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Thermoluminescence in calcium fluoride doped with terbium and gadolinium ions
Radiation Measurements 43 (2008) 455 – 458 www.elsevier.com/locate/radmeas
Thermoluminescence in calcium fluoride doped with terbium and gadolinium ions Y. Fukuda ∗ Department of Urban Environment, Osaka Sangyo University, Japan
Abstract Thermoluminescence (TL) in sintered CaF2 co-doped with Tb4 O7 and Gd2 O3 has been investigated for UV and X-ray irradiation. Three TL glow peaks for the CaF2 :Tb, Gd sample are observed at about 353, 435 K (437 K: for X-ray) and 523 K by heating the sample at a rate of 20 K min−1 after UV or X-ray irradiation at room temperature. It was found that the second peak (435 K) intensity became stronger than that of the sample singly doped with only terbium or gadolinium ions. From the TL emission spectrum and the excitation and emission spectra of photo luminescence for the CaF2 doped activators, it is concluded that the TL of Tb3+ ions is sensitized by the existence of Gd3+ ions. The 435 K TL peak may be also suitable for using it as a dosimeter. © 2007 Elsevier Ltd. All rights reserved. Keywords: Sintered CaF2 ; Tb, Gd; Thermoluminescence; TL emission spectrum
1. Introduction In a previous paper, we reported the possibility of estimating UV radiation dose using CaF2 :Tb as a thermoluminescence (TL) dosimeter (Ohtaki et al., 1994). It was found that the TL intensity from CaF2 doped with Tb4 O7 was the highest among the samples doped with various lanthanide oxides, and three TL peaks appeared at about 353, 378 and 458 K by heating the sample at a rate of 20 K min−1 after UV or X-ray irradiation at room temperature (Fukuda et al., 1996; Awata et al., 1999; Fukuda and Tomita, 1999). To improve TL sensitivity, the effect of co-doping activators has been investigated (Fukuda et al., 1994; Fukuda, 2002; Menon et al., 2005; Fukuda and Niwa, 2006; Salah and Sahare, 2006). It has been found that the second peak (378 K) intensity of the sample co-doped with Tb4 O7 and Sm2 O3 became stronger than those doped with only terbium or samarium ions, and the TL peaks of 353 K and 458 K were not observed. According to our preceding investigation, the intensity of 378 K glow peak of CaF2 triply doped with Tb4 O7 , Sm2 O3 ∗ Tel.: +81 72 875 3001.
E-mail address: [email protected]. 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.034
and Gd2 O3 was 5 times higher than that of sample co-doped with Tb4 O7 and Sm2 O3 , and the TL emission from the excited state of Tb3+ ions was observed. Therefore, it is expected that the TL of Tb3+ ions is intensified by the existence of not only Sm3+ ions but Gd3+ ions. However, in order to use for dosimetry, it is desirable to intensify the peak at high temperature region by the addition of activators, if possible, because the 378 K peak is unstable for storage at room temperature after irradiation. The purpose of the present paper is to obtain a prominent TL peak at high temperature region, and to study the effect of co-doping with Tb4 O7 and Gd2 O3 . The influence of sintering temperature is also reported. 2. Materials and methods The samples were prepared by solid state reaction. Starting materials, CaF2 (purity 99.99%), Tb4 O7 (purity 99.99%) and Gd2 O3 (purity 99.99%) were mixed in the desired ratio and pressed into pellets of about 6mm in diameter and 0.7 mm in thickness. Pellets of CaF2 : Tb, Gd were heated at 1373 K in a platinum crucible in air for 2 h followed by cooling to room temperature.
