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Thermoluminescence studies on CsCl:NO3− crystals irradiated with X-rays
Materials Chemistry and Physics, 37 (1994) 299-301
Thermoluminescence X-rays P. Christopher
Selvan
299
MATERIALS CHEMISTRYAND PHYSICS
studies on CsCl:NO,-
crystals irradiated
with
and S. Selvasekarapandian
Department of Physics, Bharathiar University Coimbatore - 641 046 (India) (Received
August
16, 1993; accepted
November
15, 1993)
Abstract Thermoluminescence of CsCI:NOScrystals irradiated by X-rays at room temperature has been studied. The presence of nitrate impurity was confirmed through optical absorption measurements. Nitrate-doped crystals exhibit two glow peaks centered at 361 and 401 K with a warming rate of 60 K min-’ and analysis indicates first order kinetics with activation energies 0.91 and 1.02 eV respectively. The first peak (361 K) originates from F centers and the 401 K peak from nitrate impurity in the host matrix.
Introduction The study of the properties of molecular impurities in different crystalline matrices has stimulated special interest in solid state research. Recent studies on molecular oxyanions such as CrOd2- and MnOd2- doped in different lattices have indicated that these type of ions can be damaged by X-rays or y-rays resulting in the formation of electron centers like CrOd3-, CrO,‘and MnO,- [l, 21. Among the molecular impurities, however there seem to be very few reports on the radiation damage of NO,- ions doped alkali halide lattices [3-6]. Thermoluminescence (TL) studies available on CsCl type crystals are still meager compared to the enormous amount of work done on f.c.c. type crystals, especially with molecular impurities [2, 61. In view of these points we have undertaken TL studies on NO,- doped CsCl single crystals irradiated by Xrays at room temperature. Experimental Single crystals of undoped and NO,- doped CsCl were grown by slow evaporation of their saturated solutions at room temperature. About 3.145 X lop4 mole fraction of CsNO, was added to obtain impurity-doped crystals. Optical absorption spectra were recorded using a Hitachi-3010 spectrophotometer. Irradiation was done
0254-0584/94/$07.00 0 1994 Elsevier SSDI 0254-0584(93)01334-Y
Sequoia. All rights reserved
by X-rays from a copper target operated at 30 kV and 10 mA with a dose rate of 600 Rad min-‘. The glow curves were recorded at a heating rate of 60 K min? using apparatus described elsewhere [7]. The sample sizes were typically of 3 mm3. F-bleaching studies were carried out using an Ilford filter (603 nm) and a tungsten filament lamp (60 W). The TL emission spectra were recorded using a Jarrel-Ash monochromator with omnidrive and a R955 photomultiplier tube in the manner described by Halperin et al. [S]. The emissions given here were not corrected for the spectral response of the photomultiplier tube. Results and discussion TL glow and optical absorption Thermoluminescence (TL) spectra of undoped samples exhibit two glow peaks, at 341 and 361 K. On increasing the irradiation time to 2 hours a new peak develops at 378 K [9]. The typical TL behaviour of NO,- doped cesium chloride crystal is illustrated in Fig. 1. In the as-grown samples illuminated with X-rays for 10 minutes, two glow peaks are recorded, a well defined peak at 361 K and a shoulder at 401 K (curve (a), Fig. 1). On increasing the exposure time the first peak soon saturates and declines whereas the second one grows in intensity. On comparing the spectrum of the doped samples with
1
303
323
343
363 TEMPERATURE
383
403
423
44:
CR)
Fig. 1. TL glow curve of CsCI:NOJ crystals. a) X-irradiated for 10 min; b) TL glow curve after bleaching with F-light for 5 min subsequent to X-irradiation for 10 min. Inset (left) shows growth of glow peaks with exposure time; inset (right) shows the fall in intensity of glow peaks with F-bleaching time.
that of undoped material, it is observed that doping decreases the total TL output by about one order. Quenching the samples at a temperature of 473 K for 15 minutes also suppresses the intensity of these peaks about one order. Bleaching studies were undertaken to investigate the centers responsible for TL glow. The respective samples are radiated with X-rays for 10 minutes and then bleached with F-light (603 nm) for 1, 2, 3, . . . minutes and repeated for increasing times of radiation as well as bleaching. Curve (b) in Fig. 1 shows one such typical glow curve. There is an enormous decrease in the intensity after optical bleaching with F-light for the peak at 361 K and eventually it vanishes completely, whereas the 401 K peak is not much affected. Figure 2 gives the optical absorption spectra of doped crystals. A significant absorption band peaking at 267 nm is seen (curve a). On X-irradiation for 10 minutes, this band decreases in intensity and a broad band develops between 220 and 260 nm in addition to a weak F-band at 603 nm (curve b). The optical absorption band appearing at 267 nm in the doped samples only is attributed to the molecular anionic nitrate impurity in the CsCl lattice and the reduction in intensity on irradiation with X-rays is due to the formation of NOS2- from NO,- by capturing electrons that are produced on irradiation, as follows
141:
4
I . 200
300
400
500
WAVELENGTH
600
700
(nm)
Fig. 2. Optical absorption spectra of CsCl:N03 crystals: a) before irradiation; b) after irradiation for 10 min.
