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UV excitation spectra of thermoluminescence in quartz
Solid State Communications, Vol. 48, No. 7, pp. 621-623, 1983. Printed in Great Britain.
0038-1098/83 $3.00 + .00 Pergamon Press Ltd.
UV EXCITATION SPECTRA OF THERMOLUMINESCENCE IN QUARTZ N. Kristianpoller Department of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
(Received 19 May 1983 by R. Fieschi) Thermoluminescence was excited at 300 K in natural quartz crystals by monochromatic ultraviolet radiation. The excitation spectra of the main glow peaks were measured in the spectral region 1150-2000 A. A strong excitation maximum appeared for all measured glow peaks in the region of high absorbance on the long wavelength tail of a sharp reflectance peak at 1275 A. Some glow peaks showed excitation maxima also at photon energies smaller than the absorption edge of the material. The dependence of the TL intensities on the dose of the exciting radiation was investigated for various glow peaks and excitation wavelengths. A sublinear dependence was recorded for some peaks by excitation at 1275 A, while the same peaks showed a strictly linear dependence up to relatively high radiation doses, when excited at 1600 A.
1. INTRODUCTION THE THERMOLUMINESCENCE (TL) induced in crystalline quartz and fused silica by higher energy radiations, such as neutrons, X,/~ or 3' rays, has been studied intensively in the past (e.g. [1,2]). However, the excitation of TL by UV has only rarely been investigated in these materials (e.g. [3 ]). In ionic crystals such as alkali halides the TL induced by UV light has been studied more frequently. In some previous studies in this laboratory, the UV excitation spectra and dose dependences of the main glow peaks were measured in various alkali halides and alkaline-earth fluorides and revealed significant information regarding the processes of defect formation in these crystals (e.g. [4]). In the present work these methods were applied to the study of natural quartz. 2. EXPERIMENTAL TECHNIQUES Single crystals of Norwegian quartz were mounted in a windowless vacuum cryostat, attached to a normal incident vacuum UV monochromator (McPherson 225). The samples were irradiated at room temperature with monochromatic UV radiation in the spectral region from about 1150 to 2000 A. As a light source a 1000 watt hydrogen arc was used. The photon flux of the incident radiation was monitored with a sodium salicylate screen. During heating the TL signal was detected, and recorded with a standard TL recording system. A schematic diagram of the experimental set.up is shown in Fig. 1. The same monochromator was also applied for measurements of absorption spectra in the vacuum UV region. For these measurements, however, a special cryostat and
double beam attachment, as previously described by Schlesinger and Szczureck, was used [5]. The slit width of the monochromator was for the absorption measurement 0.2 mm while for the excitation of TL the slits were up to 2 mm. For the absorption measurements 0.2-0.3 mm thick specimens were used. In the longer wavelength region the absorption was measured with a Backman DK spectrophotometer. 3. RESULTS AND DISCUSSION Crystalline a-quark is known to be transparent in a wide spectral range from about 0.46 to 8.3 eV. Below 1500 A the material becomes practically opaque. The absorption was measured at 300 as well as at 80 K before and after the UV radiation. No measurable additional absorption bands could be detected, even after prolonged UV irradiations; however, during heating of the irradiated specimens a notable TL was emitted. Figure 2 shows the glow curves of a quartz crystal, recorded after irradiation at room temperature. Curve (a) was recorded after UV irradiation at a wavelength of 1275 A, and curve (b) at 1650 A. The incident photon flux and irradiation time were the same for both of the measurements. Main glow peaks appeared in both cases at about 380,450 and 500 K. The relative intensities and excitation efficiencies were, however, found to be different for the various glow peaks and excitation wavelengths. As can be seen, that by excitation with 1275 A the 450 K peak is the strongest one, while by excitation at 1650 A the 380 K peak becomes dominant. For comparison the crystals were also irradiated with a 9°Sr beta source of about 10 m Ci. The TL, recorded after beta irradiation at 300 K is shown by curve (c) of Fig. 2; this
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Fig. 3. I - U V excitation spectra of the: (a) 380 K, and (b) 450 K glow peaks, II - (c) Absorption spectrum of our natural unirradiated crystal measured at 80 K; (d) Reflectance spectrum of a natural quartz crystal (after Loh [6]).
