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Radioluminescence and thermoluminescence properties of X-ray-irradiated pure CsI
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Journal of Luminescence 128 (2008) 1191–1196 www.elsevier.com/locate/jlumin
Radioluminescence and thermoluminescence properties of X-ray-irradiated pure CsI Z. Wua, B. Yangb,c,, P.D. Townsendc a
The Test and Analysing Center, Beijing Normal University, Beijing 100875, China b Department of Physics, Beijing Normal University, Beijing 100875, China c Science and Technology, University of Sussex, Brighton BN1 9QH, UK
Received 18 March 2007; received in revised form 28 October 2007; accepted 29 November 2007 Available online 5 December 2007
Abstract The 25–280 K radioluminescence (RL) and thermoluminescence (TL) spectra in a nominally pure CsI have been studied. Strong emissions at 250–400 nm consist of two bands at 305 and 340 nm associated with the VK+e and H+F-type self-trapped excitons (STEs), respectively. There are some weak extrinsic signals in RL. The temperature dependence and the response of the two main bands to the X-ray tube voltage have been studied. It has been found that the dominant TL signals are associated with contamination of the sample, though the trapping levels are still characterised by the host lattice. A temperature shift between the intrinsic and extrinsic TL peaks has been observed in the thermal emissions at 55 and 82 K. r 2007 Elsevier B.V. All rights reserved. PACS: 78.60.K; 61.80 Keywords: Caesium iodide; Radioluminescence; Thermoluminescence; Self-trapped exciton, Contamination
1. Introduction The luminescence properties of CsI crystals have been extensively investigated, because under excitation they emit 300 and/or 340 nm luminescence over a wide temperature range, at least from liquid helium temperature (LHT) to room temperature (RT). These signals have short decay times and then the CsI sample has possible applications as a scintillator with fast timing characteristics. The CsI crystal can also be used as a radiation detector by doping with activators, such as thallium, lead, etc., to obtain emissions with longer wavelengths. Experimental data for the luminescence in pure and doped CsI excited by UV light, g-ray, neutrons and X-ray sources have been reported [1–6] with studies on the defect creation and emission Corresponding author at: Department of Physics, Beijing Normal University, Beijing 100875, China. Tel.: +86 10 62 208 417; fax: +86 10 62 200 141. E-mail address: [email protected] (B. Yang).
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.11.091
mechanisms. Theoretical works in this area are also in progress [2,7]. It has been suggested by Iida et al. in 1980 [7] that both the 300 and 340 nm emissions in CsI are associated with the radiative decay of the self-trapped excitons (STEs). In the later reports from Nishimura et al. [2], the 300 and 340 nm emissions were suggested to be related with the decay of the on-centre-type STE (VK+e) and off-centretype STE (H+F), respectively. Thermoluminescence (TL) in the pure or doped CsI has also been reported [1,6,8,9]. The 300 and 340 nm emissions were not dominant in the TL signals, indeed they were not even observed in some cases. The origin of the TL signals and the emission mechanisms, especially for those in the undoped samples, are still unclear. The 25–280 K radioluminescence (RL) and TL spectra in the nominally pure CsI were studied in this work. The RL signals were recorded during a controlled cooling. Therefore, no TL-type features were mixed in with the temperature-dependent RL data.
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2. Experiments Since the literature suggests sensitivity to impurities, material was obtained from more than one supplier. The single-crystal CsI samples used in this work were provided by the Harshaw Company, USA and the Zhongjing Company, Beijing, China. The samples were cut into slices with a thickness of 1 mm for the RL and TL measurements. The spectra were recorded on the Sussex high-sensitivity wavelength multiplexed TL system, which has two detectors covering the wavelength ranges 200–480 and 480–800 nm. (Therefore, there may be a trace in the figures of the matching point of the two detectors.) Signals with different wavelengths were recorded simultaneously with time steps of 3 s. Spectra were corrected for the spectral sensitivity of the recording system. The resolution used in this work was 5 nm. The X-ray irradiation was performed in vacuum at low temperature with an in situ Philips tungsten X-ray tube. The voltage and current used in this work were 11–40 kV and 4–15 mA, respectively. The rate of both heating and cooling was 6 K/min. The integration time for data collection was unchanged when different voltages were used.
