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Defect states induced by UV–laser irradiation in scintillating glasses
Nuclear Instruments and Methods in Physics Research B 191 (2002) 366–370 www.elsevier.com/locate/nimb
Defect states induced by UV–laser irradiation in scintillating glasses G.P. Pazzi a,*, P. Fabeni a, C. Susini a, M. Nikl b, E. Mihokova b, N. Solovieva b, K. Nitsch b, M. Martini c, A. Vedda c, S. Baccaro d, A. Cecilia d, V. Babin e a
IROE – CNR N. Carrara, Via Panciatichi 64, 50127 Firenze, Italy Institute of Physics AS CR, Cukrovarnicka 10, 162 53 Prague, Czech Republic INFM and Department of Materials Science, University of Milano-Bicocca, Via Cozzi 53, 20125 Milano, Italy d ENEA, TEC/IRR, Via Anguillarese 301, 00060 S. Maria di Galeria, Roma, Italy e Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia b
c
Abstract Emission intensity variation of the leading 542 nm emission line of Tb3þ centre in Na–Gd phosphate glasses under 308 nm excimer laser line irradiation was studied. Furthermore, induced optical absorption measurements after 308 nm laser, X-ray and c-ray irradiations were performed. Thermoluminescence (TSL) above room temperature was also studied after 308 nm and X-ray irradiations at 295 K. The observed phenomenology evidences the presence of traps, which compete for carrier capture from the Gd3þ energy guiding sublattice with Tb3þ emission centres. The radiation induced optical absorption features are explained as due to both Tb4þ and host lattice-related hole colour centres. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Tb3þ -doped phosphate glasses; Thermoluminescence; Radiation induced absorption; Colour centres
1. Introduction Scintillators based on glassy matrices are interesting materials due to their low production cost, easy shaping of elements and possibility to incorporate activator ions at high concentrations; however their light yield is usually lower with respect to crystals [1]. In fact, the lack of long range order and the presence of many point defects give rise to trapping sites responsible for non-radiative
*
Corresponding author. Tel.: +39-055-42351; fax: +39-055410893/4235257. E-mail address: speclab@iroe.fi.cnr.it (G.P. Pazzi).
recombinations. Recently a new concept of energy transfer towards emission centres in glasses was introduced. Namely, an energy guiding sublattice based on Gd3þ ions has been realised in a phosphate glass by mixing about 30 mol% of GdPO3 into the glass matrix. Energy migration through this sublattice results in more efficient radioluminescence of Ce3þ or Tb3þ emission centres with respect to Gd-free phosphate glass [2,3]. An interesting phenomenon was noticed in Tb3þ -doped phosphate glass: during irradiation by the intense excimer laser pulses at 308 nm (in resonance with 8 S ! 6 P5=2 absorption transition of Gd3þ ) a noticeable intensity increase of the 542 nm emission of Tb3þ was detected [4]. Such an effect indicates
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 5 3 8 - 4
G.P. Pazzi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 366–370
possible energy exchange between the Gd3þ energy guiding sublattice and phosphate glass matrix, which is an unwanted phenomenon, if practical applications are to be considered. In this study, we present a systematic characterisation of Tb3þ -doped phosphate-based scintillating glasses, which show the mentioned variations of the luminescence efficiency, and the creation of induced optical absorption bands under the intense 308 nm excimer laser irradiation. Induced optical absorption is studied also after X-ray or c-ray irradiations and thermoluminescence (TSL) measurements above room temperature (RT) have been performed as well in order to put in evidence localised trap levels.
