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Photochromic effect and its influence on scintillation properties of CdWO4 and PbWO4 crystals
NUCLEAR INSTRUMENTS a METHooS IN PHVSICS RESEARCH EUEWIER
Nuclear
Instruments
Photochromic
in Physics Research A 411 (1998)376-382
and Methods
SectIonA
effect and its influence on scintillation properties of CdW04 and PbW04 crystals
V.G. Bondara, S.F. Burachasa, K.A. Katrunova, V.P. Martinova, V.D. Ryzhikov”, V.I. Mankob, H.H. Gutbrod”, G. Tamulaitisd,* aSTC RI of the Institute of Single Crystals, Ukrainian Academy of Sciences, pr. Lenina 60, KharkoL? 310001, Ukraine b Kurchatov National Center, Moscow, Kurchatov sq. I, Moscow 123182. Russian Federation Ecole des Mines de Nantes. Laboratoire de physique subatomique et des technologies associees, 4 rue Arfed F-44070 Names Cedex 03. France ‘Institute of Material Sciences and Applied Research, Vilnius University. 24 Naugarduko, Vilnius 2006. Lithuania
’ SUBATECH,
Kastler.
Received 17 December 1997
Abstract Absorption induced in PbW04 and CdW04 scintillation crystals by UV and y-ray irradiation was investigated by observing the dynamics of the absorption spectra after the irradiation of the samples with quanta of different energies and time evolution of the darkening effect at different rates of the irradiation. Dependence of the irradiation-induced absorption on growth conditions of the crystals, as well as bleaching of the samples by heating and illumination with visible light were studied. Trapping of free carriers at structural defects related with oxygen content in the lattice was shown to result in the creation of color centers in both tungstates under investigation. C:I 1998 Elsevier Science B.V. All rights reserved. PACS: 29.40.M~; 61.82.M~ Keywords:
Scintillation
detectors; PWO; CWO; Radiation hardness; Photochromic
1. Introduction Radiation hardness is, as a rule, one of the most important properties of scintillation materials. Beside possible influence of the radiation on the emission centers, there are the radiation-induced changes that usually deteriorates the light yield of the
*Corresponding author. Tel: + 370 2 636022, fax: 263417, e-mail: [email protected].
+ 370 2
0168-9002/98/$19.00 c.1 1998 Elsevier Science B.V. All rights reserved. PII: SO168-9002(98)00346-5
effect; Optical absorption
scintillation crystal exposed to intense flux of UV, X-ray or y-ray quanta. A reversible change of color under the influence of the activating radiation (the photochromic effect) is a property peculiar to nearly all inorganic wide-band-gap crystals (E, > 3 eV). The atomic structure and creation mechanisms of the color centers are well established for alkali halides or alkaline earth fluorides [l-3]. However, the origin of the color centers in wide-bandgap oxide scintillation crystals are far less understood in spite of the importance of this issue for
I’.G. Bondar
et al. /Nucl.
Instr.
and Meth.
application purposes. In particular, intensive attempts to improve radiation hardness of PbW04 (PWO), a fast scintillator crystal utilized for detection of high-energy quanta. were stimulated by the application of this material in radiation detectors of the Large Hadron Calorimeter at CERN [4,5]. It has been observed that the radiation-induced changes of absorption vary in different crystals [6,7], and even along the growth axis of the ingot [4], in a rather wide range. The induced absorption saturates after a certain dose of radiation, usually a few krad, is accumulated [4,7]. The spectrum of the induced absorption consists of several bands and correlates with thermoluminescence JTSL) glow curves [8,9]. It is reported that doping of the PWO by Nd [4] and La [lO,ll] improves the radiation hardness. On the other hand, PWO crystals with exceptionally high radiation hardness do exist without intentional doping [7] and may be grown by optimizing conditions of their growth and thermal annealing [12]. In the present paper, peculiarities of formation of the color centers as a function of growth conditions causing various content of impurities and structural defects as well as the influence of these color centers on scintillation properties of the crystals were investigated. Absorption induced by UV and y-ray irradiation and subsequent bleaching of the samples by heating and illumination with visible light were studied. In order to reveal some general features of the photochromic effect in tungstate crystals, experimental results obtained for two materials, the PbW03 (PWO) and CdWOS (CWO), another scintillator used in tomography and introscopy, are compared.
