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Study of growth imperfections, optical absorption, thermoluminescence and radiation hardness of CdWO4 crystals
Journal of Crystal Growth 200 (1999) 191—198
Study of growth imperfections, optical absorption, thermoluminescence and radiation hardness of CdWO crystals S.C. Sabharwal*, Sangeeta Crystal Technology Section, TPPED, BARC, Trombay, Mumbai 400 085, India Received 10 October 1998; accepted 28 November 1998
Abstract The effect of starting charge composition used for growth on the crystalline perfection, optical, thermoluminescent and radiation hardness of CdWO (CWO) single crystals has been investigated. One of the major problems faced with the growth of defect free crystals is the presence of a core-like defect. The loss of CdO by evaporation, which makes the melt progressively rich in WO , is found to be the main cause of core formation. This problem has been overcome by employing a starting charge rich in CdO, the pull rates in the range of 2—3 mm/h and maintaining a convex solid—melt interface during the growth. The crystals grown from starting charges containing &53 mol% CdO exhibit excellent optical transmission, high radiation hardness towards gamma rays and extremely weak thermoluminescence (TL) emission, the characteristics which are the most desirable for any scintillator crystal. In the TL glow curves recorded for stoichiometric crystals, only one glow peak around 148°C is observed. Minute stoichiometric deviations are found to shift the glow peak temperature towards higher temperature side and enhance the TL output considerably. The stoichiometric crystals are found not to develop coloration for gamma exposures of 10 Mrad. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Cadmium tungstate (CdWO ) crystal is a useful scintillator due to its nonhygroscopic nature, high density (7.9 g cm\) and efficient scintillation output, which is about 40% that of the NaI(Tl) [1—3]. The scintillation emission from CdWO (CWO) is known to be intrinsic in nature and it peaks around 490 nm [2]. This spectral range allows the scintil-
* Corresponding author. Fax: #91 22 550 5151; e-mail: [email protected].
lator to be coupled to both photomultiplier tube and photo diodes. All these features make CWO a very useful crystal for high-energy nuclear spectroscopy, oil well logging, dosimetry and computer tomography applications. In order to obtain efficient scintillation emission, crystals of high optical quality and radiation hardness towards gamma rays are required. However, the single crystal growth of this material is encountered with some serious problems. The loss of one of the crystal constituents namely CdO is found to be very high at the growth temperature and this gives rise to compositional changes and the presence of a
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defective core in the grown crystal. These growth imperfections adversely affect the optical transmission characteristics and hence the scintillation output of the crystal. For the system CdO/WO , the liquidus curve around 52 mol% CdO is found to be nearly flat [4]. This permits the growth of CWO crystals of stoichiometric composition from a CdO rich charge. Apparently, a starting charge of composition higher than 50 mol% CdO should be suitable for the purpose of crystal growth. It may be pointed out that the relationship between the starting charge composition and the crystal properties of interest like optical, thermoluminescence and radiation hardness has not been reported in literature. Such an investigation is thought to be important from the point of view of both the single crystal growth and the applications of this crystal as a scintillator. In this paper, we investigate the influence of different growth parameters which affect the formation of core. The optical absorption, thermoluminescence and the effect of gamma irradiation for crystals grown under optimized conditions but employing starting charges of different purity and compositions have also been presented here for the first time. Since, the raw materials used are of two different purity levels viz. 99.9 and 99.995%, the results obtained can perhaps be generalized. The results show that the core is influenced by the shape of the solid—melt interface maintained during the growth. A starting charge composition of &53 mol% CdO is found to be the most suitable from the point of view of the optical quality and radiation hardness of CWO crystal. The normalized TL output and glow peak temperatures are found to be affected by the stoichiometric deviations. The results suggest that this fact can be used for the detection of minute stoichiometric deviations in CWO crystals.
2. Experimental procedure The crystal growth was effected from melt by Czocharalski method and employing crystal weighing technique to grow crystals in uniform diameter mode. A Pt crucible containing the material and surrounded by zirconia grog and magnesia refrac-
tory was coupled to a 50 kW RF generator for melting and holding the charge at a desired temperature during growth. In order to provide a moderate temperature gradient to the grown crystal, a refractory cover at the top of the crucible was provided. The TL glow curves were recorded using the setup described elsewhere [5]. A linear heating rate of 100°C/min was employed to record the glow curves over the temperature range 30—300°C. The crystals were irradiated with gamma rays in a Co gamma chamber. For recording the TL emission spectra, a Jobin 0.25 m monochromator coupled with a photomultiplier tube (IP-28) was used. The optical transmission spectra of the crystals were recorded, over the wavelength range 200—1100 nm, by a Chemito spectrophotometer model 2500.