NO,- + e- -
NO,‘-
The broad UV band may be a combination of two bands at 233 and 252 nm and are attributed to V, and V, centers produced on irradiation at RT [lo].
301
It may be mentioned that the peak at 401 K is prominently developed only in NO,- doped crystals. The undoped crystal produces two glow peaks at 341 K and 361 K which are attributed to thermal decay of F centers [9]. Comparing these TL spectra with those of undoped samples, the prominent glow peak at 361 K can be completely bleached optically in the doped material, indicating that F-centers are involved in this process. In the doped product the 341 K glow peak (observed in undoped CsCl) appears to be hidden or overshadowed owing to the growth of the 361 K glow peak, and hence is not well resolved. The latter one at 401 K which is not affected much on F-bleaching can probably be attributed to the nitrate impurity. The total decrease in the TL output may be due to the capture of electrons produced during irradiation by the nitrate complex, as explained earlier, and this process annihilates the majority of F-centers responsible for TL glow. Such type of behaviour has already been observed by Kulkarni and Garg [4] and Joshi and Garg [6] for nitrate-doped binary mixtures. They have concluded that doping by nitrate impurity decreases the TL intensity compared to the pure material. The results of nitrate-doped CsCl in this work also reveals the same phenomenon. The glow peaks observed are thermally well separated and analysed by a curve fitting method. Both peaks obey first order kinetics; trap depths are 0.91 and 1.02 eV for the two glow peaks at 361 and 401 K respectively.
4
200
400
300
WAVELENGTH
500
600
(mu)
Fig. 3. TL emission spectrum of CsCl:N03 for 10 min: a) 361 K; b) 401 K.
NO,‘- -
700
after X-irradiation
NO,- + e- + hvE2.64 eV]
The one order increase in the intensity of the 2.64 eV emission in the latter case (which is attributed to NO, impurity) indicates that only V, centers are acting as recombination centers making the emission more intense. More work is probably needed to point out the exact nature of defect centers.
TL emission Emission studies were undertaken in order to identify the recombination centers (Fig. 3). The TL emission spectra obtained at 361 K (curve a) has a recognisable emission band at 407 nm (3.05 eV) with a weak shoulder at 471 nm (2.64 eV). When the crystal is kept at 401 K the emission spectrum (curve b) showed a well resolved emission band peaking at 471 nm (2.64 eV). The 3.05 eV emission is absent in the latter case. Compared with undoped material the emission in the nitrate-doped samples is very weak. The bands at 3.05 eV and 2.64 eV for the 361 K peak correspond to the energy difference between the bottom of the V, and V, centers at 4.92 eV (252 nm) and 5.32 eV (233 nm) which can act as recombination centers for the electrons thermally released from the F-center traps. The 2.64 eV emission of the 401 K peak seems to be the most prominent emission, but is not due to the electrons released from F-centers, since the peak is not much affected on F-bleaching. These electrons may be thermally detrapped from some shallow traps, probably from the nitrate complexes, and recombining with the hole centers preferably with more stable V3 centers giving rise to 2.64 eV emission.
Acknowledgement The authors would like to thank Dr A. Natarajan of the Safety Research Laboratory, Kalpakkam, and Dr S.B.S. Sastry, IIT, Madras, for providing the necessary help regarding optical absorption and TL emission spectra. References 1 B.V.R.
Chowdary and Y. Ravisekar, Solid State Commun., 31 (1979) 453. 2 S. Radhakrishna and K. Hariharan, Phys. Status Solidi, 92 (1979)
293.
3 A.R. Jones, J. Chem. Phys., 35 (1961) 751. 4 S.P. Kulkami and A.N. Garg, ht. J. Radiat. Phys. Chem., 32 (1988) 609. S. Selvasekarapandian and K. Hariharan, Mater. Chem. Phys., 27 (1991) 213. 6 N.G. Joshi and A.N. Garg, Ind. J. Chem., 31 (1992) 915. 7 C.M. Sunta, J. Phys. C, 3 (1970) 1978. 8 A. Halperin, N. Kristianpoller and A. Ben-Zi, Phys. Rev., 116 (1959) 1081. 9 J.K. Radhakrishnan and S. Selvasekarapandian, Ctyst. Rex TechnoZ., 27 (1992) K 95. 10 Rabin and J.H. Schulman, Phys. Rev., 125 (1962) 1584.