about 140 K, and then decreased with the decrease in temperature. Starting from 180 K, a very fast increase of the 340 nm band was detected during cooling. This band predominated at the lowest temperatures. As an example of the data, an isometric plot of the RL signals recorded for 11 kV, 6 mA X-ray irradiation conditions of a sample (from Harshaw) are given in Fig. 1. Some weak luminescence signals at 400–650 nm can also be seen in this figure. The change of the intensity of these signals vs. temperature is the same as that of the 305 nm band. The 275, 145 and 30 K spectral slices from Fig. 1 are shown in Fig. 2. It is seen that almost only the 305 nm band exists in the 250–400 nm emissions at 275 K, whereas the 305 nm emissions have nearly vanished at 30 K, and in the 145 K spectrum, there are two overlapping emission bands. Some 400–650 nm emissions are also apparent in the spectra. The recorded intensities of both the 305 and 340 nm bands increased with the increase of X-ray tube voltage when the temperature is constant. However, the trend of the intensity of the 305 and 340 nm emission bands vs.
25000
3.1. Radioluminescence The 280–25 K RL spectra of the undoped CsI with various X-ray tube voltages (11–40 kV) and a fixed current (6 mA) have been recorded during cooling. In agreement with the previous reports [1,2], strong luminescence signals at 250–400 nm containing two emission bands at about 305 and 340 nm, respectively, have been observed during the X-ray irradiation. When the temperature was higher than 180 K, the 305 nm band was absolutely dominant. The intensity of the 305 nm band increased when the sample was cooled down from 280 K and reached the maximum at
Intensity (arb. unit)
3. Results
275K (x5) 145K 30K
20000 15000 10000 5000 0 200
300
400
500
600
700
Wavelength (nm)
Fig. 2. The 275, 145 and 30 K temperature slices of Fig. 1.
Fig. 1. The radioluminescence spectra of CsI recorded during cooling under the 11 kV, 6 mA X-ray radiation. The sample was produced by the Harshaw Company.
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different manufacturers and the 145 K RL spectra recorded during the 15 kV, 6 mA X-ray radiation of two samples, from Harshaw and Beijing, respectively, are presented in Fig. 4. The differences in the 400–700 nm signals in these two samples are shown in the figure. To some extent such differences can also be observed in samples from the same producer. During recording at constant temperatures for times of up to 40 s the RL intensity was nominally constant; thus ionisation damage to the crystals is of minor importance for this study.
temperature is independent of the tube voltage and the source of the sample. Surprisingly, it was found that the relative intensity of the 305 and 340 nm components in the 250–400 nm emissions changed with the tube voltage. The 145 K emission spectra recorded with different voltages are shown in Fig. 3. It is seen that the 340 nm component increases faster than the 305 nm signal when the tube voltage is increased. It seems probable that at long wavelengths impurity effects play a role, as it was also found that for the 400–650 nm emissions, the intensity and the band shape vary from sample to sample, even if the data were recorded under the same conditions. Impurity effects are probably sensitive to the source of the materials, and thus one can emphasise the variations by contrasting material from
Intensity (normalized)
1.00 0.80
3.2. Thermoluminescence The 25–280 K TL spectra of the X-ray irradiated undoped CsI look different from sample to sample, although some common behaviours could be found. The TL spectra of the samples from Harshaw and Beijing are given in Fig. 5a and b, respectively. Some TL spectra at different temperatures are given in Fig. 6 for samples from both suppliers. It is evident in Figs. 5 and 6 that (i) the strongest and the second strongest thermal peaks are recorded at about 82 and 55 K, respectively, with some weak emissions at about 130, 175 and 200 K; (ii) the emission bands at 340 nm are observed in the 55 and 82 K peaks in both the samples and disappeared at higher temperatures; and (iii) apart from the 340 nm band, other broad TL bands at 380–650 nm are also detected, but the wavelength position and the band shape are obviously different in the two samples, i.e. in Fig. 5a the 410 nm band in the 55 and 82 K peaks is much stronger than the 450–650 nm ones, but in Fig. 5b the latter is dominant.
11kV 15kV 25kV 30kV
0.60 0.40 0.20 -0.00 250
350
300
1193
400
Wavelength (nm)
Fig. 3. The 145 K RL spectra recorded during the X-ray irradiation with different tube voltages. The intensities are normalised for comparison.
3e+005
Intensity (arb. unit)
2e+005
2e+005
1e+005
5e+004 sample from Harshaw sample from Beijing
0 200
300
400
500
600
700
Wavelength (nm) Fig. 4. The 145 K RL spectra of CsI samples from different sources. Data were recorded under the X-ray radiation with the voltage of 15 kV and the current of 6 mA.
