- Email: [email protected]
Radioluminescence of red-emitting Eu-doped phosphors for fiberoptic dosimetry
Applied Radiation and Isotopes 71 (2012) 12–14
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radioluminescence of red-emitting Eu-doped phosphors for fiberoptic dosimetry P. Molina a,b,n, M. Santiago a,b, J. Marcazzo´ a,b, F. Spano c, J. Henniger d, W. Cravero e,b, E. Caselli a,f a
Instituto de Fı´sica Arroyo Seco, Universidad Nacional del Centro de la Provincia de Buenos Aires (UNICEN), Pinto 399, 7000 Tandil, Argentina Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), Rivadavia 1917, 1033 Buenos Aires, Argentina c Autoridad Regulatoria Nuclear (ARN), Av. Del Libertador 8250, 1429 Buenos Aires, Argentina d Institut f¨ ur Kern- und Teilchenphysik, Zellescher Weg 19, 01069 Dresden, Germany e Departamento de Fı´sica, Universidad Nacional Del Sur, Bahı´a Blanca, Argentina f ´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires (CICPBA), calle 526 entre 10 y 11, 1900 La Plata, Argentina Comisio b
a r t i c l e i n f o
a b s t r a c t
Available online 14 January 2012
Fiberoptic dosimetry (FOD) technique has become an attractive method for real-time dosimetry. Al2O3:C is one of the most used radioluminescence materials for FOD due to its high efficiency but it presents the drawback of emitting in the spectral region, where spurious luminescence is also important. Optical filtering is the simplest technique to remove spurious luminescence, but is useful when red-emitting scintillators are employed. In this work, the feasibility of using red-emitting Eu-doped phosphors as FOD scintillators has been investigated. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Radioluminescence Yttrium phosphors In vivo dosimetry
1. Introduction The fiberoptic dosimetry technique (FOD) requires the removal of spurious luminescence (stem effect) in order to be successfully used in radiation treatments (teletherapy and braquitherapy). The stem effect is made up of the intrinsic fluorescence of the optical fiber and Cerenkov radiation originated in the fiber when it is irradiated with ionizing radiation. The spectrum of the stem effect, which adds to the radioluminescence (RL) emission from the scintillator, dominates in the blue/green region (De Boer et al., 1993). Four stem effect removal methods have been suggested over the last past years: subtraction method (Beddar et al., 1992), optical filtering (De Boer et al., 1993), chromatic removal (Fontbonne et al., 2002), and the temporal separation technique (Clift et al., 2002). Among these methods optical filtering is the simplest and cheapest one for removing the stem effect from RL signal, but scintillator emission away from the blue/green region is needed. The main advantage of the optical filtering method is that it is compatible with any irradiation source (pulsed or continuous). So far plastic scintillators and C-doped aluminum oxide (Al2O3:C) compounds are the most promising converters of ionizing radiations to light from the viewpoint of their application
n Corresponding author at: Instituto de Fı´sica Arroyo Seco, Universidad Nacional del Centro de la Provincia de Buenos Aires (UNICEN), Pinto 399, 7000 Tandil, Argentina. E-mail address: [email protected] (P. Molina).