3. Results 3.1. Crystal growth The starting CdWO powders used for crystal growth were of two different purity viz. 99.9 and 99.995%. Both stoichiometric and CdO rich compositions were investigated in different experiments. The growth was carried out under normal atmosphere and it was initiated either using a platinum wire or the seed crystals oriented along different crystallographic directions. The crystals grown from charges of stoichiometric composition were found to be free from the presence of any macroscopic structural defects only up to a few millimeter of the pulled length. However, the crystal portion pulled beyond a certain length showed the presence of inclusions in the middle of the crystal. The extent of this central defective region called core was found to be strongly governed by the melt composition. In the case of crystals grown employing a charge of stoichiometric composition, the core commenced some time after initiation of growth and encompassed about one third of the crystal cross section. When a left over charge of a previous growth run was used, the core area was found to be greater than one half of the crystal cross section. The effect of crystal rotation rate on the core was investigated and it was found to have no significant
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influence. The application of very slow growth rates, used for the growth of several other oxide crystals, did not yield defect free crystals [6,7]. A crystal pulling rate of 2 mm h\ was found to be the optimum. While, lower pulling rates give rise to enhanced loss of CdO, higher rates result in the entrapment of WO rich composition into the crys tal as precipitates. The presence of the core was seen to be strongly influenced by the shape of the solid—melt interface. When the solid—melt interface maintained during the growth was near planar or slightly concave, the presence of core was quite prominent. During initial experiments on growth, the resultant crystals were quite often found to develop multiple cracks during cooling to room temperature. This problem could not be overcome simply by the application of very slow cooling rates. The problem of cracking was seen to be particularly more serious when the growth was initiated with a Pt wire. On the other hand, when the growth was effected using a seed which had the same orientation as that of the crystal pulled with a Pt wire, cracking along the cleavage plane was only sometimes observed. The problem of crystal cracking was found to be more serious when either a concave or a planar solid— melt interface was maintained during growth. Our experiments on the growth of crystals along different crystallographic directions revealed that crack free crystals can be obtained by effecting the growth normal to 10 1 02 axis and subsequently cooling them to room temperature at an optimum rate. The use of CdO rich starting charge compositions helped to grow defect free crystals of dimensions 35 mm dia. ;50 mm length. A polished section is shown in Fig. 1. The crystals pulled at the rate of 2 mm h\ from a charge containing higher than 52 mol% CdO did not show the presence of core. While the crystals grown using 99.9% pure charge showed light yellow appearance, those from 99.995% pure charge were colorless. The crystals pulled from a left over charge of a previous growth run carried out using 99.9% pure material showed light to dark greenish coloration, depending upon the degree of stoichiometric deviation. No detectable changes in the XRD patterns recorded for colorless and colored crystals were observed, indic-
193
Fig. 1. A polished section of the CdWO ingot.
ating that the stoichiometric deviations were smaller than the detectable limit of the measurements. A typical XRD pattern recorded for a cleaved section of the crystal is shown in Fig. 2. The pattern shows the (0 k 0) only peaks agreeing with data reported in the literature [8]. 3.2. Optical absorption The transmission spectra recorded for the crystals grown using both stoichiometric and a CdO rich starting charge composition prepared from 99.995% pure charge are reproduced in Fig. 3. For comparison, the percentage transmission obtained at some select wavelengths for the crystals grown from starting charges containing different mol% CdO are given in Table 1. While, the lower wavelength cut off is found to be most sharp in crystals grown from 53.5 mol% CdO charges, the best transmission characteristics over the wavelength range investigated are in fact obtained for 52.2 mol% CdO initial charges. Thus, the results show that the crystal transmission decreases on both sides of an optimum concentration. It may be mentioned that similar results are also obtained for the yellow colored crystals except with a difference
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Fig. 2. X-ray diffraction pattern for a cleaved section of the crystal (Cu K radiation). a
Table 1 Percentage transmission recorded at some select wavelengths for the colorless crystals grown from starting charges of different compositions CdO mol% in the starting charge
50 52.2 53.5 54.3
Transmission (%) at different wavelengths (nm) 1100
500
400
350
70.1 77.3 76.7 75.3
62.2 75 74.4 71.2
53.1 68.2 65.5 63.4
43.8 60 59.5 48
Fig. 3. Optical transmission spectra recorded for crystals grown from 99.995% pure starting charges containing different mol% CdO: (a) 50 and (b) 53.5.
that the optimum starting charge composition found in this case is 53.5 mol% CdO. This difference is possibly due to some amount of CdO deficiency inherently present in the CWO raw material of 99.9% purity used for growth. The transmission spectra recorded for the yellow colored crystal are shown in Fig. 4. It may be noted from plot (c) that
the lower wavelength cut off for crystals grown from a starting charge of optimum concentration is also not quite sharp. This implies the presence of an energy continuum close to the lower edge of the conduction band. The origin of this continuum is thought to be the presence of foreign ions in the starting powder. A comparison of the results reproduced in Figs. 3 and 4 shows that the crystal absorption below 500 nm in CWO arises due to both
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195
Fig. 4. Plots of optical transmission for CWO crystals grown from 99.9% pure starting charges containing different mol% CdO: (a) 50 (䉭), (b) 50.8 (;), (c) 52.2 () and (d) 54.32 (—).