in Phl,s. Res. A 41 I (IYW)
376-382
3-n
blocks with dimensions 22 x 22 x 50 mm cut from boules with 300 mm in length and 40 mm in diameter. To reveal the influence of UV radiation on the optical and scintillation properties of the crystals, the samples were irradiated by 365 and 313 nm lines filtered from the spectrum of a mercury lamp as well as by radiation in the region between these wavelengths selected from emission of a hydrogen lamp. Intensity of the radiation was measured by a powermeter for the optical radiation “Kvarz-001” with the calibrated silicon cathode. Gamma-radiation emitted from a “Co source was also used to irradiate the samples. Transmission spectra of the samples were measured in the spectral range from 400 to 600 nm by using the spectrophotometer KSVU-23. The time of scanning the transmission spectrum was kept as short as 1 min to avoid measurable recovery of the modified transmission during the measurement and to be able to follow the time evolution of the photomodification. The quantum yield of the CWO crystals was measured both in relative units by using the current method under X-ray irradiation with a quantum energy of 100 keV and in absolute units by using an amplitude analyzer and NaI~Tl) as a reference under irradiation emitted by the 13’Cs source. For the PWO crystals, the light yield was measured in photons/MeV. It is worth noting that a phosphorescence, induced by UV- or y-radiation and decaying within some hours and longer, deteriorates the direct measurements of the light yield by using the amplitude analyzer. So we evaluated the ratio of the intensities of spectrally integrated luminescence before and after the irradiation as a measure of change of the light yield.
2. Ex~ri~ental The crystals were grown in a platinum crucible by using the Czochralski method. Four samples of the CWO under investigation varied in their stoichiometry and content of microimpurities. Another four samples of the PWO were cut from crystals grown at different contents of oxygen in the atmosphere. The samples of the CWO were cleaved from 200 mm long boules with 50 mm diameter while the PWO sampies were prepared as polished
3. Results 3.1. PbW04
The samples of the PWO under investigation are listed and characterized in Table 1. The crystals were grown at different contents of oxygen in the atmosphere. The oxygen is shown to be of considerable importance to scintillation properties of the PWO
KG. Ba~dar et al. JNacl. Insrr. and Meth. in Ph_vs.Rex A 41 I (1998) 376-382
378
as well as to even visually observable change of color: excess oxygen in the atmosphere results in the yellow color, the crystals grown at optimal conditions are colorless, while oxygen deficiency causes the gray color of the crystals [12,13]. The BC-4 differs from the other three samples in that the pulling rate of this crystal from the melt was 10 mm/h instead of 5 mm/h for other samples. Though the oxygen content in the atmosphere during its growth was the same as that for the sample BC-3, the sample X-4 was actually exposed to the influence of the ambient oxygen for a shorter period of time than sample Be-3 was. Modification of the absorption spectra of the samples by UV- and y-irradiation is illustrated in Fig. 1. Curves 1’-4’, characterizing the corresponding samples, represent the differential spectra obtained by subtracting the transmission spectrum of the sample exposed to a dose of 4.5 krad of yradiation emitted by a 60Co source at the rate of 500 rad/min from the transmission spectrum measured before the irradiation. An induced absorption is observed in all the samples. Two kinds of spectra of this induced absorption are evident in Fig. 1. The spectra of the samples BC-1 and BC-2 have a peak in the region of 450-470 nm while the induced absorption of the other two samples increase monotonously towards the long-wavelength end of the region under investigation. The darkening of the samples may also be induced by the ultraviolet Iight. The change in transmission due to the UV irradiation (power density of 0.2 MWjcm2) at the wavelength of 365 for 30 min is illustrated in Fig. 1 by curves 1 to 4. These differential spectra are rather similar to the corresponding spectra characterizing the influence of y-radiation, the absolute values of the induced absorption being
Table 1 Characterization
the only essential difference. These values for samples treated by y-radiation can be reduced by decreasing the radiation dose. So the darkening of the samples exposed to the dose of 600 rad at the rate of 30 rad/min [14] is equivalent to the darkening caused by the UV radiation at conditions mentioned above. The time evolution of the darkening caused by the UV radiation was investigated at different power densities of the radiation (see Fig. 2). The rate of the observed decrease of transmission depends on the intensity of the UV radiation. However, sooner or later, the effect saturates. It is worth noting that the value of the saturation of the radiation-induced absorption is the same for all intensities of irradiation, at least up to 0.5 mW/cm’ used in our experiments. Furthermore, the darkening, nevertheless progressing very
400 t,
550
600
h @ml Fig. 1. Change of optical transmission induced by UV and y irradiation (curves marked with apostrophes) in four PWO samples listed in Table 1: BC-I (1. 1’). BC-2 (2.2’). BC-3 (3.3’), and BC-4 (44’).
of the PWO samples
Crystal
Wavelength of emission peak, nm
Light yield, ph/MeV
BC-1 BC-2 BC-3 BC-4
440 430 440 450
30.2 93.8 32.3 65.0
0, content in the growth atmosphere, volume % 19 0.3 < lo-‘+ < 10--j
Color
Yellowish Colorless Grayish Grayish
379
400 t (min)
500
600
700
h (nm)
Fig. 2. Time evolution of the change in optical transmission (L = 450 nm) of the PWO sample BC-2 under exposition to UV radiation at different power densities: 0.5 (I), 0.2 (21,O.l (3). 0.05 (4). and 0.005 m W/cm’ (5).