0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2012.01.005
to the FOD technique (Andersen et al., 2006; Archambault et al., 2006; Damkjaer et al., 2008; Erfurt et al., 2000). Since the spectrum of the RL emission of plastic scintillators and Al2O3:C partially overlaps with that of the stem effect, simple optical filtering techniques cannot be employed for stem effect removal. Instead the chromatic method and the temporal separation technique have been successfully employed to get rid of the Cerenkov radiation contribution (Clift et al., 2002; Fontbonne et al., 2002). Two additional drawbacks afflict the RL signal of Al2O3:C from the viewpoint of its direct application to in vivo and real-time RL dosimetry, namely, its RL response is not constant as dose accumulates and shallow trap-related phosphorescence (i.e. afterglow) is observed promptly after irradiation (Damkjaer et al., 2008; Markey et al., 1995). It is worth mentioning that Damkjaer et al. (2008) developed an algorithm, which greatly reduces the influence of shallow traps on the shape of the RL signal of Al2O3:C. Even though the effect of shallow traps on the RL response has been successfully removed employing the temporal separation technique for Al2O3:C, there is a permanent interest in developing new RL materials having low shallow trap concentration and emitting in a longer wavelength region than that where the stem effect is important (Marcazzo´ et al., 2007; Molina et al., 2010, 2011). As mentioned, simple optical filtering can be used in this case to get rid of the stem effect making the FOD technique cheaper and more robust to be used in the clinical practice. Yttrium phosphors doped with europium have been widely used in industry because of their bright red-emission light. Particularly, yttrium vanadate (YVO4:Eu3 þ ) has attracted special attention due to the wide application as red phosphor in color
P. Molina et al. / Applied Radiation and Isotopes 71 (2012) 12–14
television CRT displays and high pressure mercury lamps (Cheng et al., 2010; Riwotzki and Hasse, 2001; Takeshita et al., 2008). On the other hand, yttrium oxide phosphor (Y2O3:Eu3 þ ) is also used in high efficiency CRT and field emission displays (Hirata et al., 1997; Pons et al., 2000; Wakefield et al., 2001). Yttrium oxysulphide (Y2O2S:Eu3 þ ) has also received considerable attention due to its industrial applications since it is suitable for luminescence detection owing to its excellent spectral characteristics such as narrow line-shaped emission bands, and inherent photostability (Fu et al., 2008; Kawahara et al., 2006; Thirumalai et al., 2009). In this work the RL sensitivity and spectral emission of different yttrium phosphors doped with europium are investigated for the first time in order to assess their possible use in the framework of the FOD technique with simple optical filtering of the stem effect.
3000 2000 1000
a
0 4000 2000
b
0 2000 1000
c
0 0
2. Materials and methods Three red-emitting phosphor samples, namely Y2O2S:Eu3 þ (QKL63/F-C1), YVO4:Eu3 þ (QHK63/N-U1), and Y2O3:Eu3 þ (QK63/ F-C1), kindly provided by Phosphor Technology Ltd. (UK), were used in the present work. The three phosphors present white colored powder form, with median particle grain sizes of 6 mm. The samples were weighted and all data were normalized to the weight. RL was investigated under excitation with electrons from a 90 Sr beta source rendering a dose rate of 0.024 Gy min 1 at the sample location, at room temperature. The measurements of RL intensity as a function of time (RL curves) were performed with a lab-made RL reader featuring a Hammamatsu H9319 photon counting photomultiplier tube (300–850 nm sensitivity range). One meter of a standard PMMA optical cable with a core diameter of 980 mm was used to collect light from the sample to the photomultiplier tube. Spectra of RL emission were measured by means of a Czerny–Turner monochromator SP-2155 (Acton Research) with a focal length of 0.15 m and a resolution of 10 nm and a 600 g mm 1 grating with the blaze at 500 nm. The Hammamatsu H9319 head placed at the exit slit was employed to detect the scattered light. The samples were positioned in front of the exit slit (1 mm width) with the 90Sr source closely placed behind.
3. Results and discussion The RL curves from the phosphors investigated in this work are presented in Fig. 1 (1 h beta irradiation). No changes in RL-sensitivity are observable during irradiation as can be seen in Fig. 1. Besides no afterglow with long decay time is observable from these phosphors. Stable RL response and no long-lasting afterglow could be related to either a low concentration of shallow traps or rapid equilibrium attainment between chargetrapping and detrapping rates during irradiation (Damkjaer et al., 2008). In order to study the response stability after repeated use of the Y2O2S:Eu3 þ , YVO4:Eu3 þ , and Y2O3:Eu3 þ phosphors, their RL emissions have been recorded during 11 consecutive irradiation cycles. The exposure time from the first 10 cycles were 10 min and the 11th cycle had an exposure time of 1 h. The lapse between irradiations was 1 min. Each time the RL intensity has been recorded and the mean value Vf of the RL intensity over the last 100 s of the RL curve has been computed. In principle, the value of Vf can be regarded as proportional to the dose rate (Mones et al., 2006). Fig. 2 shows the variation of the value Vf with respect the number of the cycle, where normalization by the
13
500
1000
1500
2000
2500
3000
3500
Fig. 1. RL intensity as function of time for: (a) Y2O2S:Eu3 þ ; (b) YVO4:Eu3 þ ; and (c) Y2O3:Eu3 þ under beta radiation.