the presence of impurities and the stoichiometric deviations in the grown crystal, with the former having a major contribution. Considering that the plots in Figs. 3 and 4 represent two quite different grades of raw materials, the results show that charges containing &53 mol% CdO would in general yield crystals exhibiting the best transmission characteristics. 3.3. Thermoluminescence The TL glow curves were recorded for CWO crystals grown under different conditions viz. crystals with and without the presence of core and those which were dark greenish in appearance. Typical plots recorded for different crystals are reproduced in Fig. 5. For a core free and colorless crystal, as shown by plot (a), only one glow peak &148°C is observed. The normalized TL intensity in this case is found to be very weak. For crystal samples showing the presence of core, again a single glow peak is recorded. While, the glow peak temperature in this case is seen to remain unchanged, the normalized TL intensity is found to be about three times higher than that of the colorless samples. In the case of dark greenish color samples (lower portion of crystals grown using 99.9% pure charge and employing slow growth rates), as shown by plot (b) in Fig. 5, the TL emission is observed to be very strong and the glow peak temperature is shifted towards higher temperature side by about 10°. The results show that minute deviations from
Fig. 5. Typical TL glow curves recorded for CWO crystals of (a) stoichiometric and (b) off-stoichiometric compositions.
crystal stoichiometry have the effect of enhancing the TL output, whereas large stoichiometric deviations influence both the TL output and glow peak temperature. These results are found to be qualitatively similar to those reported for the single crystals of some other oxide materials [6,9,10]. In order to further investigate the nature of trapping and emission centers responsible for the TL emission in this crystal, the order of kinetics for TL emission was determined by peak shape method [11]. For the TL glow peak shown by plot (a) in Fig. 5, various shape parameters such as half of the peak width on the rising side q ("¹ !¹ ) or on
the fall side d ("¹ !¹ ) and full-width at half
maxima u ("¹ !¹ ) were determined. The value
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of the ratio d/u, which determines the order of kinetics of the TL emission, was found to have a value of 0.41. This result implies a first-order kinetics for the TL glow peak observed at 148°C for CWO single crystals. This result shows that the trapping and emission centers which give rise to the TL emission in this lattice are the same. The emission spectra in this case peaked around 490 nm, which matches with the spectra of the scintillation emission. It may therefore be inferred that the emission centers responsible for the TL and the scintillation emissions in CWO are the same. This is an important result which suggests that the crystals must exhibit either the absence or extremely weak TL output in order to produce efficient scintillation emission. 3.4. Radiation hardness The radiation damage in crystals grown from the starting charges of two different purity and containing different mole fractions of CdO was studied by monitoring changes in their optical transmission produced by gamma irradiation. In the case of colorless crystals grown from both stoichiometric and CdO rich charges, no measurable change in crystal absorption at 500 nm was observed for irradiation dose up to 5 Mrad. At 10 Mrad dose, the transmission of crystals grown from stoichiometric charge was reduced by nearly 6% while for crystals grown from charges containing 52.2 or higher mole percent CdO no significant change was observed. These crystals are found to be more radiation hard as compared to those reported in the literature [12]. For yellow colored crystals grown from charges containing different mol% CdO, the observed changes in transmission on gamma irradiation are shown in Fig. 6. For crystals grown from stoichiometric melt composition, the transmission is seen to increase with irradiation dose (plot (a)). In the case of stoichiometric crystals grown from 53.5 mol% CdO charge, no change in the optical transmission (plot (b)) is observed for the highest dose of 10 Mrad studied. The effect of gamma irradiation on transmission for crystals grown from charges containing 54.3 mol% CdO is shown by plot (c). At lower doses the transmission is seen to decrease by about 5%. However, after this initial
Fig. 6. The observed changes in optical transmission at 500 nm as a function of gamma dose for crystals grown from 99.9% pure starting powders of different compositions; (a) 50; (b) 53.5; and (c) 54.3 mol% CdO.
deterioration the transmission remains unaffected up to the highest dose of 10 Mrad. The increase in transmission with dose shown by plot (a) in Fig. 6 is attributed to the presence of charge traps in the crystal lattice. On irradiation, the charges released in lattice are trapped by these centers whereby they become inactive. These traps arise as a result of the nonstoichiometry and not due to the presence of any foreign ion(s) as this behavior is not observed for crystals grown from 54.3 mol% CdO charges. The results of the optical transmission and the TL measurements summarized under Sections 3.2 and 3.3 are also found to be consistent with this view.