Thermoluminescence X-rays P. Christopher
Selvan
299
MATERIALS CHEMISTRYAND PHYSICS
studies on CsCl:NO,-
crystals irradiated
with
and S. Selvasekarapandian
Department of Physics, Bharathiar University Coimbatore - 641 046 (India) (Received
August
16, 1993; accepted
November
15, 1993)
Abstract Thermoluminescence of CsCI:NOScrystals irradiated by X-rays at room temperature has been studied. The presence of nitrate impurity was confirmed through optical absorption measurements. Nitrate-doped crystals exhibit two glow peaks centered at 361 and 401 K with a warming rate of 60 K min-’ and analysis indicates first order kinetics with activation energies 0.91 and 1.02 eV respectively. The first peak (361 K) originates from F centers and the 401 K peak from nitrate impurity in the host matrix.
Introduction The study of the properties of molecular impurities in different crystalline matrices has stimulated special interest in solid state research. Recent studies on molecular oxyanions such as CrOd2- and MnOd2- doped in different lattices have indicated that these type of ions can be damaged by X-rays or y-rays resulting in the formation of electron centers like CrOd3-, CrO,‘and MnO,- [l, 21. Among the molecular impurities, however there seem to be very few reports on the radiation damage of NO,- ions doped alkali halide lattices [3-6]. Thermoluminescence (TL) studies available on CsCl type crystals are still meager compared to the enormous amount of work done on f.c.c. type crystals, especially with molecular impurities [2, 61. In view of these points we have undertaken TL studies on NO,- doped CsCl single crystals irradiated by Xrays at room temperature. Experimental Single crystals of undoped and NO,- doped CsCl were grown by slow evaporation of their saturated solutions at room temperature. About 3.145 X lop4 mole fraction of CsNO, was added to obtain impurity-doped crystals. Optical absorption spectra were recorded using a Hitachi-3010 spectrophotometer. Irradiation was done
0254-0584/94/$07.00 0 1994 Elsevier SSDI 0254-0584(93)01334-Y
Sequoia. All rights reserved
by X-rays from a copper target operated at 30 kV and 10 mA with a dose rate of 600 Rad min-‘. The glow curves were recorded at a heating rate of 60 K min? using apparatus described elsewhere [7]. The sample sizes were typically of 3 mm3. F-bleaching studies were carried out using an Ilford filter (603 nm) and a tungsten filament lamp (60 W). The TL emission spectra were recorded using a Jarrel-Ash monochromator with omnidrive and a R955 photomultiplier tube in the manner described by Halperin et al. [S]. The emissions given here were not corrected for the spectral response of the photomultiplier tube. Results and discussion TL glow and optical absorption Thermoluminescence (TL) spectra of undoped samples exhibit two glow peaks, at 341 and 361 K. On increasing the irradiation time to 2 hours a new peak develops at 378 K [9]. The typical TL behaviour of NO,- doped cesium chloride crystal is illustrated in Fig. 1. In the as-grown samples illuminated with X-rays for 10 minutes, two glow peaks are recorded, a well defined peak at 361 K and a shoulder at 401 K (curve (a), Fig. 1). On increasing the exposure time the first peak soon saturates and declines whereas the second one grows in intensity. On comparing the spectrum of the doped samples with
1
303
323
343
363 TEMPERATURE
383
403
423
44:
CR)
Fig. 1. TL glow curve of CsCI:NOJ crystals. a) X-irradiated for 10 min; b) TL glow curve after bleaching with F-light for 5 min subsequent to X-irradiation for 10 min. Inset (left) shows growth of glow peaks with exposure time; inset (right) shows the fall in intensity of glow peaks with F-bleaching time.
that of undoped material, it is observed that doping decreases the total TL output by about one order. Quenching the samples at a temperature of 473 K for 15 minutes also suppresses the intensity of these peaks about one order. Bleaching studies were undertaken to investigate the centers responsible for TL glow. The respective samples are radiated with X-rays for 10 minutes and then bleached with F-light (603 nm) for 1, 2, 3, . . . minutes and repeated for increasing times of radiation as well as bleaching. Curve (b) in Fig. 1 shows one such typical glow curve. There is an enormous decrease in the intensity after optical bleaching with F-light for the peak at 361 K and eventually it vanishes completely, whereas the 401 K peak is not much affected. Figure 2 gives the optical absorption spectra of doped crystals. A significant absorption band peaking at 267 nm is seen (curve a). On X-irradiation for 10 minutes, this band decreases in intensity and a broad band develops between 220 and 260 nm in addition to a weak F-band at 603 nm (curve b). The optical absorption band appearing at 267 nm in the doped samples only is attributed to the molecular anionic nitrate impurity in the CsCl lattice and the reduction in intensity on irradiation with X-rays is due to the formation of NOS2- from NO,- by capturing electrons that are produced on irradiation, as follows
141:
4
I . 200
300
400
500
WAVELENGTH
600
700
(nm)
Fig. 2. Optical absorption spectra of CsCl:N03 crystals: a) before irradiation; b) after irradiation for 10 min.
