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Thermoluminescence, optically stimulated luminescence and radioluminescence properties of Al2O3:C,Mg
Radiation Measurements 46 (2011) 1469e1473
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Thermoluminescence, optically stimulated luminescence and radioluminescence properties of Al2O3:C,Mg M.G. Rodriguez a, G. Denis a, M.S. Akselrod b, T.H. Underwood b, E.G. Yukihara a, * a b
Physics Department, Oklahoma State University, 145 Physical Sciences II, Stillwater, OK 74078, USA Landauer, Inc., Stillwater Crystal Growth Division, 723 1/2 Eastgate, Stillwater, OK 74074, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2010 Received in revised form 12 April 2011 Accepted 27 April 2011
The objective of this paper is to investigate the basic luminescence properties of enhanced aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg), including thermoluminescence (TL), optically stimulated luminescence (OSL), and radioluminescence (RL) to determine its potential for applications in radiation dosimetry. In general, the intensities of the TL, OSL, and RL signals of the Al2O3:C,Mg samples are comparable to those of regular carbon-doped aluminum oxide (Al2O3:C). The enhanced Al2O3:C,Mg samples show the presence of Mg-induced aggregate defects, mainly F22þ(2Mg) and F2þ(2Mg) centers, with emissions at 520 and 750 nm, respectively, in addition to an increased Fþ-center RL emission at 325 nm when compared to Al2O3:C. In Al2O3:C,Mg with empty trapping centers, the RL intensity varies less with irradiation time than Al2O3:C, showing that the RL sensitivity of Al2O3:C,Mg is less dependent on the occupancy of the trapping states than Al2O3:C. Doping with Mg also introduces three TL peaks associated with new shallow traps. The main TL peak in Al2O3:C,Mg is narrower and shifted to a lower temperature when compared to Al2O3:C. The OSL from the Al2O3:C,Mg samples was also observed to decay faster than Al2O3:C. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Optically stimulated luminescence Radioluminescence Aggregate defects Al2O3:C Al2O3:C,Mg F centers
1. Introduction Specially enhanced aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg) is a material developed for optical data storage and fluorescence nuclear track detection (FNTD) (Akselrod et al., 2006, 2003a; Sykora et al., 2007). Al2O3:C,Mg contains luminescence centers with short luminescence lifetime (<100 ns), particularly new defects such as F22þ(2Mg) and F2þ(2Mg) centers (Akselrod et al., 2003b). These defects are reported as pairs of oxygen vacancies (2Fþ 4 F22þ and F þ Fþ 4 F2þ) in a next-nearest neighbor configuration stabilized in the vicinity of a Mg2þ cation (Ramirez et al., 2007). The proximity of the Mg2þ cation is evident by the blue shift of the absorption and emission bands relative to the bands of the undisturbed oxygen vacancy pairs (Akselrod et al., 2003b). In this work the defects responsible for the emissions at 520 and 750 nm will be labeled F22þ(2Mg) and F2þ(2Mg), respectively, in agreement with the notation used by Akselrod et al. (2003b). The F22þ(2Mg) center is responsible for the 435 nm absorption band and green coloration of the crystals (Akselrod et al., 2003b). In addition, Al2O3:C,Mg also shows an increased
* Corresponding author. Tel.: þ1 405 744 5051; fax: þ1 405 744 6811. E-mail address: [email protected] (E.G. Yukihara). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.04.026
concentration of Fþ centers when compared to carbon-doped aluminum oxide (Al2O3:C), which is widely used as a thermoluminescent (TL) and optically stimulated luminescent (OSL) dosimeter (Akselrod and Kortov, 1990; Bøtter-Jensen et al., 2003). The photoluminescence properties of Al2O3:C,Mg and the possibility of converting the F22þ(2Mg) and F2þ(2Mg) centers into each other by light or radiation are relatively well-known (Akselrod et al., 2003b; Akselrod and Akselrod, 2006; Sanyal and Akselrod, 2005; Sykora, 2010). However, there is little information on the OSL, TL and radioluminescence (RL) properties of this material, which is essential when considering new applications in radiation dosimetry. Although Al2O3:C has been successfully used as a detector in TL and OSL dosimetry, particularly in the fields of personal, medical, and space dosimetry (Yukihara and McKeever, 2011), there are some limitations for applications in two-dimensional dose mapping and real-time optical fiber dosimetry. For example, in two-dimensional dose mapping the long lifetime of the main luminescence center in Al2O3:C, w35 ms at room temperature (Akselrod et al., 1998; Markey et al., 1995), prevents the readout using spot laser scanning techniques. In fiber-optic dosimetry using Al2O3:C, the dose rate is evaluated in real-time by measuring the RL signal while a post-treatment total dose evaluation can be performed by OSL readout (Aznar et al., 2004; Erfurt et al., 2000; Polf et al., 2002). However, the change in RL sensitivity with the
M.G. Rodriguez et al. / Radiation Measurements 46 (2011) 1469e1473
