronic transitions from the triplet excited state of a Tl+ ion, as it has been believed for many previous years, but it arises from the radiative decay of an exciton localized near a Tl+ ion. Indeed, the parameters of the relaxed excited states (RES) responsible for the 2.55 and 2:25 eV emissions of CsI : Tl, determined from the temperature and magnetic .eld dependences of the luminescence decay times (Nagirnyi et al., 1995, 1996), diIer by orders of magnitude from the RES parameters of Tl+ centres in alkali halides but are close to the RES parameters of the self-trapped exciton (STE) in CsI and localized exciton in CsI : Na (Babin et al., 2000a) (Fig. 1). At T ¡ 1 K the radiative transitions occur from the lowest minimum of the triplet RES. In the case of the 2:55 eV band, this is the metastable minimum, and in the case of the 2:25 eV band, the emitting minima. As the temperature increases, the upper (emitting or metastable, respectively) minimum of the triplet RES is thermally populated. It results in a change (increase or decrease, respectively) of the emission decay time. At T ¿ 30 K the singlet state of the localized exciton is thermally populated. It leads to a drastic decrease of decay times (down to 10−8 s at 295 K) and a change in the polarization characteristics of both emission bands. According to Song et al. (1990), the reverse order of the metastable and emitting minima may be observed in the systems with a weak spin–orbit interaction (where the spin–spin interaction dominates) but only in case the hole is shared almost equally between two halogen ions and the electron and hole components of the exciton are well separated. These conditions can be realized for a strong oI-centre exciton con.guration. In this case the spin–orbit interaction is essentially suppressed by an electron–phonon interaction and a defect stimulates a more symmetric location of a hole between two halogen ions. A further study has shown (Babin et al., 1998, 2000a) that the excitation of CsI : Tl in the Tl+ -induced absorption
Fig. 1. Energy levels of the singlet (s) and triplet (t) localized exciton states responsible for the 2.55 and 2:25 eV emission of CsI : Tl. Parameters of the RES responsible for some exciton emissions in CsI : the probabilities of radiative transitions from the metastable (1 ) and emitting (2 ) minima of the triplet state; the energy distances between the metastable and emitting minima of the triplet state () and between the triplet and singlet states (JEs−t ) (Nagirnyi et al., 1995; Babin et al., 2000b).
bands results in the electron transfer to a vacant 6p orbital of a Tl+ ion from the 5p orbital of an I− ion perturbed by a close Tl+ ion. As a result, an electron Tl0 centre is created, and a mobile hole appears in the valence band and migrates to a Tl0 atom. After its self-trapping near the Tl0 atom, the short-living closest pair of the type of {Tl0 ; VK } is produced. The fast tunnelling recombination of the 6p electron of Tl0 with the VK centre occurs in the {Tl0 ; VK } pair (Fu et al., 1999). The subsequent vibrational relaxation of the excited state formed in the recombination process .nally leads to the creation of the localized exciton RES whose radiative decay leads to the appearance of the well-known visible emission of CsI : Tl. It should be pointed out that the formation of the closest {Tl0 ; VK } pairs and thereafter the RES of the localized exciton occurs very quickly, as at Eexc ¡ 4:8 eV the rise time of the visible emission of CsI : Tl is less than 50 ps, and no afterglow is detected under excitation in this region. Analogous processes resulting in the appearance of the localized exciton emission (2:98 eV) should take place in CsI : Na under excitation in the 5:4 eV absorption band. On the basis of the new interpretation of the CsI : Tl visible emission, the following scintillation mechanism has been proposed for the CsI : Tl and CsI : Na scintillators (Babin et al., 1998, 2000a). An excitation of crystals by the ionizing radiation produces electrons and holes. According to Iwai et al. (1996), the correlated relaxation of associated electron–hole pairs occurs in KI and RbI. In pure crystals they recombine and .rst the one-centre excitons are created
S. Zazubovich / Radiation Measurements 33 (2001) 699–704
transforming into the two-centre STE. Unlike this case, in the crystals containing the traps for electrons, the one-centre excitons do not appear. Taking into account these data, we have suggested that also in irradiated CsI, as soon as one component of the relaxing electron–hole pair (an electron) is trapped by an impurity (A+ ) ion, the second component (a hole) is immediately self-trapped near the A0 centre producing a pair of the type of A0 –VK . The A0 electron recombination with the VK centre in this pair leads to the formation of the exciton localized near A+ , whose radiative decay is accompanied by luminescence. This mechanism explains a relatively long scintillation rise time, temperature dependence of scintillations, a large amount of Tl0 –VK and Na0 –VK pairs, high e=ciency of CsI : Tl and CsI : Na scintillators and the appearance of relatively slow afterglow under irradiation by the ionizing radiation (see Babin et al., 2000a and references therein). A deep understanding of the defects creation mechanisms in doped CsI crystals is also of great importance for the physics of scintillators as well as for successful