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Fig. 2. Thermoluminescence recorded after irradiation at room temperature with: (a) 1275 A; (b) 1650 ,~; (c)/~ rays (curve (b) is enlarged by a factor of 10). glow curve shows the same peaks as after UV irradiation and an additional peak above 600 K. In the beta induced TL the 380 K glow peak is dominant and the higher temperature peaks are relatively weak; similar to the results obtained by UV irradiations at 1650 A. The excitation spectra of the main TL glow peaks were measured independently, with irradiation time and incident photon flux kept constant. In Fig. 3 I the TL excitation spectra of a quartz crystal are shown in the spectral region 1150-1800,8,. The data of curves (a) and (b) are for the 380 and 450 K glow peaks, respectively. Both show a main excitation maximum at 1275 g. The 380 K peak had t w o additional weaker excitation maxima in this spectral region, near 1500 g and between 1650 and 1700 A; a sharp minimum appeared at 1400 A. The dependence of the TL intensities on the dose of the incident UV radiation was also investigated for the main glow peaks and for various excitation energies. In Fig. 4 the dose dependences of the 380 K and 450 K glow peaks are shown on a log-log scale. The data were taken for constant irradiation times by varying the exciting photon flux. Graphs (a) and (b) represent the dose dependences of the 380 K peak when excited by 1600 A
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Fig. 4. Dose dependence of the main glow peaks: (a) 380 K peak excited at RT by 1600 A; (b) 380 K peak excited at RT by 1275 A, and (c) 450K peak excited at RT by 1275 ,~. and 1275 A radiation, respectively; graph (c) represents the dose dependence of the 450 K glow peak, when excited by 1275 A radiation. The dashed lines give a linear dose dependence for comparison. Results show that for excitation at 1600 ,~ the dose dependence of the TL intensities was linear, although the photon flux of the exciting UV radiation was increased by more than three orders of magnitude. For excitation with a wavelength of 1275 A all glow peaks showed a linear dose dependence for relatively low doses only and a sublinear
Vol. 48, No. 7
UV EXCITATION SPECTRA OF THERMOLUMINESCENCE IN QUARTZ
dependence for higher doses, indicating a tendency for fast saturation. The measured excitation spectra were compared to the absorption and reflectance spectra of crystalline quartz in the same spectral region. The excitation maxima, recorded for some glow peaks, are in a spectral region of relatively low absorbance, while the maximum near 1500 is near the absorption edge of crystalline quartz. The main excitation maximum at 1275 A is in a region of high absorbance, where the absorbance of the crystal could not easily be measured. A comparison with reflectance spectra, reveals that this 1275 A TL excitation maximum is located on the long wavelength tail of a sharp exciton-like reflectance peak [6]. This is shown in part II of Fig. 3. The excitation of glow curves in regions of high absorbance has previously been investigated theoretically in this laboratory, and a model for the UV excitation of TL has been proposed [4, 7]. This model takes into account the radiation dose and the absorption coefficient of the crystal and therefore the penetration depth of the incident UV radiation. In a region of high absorbance, the excitation minima are predicted to coincide with absorption maxima. This fits the shape of the excitation spectrum of quartz near 1400 