Al2O3:C and enhanced Al2O3:C,Mg single crystals were grown by the Czochralski technique at Landauer Stillwater Crystal Growth Division (Stillwater, OK, USA). Al2O3:C,Mg is grown in the presence of Mg and submitted to an enhancing thermal procedure, resulting in a yellow-green coloration due to formation of aggregated defects and introduction of an optical absorption band at w435 nm. For this investigation, Al2O3:C,Mg samples with different coloration were chosen. Samples of the batches E41 and E35 (labeled Mg-E41 and Mg-E35) were selected for their low green coloration (i.e., a low extinction coefficient at 435 nm), samples of batches A195 and A146 (labeled Mg-A195 and Mg-A146) were chosen because of their intermediate green coloration, and samples from batches A194 and A136 (labeled Mg-A194 and Mg-A136) were taken because of their high green coloration. Samples of Al2O3:C were labeled CZ#60. Some samples were ground into powder using an agate mortar and sieved to obtain grain sizes inferior to 75 mm. RL measurements were performed under irradiation with X-rays (Magnum X-ray tube, model TUB00045-1, having tungsten filament, silver target and a 0.25 mm thick beryllium window, Moxtek Inc., Orem, UT, USA) produced at 40 kVp tube potential and 100 mA tube current. The light emitted by the sample was collected either by a photomultiplier tube (PMT) (model P25PC, Sens-Tech Ltd., Langley, UK) or a fiber-optic spectrometer (model USB2000, Ocean Optics Inc., Dunedin, FL, USA). A Corion 415D10 nm interference filter was used in front of the PMT. The Ocean Optics spectrometer is equipped with a linear silicon CCD array (model ILX511, Sony, Tokyo, Japan) and the light is captured via an optical fiber with an f/2 fused silica lens, 1 mm core and wavelength transmission from 200 nm to 1100 nm. The spectral sensitivity was determined and corrected for by using a light source with known irradiance. The X-ray dose rate at the sample position is estimated to be w150 mGy/s. Risø TL/OSL-DA-15 readers were used for TL and OSL measurements (Risø National Laboratory, Røskilde, Denmark; Bøtter-Jensen et al., 2010). Irradiations were performed using a 90Sr/90Y beta source delivering a dose rate of approximately 50 mGy/s (calibrated in 60Co gamma dose to water using 0.9-mm thick Al2O3:C as the calibration transfer detector). The samples were placed in stainless steel cups and the light emitted by the samples was measured by a PMT (model 9235QB, Electron Tubes Ltd., Middlesex, UK). All TL measurements were carried out at a heating rate of 1 C/s in a N2 atmosphere. A band-pass filter (BG-39, 6 mm total thickness, transmission in the 350e690 nm range, Schott AG, Mainz, Germany) was placed in front of the PMT for the TL measurements.
3. Results The RL emission spectra of Al2O3:C and Al2O3:C,Mg are shown in Fig. 1. In Al2O3:C (CZ#60), emission bands appear at 415 nm and, with much lower intensity, at 325 nm. These two bands are associated with F and Fþ centers, respectively (Akselrod and Kortov, 1990). The introduction of Mg decreases the intensity of the F-center emission and increases the intensity of the Fþ-center emission. This can be partially attributed to the conversion of some F centers into Fþ centers as a result of charge compensation for the substitutional Mg2þ ion in an Al3þ site. Mg doping also induces new broad RL bands at w520 and w750 nm, attributed to F22þ(2Mg) and F2þ(2Mg) centers, respectively (Akselrod et al., 2003b). Another effect of Mg doping is a shift of the F-center emission toward shorter wavelengths in Al2O3:C,Mg when
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2. Materials and methods
The TL emission spectra were measured by replacing the PMT with the fiber-optic spectrometer described earlier and integrating the TL signal over 5 s for each spectrum. TL spectra acquired were not corrected for the spectral sensitivity of the spectrometer. The OSL measurements were carried out under continuous green LED stimulation (w525 nm) with an irradiance of w8.5 mW/cm2 at the sample position. A UV band-pass filter (U-340, 7.5 mm total thickness, transmission in the 290e390 nm range, Hoya Corporation, Santa Clara, CA, USA) was placed in front of the PMT for the OSL measurements.
RL Signal (10 Counts)
accumulated dose of radiation is a problem that has to be mitigated by algorithms following a characterization of the dose-dependent sensitivity (e.g., Andersen et al., 2006). It is in this context that the interest in the properties of Al2O3:C,Mg arise. The higher concentration of Fþ centers, which have a luminescence lifetime <7 ns (Evans and Stapelbroek, 1978), could eliminate the problem related to the long luminescence lifetime of the F centers during OSL readout in dose mapping applications. Moreover, the different types and concentration of defects may result in properties that are more suitable for optical fiber dosimetry, such as a smaller dependence of the RL intensity with dose. At this point, these are only speculations. The objective of this paper is to investigate basic luminescence properties of Al2O3:C,Mg and compare them to the properties of Al2O3:C. Properties investigated include RL, TL and OSL of several Al2O3:C,Mg samples chosen for their different concentration of Mg-associated aggregate defects. We also investigated Al2O3:C for comparison.
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Wavelength (nm) Fig. 1. (a) RL emission spectra Al2O3:C (sample CZ#60) and Al2O3:C,Mg samples (Mg-E41, Mg-A146, and Mg-A136) integrated over 10 s of irradiation and then corrected for the spectrometer sensitivity; (b) same data normalized to the intensity of the F-center emission.