application of the systems of this type. The stable defects mainly of the type of VK and Tl0 centres are created in CsI : Tl under irradiation not only by the ionizing radiation but also by the ultraviolet light at 80 K (Chernov et al., 1998) or at 4:2 K (Babin et al., 1998). Due to the selective excitation, in the latter case it is possible to vary the relative number of various defects which allows to understand the processes of their creation. The formation of defects is evident from the appearance of an afterglow, thermally stimulated luminescence (TSL) peaks and photostimulation bands of the recombination luminescence. All these features arise from the recombination of Tl0 electrons with the VK -type centres located at diIerent distances from Tl0 . According to Barland et al. (1981) and Babin et al. (1998), the TSL peak at 60 K arises from the recombination of Tl0 electrons with VK centres in the close {Tl0 –VK } pairs, i.e., no migration of VK centres takes place. It is only the TSL peak at 90 K that is connected with the VK centres migration. At these temperatures, the 90o -jump diIusion of VK centres precedes their recombination with Tl0 (Sidler et al., 1973). The peak at 125 K arises from the recombination of the electrons, thermally released from the unperturbed Tl0 atoms, with VKA (Tl+ ) centres (Gutan et al., 1974). Recently, we have carried out a systematic study of the defects created at 4:2 K under selective irradiation of CsI : Tl in the Tl+ -related absorption bands (by photons of the 5.8–4:8 eV energy) (Babin et al., 2000b). The strong dependences of relative intensities of the recombination luminescence photostimulation bands (peaking at 2.35, 1.92, 1.33 and 0:89 eV), afterglow, and TSL peaks (near 60, 90 and 125 K) on the irradiation energy and duration, uniaxial stress (which stimulates the self-trapping of the optically created holes) and thallium concentration have been found. It allows to make conclusions on the detailed mechanisms of the processes, resulting in the appearance of defects of various types: the close or the more separated Tl0 –VK and
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Tl+ –VK pairs and the isolated VKA (Tl+ ) and Tl0 centres, and on the role of these defects in luminescence of CsI : Tl. 3. Luminescence of on-centre excitons in CsI and RbI In undoped CsI crystals as well as in CsI containing some impurity anions (e.g., Cl, Br, CO3 , etc.), the ultraviolet emissions of exciton origin are present. At 295 K their relatively fast (ns) decay is caused by the radiative transitions from the singlet exciton state populated directly or through thermal transitions from the triplet state which occur at relatively low temperatures due to a small (∼ 10−2 eV) energy distance between the exciton triplet and singlet states. As a result, at 295 K these states are in thermal equilibrium. The luminescence of the on-centre excitons is of special interest for practical applications as in this case the direct radiative transitions from the singlet state usually prevail. Even in case the triplet state is also populated, its decay probability is by about an order of magnitude higher in the on-centre con.guration than in the oI-centre con.guration due to a stronger overlapping of the electron and hole wavefunctions in the former case. Until now, only the 4:1 eV emission of a pure CsI crystal with a decay time of ∼ 10−8 s at 295 K is used in fast scintillators. The origin of this emission is not clear. Nishimura et al. (1995) have connected it with the radiative decay of the on-centre STE. Our studies have shown (Babin et al., 1999b) that at 4:2 K, besides the on-centre STE emission (4:29 eV), two additional emissions (4.32 and 4:25 eV) exist in some samples. Unlike the 4:29 eV STE emission, the 4:25 eV emission, which is often mixed up with the on-centre STE emission, is not noticeably quenched up to 80 K. Consequently, at these temperatures it is predominantly the 3:65 eV STE band and the 4:25 eV band that are observed. As the 4:25 eV emission intensity depends on the particular sample, it explains why the 3:65 eV=4:25 eV emission intensity ratio at 80 K varies strongly (up to 25 times) from report to report. The presence of the 4.32 and 4:25 eV emissions overlapping with the on-centre STE emission may inMuence the scintillation characteristics of CsI. A strong eIect of the uniaxial stress on the exciton states structure and on various stages of the electronic excitation relaxation (their migration and self-trapping, nonradiative decay accompanied with the defects creation, etc.) has been recently found. From the practical point of view, the most important result is the stress-induced enhancement of the on-centre STE emission which is especially strong in RbI due to a small energy barrier between the on-centre and the oI-centre exciton con.gurations (Fig. 2) (Babin et al., 1999a). Under the stress a reduction of various extrinsic emissions and a decrease of the radiation defects creation e=ciency have also been detected. One may suppose that the use of the uniaxial stress can considerably increase the fast=slow components ratio in the decay of exciton luminescence. The possibility of decreasing the radiation damage
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Fig. 2. Emission spectra of the unstressed (1) and uniaxially stressed (2) RbI:Tl measured at 80 K under the X-ray excitation (Babin et al., 1999a).