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Y. Fukuda / Radiation Measurements 43 (2008) 455 – 458
Samples were exposed to UV light ( = 253.7 nm, L937 low pressure Hg lamp. Hamamatsu Photonics Co. Ltd.) or X-rays (Cu K radiation; 35 KV, 20 mA). The TL was measured using a home –made apparatus with a photomultiplier (HTV-R212, Hamamatsu Photonics Co. Ltd.) and the heating rate for TL was 20 K min−1 . The excitation and the fluorescence spectra of the sample were measured using a spectrofluoro photometer (Shimadzu RF-1500). TL emission spectra were measured using a SMA (spectrometric multichannel analyzer) system (Princeton Instruments Inc.) which consisted of the personal computer, detector and its controller. 3. Results and discussion Fig. 1 shows the glow curves for CaF2 phosphor exposed UV radiation for 1 min (∼0.4 J m−2 ), where curve (a) is for the sample doped with Tb4 O7 (0.06 wt%) and Sm2 O3 (0.03 wt%), and curves (b) and (c) with 0.06 wt% Tb4 O7 . Curves (a) and (b) show the results for the sample sintered at 1473 K, while curve (c) shows the result for the sample sintered at 1373 K. In the TL glow curve of sintered CaF2 singly doped with Tb4 O7 , three TL peaks usually appear at about 353, 383 and 458 K [curves (b) and (c)], on the other hand, one TL glow peak is observed at about 378 K in the sample doped with Tb4 O7 and Sm2 O3 . The intensity of the 458 K glow peak of CaF2 :Tb sample sintered at 1473 K [curve (b)] is 1.9 times higher than that of the sample sintered at 1373 K [curve (c)]. As shown in Fig. 1, the intensity of the 378 K glow peak of the co-doped sample [curve (a)] is 12.9 times higher than that of the 458 K glow peak of the sample singly doped with Tb4 O7 [curve (b)]. However, the 378 K glow peak of CaF2 :Tb,Sm is unstable for storage at room temperature after irradiation. And pellet type sample sintered at 1473 K breaks in many cases, while using repeatedly. On the other hand, the sample sintered at 1373 K has
Fig. 1. The glow curves of TL for CaF2 phosphor exposed to UV ray radiation for 1 min (∼0.4 J m−2 ). Curve (a) is for CaF2 :Tb (0.06 wt%), Sm (0.03 wt%) sintered at 1473 K; curve (b) is for CaF2 :Tb (0.06 wt%) sintered at 1473 K; curve (c) is for CaF2 :Tb (0.06 wt%) sintered at 1373 K. ×0.14 denotes magnified by ×0.14 times.
Fig. 2. The glow curves of TL for CaF2 phosphor exposed to UV ray radiation for 1 min (∼0.4 J m−2 ). Curves (a), (b) and (c) show the result for the samples sintered at 1373 K. Curve (a) is for CaF2 :Tb (0.06 wt%); curve (b) is for CaF2 :Tb (0.06 wt%), Gd (0.05 wt%); curve (c) is for CaF2 :Gd (0.05 wt%).
Fig. 3. The glow curve for CaF2 :Tb, Gd exposed to X-ray radiation for 1 min (0.15 C kg−1 ).
few such things. Therefore, since the sample sintered at 1373 K as TLD was suitable, it investigated about the sample of CaF2 doped with Tb4 O7 and Gd2 O3 sintered at 1373 K this time. Fig. 2 shows the TL glow curves for CaF2 phosphor exposed to UV ray radiation for 1 min (∼0.4 J m−2 ). Curves (a), (b) and (c) show the result for the samples sintered at 1373 K. Where curve (a) is for the sample singly doped with 0.06 wt% Tb4 O7 , curve (b) co-doped with Tb4 O7 (0.06 wt%) and Gd2 O3 (0.05 wt%), and curve (c) with 0.05 wt% Gd2 O3 . It is seen from curves (a) and (c) that the TL glow peaks appear at about 353, 383 and 452 K. The intensity of the TL response against UV irradiation dose is increased by the co-doping of terbium and gadolinium ions, the TL peaks being found at about 358, 435 and 523 K [curve (b)]. As shown in Fig. 2, the intensity of 435 K glow peak is about 2 times higher than that of 452 K glow peak of the sample singly doped with Tb4 O7 , and about 2.5 times higher than that of 353 K grow peak of the sample singly doped with Gd2 O3 . It is noticed that by the existence of Gd2 O3 ,
Y. Fukuda / Radiation Measurements 43 (2008) 455 – 458
Fig. 4. Relationship between TL intensity and UV ray dose. The TL intensity has been estimated from the maximum intensity of the main peak.