M.G. Rodriguez et al. / Radiation Measurements 46 (2011) 1469e1473
compared to Al2O3:C (Fig. 1b). This may be due to self-absorption of F-center luminescence by the F22þ(2Mg) centers, which has an absorption band centered at 435 nm (Rodriguez, 2010). Fig. 2 shows the RL intensity as a function of the irradiation time. The data shown in Fig. 2a correspond to samples that were bleached to empty the optically active traps. Because these empty traps are filled during irradiation, there is a continuous increase in the RL signal over the 280 s irradiation period (Damkjær et al., 2008). In the case of Al2O3:C, the RL intensity increases by more than 80% over the irradiation period, whereas for Al2O3:C,Mg the increase is of the order of 15e25%. The smaller variation in RL intensity in Al2O3:C,Mg samples may be related to the higher concentration of Fþ centers in the Al2O3:C,Mg samples when compared to Al2O3:C, which may favor the Fþ þ e / F þ hn420nm recombination in comparison with charge trapping. Once the irradiation source is turned off, the RL from Al2O3:C decreases rapidly to a small value, whereas luminescence is still emitted by Al2O3:C,Mg samples. This persistent signal is due to phosphorescence associated with shallow traps induced by the Mg doping (see below). The phosphorescence intensity relatively to the RL intensity at 280 s varies from sample-to-sample in the following sequence, from higher phosphorescence intensity to lower intensity: Mg-E41, Mg-A146 and Mg-A136. This is in agreement with the measured intensity of the TL associated with shallow traps (see below).
Fig. 2b shows the RL intensity as a function of irradiation time when the trapping centers are filled (samples irradiated with w40 Gy). Because of the reduced competition for charge trapping, the variation in the RL intensity is between w5% and 15%, depending on the sample. In this case, Al2O3:C shows less variation than Al2O3:C,Mg, which may be explained by the difference in saturation dose between the samples studied here. Additional measurements (not shown here) present evidence that the OSL signal of Al2O3:C sample CZ#60 saturates around 10 Gy. Therefore, after w40 Gy all trapping centers are full and the RL intensity of Al2O3:C sample CZ#60 remains essentially constant. On the other hand, the OSL from Al2O3:C,Mg saturates at a higher dose, w100 Gy, and therefore there is still a change in the occupancy of the trapping centers over the irradiation period in Fig. 2b. Al2O3:C,Mg again exhibits phosphorescence signal after the X-rays are turned off. The TL glow curves of Al2O3:C and Al2O3:C,Mg are shown in Fig. 3. Due to the filter used (Schott BG-39), the TL glow curves consist of luminescence from F centers and, in the case of Al2O3:C,Mg, also F22þ(2Mg) centers. Whereas the TL of Al2O3:C shows a dominant TL peak centered at w198 C (FWHM of 50 C), the TL curves from Al2O3:C,Mg samples show three new TL peaks related to shallow traps with intensity maxima at 46, 75 and 109 C (Fig. 3a), in addition to the dosimetric TL peak. Samples with higher coloration seem to have lower concentration of shallow traps. Furthermore, the dosimetric TL peak of the Al2O3:C,Mg samples is shifted to lower temperatures (w178 C) and is narrower (FWHM of
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Time (s) Fig. 2. RL signal associated with the F-center emission of Al2O3:C (CZ#60) and Al2O3:C,Mg (samples Mg-E41, Mg-A146, and Mg-A136), normalized to the value at 280 s of X-rays irradiation, for: (a) samples bleached for 5 min with green LED and, (b) same samples after the previous experiment (accumulated dose of w40 Gy). The signal was detected with a 415D10 nm interference filter in front of the PMT. Each data point corresponds to an integration period of 0.05 s.
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Fig. 3. (a) Absolute and (b) normalized TL glow curves of Al2O3:C (sample CZ#60) and Al2O3:C,Mg (Mg-E41, Mg-A146, and Mg-A136) following beta irradiation with 176 mGy. Figure (a) also shows a first-order TL curve (RW) for s ¼ 2.92 1013 s1 and E ¼ 1.30 eV (see McKeever, 1985).
M.G. Rodriguez et al. / Radiation Measurements 46 (2011) 1469e1473
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34 C) in comparison to the main dosimetric peak of Al2O3:C (Fig. 3b). The different characteristics of the main TL peak in Al2O3:C and Al2O3:C,Mg may be related to the different trapping center distributions, since the dosimetric peak in Al2O3:C is reported as being an overlap of several components (e.g., Akselrod and Akselrod, 2002). The main TL peak in Al2O3:C,Mg can be described by a first-order TL peak, although the agreement is not perfect and thermal quenching was not taken into account (see Fig. 3a). Moreover, the shape and maximum intensity of both 46 C and dosimetric TL peak vary little within the Al2O3:C,Mg samples, whereas the intensities of the 75 C and 109 C TL peaks are too weak to draw any conclusion. The TL emission spectra of Al2O3:C and Al2O3:C,Mg are compared in Fig. 4a and b, respectively. In Al2O3:C, the dosimetric TL peak is characterized by a broad emission band centered at 415 nm due to F centers (Akselrod and Kortov, 1990). In Al2O3:C,Mg, the dosimetric TL peak is a result of emissions at 325, 415, 520 and 750 nm, likely corresponding to Fþ, F, F22þ(2Mg) and F2þ(2Mg) centers, respectively. Shallow trap TL peaks are not perceptible because of the low sensitivity of the fiber-optic spectrometer. The CW-OSL curves of Al2O3:C and Al2O3:C,Mg are shown in Fig. 5. Since we used Hoya U-340 filters, the OSL curves correspond only to the luminescence from the Fþ and F centers. The OSL intensity of the Al2O3:C,Mg samples is comparable to the OSL intensity of Al2O3:C in these experimental conditions, but the OSL curve decays faster (see normalized curves in Fig. 5b). This may be related to the fact that the main TL peak in Al2O3:C,Mg appears at a lower temperature than the one in Al2O3:C. As discussed by
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Stimulation time (s) Fig. 5. (a) Absolute and (b) normalized CW-OSL curves of Al2O3:C (sample CZ#60) and Al2O3:C,Mg (Mg-E35, Mg-A194, and Mg-A195) powders (w10 mg) following beta irradiation with 700 mGy.