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4. Discussion
Fig. 5. The TL spectra of the nominally pure CsI crystal from Harshaw (a) and Beijing (b). Samples were irradiated by 4 Gy X-ray at 25 K.
sample from Harshaw sample from Beijing 0.9
T = 55K
0.6 0.3
Intensity (normalized)
0.0
0.9
T = 82K
0.6 0.3 0.0
0.9
T = 175K
0.6 0.3 0.0 300
400
500
600
700
Wavelength (nm) Fig. 6. Comparison of the normalised TL spectra in the samples from Harshaw and Beijing: (a)55 K; (b)82 K; and (c)175 K.
The RL data were recorded during the X-ray irradiation. Point defects such as F, H, VK centres could be produced within 10 13–10 11 s when the sample was irradiated and then the STEs would be formed in a very short time (51 ns). There are two types of STEs in the pure CsI: the on-centre-type STE, i.e. VK+e and the off-centre one, i.e. H+F. The impurity perturbed VK+e and H+F can also be formed in the doped samples. As described above, the 300 nm emissions in CsI, i.e. the 305 nm band observed in this work, and the 340 nm one have been considered to be the result of the decay of VK+e and H+F-type STEs, respectively. The possible conversion from H+F to VK+e when the temperature was raised from a low temperature to RT has also been suggested [2]. The RL data were recorded during cooling in this work to avoid such thermal effects and any additions from TL. It is suggested from the result presented in Fig. 1 that there is a distribution of the creation rate of the two different type STEs in the irradiated sample. This distribution is a function of temperature, because the creation rate of the defects such as VK, H and F centres and the mobility of these defects are all dependent on the radiation temperature. For example, both VK and H centres can be produced in CsI when the sample is exposed to X-ray at very low temperatures, but the VK centres are more stable than the H centres in this temperature range [7]. As a result, the 305 nm emissions are negligible at these low temperatures. The effects are similar to those observed in the RL measurements of doped LiF [10]. The distribution of the creation rate of the two different type STEs is based on the result shown in Fig. 3, assumed also to be associated with the X-ray tube voltage. It is shown in Fig. 3 that the relative intensity between the 340 and the 305 nm bands increases with the increase of the voltage. It might be the result that the production rates of the H centre increases faster than that of the VK centre. As a result of increasing the tube voltage, the average (and maximum) energy of the generated X-ray photons is increased. The energy-dependent data of the emission spectra shown in Fig. 3 are interesting, since the changes in the spectra do not show a monotonic change with voltage but separate into low- and high-voltage spectral patterns. In this example, changes could be influenced by the maximum X-ray energy. Since there is a clear shift between spectra for low- and high-energy X-rays, this may indicate that a critical threshold energy is involved which alters the balance between the different ionisation or damage processes. Inner shell absorption processes that influence the ionisation rate have been considered in the past but here the energy range is between 15 and 25 kV. If this is a resonance with a K edge, then the only options are for Al, Si, P or S with edges ranging from 14.9 to 23 kV. These are not elements of the target although Al is present in the construction of the target housing. Further discussion requires more data.