Fig. 3. Residue of the UV-induced darkening of the PWO sample BC-2 at 500 nm versus the wavelength of the decolorating light after 30min of exposition at power density of 0.2 mW!cmL.
slowly, was observed even at irradiation intensities down to 0.005 mW/cmz. This observation implies that there is no threshold for the darkening effect to occur. The quantum yield of the photomodified PWO samples was measured and was found to decrease by 2535%. These values correspond to the values calculated for the samples by taking into account only the change in reabsorption of the luminescence caused by the increased absorption coefficient. No modification of the luminescence spectra due to the UV irradiation was observed. So the emission centers of the PWO seem to be unaffected by the irradiation. The transmission of the photomodified samples recovers exhibiting very strong temperature dependence of the rate of the recovering process. For example, the photomodified sample BC-2 stored at the temperature of 6°C in darkness for 21 days recovers only IO-15% of its induced absorption while this time is sufficient for the complete recovery if the temperature is increased to 20°C. At 3o”C, the time necessary for complete recovery of the sample transmission is as short as 24 h. Fast recovery of the transmission can be achieved not only by heating the crystals above the room temperature (e.g. up to 200°C as reported in [7]) but also by irradiating them by light in the spectral region of the edge of their absorption. The efficiency of the bleaching effect has a non-
monotonous dependence on the wavelength of the light, as illustrated in Fig. 3. The sample BC-2 was initially exposed to the UV irradiation causing a change of its absorption at 500 nm by 7O/a.Then the photoinduced absorption at 500 nm, still remaining after illumination with light of different wavelengths at 15 mW/cm’ for 30 min, was measured. So the light with the wavelength of 547 nm was found to be the most efficient in stimulating the recovery of the initial transmission of the sample. 3.2. CdWOj The photochromic effect in the CWO crystals was investigated by analyzing four samples with different stoichiometry and impurity content. Some parameters characterizing their structural, optical and scintiIiation properties are presented in Table 2. Note that the stoichiometric composition of the CdWQ+ corresponds to 35.65% of Cd0 and 64.35% of WOj. Furthermore, except for the data presented in TabIe 2, the content of some other impurities in the samples was analyzed and found to be as low as 2 x 10m5% for Mn, 5 x 10M5% for Ni and Ti, 10e4% for Mg and Al, and 2 x 10wJ% for Pb in all the four samples. Like the crystals of PWO, the CWO exhibits the photochromic effect. The relative change in optical transmission of the four samples listed in Table 2 after their exposition to UV radiation at
Table 2 Stoichiometry. content of mi~roimpurities obtained by chemical analysis, relative quantum yields measured under 75 keV X-ray excitation (C,) and under 662 keV excitation by lA7Cssource CC,),relative portion of the spectrally integrated luminescence emitted with a delay of more than 20 ms after excitation, and color of the CWO crystals under investigation Crystal
BK-1 BK-2 BK-3 BK-4
Stoichiometry, %
Content of micro impurities, 10U5%
Cd0
WO,
Fe
cu
Ga
Bi
35.70 35.78 35.84 35.79
64.30 64.21 64.15 64.2
20 50 40 40
2 20 2 2
5 100 5 50
5 100 5 50
0
400
450
500
550
C,, relunits
C,, ml. units Afterglow
0.95 0.74 0.63 0.65
0.34 0.29 0.17 0.23
600
R = 365 nm (the cw power density was 0.2 mW/cm*) is presented in Fig. 4, The spectra of the photo-induced darkening are dominated by a broad band. Except for the sample BK-1, the band is peaked in the vicinity of 510 nm. The spectrum of the sample BK-1 is also dominated by a single band, however, the band is peaked at 460 nm and has a clearly asymmetric shape with a more gradual slope in the short-wavelength side which, possibly, reflects a certain contribution of the other band peaked at 510 nm. Irrespective of whether the spectrum of the induced darkening is dominated by the band peaked at 460 nm or 510 nm, the photochromic effect saturates after exposition of the sample to a certain dose of the UV irradiation. The kinetics of the change of transmission is presented in Fig. 5 indicting the saturation effect for the darkening of the sample at
Yellowish Yellowish brown Reddish brown Light brown
t (min)
h (rm-0 Fig. 4. Change of optical transmission induced by UV in four CWO samples listed in Table 2: BK-1 (1). BK-2 (I!), BK-3 (3), and BK-4 (4).
< 0.05 0.44 < 0.05 < 0.05
Color
Fig. 5. Time evolution of the change in optical transmission (I = 450 nm) of the CWO sample BK-1 under exposition to IJV radiation at different power densities: 0.5 (I), 0.2 (21,O.i (3). and 0.05 m W/cm’ (4).
any rate of irradiation. Again, the photodarkened samples of CWO are bleached by light in the visible region with wavelength-dependent efficiency peaked at 550 nm like that for PWO illustrated in Fig. 3. The light yieId of the photodarkened CWO samples is decreased by 20%.