Akselrod and Akselrod (2002), the trapping centers associated with the low temperature part of the TL peak are characterized by higher photoionization cross-section than those associated with the high temperature part of the TL peak, thus explaining the faster decay of the OSL curves in Al2O3:C,Mg. 4. Conclusion
Fig. 4. TL emission spectrum of (a) Al2O3:C (sample CZ#60) and (b) Al2O3:C,Mg (Mg-A136) following beta irradiation with 100 Gy. The spectra were not corrected for the instrument response.
The results in this study show that the RL, TL and OSL signals from Al2O3:C,Mg samples are comparable to those of Al2O3:C samples. Al2O3:C,Mg samples show a higher Fþ-center RL emission when compared to Al2O3:C, in addition to the presence of F22þ(2Mg)- and F2þ(2Mg)-center emissions. The RL signal from Al2O3:C,Mg samples shows less variation in RL intensity with irradiation time (i.e., dose) than Al2O3:C when the optically active traps are empty, but the opposite is observed when they are filled. In the case of RL and TL, all emission bands participate in the luminescence process of the Al2O3:C,Mg samples. Finally, the OSL curves of Al2O3:C,Mg samples decay faster than the ones of Al2O3:C. These results represent an initial characterization of Al2O3:C,Mg in comparison with Al2O3:C for applications in radiation dosimetry. Further studies should focus on whether these characteristics of Al2O3:C,Mg result in actual dosimetric advantages for specific applications such as neutron dosimetry or proton therapy dosimetry.
M.G. Rodriguez et al. / Radiation Measurements 46 (2011) 1469e1473
Acknowledgments This work was supported by Landauer Inc. References Akselrod, A.E., Akselrod, M.S., 2002. Correlation between OSL and the distribution of TL traps in Al2O3:C. Radiat. Prot. Dosimetry 100, 217e220. Akselrod, G.M., Akselrod, M.S., Benton, E.R., Yasuda, N., 2006. A novel Al2O3 fluorescent nuclear track detector for heavy charged particles and neutrons. Nucl. Instrum. Methods Phys. Res. 247, 295e306. Akselrod, M.S., Kortov, V.S., 1990. Thermoluminescent and exoemission properties of new high-sensitivity TLD a-Al2O3:C crystals. Radiat. Prot. Dosimetry 33, 123e126. Akselrod, M.S., Agersnap Larsen, N., Whitley, V.H., McKeever, S.W.S., 1998. Thermal quenching of F-center luminescence in Al2O3:C. J. Appl. Phys. 84, 3364e3373. Akselrod, M.S., Akselrod, A.E., Orlov, S.S., Sanyal, S., Underwood, T.H., 2003a. Fluorescent aluminum oxide crystals for volumetric optical data storage and imaging applications. J. Fluoresc. 13, 503e511. Akselrod, M.S., Akselrod, A.E., Orlov, S.S., Sanyal, S., Underwood, T.H., 2003b. New aluminum oxide single crystals for volumetric optical data storage. Proc. SPIE 5069, 244e251. Akselrod, M.S., Akselrod, A.E., 2006. New Al2O3:C,Mg crystals for radiophotoluminescent dosimetry and optical imaging. Radiat. Prot. Dosimetry 119, 218e221. Andersen, C.E., Marckmann, C.J., Aznar, M.C., Bøtter-Jensen, L., Kjær-Kristoffersen, F., Medin, J., 2006. An algorithm for real-time dosimetry in intensity-modulated radiation therapy using the radioluminescence signal from Al2O3:C. Radiat. Prot. Dosimetry 120, 7e13. Aznar, M.C., Andersen, C.E., Bøtter-Jensen, L., Bäck, S.Å.J., Mattsson, S., KjærKristoffersen, F., Medin, J., 2004. Real-time optical-fibre luminescence dosimetry for radiotherapy: physical characteristics and applications in photon beams. Phys. Med. Biol. 49, 1655e1669.
