- Email: [email protected]
Themoluminescence properties of nanocrystalline of BaSO4:Dy,Tb irradiated with gamma rays
Journal of Luminescence 137 (2013) 230–236
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Themoluminescence properties of nanocrystalline of BaSO4:Dy,Tb irradiated with gamma rays Kadijeh Rezaee Ebrahim Saraee a,n, Amin Aghay Kharieky a, Mohsen Khosravi b, Mohammad Reza Abdi c, Hossien Zamani Zeinali d a
Department of Nuclear Engineering, Faculty of Advance Sciences and Technologies, University of Isfahan, Isfahan 81746-73441, Iran Department of Nanotechnology, Faculty of Advance Science and Technology, University of Isfahan, Isfahan 81746-73441, Iran c Department of Physics, Faculty of Science, University of Isfahan, Isfahan 81746-73441, Iran d Agricultural, Medical and Industrial Research School, Nuclear Science and Technology Research Institute, Karaj, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2012 Received in revised form 14 December 2012 Accepted 9 January 2013 Available online 20 January 2013
Nanocrystalline of BaSO4 and BaSO4:Dy,Tb of grain size 45–55 nm has been prepared by the coprecipitation method and its thermoluminescence characteristics have been studied. The formation of the material was confirmed by the X-ray diffraction (XRD) and UV–visible spectroscopy. Shape and size of the prepared nanocrystalline powder was observed by a scanning electron microscope (SEM). The TL glow curve of the nanocrystalline pellets of BaSO4:Dy,Tb shows a prominent single peak at 530 K along with another peak of lesser intensity at around 420 K and 560 K. On the contrary, the nanocrystalline pellets of BaSO4 show a peak of low intensity at 500 K and prominent peak around 460 K. The glow curve structure does not change at the range of 0.1–15 kGy and some new peaks appear at high doses but have low intensities. The 530 K of the nanocrystalline pellets of BaSO4:Dy,Tb shows a linear response with exposure increasing up to very high values (as high as 7 kGy), where the other doped BaSO4 show saturation. & 2013 Elsevier B.V. All rights reserved.
Keywords: BaSO4:Dy,Tb Luminescence Nanocrystalline Rare earth
1. Introduction The nanomaterials word is used for materials whose length scale is within the nanometric range, i.e., from 1 nm to 100 nm. Within this length scale, it has been noticed that this materials have different properties from the individual atoms, molecules and bulk materials. The physical, chemical, electrical, optical properties and potential applications of these nanomaterials are dependent on their size and shape and they often display different properties in comparison with bulk materials [1,2]. There properties have attracted the researchers attention towards this materials. Luminescence is one of the fields that has attracted several researchers. Themoluminescence (TL) is a very common technique that is widely used in dose measurement of ionizing radiation such as gamma rays, X-rays, UV, b-rays, a-particles, energetic ions etc. By heating the intensity of light emitted by the phosphors, reflects the dose given to it [3–6]. Understanding the importance of thermoluminescence with nanometric structure became obvious when researchers understand that this materials have a potential application in dosimetry of ionizing radiations for the measurements of high doses where microcrystalline
n
Corresponding author. Tel: þ98 9131015519; fax: þ 98 3117934215. E-mail address: [email protected] (Kh. Rezaee Ebrahim Saraee).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.01.011
phosphors will saturate [7,8]. Many works have been conducted which show that doped phosphors are important from their luminescent, dielectric and semiconductor properties point of view [4,7–10]. In the literatures, there are studies on nano and micro-crystals of both pure and doped with different rare earth of the BaSO4 [8,11] but there are no reports on nanocrystalline of BaSO4:Dy,Tb. In this work, the nanostructure of the BaSO4 and BaSO4:Dy,Tb are prepared by the chemical co-precipitation method. The dimension property such as shape and formation is observed by a scanning electron microscope (SEM). Formation of compound and size are confirmed by EDS and X-ray diffraction (XRD). Also, the energy band gaps of the BaSO4 and BaSO4:Dy,Tb is obtained by UV–visible spectrometer. Finally, the TL response of nanocrystalline of BaSO4 and BaSO4:Dy,Tb have been studied and the results are compared with commercial thermoluminescence dosimeters (TLD-100).