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The 400–650 nm emissions in the RL spectra of CsI are suggested to be related to the trace impurities, as the intensity and band structure of those emissions vary from sample to sample and the wavelengths are in agreement with those of the luminescence associated with the cation impurity in the doped samples, e.g. the broad emission band at about 401 and 550 nm observed in CsI:Tl [6], 481 and 532 nm emissions in CsI:Pb [6] and the 420 nm ones in CsI:Na [11]. Some cation impurities in CsI are called ‘activators’. Luminescence can be induced by only a few of such activators. The contamination of the samples from different sources or from the same source, but produced in different time, could be different. It is suggested for the same reasons that the 380–650 nm TL feature in the two samples, as shown in Figs. 5 and 6, are also concerned with the trace impurities. In contrast to the RL case, the TL emissions from the impurityassociated defects are much stronger than those from the decay of un-perturbed STEs. It has also been found in our previous work, e.g. in the rare-earth-doped LaF3 [12], that the intrinsic TL signals can be prohibited by the impurities. With the study on the 77–270 K TL glow curves of the CsI doped with various activators, it has been reported by Martinez et al. [13] that the trap depth is a characteristic of the host lattice, regardless of the type of the impurities. Sidler et al. [9] reported that the 60 K thermal peak in the doped CsI was associated with the 01-diffusion of the I2 centres and the peak at about 90 K corresponds to the 901 rotations of that centre. These assumptions have been supported by Babin et al. [6]. In this work, the main TL signals were detected in the wavelength range of the impurity-associated emissions and the contaminations of the two samples from Harshaw and Beijing, respectively, apparently differ. So, different TL features were observed in the two samples as presented in Fig. 5. Even then, the peak temperatures are nearly the same in these two samples, which is in agreement with the experimental results in the previous reports [6,13]. The 25–120 K glow curves of the 340, 430 and 550 nm emissions extracted from Fig. 5b are given in Fig. 7. Note
340nm 430nm (50%) 550nm
Intensity (arb. unit)
5500 4400 3300
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that the temperature shift between the 340 nm STE emission and the 380–650 nm impurity-associated emissions can be found in both the thermal peaks at about 55 and 82 K. The peak temperature of the STE emission is about 6 K higher than that of the impurity-associated emission in the so-called 55 K peak, but about 5 K lower in the so-called 82 K one. Similar results have been obtained in the glow curves from Fig. 5a. It has been reported [14] that in CsI:Tl and CsI:Pb excitation in the exciton absorption band could induce the luminescence from the impurity ions. Martinez et al. [8] observed that in the CsI crystal doped with traces of thallium, the intensity of the fluorescence from STEs decreased but that from the thallium ions increased at 70–120 K. It was therefore suggested by the authors that there could be a charge transfer and/or energy transfer between the intrinsic and the extrinsic defects when the sample was excited. It is found in Fig. 7 that from 52 to 58 K, the STE emissions increase while the impurityassociated emissions decrease, but from 79 to 84 K, just being opposite. Therefore, the energy and/or charge transfer in those two temperature ranges might be essentially different. Detailed studies about that will be given in the following report. The temperature shift between the intrinsic TL peak and the extrinsic ones was also found in the rare-earth-doped LaF3 crystal [12], but the origin of that is still unclear. 5. Conclusions Emissions from the decay of the STEs and those associated with the trace impurities have been detected in both RL and TL measurements in the nominally pure CsI crystal at 25–280 K. The strong STE emissions including two bands at 305 and 340 nm associated with the VK+e and H+F-type STEs, respectively, are observed with some weak extrinsic signals in RL. The temperature dependence and the response of these two bands to the X-ray tube voltage have been studied. The impurity-associated signals are dominated in TL. But the trap depth has not been found to be effected by the contamination. The temperature shift of the intrinsic and extrinsic TL peaks has been noticed. References
2200 1100 0 20
30
40
50
60
70
80
90
100 110 120
Temperature (K)
Fig. 7. The 340, 430 and 550 nm TL glow curves in a sample from Beijing.
[1] M. Okada, M. Nakagawa, K. Atobe, N. Itatani, K. Ozawa, Phys. Stat. Sol. (a) 167 (1998) 253. [2] H. Nishimura, M. Sakata, T. Tsujimoto, M. Nakayama, Phys. Rev. B. 51 (1994) 2167. [3] A.N. Panova, B.V. Grinev, F.F. Lavrent’ev, L.M. Soifer, K.V. Shakhova, N.N. Kosinov, A.I. Mitichkin, S.P. Korsunova, J. Appl. Spectrosc. 71 (2004) 546. [4] V. Babin, K. Kalder, A. Krasnikov, S. Zazubovich, J. Lumin. 96 (2002) 75. [5] V. Babin, A. Krasnikov, H. Wieczorek, S. Zazubovich, Nucl. Instrum. Methods—Phys. Res. A 486 (2002) 486.