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Bøtter-Jensen, L., McKeever, S.W.S., Wintle, A.G., 2003. Optically Stimulated Luminescence Dosimetry. Elsevier, Amsterdam. Bøtter-Jensen, L., Thomsen, K.J., Jain, M., 2010. Review of optically stimulated luminescence (OSL) instrumental developments for retrospective dosimetry. Radiat. Meas. 45, 253e257. Damkjær, S.M.S., Andersen, C.E., Aznar, M.C., 2008. Improved real-time dosimetry using the radioluminescence signal from Al2O3:C. Radiat. Meas. 43, 893e897. Erfurt, G., Krbetschek, M.R., Trautmann, T., Stolz, W., 2000. Radioluminescence (RL) behaviour of Al2O3:C e potential for dosimetric applications. Radiat. Meas. 32, 735e739. Evans, B.D., Stapelbroek, M., 1978. Optical properties of the Fþ center in crystalline Al2O3. Phys. Rev. B 18, 7089e7098. Markey, B.G., Colyott, L.E., McKeever, S.W.S., 1995. Time-resolved optically stimulated luminescence from a-Al2O3:C. Radiat. Meas. 24, 457e463. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, Cambridge. Polf, J.C., McKeever, S.W.S., Akselrod, M.S., Holmstrom, S., 2002. A real-time, fibre optic dosimetry system using Al2O3 fibres. Radiat. Prot. Dosimetry 100, 301e304. Ramirez, R., Tardio, M., Gonzalez, R., Santiuste, J.E.M., 2007. Optical properties of vacancies in thermochemically reduced Mg-doped sapphire single crystals. J. Appl. Phys. 101. Rodriguez, M.G., 2010. Characterization of Aluminum Oxide Doped with Carbon and Magnesium for Radiation Detection. M.S. thesis, Oklahoma State University. Sanyal, S., Akselrod, M.S., 2005. Anisotropy of optical absorption and fluorescence in Al2O3:C,Mg crystals. J. Appl. Phys. 98. Sykora, G., Akselrod, M.S., Salasky, M., Marino, S.A., 2007. Novel Al2O3:C,Mg fluorescent nuclear track detectors for passive neutron dosimetry. Radiat. Prot. Dosimetry 126, 278e283. Sykora, G., 2010. Investigation of Radiation Induced Transformation of Al2O3:C,Mg Fluorescent Nuclear Track Detectors Applied to Mixed Neutron-gamma Fields. Oklahoma State University. Yukihara, E.G., McKeever, S.W.S., 2011. Optically Stimulated Luminescence: Fundamentals and Applications. Wiley & Sons, Chichester, West Sussex, UK.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Thermoluminescence, optically stimulated luminescence and radioluminescence properties of Al2O3:C,Mg M.G. Rodriguez a, G. Denis a, M.S. Akselrod b, T.H. Underwood b, E.G. Yukihara a, * a b
Physics Department, Oklahoma State University, 145 Physical Sciences II, Stillwater, OK 74078, USA Landauer, Inc., Stillwater Crystal Growth Division, 723 1/2 Eastgate, Stillwater, OK 74074, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2010 Received in revised form 12 April 2011 Accepted 27 April 2011
The objective of this paper is to investigate the basic luminescence properties of enhanced aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg), including thermoluminescence (TL), optically stimulated luminescence (OSL), and radioluminescence (RL) to determine its potential for applications in radiation dosimetry. In general, the intensities of the TL, OSL, and RL signals of the Al2O3:C,Mg samples are comparable to those of regular carbon-doped aluminum oxide (Al2O3:C). The enhanced Al2O3:C,Mg samples show the presence of Mg-induced aggregate defects, mainly F22þ(2Mg) and F2þ(2Mg) centers, with emissions at 520 and 750 nm, respectively, in addition to an increased Fþ-center RL emission at 325 nm when compared to Al2O3:C. In Al2O3:C,Mg with empty trapping centers, the RL intensity varies less with irradiation time than Al2O3:C, showing that the RL sensitivity of Al2O3:C,Mg is less dependent on the occupancy of the trapping states than Al2O3:C. Doping with Mg also introduces three TL peaks associated with new shallow traps. The main TL peak in Al2O3:C,Mg is narrower and shifted to a lower temperature when compared to Al2O3:C. The OSL from the Al2O3:C,Mg samples was also observed to decay faster than Al2O3:C. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Optically stimulated luminescence Radioluminescence Aggregate defects Al2O3:C Al2O3:C,Mg F centers
1. Introduction Specially enhanced aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg) is a material developed for optical data storage and fluorescence nuclear track detection (FNTD) (Akselrod et al., 2006, 2003a; Sykora et al., 2007). Al2O3:C,Mg contains luminescence centers with short luminescence lifetime (<100 ns), particularly new defects such as F22þ(2Mg) and F2þ(2Mg) centers (Akselrod et al., 2003b). These defects are reported as pairs of oxygen vacancies (2Fþ 4 F22þ and F þ Fþ 4 F2þ) in a next-nearest neighbor configuration stabilized in the vicinity of a Mg2þ cation (Ramirez et al., 2007). The proximity of the Mg2þ cation is evident by the blue shift of the absorption and emission bands relative to the bands of the undisturbed oxygen vacancy pairs (Akselrod et al., 2003b). In this work the defects responsible for the emissions at 520 and 750 nm will be labeled F22þ(2Mg) and F2þ(2Mg), respectively, in agreement with the notation used by Akselrod et al. (2003b). The F22þ(2Mg) center is responsible for the 435 nm absorption band and green coloration of the crystals (Akselrod et al., 2003b). In addition, Al2O3:C,Mg also shows an increased
* Corresponding author. Tel.: þ1 405 744 5051; fax: þ1 405 744 6811. E-mail address: [email protected] (E.G. Yukihara). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.04.026