2. Experimental Samples of Tb-doped phosphate glass were prepared using NaPO3 , GdPO4 and TbPO4 of 99.9% purity as starting materials (for preparation details see [2,3]). In the following, the samples are labelled as Nax Gdy Tbz , where x, y, z denote the molar percentage of the corresponding starting materials in the melt. The compositions of the sample here studied are the following: Na77Gd20Tb3, Na67Gd30Tb3, Na65Gd30Tb5, Na60Gd30Tb10, Na70Gd30Tb0, Na97Gd0Tb3. Polished plates of 1 mm thickness were used for luminescence, TSL and UV/X-ray induced absorption measurements, while for c-ray (60 Co irradiation, for the details see [5]) induced absorption (IA) experiment bulk polished samples of 10 10 20 mm3 were prepared. Excited luminescence was measured by using an excimer laser (XeCl mixture, frep ¼ 10 Hz, Epulse ðmaxÞ ¼ 8 mJ, kem ¼ 308 nm, FWHM ¼ 10 ns). The samples were placed in a closed cycle cryogenerator (10–300 K). The laser beam was focused on the sample surface in a pencil-shape beam (power density of the order of 105 W/mm2 ) and the emission was observed at right angle by a spectrometer coupled to an optical multichannel analyser or to a photomultiplier connected to a digital oscilloscope (for details see also [2]). IA spectra were reconstructed from transmission (T) (or absorption (A)) spectra measured mostly by a
367
Perkin Elmer k19 transmission spectrometer and obtained prior (index 0) and after (index irr) irradiation by X- or c-rays by using the formula IA ¼ ð1=dÞ lnðT0 =Tirr Þ
ð1Þ
or IA ¼ ð1=dÞðAirr A0 Þ;
ð2Þ
respectively, where d stands for sample thickness; for details see also [5]. For UV laser light a different approach was used, as explained in Section 3. TSL measurements were performed after X-ray (Machlett OEG 50 X-ray tube operated at 30 kV) or 308 nm laser excitations, from RT up to 250 °C, using a heating rate of 1 °C/s. Spectrally unresolved TSL emitted light was detected in photon counting mode by means of EMI 9635 QB photomultiplier tube.
3. Experimental results and discussion In Fig. 1 the dependence of the leading Tb3þ 542 nm emission line intensity versus the laser
Fig. 1. Dependence of the Tb3þ 542 nm emission line intensity versus the irradiation time under excitation by the 308 nm XeCl excimer laser line. Samples displayed: (1) Na77Gd20Tb3, (2) Na67Gd30Tb3, (3) Na65Gd30Tb5, (4) Na60Gd30Tb10, (5) Na70Gd30Tb0 (312 nm emission of Gd3þ ), (6) Na97Gd0Tb3.
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G.P. Pazzi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 366–370
Fig. 2. IA after 120 min irradiation by 308 nm laser line: (1) Na60Gd30Tb10, (2) Na70Gd30Tb0 and (3) Na65Gd30Tb5 samples. In the inset the absorption transitions related to Tb3þ (5 D2;3 , 5 Lx and 5 D4 ) and Gd3þ (6 Px , 6 Ix and 6 Dx ) centres are displayed.
Fig. 3. IA spectra of (1) Na70Gd30Tb0 and (2) Na60Gd30Tb10 samples after c-ray irradiation (60 Co isotope, dose 10 Gy in air). Solid line is the single gaussian fit of sample (2): E ¼ 3:357 eV (369 nm), FWHM ¼ 1:22 eV.
irradiation time is given for all the samples. In the case of Na70Gd30Tb0, the 312 nm emission line of the Gd3þ centre was followed. As a rule, in all the samples these emission intensities were getting higher with respect to the initial values after sufficiently long irradiation time, even if the relative enhancement is clearly sample dependent. For such 308 nm irradiated samples, transmission was measured in the irradiated pencil-shaped area (a narrow spot on the surface was produced) and well outside it and the IA coefficient was evaluated using Eq. (1); in Fig. 2 the spectra of selected samples are reported. Even if the obtained spectra were broad and possibly affected by surface damage effects, characteristic IA can be clearly noticed around 240–260 nm (5 eV) and between 300 and 400 nm (3–4 eV), this last is more evident for the sample Na65Gd30Tb5. The inset of Fig. 2 shows the absorption transitions related to Tb3þ (5 D2;3 , 5 Lx and 5 D4 ) and Gd3þ (6 Px , 6 Ix and 6 Dx ) centres, which were present in all transmission spectra of the samples but which almost annul each other in the evaluation of the IA. The broad underlying absorption round 250 nm and sharp absorption increase below 230 nm are most probably related to 4f ! 5d transition of Tb3þ ion [6]. In
Fig. 3 the c-ray IA characteristics are shown for Na60Gd30Tb10 and Na70Gd30Tb0 samples, evaluated using Eq. (2). It is worth noting that qualitatively very similar spectra (with much higher amplitude of the IA) were also obtained after a 1000 Gy, 30 keV X-ray irradiation dose. In all Tb3þ -containing samples, a characteristic IA band at around 360 nm and less structured and broad IA at the high energy side were observed. The rising edge of the 360 nm band can be very well fitted by a gaussian function peaking at 3.357 eV (369 nm) and with a halfwidth of about 1.22 eV. The absorption band at 364 nm (3.405 eV) with a halfwidth of 1.32 eV was attributed to the Tb4þ colour centre in Tb-doped metaphosphate glasses (based on AðPO3 Þ2 , A ¼ Ca, Sr, Ba or Na2 ) [7], so that the same assignment can be made in the present study. In the Tb-free sample, other IA structures appeared at lower energy, which were extended down to 600 nm. Taking into account the reported results concerning P-doped silica glass [8], such features could be ascribed to phosphorus group-related hole traps of the host glass matrix; similar intrinsic defects are probably responsible also for the IA observed at about 250 nm (5 eV) [8].