101 100 99 101 100 99 101 100 99 0
2
4
6
8
10
12
Fig. 2. Repeatability of the RL yield (Vf) from the phosphors (a) Y2O2S:Eu3 þ , (b) YVO4:Eu3 þ , and (c) Y2O3:Eu3 þ under beta radiation excitation. Normalization by the mean value Vf ¼ 100% has been applied. Ten minutes of exposure each cycle except the last one, which received 1 h of exposure (1 min elapsed between consecutive measurements).
mean Vf value (regarded as 100%) has been applied. The error bars in Fig. 2, which have been computed as the standard deviation from the average of the RL readings, only amount to roughly 0.1 of Vf. It is apparent from Fig. 2 that the RL response does not increase as function of accumulated dose. It is remarkable that the variations of Vf lies with in 1.5%. In Fig. 3 the RL spectra of Y2O2S:Eu3 þ , YVO4:Eu3 þ , and Y2O3:Eu3 þ are shown. Also the RL spectra from an optical fiber without a scintillator (stem effect) is shown for comparison. As can be seen, the RL spectrum from these red phosphors shows characteristic fluorescence 5D 0 -7 FJ (J ¼0–4) indicating that Eu3 þ sites act as luminescence centers under ionizing irradiation excitation, with strong red emission near 625 nm, assigned to 5 D0 -7 F2 (Chakradhar et al., 2011; Lin et al., 2003; Jo et al., 2011). It is apparent from Fig. 3 that strong red emission of the investigated compounds is located at wavelengths where the stem effect is negligible. For this reason the light emitted by these phosphors is expected to be less affected by the stem effect emission if they are employed as FOD scintillators. Using a 575 nm long pass filter, reduction of the RL signal of 13%, 6%, and 6% for Y2O2S:Eu3 þ , YVO4:Eu3 þ , and Y2O3:Eu3 þ respectively is expected while a significant 88% reduction of the stem effect can be realized in this case.
14
P. Molina et al. / Applied Radiation and Isotopes 71 (2012) 12–14
1.0 0.5 0.0 1.0 0.5 0.0 1.0 0.5 0.0 1.0 0.5 0.0 300
400
500
600
700
800
Fig. 3. RL spectra from: (a) optical fiber (stem effect), (b) Y2O2S:Eu3 þ , (c) YVO4:Eu3 þ , and (d) Y2O3:Eu3 þ under beta radiation excitation.
4. Conclusions The results of this work demonstrate the feasibility of using Y2O3:Eu3 þ , Y2O2S:Eu3 þ , and YVO4:Eu3 þ phosphors as scintillators in the FOD technique. Efficient RL from these phosphors in the red spectral region allows using simple optical filtering methods to get rid of the spurious luminescence caused by the stem effect. Moreover, these yttrium europium-doped phosphors show no changes in RL response as dose accumulates and no afterglow decay. Summarizing, this work has demonstrated that Y2O2S:Eu3 þ , YVO4:Eu3 þ , and Y2O3:Eu3 þ are promising compounds to be employed as FOD RL phosphors deserving further investigations.
Acknowledgements This research has been supported by Grant PICT Nr. 1907 from Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (ANPCyT, Argentina). References Andersen, C.E., Marckmann, C.J., Aznar, M.C., Botter-Jensen, L., 2006. An algorithm for real-time dosimetry in intensity-modulated radiation therapy using the radioluminescence signal from Al 2 O 3 :C. Radiat. Prot. Dosim. 120 (1–4), 7–13. Archambault, L., Beddar, A., Gingras, L., 2006. Measurement accuracy and Cerenkov removal for high performance, high spatial resolution scintillation dosimetry. Med. Phys. 33, 128.