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[6] V. Babin, P. Fabeni, K. Kalder, M. Nikl, G.P. Pazzi, S. Zazabovich, Radiat. Meas. 29 (1998) 333. [7] T. Iida, Y. Nakaoka, J.P. von de Weid, M.A. Aegerter, J. Phys. C: Solid State Phys. 13 (1980) 983. [8] P. Martinez, E.F. Senftle, M. Page, Phys. Rev. Lett. 12 (1964) 369. [9] T. Sidler, J.P. Pellaux, A. Nouaijhat, M.A. Aegerter, Solid State Commun. 13 (1973) 479. [10] K. Kurt, V.K. Mathur, S.W.S. McKeever, P.D. Townsend, L. Valberg, in: Proceedings of the 14th International Conference on
[11] [12] [13] [14]
Solid State Dosimetry, Yale University, Connecticut, USA, 2004 June 27–July 2. C.R. Fu, L.F. Chen, K.S. Song, J. Phys.: Condens. Matter 11 (1999) 5517. B. Yang, P.D. Townsend, A.P. Rowlands, Phys. Rev. B 57 (1998) 178. P. Martinez, F.E. Senftle, M. Page, Phys. Rev. Lett. 12 (1964) 369. S. Zazubovich, R. Aceves, M.F. Barboza, P. Karner, T. Pazzi, G.P. Perez, R. Salas, J. Phys.: Condens. Matter 9 (1997) 7249.
Journal of Luminescence 128 (2008) 1191–1196 www.elsevier.com/locate/jlumin
Radioluminescence and thermoluminescence properties of X-ray-irradiated pure CsI Z. Wua, B. Yangb,c,, P.D. Townsendc a
The Test and Analysing Center, Beijing Normal University, Beijing 100875, China b Department of Physics, Beijing Normal University, Beijing 100875, China c Science and Technology, University of Sussex, Brighton BN1 9QH, UK
Received 18 March 2007; received in revised form 28 October 2007; accepted 29 November 2007 Available online 5 December 2007
Abstract The 25–280 K radioluminescence (RL) and thermoluminescence (TL) spectra in a nominally pure CsI have been studied. Strong emissions at 250–400 nm consist of two bands at 305 and 340 nm associated with the VK+e and H+F-type self-trapped excitons (STEs), respectively. There are some weak extrinsic signals in RL. The temperature dependence and the response of the two main bands to the X-ray tube voltage have been studied. It has been found that the dominant TL signals are associated with contamination of the sample, though the trapping levels are still characterised by the host lattice. A temperature shift between the intrinsic and extrinsic TL peaks has been observed in the thermal emissions at 55 and 82 K. r 2007 Elsevier B.V. All rights reserved. PACS: 78.60.K; 61.80 Keywords: Caesium iodide; Radioluminescence; Thermoluminescence; Self-trapped exciton, Contamination
1. Introduction The luminescence properties of CsI crystals have been extensively investigated, because under excitation they emit 300 and/or 340 nm luminescence over a wide temperature range, at least from liquid helium temperature (LHT) to room temperature (RT). These signals have short decay times and then the CsI sample has possible applications as a scintillator with fast timing characteristics. The CsI crystal can also be used as a radiation detector by doping with activators, such as thallium, lead, etc., to obtain emissions with longer wavelengths. Experimental data for the luminescence in pure and doped CsI excited by UV light, g-ray, neutrons and X-ray sources have been reported [1–6] with studies on the defect creation and emission Corresponding author at: Department of Physics, Beijing Normal University, Beijing 100875, China. Tel.: +86 10 62 208 417; fax: +86 10 62 200 141. E-mail address: [email protected] (B. Yang).
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.11.091
mechanisms. Theoretical works in this area are also in progress [2,7]. It has been suggested by Iida et al. in 1980 [7] that both the 300 and 340 nm emissions in CsI are associated with the radiative decay of the self-trapped excitons (STEs). In the later reports from Nishimura et al. [2], the 300 and 340 nm emissions were suggested to be related with the decay of the on-centre-type STE (VK+e) and off-centretype STE (H+F), respectively. Thermoluminescence (TL) in the pure or doped CsI has also been reported [1,6,8,9]. The 300 and 340 nm emissions were not dominant in the TL signals, indeed they were not even observed in some cases. The origin of the TL signals and the emission mechanisms, especially for those in the undoped samples, are still unclear. The 25–280 K radioluminescence (RL) and TL spectra in the nominally pure CsI were studied in this work. The RL signals were recorded during a controlled cooling. Therefore, no TL-type features were mixed in with the temperature-dependent RL data.