G.P. Pazzi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 366–370
Fig. 4. TSL glow curves after irradiation at RT of Na60Gd30Tb10 and Na67Gd30Tb3 samples: curves (a) refer to 120 min irradiation by 308 nm laser line, after which the samples were kept at RT and the measurements were performed 5 days after laser irradiation; curves (b) refer to irradiation by Xrays (dose ¼ 1000 Gy) and the measurements were performed immediately at the end of the irradiation.
TSL of selected samples was measured after 308 nm laser irradiation, and after X-ray irradiation (Fig. 4). In the former case, after irradiation the samples were kept at RT and the measurements were performed 5 days later (due to unavoidable sample transport between two laboratories); in the latter case the measurements were performed immediately after irradiation. Glow curves of similar shape were obtained in the high temperature region, where a broad peak extending from approximately 100 °C up to temperatures greater than the upper limit of the measurement was observed. On the other hand, lower temperature traps detected after X-irradiation were not evidenced in the laser induced glow curves, probably due to spontaneous recovery enabled by longer time delay between irradiation and TSL measurement. The overall observed phenomenology can be qualitatively explained under the assumption that, possibly by a two-photon absorption mechanism, 308 nm laser irradiation induces band to band transitions in the considered glasses leading to the generation of free carriers. So, defect-related traps
369
were populated during the process of laser irradiation, and became progressively less efficient competitors with Tb3þ emission centres in carrier capture. This could justify the monotonic increase of the Tb3þ luminescence after laser irradiation displayed in Fig. 1. The existence of such traps was evidenced by TSL measurements. At the same time, the laser production of free carriers, later undergoing trapping at different defect sites, was also demonstrated by the creation of laser induced optical absorption features, which showed similarities with those induced by c- or X-rays. However, the direct and quantitative comparison between the laser and c- (X-) IA spectra is not straightforward: while X- and c-irradiation provided very coherent results on the shape of the IA spectra, and the IA band of Tb4þ could also be clearly identified, the 308 nm irradiation resulted in much less structured IA spectra, even if IA was created in the same spectral region (1.5–5 eV). Such ‘‘spectra smoothing’’ can be at least partly due to possible surface damage of the samples by laser pulses as in some cases even visible ‘‘fingerprint’’ of the laser spot on the sample surface was noticed after irradiation. The inspection under optical microscope indicated possible darkening of the surface layer up to 0.1 mm thickness. However, such differences might also indicate that the induced colour centres are not completely identical to those created under c (X)-ray irradiation. Acknowledgements The support of Italian INFN Newlumen project and of NATO SfP project no. 973510 Scintillators is gratefully acknowledged. V.B. would like to thank Italian CNR for a CNR–NATO Guest Fellowship.
References [1] E. Auffray et al., CC collaboration, Nucl. Instr. and Meth. A 380 (1996) 524. [2] S. Baccaro et al., J. Lumin. 87–89 (2000) 673. [3] M. Nikl et al., Appl. Phys. Lett. 77 (2000) 2159.
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G.P. Pazzi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 366–370
[4] P. Fabeni, G.P. Pazzi, M. Martini, A. Vedda, M. Nikl, K. Nitsch, S. Baccaro, Rad. Meas. 33 (2001) 721 (Proceedings of LUMDETR 2000, August 2000, Riga, Latvia). [5] S. Baccaro, A. Cecilia, A. Cemmi, G. Chen, E. Mihokova, M. Nikl, IEEE Trans. Nucl. Sci. 48 (3) (2001) 360.
[6] Ch.K. Jorgensen, R. Pappalardo, E. Rittershaus, Z. Naturforschg. A 20 (1965) 54. [7] H. Ebendorff-Heidepriem, D. Ehrt, Opt. Mater. 15 (2000) 7. [8] D.L. Griscom, E. Friebele, J. Long, J.W. Fleming, J. Appl. Phys. 54 (1983) 3743.