Beddar, A.S., Mackie, T.R., Attix, F.H., 1992. Cerenkov light generated in optical fibres and other light pipes irradiated by electrons beams. Phys. Med. Biol. 37, 925–935. Chakradhar, R.P.S., Basu, B.J., Lakshmi, R.V., 2011. Effect of particle size and dopant concentration on photophysical properties of Eu 3 þ -doped rare earth oxysulphide phosphor coatings. Spectrochim. Acta Part A 78, 783–787. Cheng, Z., Xing, R., Hou, Z., Huang, S., Lin, J., 2010. Patterning of light-emitting YVO 3þ thin films via inkjet printing. J. Phys. Chem. C 114, 9883–9888. 4 :Eu Clift, M.A., Johnston, P.N., Webb, D.V., 2002. A temporal method of avoiding the Cerenkov radiation generated in organic scintillator dosimeters by pulsed mega-voltage electron and photon beams. Phys. Med. Biol. 47, 1421–1433. Damkjaer, S.M.S., Andersen, C.E., Aznar, M.C., 2008. Improved real-time dosimetry using the radioluminescence signal from Al2O3:C. Radiat. Meas. 43, 893–897. De Boer, S.F., Beddar, A.S., Rawlinson, J.A., 1993. Optical filtering and spectral measurements of radiation-induced light in plastic scintillation dosimetry. Phys. Med. Biol. 38, 945–958. Erfurt, G., Krberschek, M.R., Trautmann, T., Stolz, W., 2000. Radioluminescence (RL) behaviour of Al2O3:C—potential for dosimetric applications. Measurements 32, 735–739. Fontbonne, J.M., Iltis, G., Ban, G., Battala, A., Vernhes, J.C., Tillier, J., Bellaize, N., Le Brun, C., Tamain, B., Mercier, K., Motin, J.C., 2002. Scintillating fiber dosimeter for radiation therapy accelerator. IEEE Trans. Nucl. Sci. 49 (5), 2223–2227. Fu, Z., Geng, Y., Chen, H., Zhou, S., Yang, H.K., Jeong, J.H., 2008. Combustion synthesis and luminescent properties of the Eu3 þ -doped yttrium oxysulfide nanocrystalline. Opt. Mater. 31, 58–62. Hirata, G.A., McKittrick, J., Devlin, D., Siqueiros, J.M., 1997. Physical properties of Y2O3:Eu luminescent films grown by MOCVD and laser ablation. Appl. Surf. Sci. 113–114, 509–514. Jo, D.S., Luo, Y.Y., Senthil, K., Masaki, T., Yoon, D.H., 2011. Synthesis of high efficient nanosized Y(V,P)O4:Eu3 þ red phosphors by a new technique. Opt. Mater. 33, 1190–1194. Kawahara, Y., Petrykin, V., Ichihara, T., Kijima, N., Kakihana, M., 2006. Synthesis of high-brightness sub-micrometer Y2O2S red phosphor powders by complex homogeneous precipitation method. Chem. Mater. 18, 6303–6307. Lin, Y., Nan, C.-W., Cai, N., Zhou, X., Wang, H., Chen, D., 2003. Anomalous afterglow from Y2O3-based phosphor. J. Alloys Compd. 361, 92–95. Marcazzo´, J., Henniger, J., Khaidukov, N.M., Makhov, V.N., Caselli, E., Santiago, M., 2007. Efficient crystal radiation detectors based on Tb 3 þ -doped fluorides for radioluminescence dosimetry. J. Phys. D: Appl. Phys. 40, 5055–5060. Markey, B.G., Colyott, L.E., McKeever, S.W.W., 1995. Time-resolved optically stimulated luminescence from a-Al2O3:C. Radiat. Meas. 24, 457–463. Molina, P., Prokic, M., Marcazzo´, J., Santiago, M., 2010. Characterization of a fiberoptic radiotherapy dosimetry probe based on Mg2SiO4:Tb. Radiat. Meas. 45, 78–82. Molina, P., Santiago, M., Marcazzo´, J., Spano, F., Khaidukov, N., Caselli, E., 2011. Radioluminescence of rare-earth doped potassium yttrium fluorides crystals. Radiat. Meas. 46, 1361–1364. Mones, E., Veronese, I., Moretti, F., 2006. Feasibility study for the use of Ce3 þ doped optical fibres in radiotherapy. Nucl. Instrum. Methods Phys. Res. A 562, 449–455. Pons, O., Moll, Y., Huignard, A., Antic-Fidancev, E., Aschehoug, P., Viana, B., Millon, E., Perrie re, J., Garapon, C., Mugnier, J., 2000. Eu3 þ - and Tm3 þ -doped yttrium oxide thin films for optical applications. J. Lumin. 87–89, 1115–1117. Riwotzki, K., Hasse, M., 2001. Colloidal YVO4:Eu and YP0.95V0.05O4:Eu nanoparticles: luminescence and energy transfer processes. J. Phys. Chem. B 105, 12709–12713. Takeshita, S., Isobe, T., Niikura, S., 2008. Low-temperature wet chemical synthesis and photoluminescence properties of YVO4:Bi3 þ , Eu3 þ nanophosphors. J. Lumin. 128, 1515–1522. Thirumalai, J., Chandramohan, R., Auluck, S., Mahalingam, T., Srikumar, S.R., 2009. Controlled synthesis, optical and electronic properties of Eu3 þ doped yttrium oxysulfide (Y2O2S) nanostructures. J. Colloid Interface Sci. 336, 889–897. Wakefield, G., Holland, E., Dobson Hutchison, P.J.J.L., 2001. Luminescence properties of nanocrystalline Y2O3:Eu. Adv. Mater. 13, 1557–1560.