concentration of Fþ centers when compared to carbon-doped aluminum oxide (Al2O3:C), which is widely used as a thermoluminescent (TL) and optically stimulated luminescent (OSL) dosimeter (Akselrod and Kortov, 1990; Bøtter-Jensen et al., 2003). The photoluminescence properties of Al2O3:C,Mg and the possibility of converting the F22þ(2Mg) and F2þ(2Mg) centers into each other by light or radiation are relatively well-known (Akselrod et al., 2003b; Akselrod and Akselrod, 2006; Sanyal and Akselrod, 2005; Sykora, 2010). However, there is little information on the OSL, TL and radioluminescence (RL) properties of this material, which is essential when considering new applications in radiation dosimetry. Although Al2O3:C has been successfully used as a detector in TL and OSL dosimetry, particularly in the fields of personal, medical, and space dosimetry (Yukihara and McKeever, 2011), there are some limitations for applications in two-dimensional dose mapping and real-time optical fiber dosimetry. For example, in two-dimensional dose mapping the long lifetime of the main luminescence center in Al2O3:C, w35 ms at room temperature (Akselrod et al., 1998; Markey et al., 1995), prevents the readout using spot laser scanning techniques. In fiber-optic dosimetry using Al2O3:C, the dose rate is evaluated in real-time by measuring the RL signal while a post-treatment total dose evaluation can be performed by OSL readout (Aznar et al., 2004; Erfurt et al., 2000; Polf et al., 2002). However, the change in RL sensitivity with the
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Al2O3:C and enhanced Al2O3:C,Mg single crystals were grown by the Czochralski technique at Landauer Stillwater Crystal Growth Division (Stillwater, OK, USA). Al2O3:C,Mg is grown in the presence of Mg and submitted to an enhancing thermal procedure, resulting in a yellow-green coloration due to formation of aggregated defects and introduction of an optical absorption band at w435 nm. For this investigation, Al2O3:C,Mg samples with different coloration were chosen. Samples of the batches E41 and E35 (labeled Mg-E41 and Mg-E35) were selected for their low green coloration (i.e., a low extinction coefficient at 435 nm), samples of batches A195 and A146 (labeled Mg-A195 and Mg-A146) were chosen because of their intermediate green coloration, and samples from batches A194 and A136 (labeled Mg-A194 and Mg-A136) were taken because of their high green coloration. Samples of Al2O3:C were labeled CZ#60. Some samples were ground into powder using an agate mortar and sieved to obtain grain sizes inferior to 75 mm. RL measurements were performed under irradiation with X-rays (Magnum X-ray tube, model TUB00045-1, having tungsten filament, silver target and a 0.25 mm thick beryllium window, Moxtek Inc., Orem, UT, USA) produced at 40 kVp tube potential and 100 mA tube current. The light emitted by the sample was collected either by a photomultiplier tube (PMT) (model P25PC, Sens-Tech Ltd., Langley, UK) or a fiber-optic spectrometer (model USB2000, Ocean Optics Inc., Dunedin, FL, USA). A Corion 415D10 nm interference filter was used in front of the PMT. The Ocean Optics spectrometer is equipped with a linear silicon CCD array (model ILX511, Sony, Tokyo, Japan) and the light is captured via an optical fiber with an f/2 fused silica lens, 1 mm core and wavelength transmission from 200 nm to 1100 nm. The spectral sensitivity was determined and corrected for by using a light source with known irradiance. The X-ray dose rate at the sample position is estimated to be w150 mGy/s. Risø TL/OSL-DA-15 readers were used for TL and OSL measurements (Risø National Laboratory, Røskilde, Denmark; Bøtter-Jensen et al., 2010). Irradiations were performed using a 90Sr/90Y beta source delivering a dose rate of approximately 50 mGy/s (calibrated in 60Co gamma dose to water using 0.9-mm thick Al2O3:C as the calibration transfer detector). The samples were placed in stainless steel cups and the light emitted by the samples was measured by a PMT (model 9235QB, Electron Tubes Ltd., Middlesex, UK). All TL measurements were carried out at a heating rate of 1 C/s in a N2 atmosphere. A band-pass filter (BG-39, 6 mm total thickness, transmission in the 350e690 nm range, Schott AG, Mainz, Germany) was placed in front of the PMT for the TL measurements.
3. Results The RL emission spectra of Al2O3:C and Al2O3:C,Mg are shown in Fig. 1. In Al2O3:C (CZ#60), emission bands appear at 415 nm and, with much lower intensity, at 325 nm. These two bands are associated with F and Fþ centers, respectively (Akselrod and Kortov, 1990). The introduction of Mg decreases the intensity of the F-center emission and increases the intensity of the Fþ-center emission. This can be partially attributed to the conversion of some F centers into Fþ centers as a result of charge compensation for the substitutional Mg2þ ion in an Al3þ site. Mg doping also induces new broad RL bands at w520 and w750 nm, attributed to F22þ(2Mg) and F2þ(2Mg) centers, respectively (Akselrod et al., 2003b). Another effect of Mg doping is a shift of the F-center emission toward shorter wavelengths in Al2O3:C,Mg when
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2. Materials and methods
The TL emission spectra were measured by replacing the PMT with the fiber-optic spectrometer described earlier and integrating the TL signal over 5 s for each spectrum. TL spectra acquired were not corrected for the spectral sensitivity of the spectrometer. The OSL measurements were carried out under continuous green LED stimulation (w525 nm) with an irradiance of w8.5 mW/cm2 at the sample position. A UV band-pass filter (U-340, 7.5 mm total thickness, transmission in the 290e390 nm range, Hoya Corporation, Santa Clara, CA, USA) was placed in front of the PMT for the OSL measurements.