4. Discussion General features of the observed photochromic effect are common for both PWO and CWO crystals. First of all, the decrease in light yield after irradiation can be quantitatively explained by the observed increase of absorption, Furthermore, the Iuminescence spectrum is not affected by the irradiation. This implies that the irradiation results
V.G. Bondar et al. JNucl. Instr. and Meth. in Phvs. Res. A 4/l
in the modification or formation of new absorption centers without any considerable influence on the emission centers. This result is consistent with the conclusion made in [7] and does not support the conjecture about possible modification of emission centers in the PWO [15]. We can conclude that both UV and y irradiation cause formation of the same absorption centers resulting in irradiation-induced absorption with essentially identical spectrum in both cases. Note, however, that the spectrum of the induced absorption is sample dependent. At least two characteristic bands can be distinguished in the absorption spectrum. The first band, peaked at ~460 nm, seems to be observed in both PWO and CWO while the peak of the other band is at ~510 nm for CWO and shifted to longer wavelengths in PWO. It is worth noting that the UV-induced absorption saturates at a certain value after a certain dose of radiation is accumulated irrespective of the accumulation rate. Such a saturation seems to exclude the explanation of the photochromic effect by formation of new absorption centers due to decay of a self-trapped exciton into charged structural defects, like it has been reported for the formation of separated pairs of F and H centers in alkali halides (see e.g. Cl.21 for review) or alkaline earth fluorides [3,16]. Such a decay takes place at a regular site of the lattice, the self-trapped exciton being a precursor for the distortion of the lattice resulting in formation of the structural defect. In the tungstate crystals under investigation, some kind of defects seem to exist in the lattice before the irradiation which transform after irradiation these defects into absorption centers. So the density of the defects determines the density of the color centers to be created by the UV irradiation. It is worth noting that the increase of absorption due to y-irradiation does not saturate, however, even at the highest rates of y-radiation, the spectrum of the y-rayinduced absorption is the same as that induced by UV irradiation. This implies that intense y-radiation creates new color centers identical to those existing before the irradiation. Consequently, it is plausible to suppose that these centers are related rather with structural defects than with impurities. Naturally, the presence of impurities or disloca-
(1998) 376-382
381
tions in the crystal may facilitate the formation of color centers through distortion of the lattice. The available data indicate that the photochromic effect may be related with deviations from the stoichiometric fraction of oxygen atoms in the crystals. So, comparison of the results presented in Fig. 1 with the characteristics of the PWO samples in Table 1 implies that a higher content of oxygen in atmosphere during the growth process increases the photoinduced absorption. For CWO, the photochromic effect seems to grow rather with deviation from the stoichiometric composition in favor of Cd0 than with density of impurities under investigation. Again, this may be relevant to certain oxygen-related structural defects. The two different types of spectra in both PWO and CWO imply that at least two kinds of such defects should exist, possibly, related to oxygen at different positions in the lattice, Other characteristics of the PWO crystals, such as luminescence and absorption spectra, light yield, were also shown to be strongly dependent on oxygen-related structural defects [4,12,13,17]. At the present state of investigation we can only suppose that the photoinduced absorption is caused by trapping of an electron or hole generated by UV or y-radiation. A model for hole trapping at regular oxygen sites, i.e. formation of O- centers, and stabilization of these centers by point defects is proposed in [ 131. The available experimental results are consistent with such a model, however, an unambiguous interpretation of atomic structure of the defects responsible for the photochromic effect in PWO and CWO needs further investigations.
5. Conclusion General features of radiation-induced absorption are common for both tungstates, PWO and CWO. Excitation of the crystals with quanta in a wide energy range from UV light to y-rays causes the formation of the same color centers. Variety of such centers resulting in different absorption spectra depends on the growth conditions of the crystal. The absolute value of the radiation-induced absorption correlates with oxygen content in the lattice and is determined by the density of the structural defects available in the crystal before the
V.G. Bondur et al. /Nucl. Instr. and Meth. in Phys. Res. A 411 (1996ri 376-382
382
irradiation. Trapping of carriers is the most probable mechanism of formation of the centers while y-quanta are capable to create additional structural defects (of the same kind as the native defects) serving as precursors to the color centers or as their stabilizers. The induced absorption may be bteached either by heating the crystal or by illumination with light far below the edge of band-to-band absorption. Efficiency of the bleaching has its maximum value for light with a wavelength of 550 nm.
Samples provided by L. Nagornaya are gratefully acknowledged. The authors are also very grateful to O.V. Zalenskaya for measuring the light yield. References [l] C.B. Lushchik, J.K. Vital, MA. Eiango, Soviet Phys. Usp. 20 f 1977) 489.