A. As the absorbance and reflectance decrease, a greater part of the UV radiation will penetrate, but will still be absorbed by the crystal and contribute to the processes responsible for the defect formation and TL excitation. While at the peaks of absorption and reflection very few photons will penetrate into the crystal, at zero absorption the penetrating photons are not effective. In spectral regions, where the absorbance and reflectance vary from nearly zero to very high values, as at the long wavelength tail of an exciton band, the photon energies, most effective for the formation of defects and excitation of TL, will apparently be between these two extremes. But in a region of relatively low absorbance, as at weak
623
absorption bands due to pre-existing defects or casual impurities, the excitation maxima may coincide with the maxima of these weak absorption bands. The measured UV excitation spectra of quartz appear to agree with these predictions. This is also supported by tb_e dose dependences, recorded for this crystal. The facts that the same main 380 K glow peak appeared after UV irradiation at 1600 A as after beta irradiation and that the dose dependence remained linear up to high UV doses, indicates that the same processes are reponsible for the TL in both cases, and that these processes take place also in the bulk of the crystal. On the other hand, the excitation at 1275 A was more efficient, but showed a sublinear dose dependence and tendency to saturation for relatively low radiation doses. This is in agreement with the assumption that in a region of high absorbance all effective processes are restricted to a very thin surface layer, which contains apparently a high concentration of trapping centers. The experimental results showed also that the glow peak at 450 K, which was very weak after UV irradiation at 1600 A and even after beta irradiations, became dominant after irradiation at 1275 A. This TL peak is therefore attributed to the thermal release of carriers trapped at sites close to the cyrstal surface. REFERENCES 1. 2. 3. 4. 5.
6. 7.
P.L. Mattern, K. Lengweiler & P.W. Levy, Rad. Effects 26, 237 (1975). A. Halperin, A.A. Braner & J. Schapira, J. Luminesc. 1,385 (1970). W.L. Medlin, J. Chem. Phys. 38, 1132 (1963). N. Kristianpoller & M. Israeli, Phys. Rev. B2, 2175 (1970). M. Schlesinger & T. Szczurek, Rev. ScL Inst. 44, 1720 (1973). E. Loh, Solid State Commun. 2,269 (1964). R. Chen, M. Israeli & N. Kristianpoller, Chem. Phys. Lett. 7, 171 (1970).
0038-1098/83 $3.00 + .00 Pergamon Press Ltd.
UV EXCITATION SPECTRA OF THERMOLUMINESCENCE IN QUARTZ N. Kristianpoller Department of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
(Received 19 May 1983 by R. Fieschi) Thermoluminescence was excited at 300 K in natural quartz crystals by monochromatic ultraviolet radiation. The excitation spectra of the main glow peaks were measured in the spectral region 1150-2000 A. A strong excitation maximum appeared for all measured glow peaks in the region of high absorbance on the long wavelength tail of a sharp reflectance peak at 1275 A. Some glow peaks showed excitation maxima also at photon energies smaller than the absorption edge of the material. The dependence of the TL intensities on the dose of the exciting radiation was investigated for various glow peaks and excitation wavelengths. A sublinear dependence was recorded for some peaks by excitation at 1275 A, while the same peaks showed a strictly linear dependence up to relatively high radiation doses, when excited at 1600 A.