by the uniaxial stress means that radiation hardness of scintillators can be controlled. 4. Luminescence of CsPbX3 nanocrystals dispersed in CsX crystals Some years ago it was found by Nikl et al. (1995) that ◦ a prolonged annealing of a CsCl : Pb crystal at 150 –250 C results in the creation of small (few nm) semiconducting aggregates of the type of CsPbCl3 with a perovskite structure dispersed in a CsCl matrix. Their intense emission has been ascribed to the radiative decay of the Wannier-type free cation excitons. The luminescence characteristics of the aggregates are similar to those of CsPbCl3 single crystals. The decay time of the CsPbCl3 crystal emission is ∼ 10−10 s, which allows to consider CsPbCl3 as a perspective material for fast scintillators. A much shorter decay time (∼ 10−11 s) has been obtained for the free exciton emission of the CsPbCl3 nanocrystals and explained as an appearance of the quantum con.nement eIect in the aggregates of a very small size. Similar results are obtained for CsPbBr 3 aggregates in CsBr. It has been found that the position and halfwidth of the free exciton emission band, and its decay kinetics depend on the aggregate size. The size increases as the annealing temperature and duration increase and the lead content decreases. Previously created aggregates can be destroyed by a fast quenching of a sample down from ◦ 590 C. Thereafter, the sample is ready for the next thermal treatment. Owing to an intense and fast emission, high stability, a simple and easily controlled preparation procedure of the nanocrystals of selected size, the systems of this type are of great interest in view of their possible application in fast scintillators. Recently, the luminescence of the CsPbCl3 aggregates has been studied by Voloshinovskii et al. (2001) under excitation by the synchrotron radiation in the energy range of 4 –20 eV. It has been found that the fast luminescence of CsPbCl3 aggregates is most eIectively excited in the CsPbCl3 absorption region as
Fig. 3. Excitation spectrum measured at 10 K for the fast luminescence decay component of the CsPbCl3 aggregates dispersed in a CsCl crystal matrix (Voloshinovskii et al., 2001).
well as by photons of the Eexc ¿ 14 eV energy (Fig. 3). In the latter case it appears due to the reabsorption of the CsCl core-valence luminescence by CsPbCl3 aggregates. However, the e=ciency of the energy transfer from a CsCl matrix to the CsPbCl3 aggregates is found to be small. Being excited in the absorption bands of Pb2+ centres, which were not completely destroyed in the course of annealing, the CsPbCl3 emission is slow due to the reabsorption of Pb2+ emission by the CsPbCl3 aggregates. These circumstances may inMuence the possibility of application of these systems. 5. Luminescence of lead halide crystals Recent studies have shown that lead halides may also be considered as possible materials for high energy radiation detectors. From this point of view it was of interest to consider brieMy these systems as well. For a long time the luminescence of lead halides, arising from the radiative decay of cation excitons, has been ascribed to the electronic transitions from the excited states of a single Pb2+ ion. Recently, the self-trapping of an electron on the two nearest-neighbouring Pb2+ ions in PbCl2 has been found (Nistor et al., 1993). One may assume that under irradiation of lead halides in the exciton absorption band an electron is immediately self-trapped at two Pb2+ ions. The subsequent recombination of a mobile hole with the self-trapped electron results in the creation of the STE of the type of (Pb2+ Pb2+ )∗ whose radiative decay is accompanied by the STE luminescence. Thus, the RES responsible for the exciton emission of lead halides has to be similar not to the RES of single Pb2+ centres in alkali halides, but to the RES of dimer Pb2+ Pb2+ centres. Indeed, the decay kinetics of all the exciton emission bands (denoted as UV, B, BG) of lead halides is strongly diIerent from that characteristic of Pb2+ centres in alkali halides. In the decay kinetics of Pb2+ emission, two components (slow and fast) are observed at 4:2 K with the decay times 10−3 and 10−8 s. The slow component arises from the radiative decay of the metastable minima of the triplet RES. Due to a large
S. Zazubovich / Radiation Measurements 33 (2001) 699–704
Fig. 4. Decay kinetics of the UV emission in a PbCl2 crystal. Eexc = 4:5 eV, Eem = 3:75 eV.