4. Discussion The X-ray diffraction pattern shown in Fig. 2 for a single crystal matches well with the reported data [8]. The powder diffraction patterns recorded for the crystals showing both the presence and absence of some absorption below 500 nm were found to be essentially identical. This result shows that the deviation from stoichiometry is not large enough (i.e. within a few percent) so as to be revealed by the XRD measurements but it is sufficiently large to
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affect the optical properties of the crystal. Since, the preferential loss of one of the crystal constituent viz. CdO is high and continuous at the operating temperature, it is therefore desirable to maintain relatively higher growth rate so that maximum fraction of the molten charge can be transformed into a single crystal of stoichiometric composition. Further, the use a starting charge composition slightly rich in cadmium oxide, as also mentioned elsewhere, is found beneficial [4]. The results show that the core-like defect can be minimized by maintaining a convex solid—melt interface. In this case, the excess component of the crystal constituent remaining in the melt namely tungsten oxide has a better chance of mixing with the entire melt rather than accumulating at the growth interface and eventually getting trapped into the growing crystal. The crystal cracking in the present case is found to be a very serious problem due to the presence of 10 1 02 cleavage plane. When the growth is seeded using a Pt wire, crystal shows high tendency of developing multiple cracks even during the growth. The growth in this case is observed to be predominately along 11 0 02 direction. However, when the growth is carried out using a properly oriented 11 0 02 seed crystal and an optimum cooling rate is employed, crack free crystals can be obtained. These results suggest that cracking of the crystals observed when the growth is seeded with a Pt wire could not be just due to the growth direction. Since, the application of Pt wire would enhance the axial gradient, it therefore appears that the maximum acceptable axial gradient for CWO crystal is quite small. The problem of cracking of CZ grown crystals has been studied by several authors [13—15]. According to these studies, there exists a maximum acceptable axial temperature gradient whose value, besides crystal diameter, depends upon other physical properties like breaking strain, cooling constant and the anisotropic expansion of the crystal. The fact that we have been able to overcome the problem of crystal cracking by effecting the growth along 11 0 02 direction suggests that, among other factors, the expansion coefficient is an important parameter. The measurements on thermal expansion of CWO along different directions are required to further support this inference.
197
The results of the optical absorption measurements performed on crystals grown from CdO rich starting charges of different compositions have been summarized in Table 1. It is seen that crystal transmission is a function of the starting charge composition. The best results are obtained when the starting charge consists of &53% CdO. The starting charges containing higher molar fractions of CdO are found to adversely affect the optical transmission characteristics of the crystals. These results have been understood as follows. When a charge of stoichiometric composition is used, the loss of CdO during growth gives rise to the stoichiometric deviations in an ingot. This is clearly borne out by the TL measurements which show that the normalized TL intensity in this case is enhanced. When the CdO fraction in the charge lies close to 53 mol%, the excess amount of CdO present in the melt compensates for the evaporation losses during growth. The normalized TL output measured in this case is quite low and the glow peak temperature is centered around 148°C, suggesting that the crystals, within the limits of experimental error, are of stoichiometric composition. They exhibit excellent optical transmission characteristics, sharp short wavelength cutoff and high radiation hardness. In the case of crystals grown from charges containing higher than 53.5 mol% CdO, the optical absorption over the entire wavelength range investigated is found to be higher than that for the crystals of stoichiometric composition. This result suggests that the increase in absorption is due to changes in the crystal lattice as a whole and not because of the change in the charge state of any particular ion. The latter mechanism would be expected to give rise to selective absorption in the crystal lattice. Summarizing, the results show that the growth parameters for CWO crystals are quite different from those of another tungstate material namely lead tungstate [6]. For the growth of PbWO crys tals, the application of low growth rates (less than a millimeter) helps to purify the starting material for the stoichiometric deviations and the impurities present in the starting powders. Whereas, the growth of CWO crystal must be carried out at the rate of several millimeters per hour to avoid stoichiometric deviations in the ingots due to
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considerable vaporization of CdO. The starting powders containing &53 mol% CdO are found to be most suitable for the growth of defect free crystals which do not show any loss of transmission at 500 nm for gamma exposure of 10 Mrad. In order to avoid cracking of the grown crystals, the growth along 11 0 02 direction is preferred. Minute stoichiometric deviations are found to enhance the TL output, while large deviations not only enhance the TL output but also cause a shift in the glow peak temperature towards higher temperature side. This fact can be used to detect minute deviations from stoichiometry.
Acknowledgements The authors are grateful to Dr. V.C. Sahni for his support to this work. Thanks are also due to Mrs. K.S. Keshwani and Ms. M.N. Ahuja for their help in carrying out the crystal growth runs and subsequent processing of the ingots.
References [1] M.R. Farukhi, IEEE Trans. Nucl. Sci. NS-29 (1982) 1237. [2] B.C. Grabmaier, IEEE Trans. Nucl. Sci. NS-31 (1984) 372.