2. Experimental 2.1. Method of preparation The nanocrystalline form of BaSO4 and BaSO4:Dy,Tb was prepared by the chemical co-precipitation method. In this research, reagents of BaCl2d2H2O 99.9% (Merck), (NH4)2SO4 99.9% (Merck),
Kh. Rezaee Ebrahim Saraee et al. / Journal of Luminescence 137 (2013) 230–236
231
Dy2O3 99.9% (Merck), HCl 46 M (Merck), ethanol 99.99% (Merck) and TbCl3d6H2O 99.9% (sigma-Aldrich) are used as a raw material. At first step, BaCl2d2H2O is dissolved in the first super ionized sterilized distilled water. Dy2O3 (0.1 mol%) is dissolved in diluted HCl and also TbCl3d6 H2O which is dissolved in a solution of BaCl2 with little water that contain ethanol. Then solution of (NH4)2SO4 is added dropwise to above solution till precipitation is completed. Then the precipitation is centrifuged and is washed several times using sterilized water and is centrifuged again. Ultimately, the nanophosphor is obtained by drying the precipitate at 378 K for 2 h and then the nanocrystalline powder is further annealed in ceramic crucible at 1123 K for 1 h in argon atmosphere. For preparation of pure BaSO4 all processes mentioned above is repeated; only Dy2O3 and TbCl3d6H2O is not added to compound.
show that the range of particles size is from 45 to 55 nm. Formation of the BaSO4 and BaSO4:Dy,Tb is confirmed by studying the XRD pattern. Fig. 2a and b shows the XRD pattern of the BaSO4 and BaSO4:Dy,Tb, respectively. The high purity and high crystallinity of BaSO4 is caused all diffraction peak be relatively sharp and these peaks are in good agreement with the JCPDS card (JCPDS card no. 24-1035). The XRD pattern is fitted well with orthorhombic structure of BaSO4. Broadening in the XRD lines in the samples of BaSO4 and BaSO4:Dy,Tb (Fig.2a and b) are utilized to determine the dimension of prepared nanocrystalline powder by Debye Scherer formula, D¼0.9l/ß cosy, where D is the average grain size of the nanocrystalline; l is the wavelength of incident radiation (1.5406 A); b (in radian) is the full width at half maximum (FWHM) and y is Bragg angle.
2.2. Characterization
3.2. Impurity concentrations
In order to determine the shape and size of nanocrystalline powder, SEM images is used. SEM images were obtained by VEGA, TESCAN SEM. To confirm the size and structure of the compound of nanocrystalline, XRD pattern is studied at room temperature for BaSO4 and BaSO4:Dy,Tb using Cu target (Cu-Ka, l ¼1.5406 A with Ni filter) on Bruker, model D8ADVANCE, Germany XRD machine and matched with the standard data available (JCPDS card no. 24-1035). The band gap of nanocrystalline powder of BaSO4 and BaSO4:Dy,Tb is also obtained using UV–visible absorption spectra with which this spectra is obtained by V-670 spectrophotometers (Jasco Company of Japan). To facilitate easy handling, the nanocrystalline powder with Na2SO4 99.9% (Merck) for the binder and the parts are mixed in the ratio 2 (Na2SO4):1 (powdered sample). The dimensions of pellets are 5 mm diameter and 0.8 mm thickness. The obtained pellets are irradiated with different doses of 60Co gamma rays from 0.1 Gy to 15 kGy at room temperature. TL glow curves are recorded by an IAP TLD reader (7102 model). The heating rate was 2 K/s. In order to more interoperate our results regarding the structure of glow curve, the TLD-100 is also irradiated under identical condition and recorded.