PI N. Itoh, Adv. Phys. 31 (1982) 491. E31KS. Song, R.T. Williams, Self-Trapped Excitons, Springer Ser. Solid State Sci., Springer, Berlin/Heidelberg 1993. c41P. Lecoq et al.. Nucl. Instr. and Meth. A 365 (1995) 291. c-51P. Lecoq, CMS Collaboration Meeting, Aachen, Germany. 1l-12 September 1996. C61L.L. Nagornaya et al., Rad. Meas. 24 (1995) 375. c71R.Y. Zhu et al., Nucl. Instr. and Meth. A 376 (1996) 319. PI S. Baccaro et al., Book of Abstracts SCINT-97 Int. Conf., Shanghai, Sept. 22-25, 1997, p. 80. c91L. Nagornaya et al., Book of Abstracts SCINT-97 Int. Conf.. Shanghai, Sept. 22-25, f997, p. 109. CNI M. Kobayashi et al., to be published in Nucl. Ins&. and Meth. A. rJ11S. Baccaro, P. Bohacek, A. Vedda, Phys. Stat. Sol. A 160 (1997) R5. Cl23S.Burachas et al., J. Crystal Growth 186 (1998) 175. u31 M. Nikl et al., Phys. Stat. Sol. B 196 (1996) K7. Cl41G. Britwitch et al., Proc. IEEE Nuclear Science Symp., Anaheim, USA, 3-9 November 1996. G. Chen et al.. Book of Abstracts SCINT-97 Int. Conf.. La151 Shanghai, September 22-25, 1997, p. 55. Cl61K. Tanimura, N. Itoh, J. Appl. Phys. 69 (1991) 7831. P71 G. Tamulaitis et al., Phys. Stat. Sol. A 157 (1996) 187.
Nuclear
Instruments
Photochromic
in Physics Research A 411 (1998)376-382
and Methods
SectIonA
effect and its influence on scintillation properties of CdW04 and PbW04 crystals
V.G. Bondara, S.F. Burachasa, K.A. Katrunova, V.P. Martinova, V.D. Ryzhikov”, V.I. Mankob, H.H. Gutbrod”, G. Tamulaitisd,* aSTC RI of the Institute of Single Crystals, Ukrainian Academy of Sciences, pr. Lenina 60, KharkoL? 310001, Ukraine b Kurchatov National Center, Moscow, Kurchatov sq. I, Moscow 123182. Russian Federation Ecole des Mines de Nantes. Laboratoire de physique subatomique et des technologies associees, 4 rue Arfed F-44070 Names Cedex 03. France ‘Institute of Material Sciences and Applied Research, Vilnius University. 24 Naugarduko, Vilnius 2006. Lithuania
’ SUBATECH,
Kastler.
Received 17 December 1997
Abstract Absorption induced in PbW04 and CdW04 scintillation crystals by UV and y-ray irradiation was investigated by observing the dynamics of the absorption spectra after the irradiation of the samples with quanta of different energies and time evolution of the darkening effect at different rates of the irradiation. Dependence of the irradiation-induced absorption on growth conditions of the crystals, as well as bleaching of the samples by heating and illumination with visible light were studied. Trapping of free carriers at structural defects related with oxygen content in the lattice was shown to result in the creation of color centers in both tungstates under investigation. C:I 1998 Elsevier Science B.V. All rights reserved. PACS: 29.40.M~; 61.82.M~ Keywords:
Scintillation
detectors; PWO; CWO; Radiation hardness; Photochromic
1. Introduction Radiation hardness is, as a rule, one of the most important properties of scintillation materials. Beside possible influence of the radiation on the emission centers, there are the radiation-induced changes that usually deteriorates the light yield of the
*Corresponding author. Tel: + 370 2 636022, fax: 263417, e-mail: [email protected].
+ 370 2
0168-9002/98/$19.00 c.1 1998 Elsevier Science B.V. All rights reserved. PII: SO168-9002(98)00346-5
effect; Optical absorption
scintillation crystal exposed to intense flux of UV, X-ray or y-ray quanta. A reversible change of color under the influence of the activating radiation (the photochromic effect) is a property peculiar to nearly all inorganic wide-band-gap crystals (E, > 3 eV). The atomic structure and creation mechanisms of the color centers are well established for alkali halides or alkaline earth fluorides [l-3]. However, the origin of the color centers in wide-bandgap oxide scintillation crystals are far less understood in spite of the importance of this issue for
I’.G. Bondar
et al. /Nucl.
Instr.
and Meth.
application purposes. In particular, intensive attempts to improve radiation hardness of PbW04 (PWO), a fast scintillator crystal utilized for detection of high-energy quanta. were stimulated by the application of this material in radiation detectors of the Large Hadron Calorimeter at CERN [4,5]. It has been observed that the radiation-induced changes of absorption vary in different crystals [6,7], and even along the growth axis of the ingot [4], in a rather wide range. The induced absorption saturates after a certain dose of radiation, usually a few krad, is accumulated [4,7]. The spectrum of the induced absorption consists of several bands and correlates with thermoluminescence JTSL) glow curves [8,9]. It is reported that doping of the PWO by Nd [4] and La [lO,ll] improves the radiation hardness. On the other hand, PWO crystals with exceptionally high radiation hardness do exist without intentional doping [7] and may be grown by optimizing conditions of their growth and thermal annealing [12]. In the present paper, peculiarities of formation of the color centers as a function of growth conditions causing various content of impurities and structural defects as well as the influence of these color centers on scintillation properties of the crystals were investigated. Absorption induced by UV and y-ray irradiation and subsequent bleaching of the samples by heating and illumination with visible light were studied. In order to reveal some general features of the photochromic effect in tungstate crystals, experimental results obtained for two materials, the PbW03 (PWO) and CdWOS (CWO), another scintillator used in tomography and introscopy, are compared.