[3] L. Nagornaya, V. Ryzhikov, I.S.C. Kharkov, in: F. De Notaristefani, P. Lecoq, M. Schneegans (Eds.), Heavy Scintillators for Scientific and Industrial Applications, Editions Frontieres, Dreux, 1993, p. 367. [4] D.S. Robertson, I.M. Young, J.R. Telfer, J. Mater. Sci. 14 (1979) 2974. [5] Sangeeta, S.C. Sabharwal, B. Ghosh, M.K. Gupta, Phys. Stat. Sol. (a) 121 (1990) 657. [6] S.C. Sabharwal, Sangeeta, D.G. Desai, S.C. Karandikar, A.K. Chauhan, A.K. Sangiri, K.S Keshwani, M.N. Ahuja, J. Crystal Growth 169 (1996) 304. [7] Sangeeta, S.C. Sabharwal, M.K. Gupta, Proc. Indian. Natl. Sci. Acad. 57 (1991) 153. [8] Powder diffraction file No. 14-676, JCPDS-ICDD, USA, 1995. [9] Sangeeta, Hema Prasad, S.C. Sabharwal, J. Crystal Growth 118 (1992) 396. [10] S.C. Sabharwal, Hema Prasad, D.G. Desai, Sangeeta, S.C. Karandikar, M.K. Gupta, Nucl. Instr. and Meth. A 329 (1993) 179. [11] R. Chen, J. Appl. Phys. 40 (1969) 570. [12] C.W.E. van Eijk, in: F. De Notaristefani, P. Lecoq, M. Schneegans (Eds.), Heavy Scintillators for Scientific and Industrial Applications, Editions Frontieres, Dreux, 1993, p. 601. [13] E. Billig, Proc. Roy. Soc. (London), A 235 (1956) 37. [14] V.L. Indenbom, I.S. Zhitomirskii, T.S. Chebanova, Growth Cryst. 8 (1969) 249. [15] J.C. Brice, J. Crystal Growth 42 (1977) 427.
Study of growth imperfections, optical absorption, thermoluminescence and radiation hardness of CdWO crystals S.C. Sabharwal*, Sangeeta Crystal Technology Section, TPPED, BARC, Trombay, Mumbai 400 085, India Received 10 October 1998; accepted 28 November 1998
Abstract The effect of starting charge composition used for growth on the crystalline perfection, optical, thermoluminescent and radiation hardness of CdWO (CWO) single crystals has been investigated. One of the major problems faced with the growth of defect free crystals is the presence of a core-like defect. The loss of CdO by evaporation, which makes the melt progressively rich in WO , is found to be the main cause of core formation. This problem has been overcome by employing a starting charge rich in CdO, the pull rates in the range of 2—3 mm/h and maintaining a convex solid—melt interface during the growth. The crystals grown from starting charges containing &53 mol% CdO exhibit excellent optical transmission, high radiation hardness towards gamma rays and extremely weak thermoluminescence (TL) emission, the characteristics which are the most desirable for any scintillator crystal. In the TL glow curves recorded for stoichiometric crystals, only one glow peak around 148°C is observed. Minute stoichiometric deviations are found to shift the glow peak temperature towards higher temperature side and enhance the TL output considerably. The stoichiometric crystals are found not to develop coloration for gamma exposures of 10 Mrad. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Cadmium tungstate (CdWO ) crystal is a useful scintillator due to its nonhygroscopic nature, high density (7.9 g cm\) and efficient scintillation output, which is about 40% that of the NaI(Tl) [1—3]. The scintillation emission from CdWO (CWO) is known to be intrinsic in nature and it peaks around 490 nm [2]. This spectral range allows the scintil-
* Corresponding author. Fax: #91 22 550 5151; e-mail: [email protected].
lator to be coupled to both photomultiplier tube and photo diodes. All these features make CWO a very useful crystal for high-energy nuclear spectroscopy, oil well logging, dosimetry and computer tomography applications. In order to obtain efficient scintillation emission, crystals of high optical quality and radiation hardness towards gamma rays are required. However, the single crystal growth of this material is encountered with some serious problems. The loss of one of the crystal constituents namely CdO is found to be very high at the growth temperature and this gives rise to compositional changes and the presence of a
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defective core in the grown crystal. These growth imperfections adversely affect the optical transmission characteristics and hence the scintillation output of the crystal. For the system CdO/WO , the liquidus curve around 52 mol% CdO is found to be nearly flat [4]. This permits the growth of CWO crystals of stoichiometric composition from a CdO rich charge. Apparently, a starting charge of composition higher than 50 mol% CdO should be suitable for the purpose of crystal growth. It may be pointed out that the relationship between the starting charge composition and the crystal properties of interest like optical, thermoluminescence and radiation hardness has not been reported in literature. Such an investigation is thought to be important from the point of view of both the single crystal growth and the applications of this crystal as a scintillator. In this paper, we investigate the influence of different growth parameters which affect the formation of core. The optical absorption, thermoluminescence and the effect of gamma irradiation for crystals grown under optimized conditions but employing starting charges of different purity and compositions have also been presented here for the first time. Since, the raw materials used are of two different purity levels viz. 99.9 and 99.995%, the results obtained can perhaps be generalized. The results show that the core is influenced by the shape of the solid—melt interface maintained during the growth. A starting charge composition of &53 mol% CdO is found to be the most suitable from the point of view of the optical quality and radiation hardness of CWO crystal. The normalized TL output and glow peak temperatures are found to be affected by the stoichiometric deviations. The results suggest that this fact can be used for the detection of minute stoichiometric deviations in CWO crystals.