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2. Experiments Since the literature suggests sensitivity to impurities, material was obtained from more than one supplier. The single-crystal CsI samples used in this work were provided by the Harshaw Company, USA and the Zhongjing Company, Beijing, China. The samples were cut into slices with a thickness of 1 mm for the RL and TL measurements. The spectra were recorded on the Sussex high-sensitivity wavelength multiplexed TL system, which has two detectors covering the wavelength ranges 200–480 and 480–800 nm. (Therefore, there may be a trace in the figures of the matching point of the two detectors.) Signals with different wavelengths were recorded simultaneously with time steps of 3 s. Spectra were corrected for the spectral sensitivity of the recording system. The resolution used in this work was 5 nm. The X-ray irradiation was performed in vacuum at low temperature with an in situ Philips tungsten X-ray tube. The voltage and current used in this work were 11–40 kV and 4–15 mA, respectively. The rate of both heating and cooling was 6 K/min. The integration time for data collection was unchanged when different voltages were used.
about 140 K, and then decreased with the decrease in temperature. Starting from 180 K, a very fast increase of the 340 nm band was detected during cooling. This band predominated at the lowest temperatures. As an example of the data, an isometric plot of the RL signals recorded for 11 kV, 6 mA X-ray irradiation conditions of a sample (from Harshaw) are given in Fig. 1. Some weak luminescence signals at 400–650 nm can also be seen in this figure. The change of the intensity of these signals vs. temperature is the same as that of the 305 nm band. The 275, 145 and 30 K spectral slices from Fig. 1 are shown in Fig. 2. It is seen that almost only the 305 nm band exists in the 250–400 nm emissions at 275 K, whereas the 305 nm emissions have nearly vanished at 30 K, and in the 145 K spectrum, there are two overlapping emission bands. Some 400–650 nm emissions are also apparent in the spectra. The recorded intensities of both the 305 and 340 nm bands increased with the increase of X-ray tube voltage when the temperature is constant. However, the trend of the intensity of the 305 and 340 nm emission bands vs.
25000
3.1. Radioluminescence The 280–25 K RL spectra of the undoped CsI with various X-ray tube voltages (11–40 kV) and a fixed current (6 mA) have been recorded during cooling. In agreement with the previous reports [1,2], strong luminescence signals at 250–400 nm containing two emission bands at about 305 and 340 nm, respectively, have been observed during the X-ray irradiation. When the temperature was higher than 180 K, the 305 nm band was absolutely dominant. The intensity of the 305 nm band increased when the sample was cooled down from 280 K and reached the maximum at
Intensity (arb. unit)
3. Results
275K (x5) 145K 30K
20000 15000 10000 5000 0 200
300
400
500
600
700
Wavelength (nm)
Fig. 2. The 275, 145 and 30 K temperature slices of Fig. 1.
Fig. 1. The radioluminescence spectra of CsI recorded during cooling under the 11 kV, 6 mA X-ray radiation. The sample was produced by the Harshaw Company.
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different manufacturers and the 145 K RL spectra recorded during the 15 kV, 6 mA X-ray radiation of two samples, from Harshaw and Beijing, respectively, are presented in Fig. 4. The differences in the 400–700 nm signals in these two samples are shown in the figure. To some extent such differences can also be observed in samples from the same producer. During recording at constant temperatures for times of up to 40 s the RL intensity was nominally constant; thus ionisation damage to the crystals is of minor importance for this study.
temperature is independent of the tube voltage and the source of the sample. Surprisingly, it was found that the relative intensity of the 305 and 340 nm components in the 250–400 nm emissions changed with the tube voltage. The 145 K emission spectra recorded with different voltages are shown in Fig. 3. It is seen that the 340 nm component increases faster than the 305 nm signal when the tube voltage is increased. It seems probable that at long wavelengths impurity effects play a role, as it was also found that for the 400–650 nm emissions, the intensity and the band shape vary from sample to sample, even if the data were recorded under the same conditions. Impurity effects are probably sensitive to the source of the materials, and thus one can emphasise the variations by contrasting material from
Intensity (normalized)
1.00 0.80
3.2. Thermoluminescence The 25–280 K TL spectra of the X-ray irradiated undoped CsI look different from sample to sample, although some common behaviours could be found. The TL spectra of the samples from Harshaw and Beijing are given in Fig. 5a and b, respectively. Some TL spectra at different temperatures are given in Fig. 6 for samples from both suppliers. It is evident in Figs. 5 and 6 that (i) the strongest and the second strongest thermal peaks are recorded at about 82 and 55 K, respectively, with some weak emissions at about 130, 175 and 200 K; (ii) the emission bands at 340 nm are observed in the 55 and 82 K peaks in both the samples and disappeared at higher temperatures; and (iii) apart from the 340 nm band, other broad TL bands at 380–650 nm are also detected, but the wavelength position and the band shape are obviously different in the two samples, i.e. in Fig. 5a the 410 nm band in the 55 and 82 K peaks is much stronger than the 450–650 nm ones, but in Fig. 5b the latter is dominant.