457
(0.15 C kg−1 ). Two main TL glow peaks are observed at about 362 and 437 K. To examine the TL responses against the irradiation of UV and X-ray irradiation, the second glow peak intensity of CaF2 :Tb, Gd (435 K for UV; 437 K for X-ray) were plotted against the UV and X-ray dose in Figs. 4 and 5, respectively. As shown in Fig. 4, the TL response of 435 K peak increases up to 120 s (∼0.8 J m−2 ) with second power of the dose of 253.7 nm UV radiation, and attains saturation, while it shows a linear relation against the X-ray dose (Fig. 5). The supralinearity in CaF2 :Tb, Gd irradiated with UV ray may also be explained by multiple-stage excitation (Harperin and Chen, 1996; Chen and Fogel, 1993). The excitation and emission spectra of photoluminescence from CaF2 :Tb, Gd were measured on condition that the range of excitation and emission were 10 nm (Fig. 6). The excitation spectrum (a) was detected at 380 nm and the emission spectrum (b) was measured with the excitation of 285 nm. The excitation peak appeared at 285 nm, and emission peaks 380, 415, 435, 485, 540 and 590 nm were assigned to 5 D3 → 7 F6 , 5 D3 → 7 F , 5 D → 7 F , 5 D → 7 F , 5 D → 7 F and 5 D → 7 F 3 4 3 2,1 4 3 4 5 5 of the inner transition of Tb3+ ions, respectively. The temperature resolved TL spectrum for CaF2 :Tb, Gd is shown in Fig. 7. After X-ray irradiation (0.30 C kg−1 ), the TL was measured at 437 K. Six emission peaks appeared at 380, 415, 435, 485, 540 and 590 nm. It is noticed from Figs. 6 and 7 that by the existence of Gd3+ ions, the intensity of the 380, 415, 435, 485, 540 and 590 nm bands of Tb3+ ions are increased. However, the emission bands of Gd3+ ion (314, 315 and 319 nm) are not observed, since their intensities are weak compared with that of Tb3+ ion (Fig. 7). The fact that the TL intensity is intensified by the co-doping of Tb3+ and Gd3+ ions may be explained by considering the energy transfer between them. As for TL emission from the excited state of Tb3+ ions, it is considered that trivalent terbium ions are reduced after UV or X-ray irradiation in the appropriate wavelength range (Merz and Pershan, 1967a, b), then the reduced terbium ions are oxidized by thermal excitation, which leads to TL emission due to Tb3+ ions. TL emission is considered to follow the reaction: Tb2+ + hole → Tb3+∗ → Tb3+ + h (380, 415, 435, 485, 540 and 590 nm),
Fig. 5. Relationship between TL intensity and X-ray dose. The TL intensity has been estimated from the maximum intensity of the main peak.
where the holes are supplied from traps by thermal excitation. 4. Conclusion
the TL intensity of CaF2 doped singly doped with Tb4 O7 is intensified. It is also found that the glow peak intensity of 435 K is 2–3 times as high as that of other peaks (358 and 523 K) in curve (b). Fig. 3 shows the TL glow curve for CaF2 :Tb4 O7 (0.06 wt%), Gd2 O3 (0.05 wt%) exposed to X-ray radiation for 1 min
From the data above, it has been found that the intensity of second TL peak (435 K peak for UV ray and 437 K peak for X-ray) of the sample which was co-doped with Tb and Gd ions shifted to high temperature region compared with that of codoped with Tb and Sm ions, and became stronger than that of
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Y. Fukuda / Radiation Measurements 43 (2008) 455 – 458
Fig. 6. Excitation and emission spectra of photoluminescence from CaF2:Tb (0.06 wt%), Gd (0.05 wt%). The excitation spectrum (a) was detected at 380 nm, and the emission spectrum (b) was excited at 285 nm.
Fig. 7. TL spectrum for CaF2 :Tb (0.06 wt%), Gd (0.05 wt%). TL spectrum was measured at 437 K after X-ray irradiation for 2 min (0.3 C kg−1 ).
the sample singly doped with Tb or Gd ions. And it is also found that the TL of Tb3+ ions is sensitized by the existence of Gd3+ ions. The mechanism causing the phenomenon is, however, at present still left unclear. The second TL peak 435 K (437 K; for X-ray) may be also suitable for using it as a dosimeter. References Awata, S., Tanaka, T., Fukuda, Y., 1999. Thermoluminescence and thermally stimulated exoelectron emission CaF2 /CaO dual phases doped with lanthanide oxides for UV-ray irradiation. Phys. Status Solidi (a) 174, 541–549. Chen, R., Fogel, G., 1993. Superlinearity in thermoluminescence revisited. Radiat. Prot. Dosim. 47, 23–26. Fukuda, Y., 2002. Thermoluminescence in sintered CaF2 :Tb. J. Radiat. Res. 43 (Suppl.), S67–S69. Fukuda, Y., Tomita, A., 1999. Thermoluminescence in CaF2 :Tb and CaO:Tb. Radiat. Prot. Dosim. 85, 269–272.