2. Experimental procedure The crystal growth was effected from melt by Czocharalski method and employing crystal weighing technique to grow crystals in uniform diameter mode. A Pt crucible containing the material and surrounded by zirconia grog and magnesia refrac-
tory was coupled to a 50 kW RF generator for melting and holding the charge at a desired temperature during growth. In order to provide a moderate temperature gradient to the grown crystal, a refractory cover at the top of the crucible was provided. The TL glow curves were recorded using the setup described elsewhere [5]. A linear heating rate of 100°C/min was employed to record the glow curves over the temperature range 30—300°C. The crystals were irradiated with gamma rays in a Co gamma chamber. For recording the TL emission spectra, a Jobin 0.25 m monochromator coupled with a photomultiplier tube (IP-28) was used. The optical transmission spectra of the crystals were recorded, over the wavelength range 200—1100 nm, by a Chemito spectrophotometer model 2500.
3. Results 3.1. Crystal growth The starting CdWO powders used for crystal growth were of two different purity viz. 99.9 and 99.995%. Both stoichiometric and CdO rich compositions were investigated in different experiments. The growth was carried out under normal atmosphere and it was initiated either using a platinum wire or the seed crystals oriented along different crystallographic directions. The crystals grown from charges of stoichiometric composition were found to be free from the presence of any macroscopic structural defects only up to a few millimeter of the pulled length. However, the crystal portion pulled beyond a certain length showed the presence of inclusions in the middle of the crystal. The extent of this central defective region called core was found to be strongly governed by the melt composition. In the case of crystals grown employing a charge of stoichiometric composition, the core commenced some time after initiation of growth and encompassed about one third of the crystal cross section. When a left over charge of a previous growth run was used, the core area was found to be greater than one half of the crystal cross section. The effect of crystal rotation rate on the core was investigated and it was found to have no significant
S.C. Sabharwal, Sangeeta / Journal of Crystal Growth 200 (1999) 191–198
influence. The application of very slow growth rates, used for the growth of several other oxide crystals, did not yield defect free crystals [6,7]. A crystal pulling rate of 2 mm h\ was found to be the optimum. While, lower pulling rates give rise to enhanced loss of CdO, higher rates result in the entrapment of WO rich composition into the crys tal as precipitates. The presence of the core was seen to be strongly influenced by the shape of the solid—melt interface. When the solid—melt interface maintained during the growth was near planar or slightly concave, the presence of core was quite prominent. During initial experiments on growth, the resultant crystals were quite often found to develop multiple cracks during cooling to room temperature. This problem could not be overcome simply by the application of very slow cooling rates. The problem of cracking was seen to be particularly more serious when the growth was initiated with a Pt wire. On the other hand, when the growth was effected using a seed which had the same orientation as that of the crystal pulled with a Pt wire, cracking along the cleavage plane was only sometimes observed. The problem of crystal cracking was found to be more serious when either a concave or a planar solid— melt interface was maintained during growth. Our experiments on the growth of crystals along different crystallographic directions revealed that crack free crystals can be obtained by effecting the growth normal to 10 1 02 axis and subsequently cooling them to room temperature at an optimum rate. The use of CdO rich starting charge compositions helped to grow defect free crystals of dimensions 35 mm dia. ;50 mm length. A polished section is shown in Fig. 1. The crystals pulled at the rate of 2 mm h\ from a charge containing higher than 52 mol% CdO did not show the presence of core. While the crystals grown using 99.9% pure charge showed light yellow appearance, those from 99.995% pure charge were colorless. The crystals pulled from a left over charge of a previous growth run carried out using 99.9% pure material showed light to dark greenish coloration, depending upon the degree of stoichiometric deviation. No detectable changes in the XRD patterns recorded for colorless and colored crystals were observed, indic-
193
Fig. 1. A polished section of the CdWO ingot.
ating that the stoichiometric deviations were smaller than the detectable limit of the measurements. A typical XRD pattern recorded for a cleaved section of the crystal is shown in Fig. 2. The pattern shows the (0 k 0) only peaks agreeing with data reported in the literature [8]. 3.2. Optical absorption The transmission spectra recorded for the crystals grown using both stoichiometric and a CdO rich starting charge composition prepared from 99.995% pure charge are reproduced in Fig. 3. For comparison, the percentage transmission obtained at some select wavelengths for the crystals grown from starting charges containing different mol% CdO are given in Table 1. While, the lower wavelength cut off is found to be most sharp in crystals grown from 53.5 mol% CdO charges, the best transmission characteristics over the wavelength range investigated are in fact obtained for 52.2 mol% CdO initial charges. Thus, the results show that the crystal transmission decreases on both sides of an optimum concentration. It may be mentioned that similar results are also obtained for the yellow colored crystals except with a difference
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Fig. 2. X-ray diffraction pattern for a cleaved section of the crystal (Cu K radiation). a
Table 1 Percentage transmission recorded at some select wavelengths for the colorless crystals grown from starting charges of different compositions CdO mol% in the starting charge
50 52.2 53.5 54.3
Transmission (%) at different wavelengths (nm) 1100
500
400
350
70.1 77.3 76.7 75.3
62.2 75 74.4 71.2
53.1 68.2 65.5 63.4
43.8 60 59.5 48
Fig. 3. Optical transmission spectra recorded for crystals grown from 99.995% pure starting charges containing different mol% CdO: (a) 50 and (b) 53.5.