RL Signal (10 Counts)
accumulated dose of radiation is a problem that has to be mitigated by algorithms following a characterization of the dose-dependent sensitivity (e.g., Andersen et al., 2006). It is in this context that the interest in the properties of Al2O3:C,Mg arise. The higher concentration of Fþ centers, which have a luminescence lifetime <7 ns (Evans and Stapelbroek, 1978), could eliminate the problem related to the long luminescence lifetime of the F centers during OSL readout in dose mapping applications. Moreover, the different types and concentration of defects may result in properties that are more suitable for optical fiber dosimetry, such as a smaller dependence of the RL intensity with dose. At this point, these are only speculations. The objective of this paper is to investigate basic luminescence properties of Al2O3:C,Mg and compare them to the properties of Al2O3:C. Properties investigated include RL, TL and OSL of several Al2O3:C,Mg samples chosen for their different concentration of Mg-associated aggregate defects. We also investigated Al2O3:C for comparison.
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Wavelength (nm) Fig. 1. (a) RL emission spectra Al2O3:C (sample CZ#60) and Al2O3:C,Mg samples (Mg-E41, Mg-A146, and Mg-A136) integrated over 10 s of irradiation and then corrected for the spectrometer sensitivity; (b) same data normalized to the intensity of the F-center emission.
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compared to Al2O3:C (Fig. 1b). This may be due to self-absorption of F-center luminescence by the F22þ(2Mg) centers, which has an absorption band centered at 435 nm (Rodriguez, 2010). Fig. 2 shows the RL intensity as a function of the irradiation time. The data shown in Fig. 2a correspond to samples that were bleached to empty the optically active traps. Because these empty traps are filled during irradiation, there is a continuous increase in the RL signal over the 280 s irradiation period (Damkjær et al., 2008). In the case of Al2O3:C, the RL intensity increases by more than 80% over the irradiation period, whereas for Al2O3:C,Mg the increase is of the order of 15e25%. The smaller variation in RL intensity in Al2O3:C,Mg samples may be related to the higher concentration of Fþ centers in the Al2O3:C,Mg samples when compared to Al2O3:C, which may favor the Fþ þ e / F þ hn420nm recombination in comparison with charge trapping. Once the irradiation source is turned off, the RL from Al2O3:C decreases rapidly to a small value, whereas luminescence is still emitted by Al2O3:C,Mg samples. This persistent signal is due to phosphorescence associated with shallow traps induced by the Mg doping (see below). The phosphorescence intensity relatively to the RL intensity at 280 s varies from sample-to-sample in the following sequence, from higher phosphorescence intensity to lower intensity: Mg-E41, Mg-A146 and Mg-A136. This is in agreement with the measured intensity of the TL associated with shallow traps (see below).
Fig. 2b shows the RL intensity as a function of irradiation time when the trapping centers are filled (samples irradiated with w40 Gy). Because of the reduced competition for charge trapping, the variation in the RL intensity is between w5% and 15%, depending on the sample. In this case, Al2O3:C shows less variation than Al2O3:C,Mg, which may be explained by the difference in saturation dose between the samples studied here. Additional measurements (not shown here) present evidence that the OSL signal of Al2O3:C sample CZ#60 saturates around 10 Gy. Therefore, after w40 Gy all trapping centers are full and the RL intensity of Al2O3:C sample CZ#60 remains essentially constant. On the other hand, the OSL from Al2O3:C,Mg saturates at a higher dose, w100 Gy, and therefore there is still a change in the occupancy of the trapping centers over the irradiation period in Fig. 2b. Al2O3:C,Mg again exhibits phosphorescence signal after the X-rays are turned off. The TL glow curves of Al2O3:C and Al2O3:C,Mg are shown in Fig. 3. Due to the filter used (Schott BG-39), the TL glow curves consist of luminescence from F centers and, in the case of Al2O3:C,Mg, also F22þ(2Mg) centers. Whereas the TL of Al2O3:C shows a dominant TL peak centered at w198 C (FWHM of 50 C), the TL curves from Al2O3:C,Mg samples show three new TL peaks related to shallow traps with intensity maxima at 46, 75 and 109 C (Fig. 3a), in addition to the dosimetric TL peak. Samples with higher coloration seem to have lower concentration of shallow traps. Furthermore, the dosimetric TL peak of the Al2O3:C,Mg samples is shifted to lower temperatures (w178 C) and is narrower (FWHM of
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Time (s) Fig. 2. RL signal associated with the F-center emission of Al2O3:C (CZ#60) and Al2O3:C,Mg (samples Mg-E41, Mg-A146, and Mg-A136), normalized to the value at 280 s of X-rays irradiation, for: (a) samples bleached for 5 min with green LED and, (b) same samples after the previous experiment (accumulated dose of w40 Gy). The signal was detected with a 415D10 nm interference filter in front of the PMT. Each data point corresponds to an integration period of 0.05 s.
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Fig. 3. (a) Absolute and (b) normalized TL glow curves of Al2O3:C (sample CZ#60) and Al2O3:C,Mg (Mg-E41, Mg-A146, and Mg-A136) following beta irradiation with 176 mGy. Figure (a) also shows a first-order TL curve (RW) for s ¼ 2.92 1013 s1 and E ¼ 1.30 eV (see McKeever, 1985).