The incorporation of impurities in the crystalline is confirmed by the EDS study and the EDS peaks corresponding to the two impurities could very well be seen in Fig. 3. 3.3. Energy band gap The UV–visible absorption spectra of BaSO4 and BaSO4:Dy,Tb nanocrystalline powders are shown in Fig. 4. The band gap energies of BaSO4 and BaSO4:Dy,Tb nanocrystalline powders are calculated using the Kubelka–Munk formula and the Tauc plot. ðFðRÞhnÞ1=n ¼ Aðhn2Eg Þ
ð1Þ
where hn is the photon energy, F(R) is the absorption coefficient and A is proportional constant. The absorption coefficient is calculated from the reflectance spectra. The value of Eg (band gap) is obtained by plotting (F(R)hn)1/n versus hn in high absorption range and extrapolating the linear region of the plots of (F(R)hn)1/n ¼ 0. The analysis of the present data shows that the plots of (F(R)hn)1/n ¼0 versus hn give linear relation which is the most fitted Eq. (1) with n¼ 1/2 for both BaSO4 and BaSO4:Dy,Tb nanocrystalline powder. Fig. 4 shows the band gaps of the nanocrystalline samples.
3. Result 4. Discussion 3.1. Particle size 4.1. TL glow curve The shape and size of the BaSO4 and BaSO4:Dy,Tb nanocrystalline powder are determined by SEM. The SEM photographs of BaSO4 and BaSO4:Dy,Tb is shown in Fig. 1a and b, respectively. The images
Fig. 5 shows the TL glow curves of BaSO4 and BaSO4:Dy,Tb pellets exposed to 1 Gy of gamma rays from a 60Co source. Also
Fig. 1. SEM images of BaSO4 and BaSO4:Dy,Tb nanocrystalline.
232
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Fig. 2. X-rays diffraction pattern of nanocrystalline powder samples.
TL glow curve of TLD-100 is shown as standard phosphor for comparison. The TL glow curve of nanocrystalline phosphors of the BaSO4 and BaSO4:Dy,Tb consist of main dosimetric peak at around 460 K and 530 K, respectively. Other temperature peaks with lesser intensities at around 500 K for BaSO4 and 420 K and 560 K for BaSO4:Dy,Tb is seen. Later mentioned above peaks the fluctuation of temperature dose not considered after 593 K. The TL glow curve of TLD-100 phosphor has usual structure with dosimetry peak at around 502 K. The sensitivity of nanocrystalline of BaSO4:Dy,Tb is more than that of the BaSO4 phosphor by a factor of approx. 10 for all exposure. The peak height of glow curve is used to compare of the different thermoluminescences sensitivity. However, the height of peak is proportional to the light intensity in a given temperature and is varying with type and structure of thermoluminescence. Moreover, it is proportional to the mass of thermoluminescence. The mass of pellets is also important when they are compared with commercial type (TLD-100) because the differences between
their masses can be caused differences between their emitted intensities. Then, the comparison between the commercial type (TLD-100) and prepared pellets in this research has not high precision. With exposure increasing, the intensities of main peak have been observed to increase. The shift in the glow curve of BaSO4:Dy,Tb and BaSO4 maybe is attributed to presence of impurities and structure of compound. 4.2. Dosimetric properties All the following studies are carried out using of IAP TLD reader and a liner heating rate of 2 1K S 1 and a standard pre-read out treatment of 363 K (90 1C) for 15 min in oven. Heating at temperature of 363 K is caused which repeatability of obtained measurements has been more precision. The response sums are obtained by integrating the glow curve between points of first (393 K) and final (593 K) of main peak. In order to reuse of TLDs and eliminate any kind of trapped electron and hole, all pellets anneal at 673 K for 1 h and quenched rapidly.
Kh. Rezaee Ebrahim Saraee et al. / Journal of Luminescence 137 (2013) 230–236
Fig. 3. A typical EDS pattern of the BaSO4:Dy,Tb nanocrystalline powder.
Fig. 4. The band gaps of BaSO4 and BaSO4:Dy nanocrystalline powders.