11kV 15kV 25kV 30kV
0.60 0.40 0.20 -0.00 250
350
300
1193
400
Wavelength (nm)
Fig. 3. The 145 K RL spectra recorded during the X-ray irradiation with different tube voltages. The intensities are normalised for comparison.
3e+005
Intensity (arb. unit)
2e+005
2e+005
1e+005
5e+004 sample from Harshaw sample from Beijing
0 200
300
400
500
600
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Wavelength (nm) Fig. 4. The 145 K RL spectra of CsI samples from different sources. Data were recorded under the X-ray radiation with the voltage of 15 kV and the current of 6 mA.
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4. Discussion
Fig. 5. The TL spectra of the nominally pure CsI crystal from Harshaw (a) and Beijing (b). Samples were irradiated by 4 Gy X-ray at 25 K.
sample from Harshaw sample from Beijing 0.9
T = 55K
0.6 0.3
Intensity (normalized)
0.0
0.9
T = 82K
0.6 0.3 0.0
0.9
T = 175K
0.6 0.3 0.0 300
400
500
600
700
Wavelength (nm) Fig. 6. Comparison of the normalised TL spectra in the samples from Harshaw and Beijing: (a)55 K; (b)82 K; and (c)175 K.
The RL data were recorded during the X-ray irradiation. Point defects such as F, H, VK centres could be produced within 10 13–10 11 s when the sample was irradiated and then the STEs would be formed in a very short time (51 ns). There are two types of STEs in the pure CsI: the on-centre-type STE, i.e. VK+e and the off-centre one, i.e. H+F. The impurity perturbed VK+e and H+F can also be formed in the doped samples. As described above, the 300 nm emissions in CsI, i.e. the 305 nm band observed in this work, and the 340 nm one have been considered to be the result of the decay of VK+e and H+F-type STEs, respectively. The possible conversion from H+F to VK+e when the temperature was raised from a low temperature to RT has also been suggested [2]. The RL data were recorded during cooling in this work to avoid such thermal effects and any additions from TL. It is suggested from the result presented in Fig. 1 that there is a distribution of the creation rate of the two different type STEs in the irradiated sample. This distribution is a function of temperature, because the creation rate of the defects such as VK, H and F centres and the mobility of these defects are all dependent on the radiation temperature. For example, both VK and H centres can be produced in CsI when the sample is exposed to X-ray at very low temperatures, but the VK centres are more stable than the H centres in this temperature range [7]. As a result, the 305 nm emissions are negligible at these low temperatures. The effects are similar to those observed in the RL measurements of doped LiF [10]. The distribution of the creation rate of the two different type STEs is based on the result shown in Fig. 3, assumed also to be associated with the X-ray tube voltage. It is shown in Fig. 3 that the relative intensity between the 340 and the 305 nm bands increases with the increase of the voltage. It might be the result that the production rates of the H centre increases faster than that of the VK centre. As a result of increasing the tube voltage, the average (and maximum) energy of the generated X-ray photons is increased. The energy-dependent data of the emission spectra shown in Fig. 3 are interesting, since the changes in the spectra do not show a monotonic change with voltage but separate into low- and high-voltage spectral patterns. In this example, changes could be influenced by the maximum X-ray energy. Since there is a clear shift between spectra for low- and high-energy X-rays, this may indicate that a critical threshold energy is involved which alters the balance between the different ionisation or damage processes. Inner shell absorption processes that influence the ionisation rate have been considered in the past but here the energy range is between 15 and 25 kV. If this is a resonance with a K edge, then the only options are for Al, Si, P or S with edges ranging from 14.9 to 23 kV. These are not elements of the target although Al is present in the construction of the target housing. Further discussion requires more data.