Defect states induced by UV–laser irradiation in scintillating glasses G.P. Pazzi a,*, P. Fabeni a, C. Susini a, M. Nikl b, E. Mihokova b, N. Solovieva b, K. Nitsch b, M. Martini c, A. Vedda c, S. Baccaro d, A. Cecilia d, V. Babin e a
IROE – CNR N. Carrara, Via Panciatichi 64, 50127 Firenze, Italy Institute of Physics AS CR, Cukrovarnicka 10, 162 53 Prague, Czech Republic INFM and Department of Materials Science, University of Milano-Bicocca, Via Cozzi 53, 20125 Milano, Italy d ENEA, TEC/IRR, Via Anguillarese 301, 00060 S. Maria di Galeria, Roma, Italy e Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia b
c
Abstract Emission intensity variation of the leading 542 nm emission line of Tb3þ centre in Na–Gd phosphate glasses under 308 nm excimer laser line irradiation was studied. Furthermore, induced optical absorption measurements after 308 nm laser, X-ray and c-ray irradiations were performed. Thermoluminescence (TSL) above room temperature was also studied after 308 nm and X-ray irradiations at 295 K. The observed phenomenology evidences the presence of traps, which compete for carrier capture from the Gd3þ energy guiding sublattice with Tb3þ emission centres. The radiation induced optical absorption features are explained as due to both Tb4þ and host lattice-related hole colour centres. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Tb3þ -doped phosphate glasses; Thermoluminescence; Radiation induced absorption; Colour centres
1. Introduction Scintillators based on glassy matrices are interesting materials due to their low production cost, easy shaping of elements and possibility to incorporate activator ions at high concentrations; however their light yield is usually lower with respect to crystals [1]. In fact, the lack of long range order and the presence of many point defects give rise to trapping sites responsible for non-radiative
*
Corresponding author. Tel.: +39-055-42351; fax: +39-055410893/4235257. E-mail address: speclab@iroe.fi.cnr.it (G.P. Pazzi).
recombinations. Recently a new concept of energy transfer towards emission centres in glasses was introduced. Namely, an energy guiding sublattice based on Gd3þ ions has been realised in a phosphate glass by mixing about 30 mol% of GdPO3 into the glass matrix. Energy migration through this sublattice results in more efficient radioluminescence of Ce3þ or Tb3þ emission centres with respect to Gd-free phosphate glass [2,3]. An interesting phenomenon was noticed in Tb3þ -doped phosphate glass: during irradiation by the intense excimer laser pulses at 308 nm (in resonance with 8 S ! 6 P5=2 absorption transition of Gd3þ ) a noticeable intensity increase of the 542 nm emission of Tb3þ was detected [4]. Such an effect indicates
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 5 3 8 - 4
G.P. Pazzi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 366–370
possible energy exchange between the Gd3þ energy guiding sublattice and phosphate glass matrix, which is an unwanted phenomenon, if practical applications are to be considered. In this study, we present a systematic characterisation of Tb3þ -doped phosphate-based scintillating glasses, which show the mentioned variations of the luminescence efficiency, and the creation of induced optical absorption bands under the intense 308 nm excimer laser irradiation. Induced optical absorption is studied also after X-ray or c-ray irradiations and thermoluminescence (TSL) measurements above room temperature (RT) have been performed as well in order to put in evidence localised trap levels.