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radioluminescence of red-emitting Eu-doped phosphors for fiberoptic dosimetry P. Molina a,b,n, M. Santiago a,b, J. Marcazzo´ a,b, F. Spano c, J. Henniger d, W. Cravero e,b, E. Caselli a,f a
Instituto de Fı´sica Arroyo Seco, Universidad Nacional del Centro de la Provincia de Buenos Aires (UNICEN), Pinto 399, 7000 Tandil, Argentina Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), Rivadavia 1917, 1033 Buenos Aires, Argentina c Autoridad Regulatoria Nuclear (ARN), Av. Del Libertador 8250, 1429 Buenos Aires, Argentina d Institut f¨ ur Kern- und Teilchenphysik, Zellescher Weg 19, 01069 Dresden, Germany e Departamento de Fı´sica, Universidad Nacional Del Sur, Bahı´a Blanca, Argentina f ´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires (CICPBA), calle 526 entre 10 y 11, 1900 La Plata, Argentina Comisio b
a r t i c l e i n f o
a b s t r a c t
Available online 14 January 2012
Fiberoptic dosimetry (FOD) technique has become an attractive method for real-time dosimetry. Al2O3:C is one of the most used radioluminescence materials for FOD due to its high efficiency but it presents the drawback of emitting in the spectral region, where spurious luminescence is also important. Optical filtering is the simplest technique to remove spurious luminescence, but is useful when red-emitting scintillators are employed. In this work, the feasibility of using red-emitting Eu-doped phosphors as FOD scintillators has been investigated. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Radioluminescence Yttrium phosphors In vivo dosimetry
1. Introduction The fiberoptic dosimetry technique (FOD) requires the removal of spurious luminescence (stem effect) in order to be successfully used in radiation treatments (teletherapy and braquitherapy). The stem effect is made up of the intrinsic fluorescence of the optical fiber and Cerenkov radiation originated in the fiber when it is irradiated with ionizing radiation. The spectrum of the stem effect, which adds to the radioluminescence (RL) emission from the scintillator, dominates in the blue/green region (De Boer et al., 1993). Four stem effect removal methods have been suggested over the last past years: subtraction method (Beddar et al., 1992), optical filtering (De Boer et al., 1993), chromatic removal (Fontbonne et al., 2002), and the temporal separation technique (Clift et al., 2002). Among these methods optical filtering is the simplest and cheapest one for removing the stem effect from RL signal, but scintillator emission away from the blue/green region is needed. The main advantage of the optical filtering method is that it is compatible with any irradiation source (pulsed or continuous). So far plastic scintillators and C-doped aluminum oxide (Al2O3:C) compounds are the most promising converters of ionizing radiations to light from the viewpoint of their application
n Corresponding author at: Instituto de Fı´sica Arroyo Seco, Universidad Nacional del Centro de la Provincia de Buenos Aires (UNICEN), Pinto 399, 7000 Tandil, Argentina. E-mail address: [email protected] (P. Molina).