233
234
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Fig. 5. The glow curve of BaSO4, BaSO4:Dy,Tb and TLD-100 respectively irradiated with gamma ray of
Fig. 6. The phosphor response versus dose of 1.25 MeV gamma ray,
4.2.1. Linearity The relative response versus dose is measured for each of pellets which are prepared from nanocrystalline powder and is irradiated with gamma of 60Co source. These pellets are placed in a plexiglass phantom and are placed in front of calibrated source of 60Co. Fig. 6 shows the phosphor response versus dose for each of samples, the units of dose is in Gray. As seen in the Fig. 6, BaSO4 and BaSO4:Dy,Tb is seen to be linear from about 0.1 Gy to 1 kGy and 0.1 Gy to 7 kGy, respectively. The nonlinearity response of the BaSo4 and BaSo4:Dy,Tb for above 1 kGy and 7 kGy respectively, may be attributed to saturate of dosimeter in these regions (Fig. 6). The peak height is also linear for doses between 0.1 and 1 kGy (Fig. 7). There is a tail in TL response above 593 K (Fig. 8) which is omitted because it is approximately same for all dose values and will not change with varying in the dose and temperature values.
4.2.2. Energy response The relative response versus photon energy is measured over the range 60 keV to 1.25 MeV. The filtered X-rays from an X-ray
60
60
Co at 1 Gy dose.
Co.
machine is used for producing low energy. This filter is made of copper metal to obtain effective energy. The phosphor is spread out at thin layer in order to reduce attenuation of X-rays in the phosphor itself. The irradiation with 662 keV and 1.25 MeV energy is obtained by 137Cs and 60Co gamma source. All exposure of this experiment is done for 500 mGy. Fig. 9 shows the relative photon energy response versus photon energy, normalized to unity at 1.25 MeV. The curve displays energy response for discrete energy value and same dose which obtained contiguous curve from this points is equivalent with the theoretical energy response calculated from ðmen =rÞ=ðmen =rÞair and with this assumption that light output function from only the absorbed energy by the phosphor.
5. Conclusions The nanocrystalline of BaSO4 and BaSO4:Dy,Tb have been prepared by co-precipitation method and is studied by SEM– EDS, XRD and UV–visible. This analysis showed these powders have dimension with scale of 45–55 nm. The UV–visible analysis
Kh. Rezaee Ebrahim Saraee et al. / Journal of Luminescence 137 (2013) 230–236
Fig. 7. The peak height versus dose of 1.25 MeV gamma ray,
60
Co. This peak height has been calculated for main peak of glow curve.
Fig. 8. The glow curves of BaSO4 and BaSO4:Dy,Tb.
235
236
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Fig. 9. Relative photon energy response plotted versus photon energy (MeV) which normalized to unity at energy of rhombic pictures shown energy response of TLD-100, BaSO4 and BaSO4:Dy respectively.
showed that impurity of Dy and Tb approximately decreases the band gap also. The studies of TL properties using of glow curves obtaining from irradiated pellets at dose with range of 0.1 Gy to 15 kGy showed that BaSO4:Dy,Tb is the more sensitive than BaSO4. This property causes that BaSO4:Dy,Tb be more useful for estimating of low and high dose of gamma rays.
Acknowledgment The authors thank the research fellows of the Research Institute of Agriculture, Medicine and Industry for their technical advice. The authors are also deeply grateful to Engineers Shahvar and Sharifzadeh. Also special thanks to Dr. Teymouri for his guidance.