that the optimum starting charge composition found in this case is 53.5 mol% CdO. This difference is possibly due to some amount of CdO deficiency inherently present in the CWO raw material of 99.9% purity used for growth. The transmission spectra recorded for the yellow colored crystal are shown in Fig. 4. It may be noted from plot (c) that
the lower wavelength cut off for crystals grown from a starting charge of optimum concentration is also not quite sharp. This implies the presence of an energy continuum close to the lower edge of the conduction band. The origin of this continuum is thought to be the presence of foreign ions in the starting powder. A comparison of the results reproduced in Figs. 3 and 4 shows that the crystal absorption below 500 nm in CWO arises due to both
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Fig. 4. Plots of optical transmission for CWO crystals grown from 99.9% pure starting charges containing different mol% CdO: (a) 50 (䉭), (b) 50.8 (;), (c) 52.2 () and (d) 54.32 (—).
the presence of impurities and the stoichiometric deviations in the grown crystal, with the former having a major contribution. Considering that the plots in Figs. 3 and 4 represent two quite different grades of raw materials, the results show that charges containing &53 mol% CdO would in general yield crystals exhibiting the best transmission characteristics. 3.3. Thermoluminescence The TL glow curves were recorded for CWO crystals grown under different conditions viz. crystals with and without the presence of core and those which were dark greenish in appearance. Typical plots recorded for different crystals are reproduced in Fig. 5. For a core free and colorless crystal, as shown by plot (a), only one glow peak &148°C is observed. The normalized TL intensity in this case is found to be very weak. For crystal samples showing the presence of core, again a single glow peak is recorded. While, the glow peak temperature in this case is seen to remain unchanged, the normalized TL intensity is found to be about three times higher than that of the colorless samples. In the case of dark greenish color samples (lower portion of crystals grown using 99.9% pure charge and employing slow growth rates), as shown by plot (b) in Fig. 5, the TL emission is observed to be very strong and the glow peak temperature is shifted towards higher temperature side by about 10°. The results show that minute deviations from
Fig. 5. Typical TL glow curves recorded for CWO crystals of (a) stoichiometric and (b) off-stoichiometric compositions.
crystal stoichiometry have the effect of enhancing the TL output, whereas large stoichiometric deviations influence both the TL output and glow peak temperature. These results are found to be qualitatively similar to those reported for the single crystals of some other oxide materials [6,9,10]. In order to further investigate the nature of trapping and emission centers responsible for the TL emission in this crystal, the order of kinetics for TL emission was determined by peak shape method [11]. For the TL glow peak shown by plot (a) in Fig. 5, various shape parameters such as half of the peak width on the rising side q ("¹ !¹ ) or on
the fall side d ("¹ !¹ ) and full-width at half
maxima u ("¹ !¹ ) were determined. The value
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of the ratio d/u, which determines the order of kinetics of the TL emission, was found to have a value of 0.41. This result implies a first-order kinetics for the TL glow peak observed at 148°C for CWO single crystals. This result shows that the trapping and emission centers which give rise to the TL emission in this lattice are the same. The emission spectra in this case peaked around 490 nm, which matches with the spectra of the scintillation emission. It may therefore be inferred that the emission centers responsible for the TL and the scintillation emissions in CWO are the same. This is an important result which suggests that the crystals must exhibit either the absence or extremely weak TL output in order to produce efficient scintillation emission. 3.4. Radiation hardness The radiation damage in crystals grown from the starting charges of two different purity and containing different mole fractions of CdO was studied by monitoring changes in their optical transmission produced by gamma irradiation. In the case of colorless crystals grown from both stoichiometric and CdO rich charges, no measurable change in crystal absorption at 500 nm was observed for irradiation dose up to 5 Mrad. At 10 Mrad dose, the transmission of crystals grown from stoichiometric charge was reduced by nearly 6% while for crystals grown from charges containing 52.2 or higher mole percent CdO no significant change was observed. These crystals are found to be more radiation hard as compared to those reported in the literature [12]. For yellow colored crystals grown from charges containing different mol% CdO, the observed changes in transmission on gamma irradiation are shown in Fig. 6. For crystals grown from stoichiometric melt composition, the transmission is seen to increase with irradiation dose (plot (a)). In the case of stoichiometric crystals grown from 53.5 mol% CdO charge, no change in the optical transmission (plot (b)) is observed for the highest dose of 10 Mrad studied. The effect of gamma irradiation on transmission for crystals grown from charges containing 54.3 mol% CdO is shown by plot (c). At lower doses the transmission is seen to decrease by about 5%. However, after this initial
Fig. 6. The observed changes in optical transmission at 500 nm as a function of gamma dose for crystals grown from 99.9% pure starting powders of different compositions; (a) 50; (b) 53.5; and (c) 54.3 mol% CdO.
deterioration the transmission remains unaffected up to the highest dose of 10 Mrad. The increase in transmission with dose shown by plot (a) in Fig. 6 is attributed to the presence of charge traps in the crystal lattice. On irradiation, the charges released in lattice are trapped by these centers whereby they become inactive. These traps arise as a result of the nonstoichiometry and not due to the presence of any foreign ion(s) as this behavior is not observed for crystals grown from 54.3 mol% CdO charges. The results of the optical transmission and the TL measurements summarized under Sections 3.2 and 3.3 are also found to be consistent with this view.