0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2012.01.005
to the FOD technique (Andersen et al., 2006; Archambault et al., 2006; Damkjaer et al., 2008; Erfurt et al., 2000). Since the spectrum of the RL emission of plastic scintillators and Al2O3:C partially overlaps with that of the stem effect, simple optical filtering techniques cannot be employed for stem effect removal. Instead the chromatic method and the temporal separation technique have been successfully employed to get rid of the Cerenkov radiation contribution (Clift et al., 2002; Fontbonne et al., 2002). Two additional drawbacks afflict the RL signal of Al2O3:C from the viewpoint of its direct application to in vivo and real-time RL dosimetry, namely, its RL response is not constant as dose accumulates and shallow trap-related phosphorescence (i.e. afterglow) is observed promptly after irradiation (Damkjaer et al., 2008; Markey et al., 1995). It is worth mentioning that Damkjaer et al. (2008) developed an algorithm, which greatly reduces the influence of shallow traps on the shape of the RL signal of Al2O3:C. Even though the effect of shallow traps on the RL response has been successfully removed employing the temporal separation technique for Al2O3:C, there is a permanent interest in developing new RL materials having low shallow trap concentration and emitting in a longer wavelength region than that where the stem effect is important (Marcazzo´ et al., 2007; Molina et al., 2010, 2011). As mentioned, simple optical filtering can be used in this case to get rid of the stem effect making the FOD technique cheaper and more robust to be used in the clinical practice. Yttrium phosphors doped with europium have been widely used in industry because of their bright red-emission light. Particularly, yttrium vanadate (YVO4:Eu3 þ ) has attracted special attention due to the wide application as red phosphor in color
P. Molina et al. / Applied Radiation and Isotopes 71 (2012) 12–14
television CRT displays and high pressure mercury lamps (Cheng et al., 2010; Riwotzki and Hasse, 2001; Takeshita et al., 2008). On the other hand, yttrium oxide phosphor (Y2O3:Eu3 þ ) is also used in high efficiency CRT and field emission displays (Hirata et al., 1997; Pons et al., 2000; Wakefield et al., 2001). Yttrium oxysulphide (Y2O2S:Eu3 þ ) has also received considerable attention due to its industrial applications since it is suitable for luminescence detection owing to its excellent spectral characteristics such as narrow line-shaped emission bands, and inherent photostability (Fu et al., 2008; Kawahara et al., 2006; Thirumalai et al., 2009). In this work the RL sensitivity and spectral emission of different yttrium phosphors doped with europium are investigated for the first time in order to assess their possible use in the framework of the FOD technique with simple optical filtering of the stem effect.
3000 2000 1000
a
0 4000 2000
b
0 2000 1000
c
0 0
2. Materials and methods Three red-emitting phosphor samples, namely Y2O2S:Eu3 þ (QKL63/F-C1), YVO4:Eu3 þ (QHK63/N-U1), and Y2O3:Eu3 þ (QK63/ F-C1), kindly provided by Phosphor Technology Ltd. (UK), were used in the present work. The three phosphors present white colored powder form, with median particle grain sizes of 6 mm. The samples were weighted and all data were normalized to the weight. RL was investigated under excitation with electrons from a 90 Sr beta source rendering a dose rate of 0.024 Gy min 1 at the sample location, at room temperature. The measurements of RL intensity as a function of time (RL curves) were performed with a lab-made RL reader featuring a Hammamatsu H9319 photon counting photomultiplier tube (300–850 nm sensitivity range). One meter of a standard PMMA optical cable with a core diameter of 980 mm was used to collect light from the sample to the photomultiplier tube. Spectra of RL emission were measured by means of a Czerny–Turner monochromator SP-2155 (Acton Research) with a focal length of 0.15 m and a resolution of 10 nm and a 600 g mm 1 grating with the blaze at 500 nm. The Hammamatsu H9319 head placed at the exit slit was employed to detect the scattered light. The samples were positioned in front of the exit slit (1 mm width) with the 90Sr source closely placed behind.
3. Results and discussion The RL curves from the phosphors investigated in this work are presented in Fig. 1 (1 h beta irradiation). No changes in RL-sensitivity are observable during irradiation as can be seen in Fig. 1. Besides no afterglow with long decay time is observable from these phosphors. Stable RL response and no long-lasting afterglow could be related to either a low concentration of shallow traps or rapid equilibrium attainment between chargetrapping and detrapping rates during irradiation (Damkjaer et al., 2008). In order to study the response stability after repeated use of the Y2O2S:Eu3 þ , YVO4:Eu3 þ , and Y2O3:Eu3 þ phosphors, their RL emissions have been recorded during 11 consecutive irradiation cycles. The exposure time from the first 10 cycles were 10 min and the 11th cycle had an exposure time of 1 h. The lapse between irradiations was 1 min. Each time the RL intensity has been recorded and the mean value Vf of the RL intensity over the last 100 s of the RL curve has been computed. In principle, the value of Vf can be regarded as proportional to the dose rate (Mones et al., 2006). Fig. 2 shows the variation of the value Vf with respect the number of the cycle, where normalization by the
13
500
1000
1500
2000
2500
3000
3500
Fig. 1. RL intensity as function of time for: (a) Y2O2S:Eu3 þ ; (b) YVO4:Eu3 þ ; and (c) Y2O3:Eu3 þ under beta radiation.