60
Co (1.25 MeV). The points of triangle, square and
References [1] Numan Salah, S. Habib, H. Khan, Fathi Djouider, Radiat. Phys. Chem. 80 (2011) 923. [2] L.I. Yu-Feng, Jia-Hu Ouyang, Y.U. Zhou, Xue-Song Liang, Ji-Yong Zhong, Bull. Mater. Sci. 32 (2) (2009) 149 & Indian Academy of Sciences.. [3] Puja Chawla, S.P. Lochab, Nafa Singh, J. Alloys Compd. 494 (2010) L20. [4] Mg Cu, P. Numan Salah, P.D. Sahare, A.A. Rupasov, J. Lumin. 124 (2007) 357. [5] A. Pandey, Shaila Bahl, Kanika Sharma, Ranju Ranjan, Pratik Kumar, S.P. Lochab, V.E. Aleynikov, A.G. Molokanov, Nucl. Instrum. Methods Phys. Res. B 269 (2011) 216. [6] Surender Singh, S.P. Lochab, Ravi Kumar, Nafa Singh, Physica B 406 (2011) 177. [7] Mg. Cu, P. phosphor, P.D. Sahare, J.S. Bakare, S.D. Dhole, N.B. Ingale, A.A. Rupasov, J. Lumin. 130 (2010) 258. [8] Numan Salah, S. Habib, H. Khan, Salim Al- Hamedi, S.P. Lochab, J. Lumin. 129 (2009) 192. [9] A. NecmeddinYazici, Semih Seyyidoglu, Huseyin Toktamis, Aysen Yilmaz, J. Lumin. 130 (2010) 1744. [10] Numan Salah, P.D. Sahare, S.P. Lochab, Pratik Kumar, Radiat. Meas. 41 (2006) 40. [11] R.L. Dixon, K.E. Ekstrand, Phys. Med. Biol. 19 (2) (1974) 196.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Themoluminescence properties of nanocrystalline of BaSO4:Dy,Tb irradiated with gamma rays Kadijeh Rezaee Ebrahim Saraee a,n, Amin Aghay Kharieky a, Mohsen Khosravi b, Mohammad Reza Abdi c, Hossien Zamani Zeinali d a
Department of Nuclear Engineering, Faculty of Advance Sciences and Technologies, University of Isfahan, Isfahan 81746-73441, Iran Department of Nanotechnology, Faculty of Advance Science and Technology, University of Isfahan, Isfahan 81746-73441, Iran c Department of Physics, Faculty of Science, University of Isfahan, Isfahan 81746-73441, Iran d Agricultural, Medical and Industrial Research School, Nuclear Science and Technology Research Institute, Karaj, Iran b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2012 Received in revised form 14 December 2012 Accepted 9 January 2013 Available online 20 January 2013
Nanocrystalline of BaSO4 and BaSO4:Dy,Tb of grain size 45–55 nm has been prepared by the coprecipitation method and its thermoluminescence characteristics have been studied. The formation of the material was confirmed by the X-ray diffraction (XRD) and UV–visible spectroscopy. Shape and size of the prepared nanocrystalline powder was observed by a scanning electron microscope (SEM). The TL glow curve of the nanocrystalline pellets of BaSO4:Dy,Tb shows a prominent single peak at 530 K along with another peak of lesser intensity at around 420 K and 560 K. On the contrary, the nanocrystalline pellets of BaSO4 show a peak of low intensity at 500 K and prominent peak around 460 K. The glow curve structure does not change at the range of 0.1–15 kGy and some new peaks appear at high doses but have low intensities. The 530 K of the nanocrystalline pellets of BaSO4:Dy,Tb shows a linear response with exposure increasing up to very high values (as high as 7 kGy), where the other doped BaSO4 show saturation. & 2013 Elsevier B.V. All rights reserved.
Keywords: BaSO4:Dy,Tb Luminescence Nanocrystalline Rare earth
1. Introduction The nanomaterials word is used for materials whose length scale is within the nanometric range, i.e., from 1 nm to 100 nm. Within this length scale, it has been noticed that this materials have different properties from the individual atoms, molecules and bulk materials. The physical, chemical, electrical, optical properties and potential applications of these nanomaterials are dependent on their size and shape and they often display different properties in comparison with bulk materials [1,2]. There properties have attracted the researchers attention towards this materials. Luminescence is one of the fields that has attracted several researchers. Themoluminescence (TL) is a very common technique that is widely used in dose measurement of ionizing radiation such as gamma rays, X-rays, UV, b-rays, a-particles, energetic ions etc. By heating the intensity of light emitted by the phosphors, reflects the dose given to it [3–6]. Understanding the importance of thermoluminescence with nanometric structure became obvious when researchers understand that this materials have a potential application in dosimetry of ionizing radiations for the measurements of high doses where microcrystalline
n
Corresponding author. Tel: þ98 9131015519; fax: þ 98 3117934215. E-mail address: [email protected] (Kh. Rezaee Ebrahim Saraee).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.01.011
phosphors will saturate [7,8]. Many works have been conducted which show that doped phosphors are important from their luminescent, dielectric and semiconductor properties point of view [4,7–10]. In the literatures, there are studies on nano and micro-crystals of both pure and doped with different rare earth of the BaSO4 [8,11] but there are no reports on nanocrystalline of BaSO4:Dy,Tb. In this work, the nanostructure of the BaSO4 and BaSO4:Dy,Tb are prepared by the chemical co-precipitation method. The dimension property such as shape and formation is observed by a scanning electron microscope (SEM). Formation of compound and size are confirmed by EDS and X-ray diffraction (XRD). Also, the energy band gaps of the BaSO4 and BaSO4:Dy,Tb is obtained by UV–visible spectrometer. Finally, the TL response of nanocrystalline of BaSO4 and BaSO4:Dy,Tb have been studied and the results are compared with commercial thermoluminescence dosimeters (TLD-100).