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The 400–650 nm emissions in the RL spectra of CsI are suggested to be related to the trace impurities, as the intensity and band structure of those emissions vary from sample to sample and the wavelengths are in agreement with those of the luminescence associated with the cation impurity in the doped samples, e.g. the broad emission band at about 401 and 550 nm observed in CsI:Tl [6], 481 and 532 nm emissions in CsI:Pb [6] and the 420 nm ones in CsI:Na [11]. Some cation impurities in CsI are called ‘activators’. Luminescence can be induced by only a few of such activators. The contamination of the samples from different sources or from the same source, but produced in different time, could be different. It is suggested for the same reasons that the 380–650 nm TL feature in the two samples, as shown in Figs. 5 and 6, are also concerned with the trace impurities. In contrast to the RL case, the TL emissions from the impurityassociated defects are much stronger than those from the decay of un-perturbed STEs. It has also been found in our previous work, e.g. in the rare-earth-doped LaF3 [12], that the intrinsic TL signals can be prohibited by the impurities. With the study on the 77–270 K TL glow curves of the CsI doped with various activators, it has been reported by Martinez et al. [13] that the trap depth is a characteristic of the host lattice, regardless of the type of the impurities. Sidler et al. [9] reported that the 60 K thermal peak in the doped CsI was associated with the 01-diffusion of the I2 centres and the peak at about 90 K corresponds to the 901 rotations of that centre. These assumptions have been supported by Babin et al. [6]. In this work, the main TL signals were detected in the wavelength range of the impurity-associated emissions and the contaminations of the two samples from Harshaw and Beijing, respectively, apparently differ. So, different TL features were observed in the two samples as presented in Fig. 5. Even then, the peak temperatures are nearly the same in these two samples, which is in agreement with the experimental results in the previous reports [6,13]. The 25–120 K glow curves of the 340, 430 and 550 nm emissions extracted from Fig. 5b are given in Fig. 7. Note
340nm 430nm (50%) 550nm
Intensity (arb. unit)
5500 4400 3300
1195
that the temperature shift between the 340 nm STE emission and the 380–650 nm impurity-associated emissions can be found in both the thermal peaks at about 55 and 82 K. The peak temperature of the STE emission is about 6 K higher than that of the impurity-associated emission in the so-called 55 K peak, but about 5 K lower in the so-called 82 K one. Similar results have been obtained in the glow curves from Fig. 5a. It has been reported [14] that in CsI:Tl and CsI:Pb excitation in the exciton absorption band could induce the luminescence from the impurity ions. Martinez et al. [8] observed that in the CsI crystal doped with traces of thallium, the intensity of the fluorescence from STEs decreased but that from the thallium ions increased at 70–120 K. It was therefore suggested by the authors that there could be a charge transfer and/or energy transfer between the intrinsic and the extrinsic defects when the sample was excited. It is found in Fig. 7 that from 52 to 58 K, the STE emissions increase while the impurityassociated emissions decrease, but from 79 to 84 K, just being opposite. Therefore, the energy and/or charge transfer in those two temperature ranges might be essentially different. Detailed studies about that will be given in the following report. The temperature shift between the intrinsic TL peak and the extrinsic ones was also found in the rare-earth-doped LaF3 crystal [12], but the origin of that is still unclear. 5. Conclusions Emissions from the decay of the STEs and those associated with the trace impurities have been detected in both RL and TL measurements in the nominally pure CsI crystal at 25–280 K. The strong STE emissions including two bands at 305 and 340 nm associated with the VK+e and H+F-type STEs, respectively, are observed with some weak extrinsic signals in RL. The temperature dependence and the response of these two bands to the X-ray tube voltage have been studied. The impurity-associated signals are dominated in TL. But the trap depth has not been found to be effected by the contamination. The temperature shift of the intrinsic and extrinsic TL peaks has been noticed. References
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60
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Fig. 7. The 340, 430 and 550 nm TL glow curves in a sample from Beijing.
[1] M. Okada, M. Nakagawa, K. Atobe, N. Itatani, K. Ozawa, Phys. Stat. Sol. (a) 167 (1998) 253. [2] H. Nishimura, M. Sakata, T. Tsujimoto, M. Nakayama, Phys. Rev. B. 51 (1994) 2167. [3] A.N. Panova, B.V. Grinev, F.F. Lavrent’ev, L.M. Soifer, K.V. Shakhova, N.N. Kosinov, A.I. Mitichkin, S.P. Korsunova, J. Appl. Spectrosc. 71 (2004) 546. [4] V. Babin, K. Kalder, A. Krasnikov, S. Zazubovich, J. Lumin. 96 (2002) 75. [5] V. Babin, A. Krasnikov, H. Wieczorek, S. Zazubovich, Nucl. Instrum. Methods—Phys. Res. A 486 (2002) 486.
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