in Phl,s. Res. A 41 I (IYW)
376-382
3-n
blocks with dimensions 22 x 22 x 50 mm cut from boules with 300 mm in length and 40 mm in diameter. To reveal the influence of UV radiation on the optical and scintillation properties of the crystals, the samples were irradiated by 365 and 313 nm lines filtered from the spectrum of a mercury lamp as well as by radiation in the region between these wavelengths selected from emission of a hydrogen lamp. Intensity of the radiation was measured by a powermeter for the optical radiation “Kvarz-001” with the calibrated silicon cathode. Gamma-radiation emitted from a “Co source was also used to irradiate the samples. Transmission spectra of the samples were measured in the spectral range from 400 to 600 nm by using the spectrophotometer KSVU-23. The time of scanning the transmission spectrum was kept as short as 1 min to avoid measurable recovery of the modified transmission during the measurement and to be able to follow the time evolution of the photomodification. The quantum yield of the CWO crystals was measured both in relative units by using the current method under X-ray irradiation with a quantum energy of 100 keV and in absolute units by using an amplitude analyzer and NaI~Tl) as a reference under irradiation emitted by the 13’Cs source. For the PWO crystals, the light yield was measured in photons/MeV. It is worth noting that a phosphorescence, induced by UV- or y-radiation and decaying within some hours and longer, deteriorates the direct measurements of the light yield by using the amplitude analyzer. So we evaluated the ratio of the intensities of spectrally integrated luminescence before and after the irradiation as a measure of change of the light yield.
2. Ex~ri~ental The crystals were grown in a platinum crucible by using the Czochralski method. Four samples of the CWO under investigation varied in their stoichiometry and content of microimpurities. Another four samples of the PWO were cut from crystals grown at different contents of oxygen in the atmosphere. The samples of the CWO were cleaved from 200 mm long boules with 50 mm diameter while the PWO sampies were prepared as polished
3. Results 3.1. PbW04
The samples of the PWO under investigation are listed and characterized in Table 1. The crystals were grown at different contents of oxygen in the atmosphere. The oxygen is shown to be of considerable importance to scintillation properties of the PWO
KG. Ba~dar et al. JNacl. Insrr. and Meth. in Ph_vs.Rex A 41 I (1998) 376-382
378
as well as to even visually observable change of color: excess oxygen in the atmosphere results in the yellow color, the crystals grown at optimal conditions are colorless, while oxygen deficiency causes the gray color of the crystals [12,13]. The BC-4 differs from the other three samples in that the pulling rate of this crystal from the melt was 10 mm/h instead of 5 mm/h for other samples. Though the oxygen content in the atmosphere during its growth was the same as that for the sample BC-3, the sample X-4 was actually exposed to the influence of the ambient oxygen for a shorter period of time than sample Be-3 was. Modification of the absorption spectra of the samples by UV- and y-irradiation is illustrated in Fig. 1. Curves 1’-4’, characterizing the corresponding samples, represent the differential spectra obtained by subtracting the transmission spectrum of the sample exposed to a dose of 4.5 krad of yradiation emitted by a 60Co source at the rate of 500 rad/min from the transmission spectrum measured before the irradiation. An induced absorption is observed in all the samples. Two kinds of spectra of this induced absorption are evident in Fig. 1. The spectra of the samples BC-1 and BC-2 have a peak in the region of 450-470 nm while the induced absorption of the other two samples increase monotonously towards the long-wavelength end of the region under investigation. The darkening of the samples may also be induced by the ultraviolet Iight. The change in transmission due to the UV irradiation (power density of 0.2 MWjcm2) at the wavelength of 365 for 30 min is illustrated in Fig. 1 by curves 1 to 4. These differential spectra are rather similar to the corresponding spectra characterizing the influence of y-radiation, the absolute values of the induced absorption being
Table 1 Characterization
the only essential difference. These values for samples treated by y-radiation can be reduced by decreasing the radiation dose. So the darkening of the samples exposed to the dose of 600 rad at the rate of 30 rad/min [14] is equivalent to the darkening caused by the UV radiation at conditions mentioned above. The time evolution of the darkening caused by the UV radiation was investigated at different power densities of the radiation (see Fig. 2). The rate of the observed decrease of transmission depends on the intensity of the UV radiation. However, sooner or later, the effect saturates. It is worth noting that the value of the saturation of the radiation-induced absorption is the same for all intensities of irradiation, at least up to 0.5 mW/cm’ used in our experiments. Furthermore, the darkening, nevertheless progressing very
400 t,
550
600
h @ml Fig. 1. Change of optical transmission induced by UV and y irradiation (curves marked with apostrophes) in four PWO samples listed in Table 1: BC-I (1. 1’). BC-2 (2.2’). BC-3 (3.3’), and BC-4 (44’).
of the PWO samples
Crystal
Wavelength of emission peak, nm
Light yield, ph/MeV
BC-1 BC-2 BC-3 BC-4
440 430 440 450
30.2 93.8 32.3 65.0
0, content in the growth atmosphere, volume % 19 0.3 < lo-‘+ < 10--j
Color
Yellowish Colorless Grayish Grayish
379
400 t (min)
500
600
700
h (nm)
Fig. 2. Time evolution of the change in optical transmission (L = 450 nm) of the PWO sample BC-2 under exposition to UV radiation at different power densities: 0.5 (I), 0.2 (21,O.l (3). 0.05 (4). and 0.005 m W/cm’ (5).