spin–orbit interaction, the energy distance between the metastable and emitting minima is ∼ 10−2 eV, so thermally stimulated transitions, resulting in the decrease of the slow component decay time, can occur only at T ¿ 50 K. Unlike this case, in PbCl2 the slow component decay time at 4:2 K is ∼ 10−5 s for the UV and B bands and ∼ 10−4 s for the BG band (Liidja et al., 1972; Nikl et al., 1991) and do not practically change down to 0:46 K (see, e.g., Fig. 4). Thus, the states responsible for all the exciton emissions of PbCl2 are really diIerent from the states of Pb2+ . We assume that the UV and B bands of the PbCl2 crystal arise from the radiative transitions from two minima of the same RES, similar to the RES of dimer (Pb2+ Pb2+ ) centres in alkali halides and formed as a result of an immediate recombination of a mobile hole with the electron self-trapped at two Pb2+ ions. The relatively slow BG emission may arise from tunnelling recombinations between the closely located self-trapped hole and self-trapped electron (Kitaura and Nakagawa, 1996). Acknowledgements The work was supported by the Estonian Science Foundation Grant No. 3875. References Babin, V., Bekeshev, A., Elango, A., Kalder, K., Maaroos, A., Shunkeev, K., Vasil’chenko, E., Zazubovich, S., 1999a. EIect of uniaxial stress on luminescence of undoped and thallium-doped KI and RbI crystals. J. Phys.: Condens. Matter 11, 2303–2317. Babin, V., Elango, A., Kalder, K., Zazubovich, S., 1999b. EIect of uniaxial stress on exciton luminescence in CsI crystals. Phys. Stat. Sol. B 212, 185–198. Babin, V., Fabeni, P., Kalder, K., Nikl, M., Pazzi, P.G., Zazubovich, S., 1998. Photo- and thermally stimulated luminescence and defects in UV-irradiated CsI : Tl and CsI : Pb crystals. Radiat. Meas. 29, 333–335. Babin, V., Fabeni, P., Mihokova, E., Nikl, M., Stolovich, A., Pazzi, G.P., Zazubovich, S., 2000a. Time-resolved spectroscopy
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of exciton states in undoped and doped CsI crystals. In: V. Mikhailin (Ed.), Proceedings of the Fifth International Conference on Inorganic Scintillators and Their Applications. Moscow University Press, Moscow, Russia, p. 544. Babin, V., Kalder, K., Krasnikov, A., Zazubovich, S., 2000b. Luminescence and defects creation under photoexcitation of CsI : Tl crystal in Tl+ -related absorption bands. Book of Abstracts of fourth Euroconference on Luminescent Detectors and Transformers of Ionizing Radiation, Riga, August 14 –17 p. 61. Barland, M., Daval, E., Nouailhat, A., Aegerter, M., 1981. Migration of VK centres in CsI crystal. J. Phys. C 14, 4237–4245. Chernov, S., Trinkler, L., Popov, A., 1998. Photo-and thermo-stimulated luminescence of CsI–Tl crystal after UV light irradiation at 80 K. Radiate EI. Def. Solids 143, 345–355. Fu, C., Chen, L., Song, K.S., 1999. Self-trapped excitons in pure and Na- and Tl-doped caesium halides and the recombination luminescence. J. Phys.: Condens. Matter 11, 5517–5532. Gutan, V., Shamovskii, L., Dunina, A., Gorobets, B., 1974. Two types of luminescence centres in CsI(Tl). Opt. Spectrosc. 37, 407–410. Iwai, S., Tokizaki, T., Nakamura, A., Tanimura, K., Itoh, N., Shluger, A., 1996. One-centre small polarons as short-lived precusors in self-trapping process of holes and electron–hole pairs in alkali iodides. Phys. Rev. Lett. 76, 1691–1694. Kitaura, M., Nakagawa, H., 1996. Luminescence due to dimer type self-trapped excitons in lead halides. J. Electron Spectrosc. Relat. Phenom. 79, 171–174. Liidja, G., Plekhanov, V., Soovik, T., 1972. Decay kinetics of exciton luminescence in undoped PbCl2 single crystals. Trudy IFA AN Est. SSR 39, 303–306 (in Russian). Nagirnyi, V., Stolovich, A., Zazubovich, S., Zepelin, V., Mihokova, E., Nikl, M., Pazzi, G.P., Salvini, L., 1995. Peculiarities of the triplet relaxed excited-state structure and luminescence of a CsI : Tl crystal. J. Phys.: Condens. Matter 7, 3637–3653. Nagirnyi, V., Stolovich, A., Zazubovich, S., Zepelin, V., Pazzi, G.P., 1996. EIect of magnetic .eld on the decay kinetics of triplet luminescence of the self-trapped exciton perturbed by Tl+ ion in thallium-doped caesium halides. Solid State Commun. 100, 621–624. Nagirnyi, V., Zazubovich, S., Zepelin, V., Nikl, M., Pazzi, G.P., 1994. A new model for the visible emission of the CsI : Tl crystal. Chem. Phys. Lett. 227, 533–538. Nikl, M., 2000. Wide band gap scintillation materials: progress in the technology and material understanding. Phys. Stat. Sol. A 178, 595–620. Nikl, M., Birch, D.J.S., Polak, K., 1991. Blue and violet emission of PbCl2 . Phys. Stat. Sol. B 165, 611–619. Nikl, M., Nitsch, K., Polak, K., Pazzi, G.P., Fabeni, P., Citrin, D.S., Gurioli, M., 1995. Optical properties of the Pb2+ -based aggregated phase in a CsCl host crystal: quantum con.nement eIects. Phys. Rev. B 51, 5192–5199. Nishimura, N., Sakata, M., Tsujimoto, T., Nakayama, M., 1995. Origin of the 4.1 eV luminescence in pure CsI scintillator. Phys. Rev. B 51, 2167–2172. Nistor, S.V., Goovaerts, E., Schoemeker, D., 1993. Direct observation of electron self-trapping in PbCl2 crystals. Phys. Rev. B 48, 9575–9580. Rodnyi, P.A., 1997. Physical Processes in Inorganic Scintillators. Chemical Rubber Company Press, Cleveland, OH.