2. Experimental 2.1. Method of preparation The nanocrystalline form of BaSO4 and BaSO4:Dy,Tb was prepared by the chemical co-precipitation method. In this research, reagents of BaCl2d2H2O 99.9% (Merck), (NH4)2SO4 99.9% (Merck),
Kh. Rezaee Ebrahim Saraee et al. / Journal of Luminescence 137 (2013) 230–236
231
Dy2O3 99.9% (Merck), HCl 46 M (Merck), ethanol 99.99% (Merck) and TbCl3d6H2O 99.9% (sigma-Aldrich) are used as a raw material. At first step, BaCl2d2H2O is dissolved in the first super ionized sterilized distilled water. Dy2O3 (0.1 mol%) is dissolved in diluted HCl and also TbCl3d6 H2O which is dissolved in a solution of BaCl2 with little water that contain ethanol. Then solution of (NH4)2SO4 is added dropwise to above solution till precipitation is completed. Then the precipitation is centrifuged and is washed several times using sterilized water and is centrifuged again. Ultimately, the nanophosphor is obtained by drying the precipitate at 378 K for 2 h and then the nanocrystalline powder is further annealed in ceramic crucible at 1123 K for 1 h in argon atmosphere. For preparation of pure BaSO4 all processes mentioned above is repeated; only Dy2O3 and TbCl3d6H2O is not added to compound.
show that the range of particles size is from 45 to 55 nm. Formation of the BaSO4 and BaSO4:Dy,Tb is confirmed by studying the XRD pattern. Fig. 2a and b shows the XRD pattern of the BaSO4 and BaSO4:Dy,Tb, respectively. The high purity and high crystallinity of BaSO4 is caused all diffraction peak be relatively sharp and these peaks are in good agreement with the JCPDS card (JCPDS card no. 24-1035). The XRD pattern is fitted well with orthorhombic structure of BaSO4. Broadening in the XRD lines in the samples of BaSO4 and BaSO4:Dy,Tb (Fig.2a and b) are utilized to determine the dimension of prepared nanocrystalline powder by Debye Scherer formula, D¼0.9l/ß cosy, where D is the average grain size of the nanocrystalline; l is the wavelength of incident radiation (1.5406 A); b (in radian) is the full width at half maximum (FWHM) and y is Bragg angle.
2.2. Characterization
3.2. Impurity concentrations
In order to determine the shape and size of nanocrystalline powder, SEM images is used. SEM images were obtained by VEGA, TESCAN SEM. To confirm the size and structure of the compound of nanocrystalline, XRD pattern is studied at room temperature for BaSO4 and BaSO4:Dy,Tb using Cu target (Cu-Ka, l ¼1.5406 A with Ni filter) on Bruker, model D8ADVANCE, Germany XRD machine and matched with the standard data available (JCPDS card no. 24-1035). The band gap of nanocrystalline powder of BaSO4 and BaSO4:Dy,Tb is also obtained using UV–visible absorption spectra with which this spectra is obtained by V-670 spectrophotometers (Jasco Company of Japan). To facilitate easy handling, the nanocrystalline powder with Na2SO4 99.9% (Merck) for the binder and the parts are mixed in the ratio 2 (Na2SO4):1 (powdered sample). The dimensions of pellets are 5 mm diameter and 0.8 mm thickness. The obtained pellets are irradiated with different doses of 60Co gamma rays from 0.1 Gy to 15 kGy at room temperature. TL glow curves are recorded by an IAP TLD reader (7102 model). The heating rate was 2 K/s. In order to more interoperate our results regarding the structure of glow curve, the TLD-100 is also irradiated under identical condition and recorded.