101 100 99 101 100 99 101 100 99 0
2
4
6
8
10
12
Fig. 2. Repeatability of the RL yield (Vf) from the phosphors (a) Y2O2S:Eu3 þ , (b) YVO4:Eu3 þ , and (c) Y2O3:Eu3 þ under beta radiation excitation. Normalization by the mean value Vf ¼ 100% has been applied. Ten minutes of exposure each cycle except the last one, which received 1 h of exposure (1 min elapsed between consecutive measurements).
mean Vf value (regarded as 100%) has been applied. The error bars in Fig. 2, which have been computed as the standard deviation from the average of the RL readings, only amount to roughly 0.1 of Vf. It is apparent from Fig. 2 that the RL response does not increase as function of accumulated dose. It is remarkable that the variations of Vf lies with in 1.5%. In Fig. 3 the RL spectra of Y2O2S:Eu3 þ , YVO4:Eu3 þ , and Y2O3:Eu3 þ are shown. Also the RL spectra from an optical fiber without a scintillator (stem effect) is shown for comparison. As can be seen, the RL spectrum from these red phosphors shows characteristic fluorescence 5D 0 -7 FJ (J ¼0–4) indicating that Eu3 þ sites act as luminescence centers under ionizing irradiation excitation, with strong red emission near 625 nm, assigned to 5 D0 -7 F2 (Chakradhar et al., 2011; Lin et al., 2003; Jo et al., 2011). It is apparent from Fig. 3 that strong red emission of the investigated compounds is located at wavelengths where the stem effect is negligible. For this reason the light emitted by these phosphors is expected to be less affected by the stem effect emission if they are employed as FOD scintillators. Using a 575 nm long pass filter, reduction of the RL signal of 13%, 6%, and 6% for Y2O2S:Eu3 þ , YVO4:Eu3 þ , and Y2O3:Eu3 þ respectively is expected while a significant 88% reduction of the stem effect can be realized in this case.
14
P. Molina et al. / Applied Radiation and Isotopes 71 (2012) 12–14
1.0 0.5 0.0 1.0 0.5 0.0 1.0 0.5 0.0 1.0 0.5 0.0 300
400
500
600
700
800
Fig. 3. RL spectra from: (a) optical fiber (stem effect), (b) Y2O2S:Eu3 þ , (c) YVO4:Eu3 þ , and (d) Y2O3:Eu3 þ under beta radiation excitation.
4. Conclusions The results of this work demonstrate the feasibility of using Y2O3:Eu3 þ , Y2O2S:Eu3 þ , and YVO4:Eu3 þ phosphors as scintillators in the FOD technique. Efficient RL from these phosphors in the red spectral region allows using simple optical filtering methods to get rid of the spurious luminescence caused by the stem effect. Moreover, these yttrium europium-doped phosphors show no changes in RL response as dose accumulates and no afterglow decay. Summarizing, this work has demonstrated that Y2O2S:Eu3 þ , YVO4:Eu3 þ , and Y2O3:Eu3 þ are promising compounds to be employed as FOD RL phosphors deserving further investigations.
Acknowledgements This research has been supported by Grant PICT Nr. 1907 from Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (ANPCyT, Argentina). References Andersen, C.E., Marckmann, C.J., Aznar, M.C., Botter-Jensen, L., 2006. An algorithm for real-time dosimetry in intensity-modulated radiation therapy using the radioluminescence signal from Al 2 O 3 :C. Radiat. Prot. Dosim. 120 (1–4), 7–13. Archambault, L., Beddar, A., Gingras, L., 2006. Measurement accuracy and Cerenkov removal for high performance, high spatial resolution scintillation dosimetry. Med. Phys. 33, 128.
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