M.G. Rodriguez et al. / Radiation Measurements 46 (2011) 1469e1473
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34 C) in comparison to the main dosimetric peak of Al2O3:C (Fig. 3b). The different characteristics of the main TL peak in Al2O3:C and Al2O3:C,Mg may be related to the different trapping center distributions, since the dosimetric peak in Al2O3:C is reported as being an overlap of several components (e.g., Akselrod and Akselrod, 2002). The main TL peak in Al2O3:C,Mg can be described by a first-order TL peak, although the agreement is not perfect and thermal quenching was not taken into account (see Fig. 3a). Moreover, the shape and maximum intensity of both 46 C and dosimetric TL peak vary little within the Al2O3:C,Mg samples, whereas the intensities of the 75 C and 109 C TL peaks are too weak to draw any conclusion. The TL emission spectra of Al2O3:C and Al2O3:C,Mg are compared in Fig. 4a and b, respectively. In Al2O3:C, the dosimetric TL peak is characterized by a broad emission band centered at 415 nm due to F centers (Akselrod and Kortov, 1990). In Al2O3:C,Mg, the dosimetric TL peak is a result of emissions at 325, 415, 520 and 750 nm, likely corresponding to Fþ, F, F22þ(2Mg) and F2þ(2Mg) centers, respectively. Shallow trap TL peaks are not perceptible because of the low sensitivity of the fiber-optic spectrometer. The CW-OSL curves of Al2O3:C and Al2O3:C,Mg are shown in Fig. 5. Since we used Hoya U-340 filters, the OSL curves correspond only to the luminescence from the Fþ and F centers. The OSL intensity of the Al2O3:C,Mg samples is comparable to the OSL intensity of Al2O3:C in these experimental conditions, but the OSL curve decays faster (see normalized curves in Fig. 5b). This may be related to the fact that the main TL peak in Al2O3:C,Mg appears at a lower temperature than the one in Al2O3:C. As discussed by
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Stimulation time (s) Fig. 5. (a) Absolute and (b) normalized CW-OSL curves of Al2O3:C (sample CZ#60) and Al2O3:C,Mg (Mg-E35, Mg-A194, and Mg-A195) powders (w10 mg) following beta irradiation with 700 mGy.
Akselrod and Akselrod (2002), the trapping centers associated with the low temperature part of the TL peak are characterized by higher photoionization cross-section than those associated with the high temperature part of the TL peak, thus explaining the faster decay of the OSL curves in Al2O3:C,Mg. 4. Conclusion
Fig. 4. TL emission spectrum of (a) Al2O3:C (sample CZ#60) and (b) Al2O3:C,Mg (Mg-A136) following beta irradiation with 100 Gy. The spectra were not corrected for the instrument response.
The results in this study show that the RL, TL and OSL signals from Al2O3:C,Mg samples are comparable to those of Al2O3:C samples. Al2O3:C,Mg samples show a higher Fþ-center RL emission when compared to Al2O3:C, in addition to the presence of F22þ(2Mg)- and F2þ(2Mg)-center emissions. The RL signal from Al2O3:C,Mg samples shows less variation in RL intensity with irradiation time (i.e., dose) than Al2O3:C when the optically active traps are empty, but the opposite is observed when they are filled. In the case of RL and TL, all emission bands participate in the luminescence process of the Al2O3:C,Mg samples. Finally, the OSL curves of Al2O3:C,Mg samples decay faster than the ones of Al2O3:C. These results represent an initial characterization of Al2O3:C,Mg in comparison with Al2O3:C for applications in radiation dosimetry. Further studies should focus on whether these characteristics of Al2O3:C,Mg result in actual dosimetric advantages for specific applications such as neutron dosimetry or proton therapy dosimetry.
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Acknowledgments This work was supported by Landauer Inc. References Akselrod, A.E., Akselrod, M.S., 2002. Correlation between OSL and the distribution of TL traps in Al2O3:C. Radiat. Prot. Dosimetry 100, 217e220. Akselrod, G.M., Akselrod, M.S., Benton, E.R., Yasuda, N., 2006. A novel Al2O3 fluorescent nuclear track detector for heavy charged particles and neutrons. Nucl. Instrum. Methods Phys. Res. 247, 295e306. Akselrod, M.S., Kortov, V.S., 1990. Thermoluminescent and exoemission properties of new high-sensitivity TLD a-Al2O3:C crystals. Radiat. Prot. Dosimetry 33, 123e126. Akselrod, M.S., Agersnap Larsen, N., Whitley, V.H., McKeever, S.W.S., 1998. Thermal quenching of F-center luminescence in Al2O3:C. J. Appl. Phys. 84, 3364e3373. Akselrod, M.S., Akselrod, A.E., Orlov, S.S., Sanyal, S., Underwood, T.H., 2003a. Fluorescent aluminum oxide crystals for volumetric optical data storage and imaging applications. J. Fluoresc. 13, 503e511. Akselrod, M.S., Akselrod, A.E., Orlov, S.S., Sanyal, S., Underwood, T.H., 2003b. New aluminum oxide single crystals for volumetric optical data storage. Proc. SPIE 5069, 244e251. Akselrod, M.S., Akselrod, A.E., 2006. New Al2O3:C,Mg crystals for radiophotoluminescent dosimetry and optical imaging. Radiat. Prot. Dosimetry 119, 218e221. Andersen, C.E., Marckmann, C.J., Aznar, M.C., Bøtter-Jensen, L., Kjær-Kristoffersen, F., Medin, J., 2006. An algorithm for real-time dosimetry in intensity-modulated radiation therapy using the radioluminescence signal from Al2O3:C. Radiat. Prot. Dosimetry 120, 7e13. Aznar, M.C., Andersen, C.E., Bøtter-Jensen, L., Bäck, S.Å.J., Mattsson, S., KjærKristoffersen, F., Medin, J., 2004. Real-time optical-fibre luminescence dosimetry for radiotherapy: physical characteristics and applications in photon beams. Phys. Med. Biol. 49, 1655e1669.
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