The incorporation of impurities in the crystalline is confirmed by the EDS study and the EDS peaks corresponding to the two impurities could very well be seen in Fig. 3. 3.3. Energy band gap The UV–visible absorption spectra of BaSO4 and BaSO4:Dy,Tb nanocrystalline powders are shown in Fig. 4. The band gap energies of BaSO4 and BaSO4:Dy,Tb nanocrystalline powders are calculated using the Kubelka–Munk formula and the Tauc plot. ðFðRÞhnÞ1=n ¼ Aðhn2Eg Þ
ð1Þ
where hn is the photon energy, F(R) is the absorption coefficient and A is proportional constant. The absorption coefficient is calculated from the reflectance spectra. The value of Eg (band gap) is obtained by plotting (F(R)hn)1/n versus hn in high absorption range and extrapolating the linear region of the plots of (F(R)hn)1/n ¼ 0. The analysis of the present data shows that the plots of (F(R)hn)1/n ¼0 versus hn give linear relation which is the most fitted Eq. (1) with n¼ 1/2 for both BaSO4 and BaSO4:Dy,Tb nanocrystalline powder. Fig. 4 shows the band gaps of the nanocrystalline samples.
3. Result 4. Discussion 3.1. Particle size 4.1. TL glow curve The shape and size of the BaSO4 and BaSO4:Dy,Tb nanocrystalline powder are determined by SEM. The SEM photographs of BaSO4 and BaSO4:Dy,Tb is shown in Fig. 1a and b, respectively. The images
Fig. 5 shows the TL glow curves of BaSO4 and BaSO4:Dy,Tb pellets exposed to 1 Gy of gamma rays from a 60Co source. Also
Fig. 1. SEM images of BaSO4 and BaSO4:Dy,Tb nanocrystalline.
232
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Fig. 2. X-rays diffraction pattern of nanocrystalline powder samples.
TL glow curve of TLD-100 is shown as standard phosphor for comparison. The TL glow curve of nanocrystalline phosphors of the BaSO4 and BaSO4:Dy,Tb consist of main dosimetric peak at around 460 K and 530 K, respectively. Other temperature peaks with lesser intensities at around 500 K for BaSO4 and 420 K and 560 K for BaSO4:Dy,Tb is seen. Later mentioned above peaks the fluctuation of temperature dose not considered after 593 K. The TL glow curve of TLD-100 phosphor has usual structure with dosimetry peak at around 502 K. The sensitivity of nanocrystalline of BaSO4:Dy,Tb is more than that of the BaSO4 phosphor by a factor of approx. 10 for all exposure. The peak height of glow curve is used to compare of the different thermoluminescences sensitivity. However, the height of peak is proportional to the light intensity in a given temperature and is varying with type and structure of thermoluminescence. Moreover, it is proportional to the mass of thermoluminescence. The mass of pellets is also important when they are compared with commercial type (TLD-100) because the differences between
their masses can be caused differences between their emitted intensities. Then, the comparison between the commercial type (TLD-100) and prepared pellets in this research has not high precision. With exposure increasing, the intensities of main peak have been observed to increase. The shift in the glow curve of BaSO4:Dy,Tb and BaSO4 maybe is attributed to presence of impurities and structure of compound. 4.2. Dosimetric properties All the following studies are carried out using of IAP TLD reader and a liner heating rate of 2 1K S 1 and a standard pre-read out treatment of 363 K (90 1C) for 15 min in oven. Heating at temperature of 363 K is caused which repeatability of obtained measurements has been more precision. The response sums are obtained by integrating the glow curve between points of first (393 K) and final (593 K) of main peak. In order to reuse of TLDs and eliminate any kind of trapped electron and hole, all pellets anneal at 673 K for 1 h and quenched rapidly.