1. INTRODUCTION THE THERMOLUMINESCENCE (TL) induced in crystalline quartz and fused silica by higher energy radiations, such as neutrons, X,/~ or 3' rays, has been studied intensively in the past (e.g. [1,2]). However, the excitation of TL by UV has only rarely been investigated in these materials (e.g. [3 ]). In ionic crystals such as alkali halides the TL induced by UV light has been studied more frequently. In some previous studies in this laboratory, the UV excitation spectra and dose dependences of the main glow peaks were measured in various alkali halides and alkaline-earth fluorides and revealed significant information regarding the processes of defect formation in these crystals (e.g. [4]). In the present work these methods were applied to the study of natural quartz. 2. EXPERIMENTAL TECHNIQUES Single crystals of Norwegian quartz were mounted in a windowless vacuum cryostat, attached to a normal incident vacuum UV monochromator (McPherson 225). The samples were irradiated at room temperature with monochromatic UV radiation in the spectral region from about 1150 to 2000 A. As a light source a 1000 watt hydrogen arc was used. The photon flux of the incident radiation was monitored with a sodium salicylate screen. During heating the TL signal was detected, and recorded with a standard TL recording system. A schematic diagram of the experimental set.up is shown in Fig. 1. The same monochromator was also applied for measurements of absorption spectra in the vacuum UV region. For these measurements, however, a special cryostat and
double beam attachment, as previously described by Schlesinger and Szczureck, was used [5]. The slit width of the monochromator was for the absorption measurement 0.2 mm while for the excitation of TL the slits were up to 2 mm. For the absorption measurements 0.2-0.3 mm thick specimens were used. In the longer wavelength region the absorption was measured with a Backman DK spectrophotometer. 3. RESULTS AND DISCUSSION Crystalline a-quark is known to be transparent in a wide spectral range from about 0.46 to 8.3 eV. Below 1500 A the material becomes practically opaque. The absorption was measured at 300 as well as at 80 K before and after the UV radiation. No measurable additional absorption bands could be detected, even after prolonged UV irradiations; however, during heating of the irradiated specimens a notable TL was emitted. Figure 2 shows the glow curves of a quartz crystal, recorded after irradiation at room temperature. Curve (a) was recorded after UV irradiation at a wavelength of 1275 A, and curve (b) at 1650 A. The incident photon flux and irradiation time were the same for both of the measurements. Main glow peaks appeared in both cases at about 380,450 and 500 K. The relative intensities and excitation efficiencies were, however, found to be different for the various glow peaks and excitation wavelengths. As can be seen, that by excitation with 1275 A the 450 K peak is the strongest one, while by excitation at 1650 A the 380 K peak becomes dominant. For comparison the crystals were also irradiated with a 9°Sr beta source of about 10 m Ci. The TL, recorded after beta irradiation at 300 K is shown by curve (c) of Fig. 2; this
621
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UV EXCITATION SPECTRA OF THERMOLUMINESCENCE IN QUARTZ 5o0 z
n 2o ~
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200~-
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'°°/-7\\ / /1 \~ -
Vol. 48, No. 7
1
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4 ~
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~1 !
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,oo ,~o ,;o '~'""1
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= =°1-
Fig. 1. Experimental set-up for VUV excitation.
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WAVELENGTH
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Fig. 3. I - U V excitation spectra of the: (a) 380 K, and (b) 450 K glow peaks, II - (c) Absorption spectrum of our natural unirradiated crystal measured at 80 K; (d) Reflectance spectrum of a natural quartz crystal (after Loh [6]).
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(K)
Fig. 2. Thermoluminescence recorded after irradiation at room temperature with: (a) 1275 A; (b) 1650 ,~; (c)/~ rays (curve (b) is enlarged by a factor of 10). glow curve shows the same peaks as after UV irradiation and an additional peak above 600 K. In the beta induced TL the 380 K glow peak is dominant and the higher temperature peaks are relatively weak; similar to the results obtained by UV irradiations at 1650 A. The excitation spectra of the main TL glow peaks were measured independently, with irradiation time and incident photon flux kept constant. In Fig. 3 I the TL excitation spectra of a quartz crystal are shown in the spectral region 1150-1800,8,. The data of curves (a) and (b) are for the 380 and 450 K glow peaks, respectively. Both show a main excitation maximum at 1275 g. The 380 K peak had t w o additional weaker excitation maxima in this spectral region, near 1500 g and between 1650 and 1700 A; a sharp minimum appeared at 1400 A. The dependence of the TL intensities on the dose of the incident UV radiation was also investigated for the main glow peaks and for various excitation energies. In Fig. 4 the dose dependences of the 380 K and 450 K glow peaks are shown on a log-log scale. The data were taken for constant irradiation times by varying the exciting photon flux. Graphs (a) and (b) represent the dose dependences of the 380 K peak when excited by 1600 A