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Sidler, T., Pellaux, J-P., Nouailhat, A., Aegerter, M., 1973. Study of VK centres in CsI crystal. Solid State Commun. 13, 479–482. Song, K.S., Leung, C.H., Spaeth, J.-M., 1990. Zero-.eld splitting of the self-trapped exciton in alkali Muorides and alkaline-earth Muorides. J. Phys.: Condens. Matter 2, 6373–6379. Spaeth, J-M., Meise, W., Song, K.S., 1994. The nature of the X-ray induced luminescence and hole centres in CsI : Tl
studied by optically detected electron paramagnetic resonance. J. Phys.: Condens. Matter 6, 3999–4008. Voloshinovskii, A., Myagkota, S., Gloskovsky, A., Zazubovich, S., Zimmerer, G., 2001. Luminescence of CsPbCl3 microcrystals dispersed in a CsCl crystal under high-energy excitation. Phys. Stat. Sol. B, in press.
www.elsevier.com/locate/radmeas
Physics of halide scintillators Svetlana Zazubovich ∗ Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia Received 20 August 2000; received in revised form 2 February 2001; accepted 11 February 2001
Abstract The paper reviews the results obtained within the last 5 – 6 years in the .eld of the physics of some halide crystals which have led to a better understanding of their characteristics. The luminescence and scintillation characteristics of caesium iodides, widely used in the existing detectors, as well as the properties of some other selected materials are discussed in view of their possible applications. For all the systems concerned, the emission important for their practical applications arises from the radiative decay of excitons of various types. The following features are considered in more detail: (1) the excited states structure, the mechanism of excitation of the localized exciton emission in the impurity-induced absorption bands, the processes of defects creation, and the scintillation mechanism in CsI : Tl and CsI : Na scintillators; (2) the luminescence of on-centre excitons in CsI and RbI crystals; (3) the luminescence of free cation excitons in the CsPbX3 -type aggregates thermally created in CsX : Pb single crystals (X = Cl, Br); (4) the model of the excited states responsible for the cation exciton luminescence c 2001 Elsevier Science Ltd. All rights reserved. of lead halides. Keywords: Luminescence; Exciton; Scintillator; Halides
1. Introduction The progress in high energy physics and nuclear physics has stimulated scintillation materials science in two directions. First, there is the search and development of new perspective materials for scintillators. Second, there is a further improvement of the traditional scintillators, including alkali halides. The scintillators created on the basis of alkali iodides (CsI, CsI : Tl, CsI : Na, NaI : Tl) .nd wide applications in the areas of high energy physics, nuclear physics, medicine, industry, security and environmental control devices, geology, astrophysics, etc. Their bene.t is a very high light output, high conversion e=ciency of the ionizing radiation energy into visible light convenient for registration by the modern sensitive silicon detectors, high spatial and energy resolution. The crystals on the basis of alkali halides are often preferred to other materials due to their
∗ Tel.: +372-7-428946; fax: +372-7-383033. E-mail address: [email protected] (S. Zazubovich).
low price and the presence of the developed technologies and equipment for the growth of large single crystals of good quality. The attention paid to these materials has, in particular, increased during the last years due to the creation of high-resolution gamma detectors suitable for precise gamma cameras, positron emission tomography, single photon emission mammography and astrophysics applications. CsI : Tl scintillators are used now in several famous investigation centres (National Laboratory for High Energy Physics, Japan; Stanford Linear Accelerator Centre, USA; Fermi Lab, etc) to construct precision electromagnetic calorimeters for the next generation of high energy physics experiments. Pure CsI is used in the fast selective scintillators for nuclear physics. Besides their practical importance, these systems are also of great scienti.c interest since for these relatively simple crystals a detailed study of the mechanisms of physical phenomena, and elaboration of the theoretical models is possible. This is necessary for an understanding and prediction of the properties of more complicated materials which are or can be applied in various devices.
