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Thermoluminescence characteristics of gamma irradiated Li2B4O7:Cu nanophosphor
Nuclear Instruments and Methods in Physics Research A 717 (2013) 63–68
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Thermoluminescence characteristics of gamma irradiated Li2B4O7:Cu nanophosphor Vibha Chopra a, Lakhwant Singh a,n, S.P. Lochab b a b
Department of Physics, Guru Nanak Dev University, Amritsar, Punjab 143005, India Inter University Accelerator Centre, Aruna Asaf Ali Marg, PostBox 10502, New Delhi 110067, India
art ic l e i nf o
a b s t r a c t
Article history: Received 28 May 2012 Received in revised form 7 February 2013 Accepted 6 March 2013 Available online 30 March 2013
Nanocrystals of the Li2B4O7:Cu were synthesized by the combustion method using different concentrations of Cu. X-ray diffraction (XRD) spectra and transmission electron microscopy (TEM) image confirm the nanometric size of synthesized material. The thermal stability of phosphor was obtained by thermogravimetric analysis (TGA). TL characteristics of the synthesized Li2B4O7:Cu material doped with Cu of concentrations 1000 ppm and 2500 ppm were studied. It is observed that Li2B4O7:Cu doped with Cu (both 1000 ppm and 2500 ppm) exhibit a linear response in the range 1 100–5 103 Gy of gamma radiations. Finally the trapping parameters associated with the glow peaks were calculated using the glow curve deconvolution (GCD) glow fit method. Fading and reproducibility of phosphors were also studied and it was found that the Li2B4O7:Cu is quite suitable for radiation dosimetry. & 2013 Elsevier B.V. All rights reserved.
Keywords: Li2B4O7:Cu Thermoluminescence Tissue equivalent Trapping parameters
1. Introduction Nanocrystalline lithium tetra borate activated by copper (Li2B4O7: Cu) is one of the known thermoluminescent phosphor for high dose measurement of gamma radiations [1]. Li2B4O7 activated with copper, first developed by Takenaga et al. in 1980, has an exceptional TL characteristics close to that of an ideal TL dosimeter [2]. Later Prokic worked on Li2B4O7:Cu sintered pallets and found that their TL sensitivity is similar to that of TLD-100 [3]. But at the same time this material was found to have disadvantage of light induced fading and a degradation of dosimetric properties under high humid atmosphere [4]. Later on the TL properties of microcrystalline Li2B4O7:Cu were studied by different authors [5,6] for the improvement of these disadvantages. It is a well known fact that the sensitivity of material varies widely depending not only on the starting materials but also on the preparation method and particle size. So recently the combustion method was used for the first time by our group to synthesize nanocrystalline Li2B4O7:Cu [1]. The dosimetric characterstics of TL materials mainly depend upon the trapping parameters quantitatively describing the trapping centers responsible for TL emission [7]. There are numerous methods given in the literature [8–11] to find out the trapping parameters namely activation energy (E), order of kinetics (b) and frequency factor (s), with their own advantages and disadvantages. To the best of our knowledge, trapping parameters of nanocrystalline Li2B4O7:Cu have never been reported. The present study is
n
Corresponding author. Tel.: +91 9915893609. E-mail address: [email protected] (L. Singh).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.03.015
the extension of our previously reported work [1] in which Li2B4O7 nanophosphor was synthesized by using the combustion method. Presently, Li2B4O7:Cu samples have been synthesized using different concentrations of Cu and their TL characteristics have been studied. Further the glow curve deconvolution (GCD) glow fit method has been used to analyze the TL glow curves and hence calculating the trapping parameters for Li2B4O7 doped with Cu. These results may be helpful in the development of tissue equivalent TL nanocrystalline detectors best suited for wide high range of radiation exposures. 2. Experimental 2.1. Synthesis Li2B4O7:Cu nanophosphor was synthesized by the combustion method [1]. Lithium nitrate, boric acid, ammonium nitrate, urea and copper nitrate were the starting materials. Out of these, urea works as fuel, ammonium nitrate as an oxidizer and copper acts as an activator. The starting mixture with a molar ratio of Li(NO3): H3BO3:NH4NO3:NH2CONH2 ¼2.0:3.2:10.2:10.2 was put in a large quartz crucible and introduced in a muffle furnace preheated to temperature of 580 1C. An appropriate amount of Cu(NO3)2 3H2O was added in starting mixture to get lithium borate doped with Copper. The mixture undergoes smoldering combustion to produce the corresponding phosphor. The phosphor thus obtained was then crushed to get the powder form. Finally the nanocrystalline powder was annealed at 300 1C for 10 min in the crucible in the presence of air and was quenched by taking the crucible out of
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V. Chopra et al. / Nuclear Instruments and Methods in Physics Research A 717 (2013) 63–68
the furnace and placing it on a metal block. The synthesized samples were then used to study their TL properties. 2.2. Characterization The formation of the compound was confirmed by X-ray diffraction pattern taken at room temperature by using Cu-target (Cu-Kα1 line, λ ¼1.54056 Å) on a Bruker AXS-D8 diffractometer at a scan step of 0.011. The results obtained were matched with the standard data available (JCPDS card no. 84-2191). The particle size and shape of the concerned phosphor was analyzed by using a transmission electron microscopy (TEM) Hitachi (H-7500) operated at 100 kV. The thermal stability of the concerned phosphor was obtained by thermogravimetric analysis (TGA) using a Mettler-(4000) thermal analyzer coupled to a DSC-30 S cell, in the air atmosphere at a heating rate of 10 1C/min. To study thermoluminescence (TL) properties, the annealed samples were then irradiated with γ-rays using calibrated 137Cs source (from BARC, Mumbai, India) for low dose ranging 2 10−4–1 100 Gy and calibrated 60Co source(from BARC, Mumbai, India) for dose range 5 101–5 104 Gy at room temperature. TL glow curves were recorded using a Harshaw TLD reader (Model 3500) fitted with a 931B photo multiplier tube (PMT). The amount (5 mg) of the sample and a heating rate (5 K s−1) was kept constant for each TL recording.