4. Discussion The X-ray diffraction pattern shown in Fig. 2 for a single crystal matches well with the reported data [8]. The powder diffraction patterns recorded for the crystals showing both the presence and absence of some absorption below 500 nm were found to be essentially identical. This result shows that the deviation from stoichiometry is not large enough (i.e. within a few percent) so as to be revealed by the XRD measurements but it is sufficiently large to
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affect the optical properties of the crystal. Since, the preferential loss of one of the crystal constituent viz. CdO is high and continuous at the operating temperature, it is therefore desirable to maintain relatively higher growth rate so that maximum fraction of the molten charge can be transformed into a single crystal of stoichiometric composition. Further, the use a starting charge composition slightly rich in cadmium oxide, as also mentioned elsewhere, is found beneficial [4]. The results show that the core-like defect can be minimized by maintaining a convex solid—melt interface. In this case, the excess component of the crystal constituent remaining in the melt namely tungsten oxide has a better chance of mixing with the entire melt rather than accumulating at the growth interface and eventually getting trapped into the growing crystal. The crystal cracking in the present case is found to be a very serious problem due to the presence of 10 1 02 cleavage plane. When the growth is seeded using a Pt wire, crystal shows high tendency of developing multiple cracks even during the growth. The growth in this case is observed to be predominately along 11 0 02 direction. However, when the growth is carried out using a properly oriented 11 0 02 seed crystal and an optimum cooling rate is employed, crack free crystals can be obtained. These results suggest that cracking of the crystals observed when the growth is seeded with a Pt wire could not be just due to the growth direction. Since, the application of Pt wire would enhance the axial gradient, it therefore appears that the maximum acceptable axial gradient for CWO crystal is quite small. The problem of cracking of CZ grown crystals has been studied by several authors [13—15]. According to these studies, there exists a maximum acceptable axial temperature gradient whose value, besides crystal diameter, depends upon other physical properties like breaking strain, cooling constant and the anisotropic expansion of the crystal. The fact that we have been able to overcome the problem of crystal cracking by effecting the growth along 11 0 02 direction suggests that, among other factors, the expansion coefficient is an important parameter. The measurements on thermal expansion of CWO along different directions are required to further support this inference.
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The results of the optical absorption measurements performed on crystals grown from CdO rich starting charges of different compositions have been summarized in Table 1. It is seen that crystal transmission is a function of the starting charge composition. The best results are obtained when the starting charge consists of &53% CdO. The starting charges containing higher molar fractions of CdO are found to adversely affect the optical transmission characteristics of the crystals. These results have been understood as follows. When a charge of stoichiometric composition is used, the loss of CdO during growth gives rise to the stoichiometric deviations in an ingot. This is clearly borne out by the TL measurements which show that the normalized TL intensity in this case is enhanced. When the CdO fraction in the charge lies close to 53 mol%, the excess amount of CdO present in the melt compensates for the evaporation losses during growth. The normalized TL output measured in this case is quite low and the glow peak temperature is centered around 148°C, suggesting that the crystals, within the limits of experimental error, are of stoichiometric composition. They exhibit excellent optical transmission characteristics, sharp short wavelength cutoff and high radiation hardness. In the case of crystals grown from charges containing higher than 53.5 mol% CdO, the optical absorption over the entire wavelength range investigated is found to be higher than that for the crystals of stoichiometric composition. This result suggests that the increase in absorption is due to changes in the crystal lattice as a whole and not because of the change in the charge state of any particular ion. The latter mechanism would be expected to give rise to selective absorption in the crystal lattice. Summarizing, the results show that the growth parameters for CWO crystals are quite different from those of another tungstate material namely lead tungstate [6]. For the growth of PbWO crys tals, the application of low growth rates (less than a millimeter) helps to purify the starting material for the stoichiometric deviations and the impurities present in the starting powders. Whereas, the growth of CWO crystal must be carried out at the rate of several millimeters per hour to avoid stoichiometric deviations in the ingots due to
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considerable vaporization of CdO. The starting powders containing &53 mol% CdO are found to be most suitable for the growth of defect free crystals which do not show any loss of transmission at 500 nm for gamma exposure of 10 Mrad. In order to avoid cracking of the grown crystals, the growth along 11 0 02 direction is preferred. Minute stoichiometric deviations are found to enhance the TL output, while large deviations not only enhance the TL output but also cause a shift in the glow peak temperature towards higher temperature side. This fact can be used to detect minute deviations from stoichiometry.
Acknowledgements The authors are grateful to Dr. V.C. Sahni for his support to this work. Thanks are also due to Mrs. K.S. Keshwani and Ms. M.N. Ahuja for their help in carrying out the crystal growth runs and subsequent processing of the ingots.
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