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 0 8 6 - 5
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The characteristics of various halide scintillators were recently reviewed by Rodnyi (1997) and Nikl (2000). In the present paper, we consider only some selected systems, namely localized excitons in doped CsI crystals, on-centre self-trapped excitons in CsI and RbI, free excitons in the CsPbX3 aggregates dispersed in a CsX single crystal matrix, and cation excitons in lead halides. 2. Luminescence and defects creation in CsI : Tl and CsI : Na Despite active spectroscopic studies and long-time practical applications, many features in these systems remained unclear up to recent times. For example, the origin of the visible emission of the CsI : Tl scintillator was established only 6 years ago. On the basis of the low-temperature (down to 0:35 K) photoluminescence decay kinetics and polarization studies (Nagirnyi et al., 1994) and from ODMR experiments (Spaeth et al., 1994) it has been concluded that this emission is not caused by the electronic transitions from the triplet excited state of a Tl+ ion, as it has been believed for many previous years, but it arises from the radiative decay of an exciton localized near a Tl+ ion. Indeed, the parameters of the relaxed excited states (RES) responsible for the 2.55 and 2:25 eV emissions of CsI : Tl, determined from the temperature and magnetic .eld dependences of the luminescence decay times (Nagirnyi et al., 1995, 1996), diIer by orders of magnitude from the RES parameters of Tl+ centres in alkali halides but are close to the RES parameters of the self-trapped exciton (STE) in CsI and localized exciton in CsI : Na (Babin et al., 2000a) (Fig. 1). At T ¡ 1 K the radiative transitions occur from the lowest minimum of the triplet RES. In the case of the 2:55 eV band, this is the metastable minimum, and in the case of the 2:25 eV band, the emitting minima. As the temperature increases, the upper (emitting or metastable, respectively) minimum of the triplet RES is thermally populated. It results in a change (increase or decrease, respectively) of the emission decay time. At T ¿ 30 K the singlet state of the localized exciton is thermally populated. It leads to a drastic decrease of decay times (down to 10−8 s at 295 K) and a change in the polarization characteristics of both emission bands. According to Song et al. (1990), the reverse order of the metastable and emitting minima may be observed in the systems with a weak spin–orbit interaction (where the spin–spin interaction dominates) but only in case the hole is shared almost equally between two halogen ions and the electron and hole components of the exciton are well separated. These conditions can be realized for a strong oI-centre exciton con.guration. In this case the spin–orbit interaction is essentially suppressed by an electron–phonon interaction and a defect stimulates a more symmetric location of a hole between two halogen ions. A further study has shown (Babin et al., 1998, 2000a) that the excitation of CsI : Tl in the Tl+ -induced absorption
Fig. 1. Energy levels of the singlet (s) and triplet (t) localized exciton states responsible for the 2.55 and 2:25 eV emission of CsI : Tl. Parameters of the RES responsible for some exciton emissions in CsI : the probabilities of radiative transitions from the metastable (1 ) and emitting (2 ) minima of the triplet state; the energy distances between the metastable and emitting minima of the triplet state () and between the triplet and singlet states (JEs−t ) (Nagirnyi et al., 1995; Babin et al., 2000b).
bands results in the electron transfer to a vacant 6p orbital of a Tl+ ion from the 5p orbital of an I− ion perturbed by a close Tl+ ion. As a result, an electron Tl0 centre is created, and a mobile hole appears in the valence band and migrates to a Tl0 atom. After its self-trapping near the Tl0 atom, the short-living closest pair of the type of {Tl0 ; VK } is produced. The fast tunnelling recombination of the 6p electron of Tl0 with the VK centre occurs in the {Tl0 ; VK } pair (Fu et al., 1999). The subsequent vibrational relaxation of the excited state formed in the recombination process .nally leads to the creation of the localized exciton RES whose radiative decay leads to the appearance of the well-known visible emission of CsI : Tl. It should be pointed out that the formation of the closest {Tl0 ; VK } pairs and thereafter the RES of the localized exciton occurs very quickly, as at Eexc ¡ 4:8 eV the rise time of the visible emission of CsI : Tl is less than 50 ps, and no afterglow is detected under excitation in this region. Analogous processes resulting in the appearance of the localized exciton emission (2:98 eV) should take place in CsI : Na under excitation in the 5:4 eV absorption band. On the basis of the new interpretation of the CsI : Tl visible emission, the following scintillation mechanism has been proposed for the CsI : Tl and CsI : Na scintillators (Babin et al., 1998, 2000a). An excitation of crystals by the ionizing radiation produces electrons and holes. According to Iwai et al. (1996), the correlated relaxation of associated electron–hole pairs occurs in KI and RbI. In pure crystals they recombine and .rst the one-centre excitons are created
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transforming into the two-centre STE. Unlike this case, in the crystals containing the traps for electrons, the one-centre excitons do not appear. Taking into account these data, we have suggested that also in irradiated CsI, as soon as one component of the relaxing electron–hole pair (an electron) is trapped by an impurity (A+ ) ion, the second component (a hole) is immediately self-trapped near the A0 centre producing a pair of the type of A0 –VK . The A0 electron recombination with the VK centre in this pair leads to the formation of the exciton localized near A+ , whose radiative decay is accompanied by luminescence. This mechanism explains a relatively long scintillation rise time, temperature dependence of scintillations, a large amount of Tl0 –VK and Na0 –VK pairs, high e=ciency of CsI : Tl and CsI : Na scintillators and the appearance of relatively slow afterglow under irradiation by the ionizing radiation (see Babin et al., 2000a and references therein). A deep understanding of the defects creation mechanisms in doped CsI crystals is also of great importance for the physics of scintillators as well as for successful application of the systems of this type. The stable defects mainly of the type of VK and Tl0 centres are created in CsI : Tl under irradiation not only by the ionizing radiation but also by the ultraviolet light at 80 K (Chernov et al., 1998) or at 4:2 K (Babin et al., 1998). Due to the selective excitation, in the latter case it is possible to vary the relative number of various defects which allows to understand the processes of their creation. The formation of defects is evident from the appearance of an afterglow, thermally stimulated luminescence (TSL) peaks and photostimulation bands of the recombination luminescence. All these features arise from the recombination of Tl0 electrons with the VK -type centres located at diIerent distances from Tl0 . According to Barland et al. (1981) and Babin et al. (1998), the TSL peak at 60 K arises from the recombination of Tl0 electrons with VK centres in the close {Tl0 –VK } pairs, i.e., no migration of VK centres takes place. It is only the TSL peak at 90 K that is connected with the VK centres migration. At these temperatures, the 90o -jump diIusion of VK centres precedes their recombination with Tl0 (Sidler et al., 1973). The peak at 125 K arises from the recombination of the electrons, thermally released from the unperturbed Tl0 atoms, with VKA (Tl+ ) centres (Gutan et al., 1974). Recently, we have carried out a systematic study of the defects created at 4:2 K under selective irradiation of CsI : Tl in the Tl+ -related absorption bands (by photons of the 5.8–4:8 eV energy) (Babin et al., 2000b). The strong dependences of relative intensities of the recombination luminescence photostimulation bands (peaking at 2.35, 1.92, 1.33 and 0:89 eV), afterglow, and TSL peaks (near 60, 90 and 125 K) on the irradiation energy and duration, uniaxial stress (which stimulates the self-trapping of the optically created holes) and thallium concentration have been found. It allows to make conclusions on the detailed mechanisms of the processes, resulting in the appearance of defects of various types: the close or the more separated Tl0 –VK and
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Tl+ –VK pairs and the isolated VKA (Tl+ ) and Tl0 centres, and on the role of these defects in luminescence of CsI : Tl. 3. Luminescence of on-centre excitons in CsI and RbI In undoped CsI crystals as well as in CsI containing some impurity anions (e.g., Cl, Br, CO3 , etc.), the ultraviolet emissions of exciton origin are present. At 295 K their relatively fast (ns) decay is caused by the radiative transitions from the singlet exciton state populated directly or through thermal transitions from the triplet state which occur at relatively low temperatures due to a small (∼ 10−2 eV) energy distance between the exciton triplet and singlet states. As a result, at 295 K these states are in thermal equilibrium. The luminescence of the on-centre excitons is of special interest for practical applications as in this case the direct radiative transitions from the singlet state usually prevail. Even in case the triplet state is also populated, its decay probability is by about an order of magnitude higher in the on-centre con.guration than in the oI-centre con.guration due to a stronger overlapping of the electron and hole wavefunctions in the former case. Until now, only the 4:1 eV emission of a pure CsI crystal with a decay time of ∼ 10−8 s at 295 K is used in fast scintillators. The origin of this emission is not clear. Nishimura et al. (1995) have connected it with the radiative decay of the on-centre STE. Our studies have shown (Babin et al., 1999b) that at 4:2 K, besides the on-centre STE emission (4:29 eV), two additional emissions (4.32 and 4:25 eV) exist in some samples. Unlike the 4:29 eV STE emission, the 4:25 eV emission, which is often mixed up with the on-centre STE emission, is not noticeably quenched up to 80 K. Consequently, at these temperatures it is predominantly the 3:65 eV STE band and the 4:25 eV band that are observed. As the 4:25 eV emission intensity depends on the particular sample, it explains why the 3:65 eV=4:25 eV emission intensity ratio at 80 K varies strongly (up to 25 times) from report to report. The presence of the 4.32 and 4:25 eV emissions overlapping with the on-centre STE emission may inMuence the scintillation characteristics of CsI. A strong eIect of the uniaxial stress on the exciton states structure and on various stages of the electronic excitation relaxation (their migration and self-trapping, nonradiative decay accompanied with the defects creation, etc.) has been recently found. From the practical point of view, the most important result is the stress-induced enhancement of the on-centre STE emission which is especially strong in RbI due to a small energy barrier between the on-centre and the oI-centre exciton con.gurations (Fig. 2) (Babin et al., 1999a). Under the stress a reduction of various extrinsic emissions and a decrease of the radiation defects creation e=ciency have also been detected. One may suppose that the use of the uniaxial stress can considerably increase the fast=slow components ratio in the decay of exciton luminescence. The possibility of decreasing the radiation damage
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Fig. 2. Emission spectra of the unstressed (1) and uniaxially stressed (2) RbI:Tl measured at 80 K under the X-ray excitation (Babin et al., 1999a).
by the uniaxial stress means that radiation hardness of scintillators can be controlled. 4. Luminescence of CsPbX3 nanocrystals dispersed in CsX crystals Some years ago it was found by Nikl et al. (1995) that ◦ a prolonged annealing of a CsCl : Pb crystal at 150 –250 C results in the creation of small (few nm) semiconducting aggregates of the type of CsPbCl3 with a perovskite structure dispersed in a CsCl matrix. Their intense emission has been ascribed to the radiative decay of the Wannier-type free cation excitons. The luminescence characteristics of the aggregates are similar to those of CsPbCl3 single crystals. The decay time of the CsPbCl3 crystal emission is ∼ 10−10 s, which allows to consider CsPbCl3 as a perspective material for fast scintillators. A much shorter decay time (∼ 10−11 s) has been obtained for the free exciton emission of the CsPbCl3 nanocrystals and explained as an appearance of the quantum con.nement eIect in the aggregates of a very small size. Similar results are obtained for CsPbBr 3 aggregates in CsBr. It has been found that the position and halfwidth of the free exciton emission band, and its decay kinetics depend on the aggregate size. The size increases as the annealing temperature and duration increase and the lead content decreases. Previously created aggregates can be destroyed by a fast quenching of a sample down from ◦ 590 C. Thereafter, the sample is ready for the next thermal treatment. Owing to an intense and fast emission, high stability, a simple and easily controlled preparation procedure of the nanocrystals of selected size, the systems of this type are of great interest in view of their possible application in fast scintillators. Recently, the luminescence of the CsPbCl3 aggregates has been studied by Voloshinovskii et al. (2001) under excitation by the synchrotron radiation in the energy range of 4 –20 eV. It has been found that the fast luminescence of CsPbCl3 aggregates is most eIectively excited in the CsPbCl3 absorption region as
Fig. 3. Excitation spectrum measured at 10 K for the fast luminescence decay component of the CsPbCl3 aggregates dispersed in a CsCl crystal matrix (Voloshinovskii et al., 2001).