Fukuda, Y., Niwa, T., 2006. Thermoluminescence of terbium sensitized by samarium in CaF2 . Radiat. Prot. Dosim. 119, 153–156. Fukuda, Y., Ohtaki, H., Owaki, S., 1994. Thermoluminescence of thulium sensitized by terbium in -phase Ca3 (PO4 )2 . Phys. Status Solidi (a) 144, K107–K111. Fukuda, Y., Ohtaki, H., Tomita, A., Owaki, S., 1996. Estimation of ultraviolet radiation dose using CaF2 :Tb phosphor. Radiat. Prot. Dosim. 65, 325–328. Harperin, A., Chen, R., 1996. Thermoluminescence in semiconducting diamonds. Phys. Rev. 18, 839–845. Merz, J.L., Pershan, P.S., 1967a. Charge conversion of irradiated rare-earth ions in calcium fluoride. I. Phys. Rev. 162, 217–235. Merz, J.L., Pershan, P.S., 1967b. Charge conversion of irradiated rare-earth ions in calcium fluoride. II. Phys. Rev. 162, 235–247. Menon, S.N., Sanaye, S.S., Dhabekar, B.S., Kumar, R., Bhatt, B.C., 2005. Role of Mn as a co-dopant in CaSO4 :Mn, Pr TL phosphor. Radiat. Meas. 39, 111–114. Ohtaki, H., Kido, H., Hiratsuka, A., Fukuda, Y., Takeuchi, N., 1994. Estimation of UV radiation dose using CaF2 :Tb4 O7 as a thermoluminescence dosimeter. J. Mater. Sci. Lett. 13, 1267–1269. Salah, N., Sahare, P.D., 2006. TL, PL and energy transfer in K2 Ca2 (SO4 )3 :Eu2+ , Ce3+ . Radiat. Meas. 41, 665–670.
Thermoluminescence in calcium fluoride doped with terbium and gadolinium ions Y. Fukuda ∗ Department of Urban Environment, Osaka Sangyo University, Japan
Abstract Thermoluminescence (TL) in sintered CaF2 co-doped with Tb4 O7 and Gd2 O3 has been investigated for UV and X-ray irradiation. Three TL glow peaks for the CaF2 :Tb, Gd sample are observed at about 353, 435 K (437 K: for X-ray) and 523 K by heating the sample at a rate of 20 K min−1 after UV or X-ray irradiation at room temperature. It was found that the second peak (435 K) intensity became stronger than that of the sample singly doped with only terbium or gadolinium ions. From the TL emission spectrum and the excitation and emission spectra of photo luminescence for the CaF2 doped activators, it is concluded that the TL of Tb3+ ions is sensitized by the existence of Gd3+ ions. The 435 K TL peak may be also suitable for using it as a dosimeter. © 2007 Elsevier Ltd. All rights reserved. Keywords: Sintered CaF2 ; Tb, Gd; Thermoluminescence; TL emission spectrum
1. Introduction In a previous paper, we reported the possibility of estimating UV radiation dose using CaF2 :Tb as a thermoluminescence (TL) dosimeter (Ohtaki et al., 1994). It was found that the TL intensity from CaF2 doped with Tb4 O7 was the highest among the samples doped with various lanthanide oxides, and three TL peaks appeared at about 353, 378 and 458 K by heating the sample at a rate of 20 K min−1 after UV or X-ray irradiation at room temperature (Fukuda et al., 1996; Awata et al., 1999; Fukuda and Tomita, 1999). To improve TL sensitivity, the effect of co-doping activators has been investigated (Fukuda et al., 1994; Fukuda, 2002; Menon et al., 2005; Fukuda and Niwa, 2006; Salah and Sahare, 2006). It has been found that the second peak (378 K) intensity of the sample co-doped with Tb4 O7 and Sm2 O3 became stronger than those doped with only terbium or samarium ions, and the TL peaks of 353 K and 458 K were not observed. According to our preceding investigation, the intensity of 378 K glow peak of CaF2 triply doped with Tb4 O7 , Sm2 O3 ∗ Tel.: +81 72 875 3001.