Fig. 3. Residue of the UV-induced darkening of the PWO sample BC-2 at 500 nm versus the wavelength of the decolorating light after 30min of exposition at power density of 0.2 mW!cmL.
slowly, was observed even at irradiation intensities down to 0.005 mW/cmz. This observation implies that there is no threshold for the darkening effect to occur. The quantum yield of the photomodified PWO samples was measured and was found to decrease by 2535%. These values correspond to the values calculated for the samples by taking into account only the change in reabsorption of the luminescence caused by the increased absorption coefficient. No modification of the luminescence spectra due to the UV irradiation was observed. So the emission centers of the PWO seem to be unaffected by the irradiation. The transmission of the photomodified samples recovers exhibiting very strong temperature dependence of the rate of the recovering process. For example, the photomodified sample BC-2 stored at the temperature of 6°C in darkness for 21 days recovers only IO-15% of its induced absorption while this time is sufficient for the complete recovery if the temperature is increased to 20°C. At 3o”C, the time necessary for complete recovery of the sample transmission is as short as 24 h. Fast recovery of the transmission can be achieved not only by heating the crystals above the room temperature (e.g. up to 200°C as reported in [7]) but also by irradiating them by light in the spectral region of the edge of their absorption. The efficiency of the bleaching effect has a non-
monotonous dependence on the wavelength of the light, as illustrated in Fig. 3. The sample BC-2 was initially exposed to the UV irradiation causing a change of its absorption at 500 nm by 7O/a.Then the photoinduced absorption at 500 nm, still remaining after illumination with light of different wavelengths at 15 mW/cm’ for 30 min, was measured. So the light with the wavelength of 547 nm was found to be the most efficient in stimulating the recovery of the initial transmission of the sample. 3.2. CdWOj The photochromic effect in the CWO crystals was investigated by analyzing four samples with different stoichiometry and impurity content. Some parameters characterizing their structural, optical and scintiIiation properties are presented in Table 2. Note that the stoichiometric composition of the CdWQ+ corresponds to 35.65% of Cd0 and 64.35% of WOj. Furthermore, except for the data presented in TabIe 2, the content of some other impurities in the samples was analyzed and found to be as low as 2 x 10m5% for Mn, 5 x 10M5% for Ni and Ti, 10e4% for Mg and Al, and 2 x 10wJ% for Pb in all the four samples. Like the crystals of PWO, the CWO exhibits the photochromic effect. The relative change in optical transmission of the four samples listed in Table 2 after their exposition to UV radiation at
Table 2 Stoichiometry. content of mi~roimpurities obtained by chemical analysis, relative quantum yields measured under 75 keV X-ray excitation (C,) and under 662 keV excitation by lA7Cssource CC,),relative portion of the spectrally integrated luminescence emitted with a delay of more than 20 ms after excitation, and color of the CWO crystals under investigation Crystal
BK-1 BK-2 BK-3 BK-4
Stoichiometry, %
Content of micro impurities, 10U5%
Cd0
WO,
Fe
cu
Ga
Bi
35.70 35.78 35.84 35.79
64.30 64.21 64.15 64.2
20 50 40 40
2 20 2 2
5 100 5 50
5 100 5 50
0
400
450
500
550
C,, relunits
C,, ml. units Afterglow
0.95 0.74 0.63 0.65
0.34 0.29 0.17 0.23
600
R = 365 nm (the cw power density was 0.2 mW/cm*) is presented in Fig. 4, The spectra of the photo-induced darkening are dominated by a broad band. Except for the sample BK-1, the band is peaked in the vicinity of 510 nm. The spectrum of the sample BK-1 is also dominated by a single band, however, the band is peaked at 460 nm and has a clearly asymmetric shape with a more gradual slope in the short-wavelength side which, possibly, reflects a certain contribution of the other band peaked at 510 nm. Irrespective of whether the spectrum of the induced darkening is dominated by the band peaked at 460 nm or 510 nm, the photochromic effect saturates after exposition of the sample to a certain dose of the UV irradiation. The kinetics of the change of transmission is presented in Fig. 5 indicting the saturation effect for the darkening of the sample at
Yellowish Yellowish brown Reddish brown Light brown
t (min)
h (rm-0 Fig. 4. Change of optical transmission induced by UV in four CWO samples listed in Table 2: BK-1 (1). BK-2 (I!), BK-3 (3), and BK-4 (4).