well as by photons of the Eexc ¿ 14 eV energy (Fig. 3). In the latter case it appears due to the reabsorption of the CsCl core-valence luminescence by CsPbCl3 aggregates. However, the e=ciency of the energy transfer from a CsCl matrix to the CsPbCl3 aggregates is found to be small. Being excited in the absorption bands of Pb2+ centres, which were not completely destroyed in the course of annealing, the CsPbCl3 emission is slow due to the reabsorption of Pb2+ emission by the CsPbCl3 aggregates. These circumstances may inMuence the possibility of application of these systems. 5. Luminescence of lead halide crystals Recent studies have shown that lead halides may also be considered as possible materials for high energy radiation detectors. From this point of view it was of interest to consider brieMy these systems as well. For a long time the luminescence of lead halides, arising from the radiative decay of cation excitons, has been ascribed to the electronic transitions from the excited states of a single Pb2+ ion. Recently, the self-trapping of an electron on the two nearest-neighbouring Pb2+ ions in PbCl2 has been found (Nistor et al., 1993). One may assume that under irradiation of lead halides in the exciton absorption band an electron is immediately self-trapped at two Pb2+ ions. The subsequent recombination of a mobile hole with the self-trapped electron results in the creation of the STE of the type of (Pb2+ Pb2+ )∗ whose radiative decay is accompanied by the STE luminescence. Thus, the RES responsible for the exciton emission of lead halides has to be similar not to the RES of single Pb2+ centres in alkali halides, but to the RES of dimer Pb2+ Pb2+ centres. Indeed, the decay kinetics of all the exciton emission bands (denoted as UV, B, BG) of lead halides is strongly diIerent from that characteristic of Pb2+ centres in alkali halides. In the decay kinetics of Pb2+ emission, two components (slow and fast) are observed at 4:2 K with the decay times 10−3 and 10−8 s. The slow component arises from the radiative decay of the metastable minima of the triplet RES. Due to a large
S. Zazubovich / Radiation Measurements 33 (2001) 699–704
Fig. 4. Decay kinetics of the UV emission in a PbCl2 crystal. Eexc = 4:5 eV, Eem = 3:75 eV.
spin–orbit interaction, the energy distance between the metastable and emitting minima is ∼ 10−2 eV, so thermally stimulated transitions, resulting in the decrease of the slow component decay time, can occur only at T ¿ 50 K. Unlike this case, in PbCl2 the slow component decay time at 4:2 K is ∼ 10−5 s for the UV and B bands and ∼ 10−4 s for the BG band (Liidja et al., 1972; Nikl et al., 1991) and do not practically change down to 0:46 K (see, e.g., Fig. 4). Thus, the states responsible for all the exciton emissions of PbCl2 are really diIerent from the states of Pb2+ . We assume that the UV and B bands of the PbCl2 crystal arise from the radiative transitions from two minima of the same RES, similar to the RES of dimer (Pb2+ Pb2+ ) centres in alkali halides and formed as a result of an immediate recombination of a mobile hole with the electron self-trapped at two Pb2+ ions. The relatively slow BG emission may arise from tunnelling recombinations between the closely located self-trapped hole and self-trapped electron (Kitaura and Nakagawa, 1996). Acknowledgements The work was supported by the Estonian Science Foundation Grant No. 3875. References Babin, V., Bekeshev, A., Elango, A., Kalder, K., Maaroos, A., Shunkeev, K., Vasil’chenko, E., Zazubovich, S., 1999a. EIect of uniaxial stress on luminescence of undoped and thallium-doped KI and RbI crystals. J. Phys.: Condens. Matter 11, 2303–2317. Babin, V., Elango, A., Kalder, K., Zazubovich, S., 1999b. EIect of uniaxial stress on exciton luminescence in CsI crystals. Phys. Stat. Sol. B 212, 185–198. Babin, V., Fabeni, P., Kalder, K., Nikl, M., Pazzi, P.G., Zazubovich, S., 1998. Photo- and thermally stimulated luminescence and defects in UV-irradiated CsI : Tl and CsI : Pb crystals. Radiat. Meas. 29, 333–335. Babin, V., Fabeni, P., Mihokova, E., Nikl, M., Stolovich, A., Pazzi, G.P., Zazubovich, S., 2000a. Time-resolved spectroscopy
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