E-mail address: [email protected]. 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.034
and Gd2 O3 was 5 times higher than that of sample co-doped with Tb4 O7 and Sm2 O3 , and the TL emission from the excited state of Tb3+ ions was observed. Therefore, it is expected that the TL of Tb3+ ions is intensified by the existence of not only Sm3+ ions but Gd3+ ions. However, in order to use for dosimetry, it is desirable to intensify the peak at high temperature region by the addition of activators, if possible, because the 378 K peak is unstable for storage at room temperature after irradiation. The purpose of the present paper is to obtain a prominent TL peak at high temperature region, and to study the effect of co-doping with Tb4 O7 and Gd2 O3 . The influence of sintering temperature is also reported. 2. Materials and methods The samples were prepared by solid state reaction. Starting materials, CaF2 (purity 99.99%), Tb4 O7 (purity 99.99%) and Gd2 O3 (purity 99.99%) were mixed in the desired ratio and pressed into pellets of about 6mm in diameter and 0.7 mm in thickness. Pellets of CaF2 : Tb, Gd were heated at 1373 K in a platinum crucible in air for 2 h followed by cooling to room temperature.
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Y. Fukuda / Radiation Measurements 43 (2008) 455 – 458
Samples were exposed to UV light ( = 253.7 nm, L937 low pressure Hg lamp. Hamamatsu Photonics Co. Ltd.) or X-rays (Cu K radiation; 35 KV, 20 mA). The TL was measured using a home –made apparatus with a photomultiplier (HTV-R212, Hamamatsu Photonics Co. Ltd.) and the heating rate for TL was 20 K min−1 . The excitation and the fluorescence spectra of the sample were measured using a spectrofluoro photometer (Shimadzu RF-1500). TL emission spectra were measured using a SMA (spectrometric multichannel analyzer) system (Princeton Instruments Inc.) which consisted of the personal computer, detector and its controller. 3. Results and discussion Fig. 1 shows the glow curves for CaF2 phosphor exposed UV radiation for 1 min (∼0.4 J m−2 ), where curve (a) is for the sample doped with Tb4 O7 (0.06 wt%) and Sm2 O3 (0.03 wt%), and curves (b) and (c) with 0.06 wt% Tb4 O7 . Curves (a) and (b) show the results for the sample sintered at 1473 K, while curve (c) shows the result for the sample sintered at 1373 K. In the TL glow curve of sintered CaF2 singly doped with Tb4 O7 , three TL peaks usually appear at about 353, 383 and 458 K [curves (b) and (c)], on the other hand, one TL glow peak is observed at about 378 K in the sample doped with Tb4 O7 and Sm2 O3 . The intensity of the 458 K glow peak of CaF2 :Tb sample sintered at 1473 K [curve (b)] is 1.9 times higher than that of the sample sintered at 1373 K [curve (c)]. As shown in Fig. 1, the intensity of the 378 K glow peak of the co-doped sample [curve (a)] is 12.9 times higher than that of the 458 K glow peak of the sample singly doped with Tb4 O7 [curve (b)]. However, the 378 K glow peak of CaF2 :Tb,Sm is unstable for storage at room temperature after irradiation. And pellet type sample sintered at 1473 K breaks in many cases, while using repeatedly. On the other hand, the sample sintered at 1373 K has
Fig. 1. The glow curves of TL for CaF2 phosphor exposed to UV ray radiation for 1 min (∼0.4 J m−2 ). Curve (a) is for CaF2 :Tb (0.06 wt%), Sm (0.03 wt%) sintered at 1473 K; curve (b) is for CaF2 :Tb (0.06 wt%) sintered at 1473 K; curve (c) is for CaF2 :Tb (0.06 wt%) sintered at 1373 K. ×0.14 denotes magnified by ×0.14 times.
Fig. 2. The glow curves of TL for CaF2 phosphor exposed to UV ray radiation for 1 min (∼0.4 J m−2 ). Curves (a), (b) and (c) show the result for the samples sintered at 1373 K. Curve (a) is for CaF2 :Tb (0.06 wt%); curve (b) is for CaF2 :Tb (0.06 wt%), Gd (0.05 wt%); curve (c) is for CaF2 :Gd (0.05 wt%).