NO,- + e- -
NO,‘-
The broad UV band may be a combination of two bands at 233 and 252 nm and are attributed to V, and V, centers produced on irradiation at RT [lo].
301
It may be mentioned that the peak at 401 K is prominently developed only in NO,- doped crystals. The undoped crystal produces two glow peaks at 341 K and 361 K which are attributed to thermal decay of F centers [9]. Comparing these TL spectra with those of undoped samples, the prominent glow peak at 361 K can be completely bleached optically in the doped material, indicating that F-centers are involved in this process. In the doped product the 341 K glow peak (observed in undoped CsCl) appears to be hidden or overshadowed owing to the growth of the 361 K glow peak, and hence is not well resolved. The latter one at 401 K which is not affected much on F-bleaching can probably be attributed to the nitrate impurity. The total decrease in the TL output may be due to the capture of electrons produced during irradiation by the nitrate complex, as explained earlier, and this process annihilates the majority of F-centers responsible for TL glow. Such type of behaviour has already been observed by Kulkarni and Garg [4] and Joshi and Garg [6] for nitrate-doped binary mixtures. They have concluded that doping by nitrate impurity decreases the TL intensity compared to the pure material. The results of nitrate-doped CsCl in this work also reveals the same phenomenon. The glow peaks observed are thermally well separated and analysed by a curve fitting method. Both peaks obey first order kinetics; trap depths are 0.91 and 1.02 eV for the two glow peaks at 361 and 401 K respectively.
4
200
400
300
WAVELENGTH
500
600
(mu)
Fig. 3. TL emission spectrum of CsCl:N03 for 10 min: a) 361 K; b) 401 K.
NO,‘- -
700
after X-irradiation
NO,- + e- + hvE2.64 eV]
The one order increase in the intensity of the 2.64 eV emission in the latter case (which is attributed to NO, impurity) indicates that only V, centers are acting as recombination centers making the emission more intense. More work is probably needed to point out the exact nature of defect centers.
TL emission Emission studies were undertaken in order to identify the recombination centers (Fig. 3). The TL emission spectra obtained at 361 K (curve a) has a recognisable emission band at 407 nm (3.05 eV) with a weak shoulder at 471 nm (2.64 eV). When the crystal is kept at 401 K the emission spectrum (curve b) showed a well resolved emission band peaking at 471 nm (2.64 eV). The 3.05 eV emission is absent in the latter case. Compared with undoped material the emission in the nitrate-doped samples is very weak. The bands at 3.05 eV and 2.64 eV for the 361 K peak correspond to the energy difference between the bottom of the V, and V, centers at 4.92 eV (252 nm) and 5.32 eV (233 nm) which can act as recombination centers for the electrons thermally released from the F-center traps. The 2.64 eV emission of the 401 K peak seems to be the most prominent emission, but is not due to the electrons released from F-centers, since the peak is not much affected on F-bleaching. These electrons may be thermally detrapped from some shallow traps, probably from the nitrate complexes, and recombining with the hole centers preferably with more stable V3 centers giving rise to 2.64 eV emission.
Acknowledgement The authors would like to thank Dr A. Natarajan of the Safety Research Laboratory, Kalpakkam, and Dr S.B.S. Sastry, IIT, Madras, for providing the necessary help regarding optical absorption and TL emission spectra. References 1 B.V.R.
Chowdary and Y. Ravisekar, Solid State Commun., 31 (1979) 453. 2 S. Radhakrishna and K. Hariharan, Phys. Status Solidi, 92 (1979)
293.
3 A.R. Jones, J. Chem. Phys., 35 (1961) 751. 4 S.P. Kulkami and A.N. Garg, ht. J. Radiat. Phys. Chem., 32 (1988) 609. S. Selvasekarapandian and K. Hariharan, Mater. Chem. Phys., 27 (1991) 213. 6 N.G. Joshi and A.N. Garg, Ind. J. Chem., 31 (1992) 915. 7 C.M. Sunta, J. Phys. C, 3 (1970) 1978. 8 A. Halperin, N. Kristianpoller and A. Ben-Zi, Phys. Rev., 116 (1959) 1081. 9 J.K. Radhakrishnan and S. Selvasekarapandian, Ctyst. Rex TechnoZ., 27 (1992) K 95. 10 Rabin and J.H. Schulman, Phys. Rev., 125 (1962) 1584.