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Fig. 4. Dose dependence of the main glow peaks: (a) 380 K peak excited at RT by 1600 A; (b) 380 K peak excited at RT by 1275 A, and (c) 450K peak excited at RT by 1275 ,~. and 1275 A radiation, respectively; graph (c) represents the dose dependence of the 450 K glow peak, when excited by 1275 A radiation. The dashed lines give a linear dose dependence for comparison. Results show that for excitation at 1600 ,~ the dose dependence of the TL intensities was linear, although the photon flux of the exciting UV radiation was increased by more than three orders of magnitude. For excitation with a wavelength of 1275 A all glow peaks showed a linear dose dependence for relatively low doses only and a sublinear
Vol. 48, No. 7
UV EXCITATION SPECTRA OF THERMOLUMINESCENCE IN QUARTZ
dependence for higher doses, indicating a tendency for fast saturation. The measured excitation spectra were compared to the absorption and reflectance spectra of crystalline quartz in the same spectral region. The excitation maxima, recorded for some glow peaks, are in a spectral region of relatively low absorbance, while the maximum near 1500 is near the absorption edge of crystalline quartz. The main excitation maximum at 1275 A is in a region of high absorbance, where the absorbance of the crystal could not easily be measured. A comparison with reflectance spectra, reveals that this 1275 A TL excitation maximum is located on the long wavelength tail of a sharp exciton-like reflectance peak [6]. This is shown in part II of Fig. 3. The excitation of glow curves in regions of high absorbance has previously been investigated theoretically in this laboratory, and a model for the UV excitation of TL has been proposed [4, 7]. This model takes into account the radiation dose and the absorption coefficient of the crystal and therefore the penetration depth of the incident UV radiation. In a region of high absorbance, the excitation minima are predicted to coincide with absorption maxima. This fits the shape of the excitation spectrum of quartz near 1400 A. As the absorbance and reflectance decrease, a greater part of the UV radiation will penetrate, but will still be absorbed by the crystal and contribute to the processes responsible for the defect formation and TL excitation. While at the peaks of absorption and reflection very few photons will penetrate into the crystal, at zero absorption the penetrating photons are not effective. In spectral regions, where the absorbance and reflectance vary from nearly zero to very high values, as at the long wavelength tail of an exciton band, the photon energies, most effective for the formation of defects and excitation of TL, will apparently be between these two extremes. But in a region of relatively low absorbance, as at weak
623
absorption bands due to pre-existing defects or casual impurities, the excitation maxima may coincide with the maxima of these weak absorption bands. The measured UV excitation spectra of quartz appear to agree with these predictions. This is also supported by tb_e dose dependences, recorded for this crystal. The facts that the same main 380 K glow peak appeared after UV irradiation at 1600 A as after beta irradiation and that the dose dependence remained linear up to high UV doses, indicates that the same processes are reponsible for the TL in both cases, and that these processes take place also in the bulk of the crystal. On the other hand, the excitation at 1275 A was more efficient, but showed a sublinear dose dependence and tendency to saturation for relatively low radiation doses. This is in agreement with the assumption that in a region of high absorbance all effective processes are restricted to a very thin surface layer, which contains apparently a high concentration of trapping centers. The experimental results showed also that the glow peak at 450 K, which was very weak after UV irradiation at 1600 A and even after beta irradiations, became dominant after irradiation at 1275 A. This TL peak is therefore attributed to the thermal release of carriers trapped at sites close to the cyrstal surface. REFERENCES 1. 2. 3. 4. 5.
6. 7.
P.L. Mattern, K. Lengweiler & P.W. Levy, Rad. Effects 26, 237 (1975). A. Halperin, A.A. Braner & J. Schapira, J. Luminesc. 1,385 (1970). W.L. Medlin, J. Chem. Phys. 38, 1132 (1963). N. Kristianpoller & M. Israeli, Phys. Rev. B2, 2175 (1970). M. Schlesinger & T. Szczurek, Rev. ScL Inst. 44, 1720 (1973). E. Loh, Solid State Commun. 2,269 (1964). R. Chen, M. Israeli & N. Kristianpoller, Chem. Phys. Lett. 7, 171 (1970).