Fig. 3. The glow curve for CaF2 :Tb, Gd exposed to X-ray radiation for 1 min (0.15 C kg−1 ).
few such things. Therefore, since the sample sintered at 1373 K as TLD was suitable, it investigated about the sample of CaF2 doped with Tb4 O7 and Gd2 O3 sintered at 1373 K this time. Fig. 2 shows the TL glow curves for CaF2 phosphor exposed to UV ray radiation for 1 min (∼0.4 J m−2 ). Curves (a), (b) and (c) show the result for the samples sintered at 1373 K. Where curve (a) is for the sample singly doped with 0.06 wt% Tb4 O7 , curve (b) co-doped with Tb4 O7 (0.06 wt%) and Gd2 O3 (0.05 wt%), and curve (c) with 0.05 wt% Gd2 O3 . It is seen from curves (a) and (c) that the TL glow peaks appear at about 353, 383 and 452 K. The intensity of the TL response against UV irradiation dose is increased by the co-doping of terbium and gadolinium ions, the TL peaks being found at about 358, 435 and 523 K [curve (b)]. As shown in Fig. 2, the intensity of 435 K glow peak is about 2 times higher than that of 452 K glow peak of the sample singly doped with Tb4 O7 , and about 2.5 times higher than that of 353 K grow peak of the sample singly doped with Gd2 O3 . It is noticed that by the existence of Gd2 O3 ,
Y. Fukuda / Radiation Measurements 43 (2008) 455 – 458
Fig. 4. Relationship between TL intensity and UV ray dose. The TL intensity has been estimated from the maximum intensity of the main peak.
457
(0.15 C kg−1 ). Two main TL glow peaks are observed at about 362 and 437 K. To examine the TL responses against the irradiation of UV and X-ray irradiation, the second glow peak intensity of CaF2 :Tb, Gd (435 K for UV; 437 K for X-ray) were plotted against the UV and X-ray dose in Figs. 4 and 5, respectively. As shown in Fig. 4, the TL response of 435 K peak increases up to 120 s (∼0.8 J m−2 ) with second power of the dose of 253.7 nm UV radiation, and attains saturation, while it shows a linear relation against the X-ray dose (Fig. 5). The supralinearity in CaF2 :Tb, Gd irradiated with UV ray may also be explained by multiple-stage excitation (Harperin and Chen, 1996; Chen and Fogel, 1993). The excitation and emission spectra of photoluminescence from CaF2 :Tb, Gd were measured on condition that the range of excitation and emission were 10 nm (Fig. 6). The excitation spectrum (a) was detected at 380 nm and the emission spectrum (b) was measured with the excitation of 285 nm. The excitation peak appeared at 285 nm, and emission peaks 380, 415, 435, 485, 540 and 590 nm were assigned to 5 D3 → 7 F6 , 5 D3 → 7 F , 5 D → 7 F , 5 D → 7 F , 5 D → 7 F and 5 D → 7 F 3 4 3 2,1 4 3 4 5 5 of the inner transition of Tb3+ ions, respectively. The temperature resolved TL spectrum for CaF2 :Tb, Gd is shown in Fig. 7. After X-ray irradiation (0.30 C kg−1 ), the TL was measured at 437 K. Six emission peaks appeared at 380, 415, 435, 485, 540 and 590 nm. It is noticed from Figs. 6 and 7 that by the existence of Gd3+ ions, the intensity of the 380, 415, 435, 485, 540 and 590 nm bands of Tb3+ ions are increased. However, the emission bands of Gd3+ ion (314, 315 and 319 nm) are not observed, since their intensities are weak compared with that of Tb3+ ion (Fig. 7). The fact that the TL intensity is intensified by the co-doping of Tb3+ and Gd3+ ions may be explained by considering the energy transfer between them. As for TL emission from the excited state of Tb3+ ions, it is considered that trivalent terbium ions are reduced after UV or X-ray irradiation in the appropriate wavelength range (Merz and Pershan, 1967a, b), then the reduced terbium ions are oxidized by thermal excitation, which leads to TL emission due to Tb3+ ions. TL emission is considered to follow the reaction: Tb2+ + hole → Tb3+∗ → Tb3+ + h (380, 415, 435, 485, 540 and 590 nm),
Fig. 5. Relationship between TL intensity and X-ray dose. The TL intensity has been estimated from the maximum intensity of the main peak.