< 0.05 0.44 < 0.05 < 0.05
Color
Fig. 5. Time evolution of the change in optical transmission (I = 450 nm) of the CWO sample BK-1 under exposition to IJV radiation at different power densities: 0.5 (I), 0.2 (21,O.i (3). and 0.05 m W/cm’ (4).
any rate of irradiation. Again, the photodarkened samples of CWO are bleached by light in the visible region with wavelength-dependent efficiency peaked at 550 nm like that for PWO illustrated in Fig. 3. The light yieId of the photodarkened CWO samples is decreased by 20%.
4. Discussion General features of the observed photochromic effect are common for both PWO and CWO crystals. First of all, the decrease in light yield after irradiation can be quantitatively explained by the observed increase of absorption, Furthermore, the Iuminescence spectrum is not affected by the irradiation. This implies that the irradiation results
V.G. Bondar et al. JNucl. Instr. and Meth. in Phvs. Res. A 4/l
in the modification or formation of new absorption centers without any considerable influence on the emission centers. This result is consistent with the conclusion made in [7] and does not support the conjecture about possible modification of emission centers in the PWO [15]. We can conclude that both UV and y irradiation cause formation of the same absorption centers resulting in irradiation-induced absorption with essentially identical spectrum in both cases. Note, however, that the spectrum of the induced absorption is sample dependent. At least two characteristic bands can be distinguished in the absorption spectrum. The first band, peaked at ~460 nm, seems to be observed in both PWO and CWO while the peak of the other band is at ~510 nm for CWO and shifted to longer wavelengths in PWO. It is worth noting that the UV-induced absorption saturates at a certain value after a certain dose of radiation is accumulated irrespective of the accumulation rate. Such a saturation seems to exclude the explanation of the photochromic effect by formation of new absorption centers due to decay of a self-trapped exciton into charged structural defects, like it has been reported for the formation of separated pairs of F and H centers in alkali halides (see e.g. Cl.21 for review) or alkaline earth fluorides [3,16]. Such a decay takes place at a regular site of the lattice, the self-trapped exciton being a precursor for the distortion of the lattice resulting in formation of the structural defect. In the tungstate crystals under investigation, some kind of defects seem to exist in the lattice before the irradiation which transform after irradiation these defects into absorption centers. So the density of the defects determines the density of the color centers to be created by the UV irradiation. It is worth noting that the increase of absorption due to y-irradiation does not saturate, however, even at the highest rates of y-radiation, the spectrum of the y-rayinduced absorption is the same as that induced by UV irradiation. This implies that intense y-radiation creates new color centers identical to those existing before the irradiation. Consequently, it is plausible to suppose that these centers are related rather with structural defects than with impurities. Naturally, the presence of impurities or disloca-
(1998) 376-382
381
tions in the crystal may facilitate the formation of color centers through distortion of the lattice. The available data indicate that the photochromic effect may be related with deviations from the stoichiometric fraction of oxygen atoms in the crystals. So, comparison of the results presented in Fig. 1 with the characteristics of the PWO samples in Table 1 implies that a higher content of oxygen in atmosphere during the growth process increases the photoinduced absorption. For CWO, the photochromic effect seems to grow rather with deviation from the stoichiometric composition in favor of Cd0 than with density of impurities under investigation. Again, this may be relevant to certain oxygen-related structural defects. The two different types of spectra in both PWO and CWO imply that at least two kinds of such defects should exist, possibly, related to oxygen at different positions in the lattice, Other characteristics of the PWO crystals, such as luminescence and absorption spectra, light yield, were also shown to be strongly dependent on oxygen-related structural defects [4,12,13,17]. At the present state of investigation we can only suppose that the photoinduced absorption is caused by trapping of an electron or hole generated by UV or y-radiation. A model for hole trapping at regular oxygen sites, i.e. formation of O- centers, and stabilization of these centers by point defects is proposed in [ 131. The available experimental results are consistent with such a model, however, an unambiguous interpretation of atomic structure of the defects responsible for the photochromic effect in PWO and CWO needs further investigations.
5. Conclusion General features of radiation-induced absorption are common for both tungstates, PWO and CWO. Excitation of the crystals with quanta in a wide energy range from UV light to y-rays causes the formation of the same color centers. Variety of such centers resulting in different absorption spectra depends on the growth conditions of the crystal. The absolute value of the radiation-induced absorption correlates with oxygen content in the lattice and is determined by the density of the structural defects available in the crystal before the
V.G. Bondur et al. /Nucl. Instr. and Meth. in Phys. Res. A 411 (1996ri 376-382
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irradiation. Trapping of carriers is the most probable mechanism of formation of the centers while y-quanta are capable to create additional structural defects (of the same kind as the native defects) serving as precursors to the color centers or as their stabilizers. The induced absorption may be bteached either by heating the crystal or by illumination with light far below the edge of band-to-band absorption. Efficiency of the bleaching has its maximum value for light with a wavelength of 550 nm.
Samples provided by L. Nagornaya are gratefully acknowledged. The authors are also very grateful to O.V. Zalenskaya for measuring the light yield. References [l] C.B. Lushchik, J.K. Vital, MA. Eiango, Soviet Phys. Usp. 20 f 1977) 489.
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