Kh. Rezaee Ebrahim Saraee et al. / Journal of Luminescence 137 (2013) 230–236
Fig. 3. A typical EDS pattern of the BaSO4:Dy,Tb nanocrystalline powder.
Fig. 4. The band gaps of BaSO4 and BaSO4:Dy nanocrystalline powders.
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Fig. 5. The glow curve of BaSO4, BaSO4:Dy,Tb and TLD-100 respectively irradiated with gamma ray of
Fig. 6. The phosphor response versus dose of 1.25 MeV gamma ray,
4.2.1. Linearity The relative response versus dose is measured for each of pellets which are prepared from nanocrystalline powder and is irradiated with gamma of 60Co source. These pellets are placed in a plexiglass phantom and are placed in front of calibrated source of 60Co. Fig. 6 shows the phosphor response versus dose for each of samples, the units of dose is in Gray. As seen in the Fig. 6, BaSO4 and BaSO4:Dy,Tb is seen to be linear from about 0.1 Gy to 1 kGy and 0.1 Gy to 7 kGy, respectively. The nonlinearity response of the BaSo4 and BaSo4:Dy,Tb for above 1 kGy and 7 kGy respectively, may be attributed to saturate of dosimeter in these regions (Fig. 6). The peak height is also linear for doses between 0.1 and 1 kGy (Fig. 7). There is a tail in TL response above 593 K (Fig. 8) which is omitted because it is approximately same for all dose values and will not change with varying in the dose and temperature values.
4.2.2. Energy response The relative response versus photon energy is measured over the range 60 keV to 1.25 MeV. The filtered X-rays from an X-ray
60
60
Co at 1 Gy dose.
Co.
machine is used for producing low energy. This filter is made of copper metal to obtain effective energy. The phosphor is spread out at thin layer in order to reduce attenuation of X-rays in the phosphor itself. The irradiation with 662 keV and 1.25 MeV energy is obtained by 137Cs and 60Co gamma source. All exposure of this experiment is done for 500 mGy. Fig. 9 shows the relative photon energy response versus photon energy, normalized to unity at 1.25 MeV. The curve displays energy response for discrete energy value and same dose which obtained contiguous curve from this points is equivalent with the theoretical energy response calculated from ðmen =rÞ=ðmen =rÞair and with this assumption that light output function from only the absorbed energy by the phosphor.
5. Conclusions The nanocrystalline of BaSO4 and BaSO4:Dy,Tb have been prepared by co-precipitation method and is studied by SEM– EDS, XRD and UV–visible. This analysis showed these powders have dimension with scale of 45–55 nm. The UV–visible analysis
Kh. Rezaee Ebrahim Saraee et al. / Journal of Luminescence 137 (2013) 230–236
Fig. 7. The peak height versus dose of 1.25 MeV gamma ray,
60
Co. This peak height has been calculated for main peak of glow curve.
Fig. 8. The glow curves of BaSO4 and BaSO4:Dy,Tb.
235
236
Kh. Rezaee Ebrahim Saraee et al. / Journal of Luminescence 137 (2013) 230–236
Fig. 9. Relative photon energy response plotted versus photon energy (MeV) which normalized to unity at energy of rhombic pictures shown energy response of TLD-100, BaSO4 and BaSO4:Dy respectively.
showed that impurity of Dy and Tb approximately decreases the band gap also. The studies of TL properties using of glow curves obtaining from irradiated pellets at dose with range of 0.1 Gy to 15 kGy showed that BaSO4:Dy,Tb is the more sensitive than BaSO4. This property causes that BaSO4:Dy,Tb be more useful for estimating of low and high dose of gamma rays.
Acknowledgment The authors thank the research fellows of the Research Institute of Agriculture, Medicine and Industry for their technical advice. The authors are also deeply grateful to Engineers Shahvar and Sharifzadeh. Also special thanks to Dr. Teymouri for his guidance.
60
Co (1.25 MeV). The points of triangle, square and
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