2. Experimental Samples of Tb-doped phosphate glass were prepared using NaPO3 , GdPO4 and TbPO4 of 99.9% purity as starting materials (for preparation details see [2,3]). In the following, the samples are labelled as Nax Gdy Tbz , where x, y, z denote the molar percentage of the corresponding starting materials in the melt. The compositions of the sample here studied are the following: Na77Gd20Tb3, Na67Gd30Tb3, Na65Gd30Tb5, Na60Gd30Tb10, Na70Gd30Tb0, Na97Gd0Tb3. Polished plates of 1 mm thickness were used for luminescence, TSL and UV/X-ray induced absorption measurements, while for c-ray (60 Co irradiation, for the details see [5]) induced absorption (IA) experiment bulk polished samples of 10 10 20 mm3 were prepared. Excited luminescence was measured by using an excimer laser (XeCl mixture, frep ¼ 10 Hz, Epulse ðmaxÞ ¼ 8 mJ, kem ¼ 308 nm, FWHM ¼ 10 ns). The samples were placed in a closed cycle cryogenerator (10–300 K). The laser beam was focused on the sample surface in a pencil-shape beam (power density of the order of 105 W/mm2 ) and the emission was observed at right angle by a spectrometer coupled to an optical multichannel analyser or to a photomultiplier connected to a digital oscilloscope (for details see also [2]). IA spectra were reconstructed from transmission (T) (or absorption (A)) spectra measured mostly by a
367
Perkin Elmer k19 transmission spectrometer and obtained prior (index 0) and after (index irr) irradiation by X- or c-rays by using the formula IA ¼ ð1=dÞ lnðT0 =Tirr Þ
ð1Þ
or IA ¼ ð1=dÞðAirr A0 Þ;
ð2Þ
respectively, where d stands for sample thickness; for details see also [5]. For UV laser light a different approach was used, as explained in Section 3. TSL measurements were performed after X-ray (Machlett OEG 50 X-ray tube operated at 30 kV) or 308 nm laser excitations, from RT up to 250 °C, using a heating rate of 1 °C/s. Spectrally unresolved TSL emitted light was detected in photon counting mode by means of EMI 9635 QB photomultiplier tube.
3. Experimental results and discussion In Fig. 1 the dependence of the leading Tb3þ 542 nm emission line intensity versus the laser
Fig. 1. Dependence of the Tb3þ 542 nm emission line intensity versus the irradiation time under excitation by the 308 nm XeCl excimer laser line. Samples displayed: (1) Na77Gd20Tb3, (2) Na67Gd30Tb3, (3) Na65Gd30Tb5, (4) Na60Gd30Tb10, (5) Na70Gd30Tb0 (312 nm emission of Gd3þ ), (6) Na97Gd0Tb3.
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G.P. Pazzi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 366–370
Fig. 2. IA after 120 min irradiation by 308 nm laser line: (1) Na60Gd30Tb10, (2) Na70Gd30Tb0 and (3) Na65Gd30Tb5 samples. In the inset the absorption transitions related to Tb3þ (5 D2;3 , 5 Lx and 5 D4 ) and Gd3þ (6 Px , 6 Ix and 6 Dx ) centres are displayed.
Fig. 3. IA spectra of (1) Na70Gd30Tb0 and (2) Na60Gd30Tb10 samples after c-ray irradiation (60 Co isotope, dose 10 Gy in air). Solid line is the single gaussian fit of sample (2): E ¼ 3:357 eV (369 nm), FWHM ¼ 1:22 eV.
irradiation time is given for all the samples. In the case of Na70Gd30Tb0, the 312 nm emission line of the Gd3þ centre was followed. As a rule, in all the samples these emission intensities were getting higher with respect to the initial values after sufficiently long irradiation time, even if the relative enhancement is clearly sample dependent. For such 308 nm irradiated samples, transmission was measured in the irradiated pencil-shaped area (a narrow spot on the surface was produced) and well outside it and the IA coefficient was evaluated using Eq. (1); in Fig. 2 the spectra of selected samples are reported. Even if the obtained spectra were broad and possibly affected by surface damage effects, characteristic IA can be clearly noticed around 240–260 nm (5 eV) and between 300 and 400 nm (3–4 eV), this last is more evident for the sample Na65Gd30Tb5. The inset of Fig. 2 shows the absorption transitions related to Tb3þ (5 D2;3 , 5 Lx and 5 D4 ) and Gd3þ (6 Px , 6 Ix and 6 Dx ) centres, which were present in all transmission spectra of the samples but which almost annul each other in the evaluation of the IA. The broad underlying absorption round 250 nm and sharp absorption increase below 230 nm are most probably related to 4f ! 5d transition of Tb3þ ion [6]. In
Fig. 3 the c-ray IA characteristics are shown for Na60Gd30Tb10 and Na70Gd30Tb0 samples, evaluated using Eq. (2). It is worth noting that qualitatively very similar spectra (with much higher amplitude of the IA) were also obtained after a 1000 Gy, 30 keV X-ray irradiation dose. In all Tb3þ -containing samples, a characteristic IA band at around 360 nm and less structured and broad IA at the high energy side were observed. The rising edge of the 360 nm band can be very well fitted by a gaussian function peaking at 3.357 eV (369 nm) and with a halfwidth of about 1.22 eV. The absorption band at 364 nm (3.405 eV) with a halfwidth of 1.32 eV was attributed to the Tb4þ colour centre in Tb-doped metaphosphate glasses (based on AðPO3 Þ2 , A ¼ Ca, Sr, Ba or Na2 ) [7], so that the same assignment can be made in the present study. In the Tb-free sample, other IA structures appeared at lower energy, which were extended down to 600 nm. Taking into account the reported results concerning P-doped silica glass [8], such features could be ascribed to phosphorus group-related hole traps of the host glass matrix; similar intrinsic defects are probably responsible also for the IA observed at about 250 nm (5 eV) [8].