where the holes are supplied from traps by thermal excitation. 4. Conclusion
the TL intensity of CaF2 doped singly doped with Tb4 O7 is intensified. It is also found that the glow peak intensity of 435 K is 2–3 times as high as that of other peaks (358 and 523 K) in curve (b). Fig. 3 shows the TL glow curve for CaF2 :Tb4 O7 (0.06 wt%), Gd2 O3 (0.05 wt%) exposed to X-ray radiation for 1 min
From the data above, it has been found that the intensity of second TL peak (435 K peak for UV ray and 437 K peak for X-ray) of the sample which was co-doped with Tb and Gd ions shifted to high temperature region compared with that of codoped with Tb and Sm ions, and became stronger than that of
458
Y. Fukuda / Radiation Measurements 43 (2008) 455 – 458
Fig. 6. Excitation and emission spectra of photoluminescence from CaF2:Tb (0.06 wt%), Gd (0.05 wt%). The excitation spectrum (a) was detected at 380 nm, and the emission spectrum (b) was excited at 285 nm.
Fig. 7. TL spectrum for CaF2 :Tb (0.06 wt%), Gd (0.05 wt%). TL spectrum was measured at 437 K after X-ray irradiation for 2 min (0.3 C kg−1 ).
the sample singly doped with Tb or Gd ions. And it is also found that the TL of Tb3+ ions is sensitized by the existence of Gd3+ ions. The mechanism causing the phenomenon is, however, at present still left unclear. The second TL peak 435 K (437 K; for X-ray) may be also suitable for using it as a dosimeter. References Awata, S., Tanaka, T., Fukuda, Y., 1999. Thermoluminescence and thermally stimulated exoelectron emission CaF2 /CaO dual phases doped with lanthanide oxides for UV-ray irradiation. Phys. Status Solidi (a) 174, 541–549. Chen, R., Fogel, G., 1993. Superlinearity in thermoluminescence revisited. Radiat. Prot. Dosim. 47, 23–26. Fukuda, Y., 2002. Thermoluminescence in sintered CaF2 :Tb. J. Radiat. Res. 43 (Suppl.), S67–S69. Fukuda, Y., Tomita, A., 1999. Thermoluminescence in CaF2 :Tb and CaO:Tb. Radiat. Prot. Dosim. 85, 269–272.
Fukuda, Y., Niwa, T., 2006. Thermoluminescence of terbium sensitized by samarium in CaF2 . Radiat. Prot. Dosim. 119, 153–156. Fukuda, Y., Ohtaki, H., Owaki, S., 1994. Thermoluminescence of thulium sensitized by terbium in -phase Ca3 (PO4 )2 . Phys. Status Solidi (a) 144, K107–K111. Fukuda, Y., Ohtaki, H., Tomita, A., Owaki, S., 1996. Estimation of ultraviolet radiation dose using CaF2 :Tb phosphor. Radiat. Prot. Dosim. 65, 325–328. Harperin, A., Chen, R., 1996. Thermoluminescence in semiconducting diamonds. Phys. Rev. 18, 839–845. Merz, J.L., Pershan, P.S., 1967a. Charge conversion of irradiated rare-earth ions in calcium fluoride. I. Phys. Rev. 162, 217–235. Merz, J.L., Pershan, P.S., 1967b. Charge conversion of irradiated rare-earth ions in calcium fluoride. II. Phys. Rev. 162, 235–247. Menon, S.N., Sanaye, S.S., Dhabekar, B.S., Kumar, R., Bhatt, B.C., 2005. Role of Mn as a co-dopant in CaSO4 :Mn, Pr TL phosphor. Radiat. Meas. 39, 111–114. Ohtaki, H., Kido, H., Hiratsuka, A., Fukuda, Y., Takeuchi, N., 1994. Estimation of UV radiation dose using CaF2 :Tb4 O7 as a thermoluminescence dosimeter. J. Mater. Sci. Lett. 13, 1267–1269. Salah, N., Sahare, P.D., 2006. TL, PL and energy transfer in K2 Ca2 (SO4 )3 :Eu2+ , Ce3+ . Radiat. Meas. 41, 665–670.