G.P. Pazzi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 366–370
Fig. 4. TSL glow curves after irradiation at RT of Na60Gd30Tb10 and Na67Gd30Tb3 samples: curves (a) refer to 120 min irradiation by 308 nm laser line, after which the samples were kept at RT and the measurements were performed 5 days after laser irradiation; curves (b) refer to irradiation by Xrays (dose ¼ 1000 Gy) and the measurements were performed immediately at the end of the irradiation.
TSL of selected samples was measured after 308 nm laser irradiation, and after X-ray irradiation (Fig. 4). In the former case, after irradiation the samples were kept at RT and the measurements were performed 5 days later (due to unavoidable sample transport between two laboratories); in the latter case the measurements were performed immediately after irradiation. Glow curves of similar shape were obtained in the high temperature region, where a broad peak extending from approximately 100 °C up to temperatures greater than the upper limit of the measurement was observed. On the other hand, lower temperature traps detected after X-irradiation were not evidenced in the laser induced glow curves, probably due to spontaneous recovery enabled by longer time delay between irradiation and TSL measurement. The overall observed phenomenology can be qualitatively explained under the assumption that, possibly by a two-photon absorption mechanism, 308 nm laser irradiation induces band to band transitions in the considered glasses leading to the generation of free carriers. So, defect-related traps
369
were populated during the process of laser irradiation, and became progressively less efficient competitors with Tb3þ emission centres in carrier capture. This could justify the monotonic increase of the Tb3þ luminescence after laser irradiation displayed in Fig. 1. The existence of such traps was evidenced by TSL measurements. At the same time, the laser production of free carriers, later undergoing trapping at different defect sites, was also demonstrated by the creation of laser induced optical absorption features, which showed similarities with those induced by c- or X-rays. However, the direct and quantitative comparison between the laser and c- (X-) IA spectra is not straightforward: while X- and c-irradiation provided very coherent results on the shape of the IA spectra, and the IA band of Tb4þ could also be clearly identified, the 308 nm irradiation resulted in much less structured IA spectra, even if IA was created in the same spectral region (1.5–5 eV). Such ‘‘spectra smoothing’’ can be at least partly due to possible surface damage of the samples by laser pulses as in some cases even visible ‘‘fingerprint’’ of the laser spot on the sample surface was noticed after irradiation. The inspection under optical microscope indicated possible darkening of the surface layer up to 0.1 mm thickness. However, such differences might also indicate that the induced colour centres are not completely identical to those created under c (X)-ray irradiation. Acknowledgements The support of Italian INFN Newlumen project and of NATO SfP project no. 973510 Scintillators is gratefully acknowledged. V.B. would like to thank Italian CNR for a CNR–NATO Guest Fellowship.
References [1] E. Auffray et al., CC collaboration, Nucl. Instr. and Meth. A 380 (1996) 524. [2] S. Baccaro et al., J. Lumin. 87–89 (2000) 673. [3] M. Nikl et al., Appl. Phys. Lett. 77 (2000) 2159.
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G.P. Pazzi et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 366–370
[4] P. Fabeni, G.P. Pazzi, M. Martini, A. Vedda, M. Nikl, K. Nitsch, S. Baccaro, Rad. Meas. 33 (2001) 721 (Proceedings of LUMDETR 2000, August 2000, Riga, Latvia). [5] S. Baccaro, A. Cecilia, A. Cemmi, G. Chen, E. Mihokova, M. Nikl, IEEE Trans. Nucl. Sci. 48 (3) (2001) 360.
[6] Ch.K. Jorgensen, R. Pappalardo, E. Rittershaus, Z. Naturforschg. A 20 (1965) 54. [7] H. Ebendorff-Heidepriem, D. Ehrt, Opt. Mater. 15 (2000) 7. [8] D.L. Griscom, E. Friebele, J. Long, J.W. Fleming, J. Appl. Phys. 54 (1983) 3743.