Fig. 2. TEM image showing the formation of synthesized nanocrystalline compound.
3. Results and discussion 3.1. Particle shape and size X-ray diffraction (XRD) pattern (shown in Fig. 1) confirms the formation of synthesized nanocrystalline compound. The well known Debye–Scherrer's relation [12] was used to estimate the particle size for synthesized Li2B4O7:Cu. The average grain size was calculated to be approximately 26 nm. When the data was fitted with the powder X-ray Data Analysis System, it was revealed that the Li2B4O7:Cu compound exhibits tetragonal structure having lattice parameters a¼b¼ 9.566 Å, c¼10.445 Å and α¼ β¼γ¼901. Further the transmission electron microscopy (TEM) has been used to determine the shape and size of the particles of the concerned phosphor. The TEM photograph of Li2B4O7:Cu shown in Fig. 2 reveals that the particles are of uniform rod shape with their average diameter approximately 28 nm, which are found to be well consistent with XRD results. 3.2. Thermal stability The thermal behavior of sample was studied using thermogravimetric analysis (TGA). Fig. 3 shows the change in the weight percent
Fig. 3. TGA thermogram of synthesized nanocrystalline Li2B4O7:Cu compound.
of material with temperature. TGA curve reveals that the weight of Li2B4O7:Cu sample at 35 1C was 100%. When the sample was heated to 250 1C, it shows a weight loss of 1.2%, this weight loss was most probably due to the trapped moisture in the samples. As the temperature was increased further to 450 1C, the total weight loss of 2.3% was observed and finally when the temperature reached 900 1C (close to its melting point) in case of Li2B4O7:Cu, the total weight loss was 2.5% only. At low temperatures, most of the bonds have energy less than the threshold energy that is required to decompose the bond. However with increase in temperature the energy of bonds becomes equal to or greater than threshold value, hence the bond dissociates that may lead to weight loss of the sample. Since the weight loss is only 2.5% before the phosphor reached its melting point, the synthesized Li2B4O7:Cu is found to be thermally stable phosphor, that is the essential requirement for being a good thermoluminescence dosimeter (TLD).
3.3. Thermoluminescence (TL) studies
Fig. 1. XRD spectra showing the formation of synthesized nanocrystalline compound.
The TL studies of synthesized Li2B4O7:Cu phosphors includes the effect of concentration on TL sensitivity, effect of dose on TL glow curve, TL response, fading, reproducibility and calculations of trapping parameters.
V. Chopra et al. / Nuclear Instruments and Methods in Physics Research A 717 (2013) 63–68
3.3.1. Batch homogeneity Five batches of Li2B4O7:Cu samples were synthesized under the same conditions of preparation. The synthesized samples were then irradiated with gamma radiations of dose 1 103 kGy and their TL glow curves were noticed (shown in Fig. 4). It was observed that all the five batches of synthesized samples shows homogeneity with an experimental error of 77%. 3.3.2. Effect of concentration on TL response TL glow curves were recorded for each sample having different dopant concentrations in host Li2B4O7. Since the background is 28 nC (TL response), standard deviation is around 84 nC, so corresponding to this the lowest detectable dose for Li2B4O7:Cu (doped with 1000 ppm Cu) is 2 10−4 Gy and that for Li2B4O7:Cu (doped with 2500 ppm Cu) is 1 100 Gy. The variation of TL response (area under the curve covering the full range of temperature) with dopant concentration of samples, irradiated by gamma radiations of different doses (1 100–5 103 Gy) are shown in Table 1. TL response of Li2B4O7:Cu doped with Cu of concentration 1000 ppm is found to be maximum when irradiated with gamma dose in the range 2 10−4–1 101 Gy. Further with increase in radiation dose, the Li2B4O7:Cu doped with Cu of concentration 2500 ppm also starts giving better TL response results (as shown in Table 1). It is found that for the doses of 5 101 Gy and 1 102 Gy, the samples doped with Cu of concentrations 1000 ppm and 2500 ppm give same TL response. With further increase in dose upto 5 103 Gy, Li2B4O7:Cu doped with Cu (2500 ppm) gives the better response than that of 1000 ppm doped samples. The better response for only 1000 ppm and 2500 ppm Cu doped samples may be due to the ordering of atoms that may occur at this particular concentration of dopant, which may leads to the enhancement of TL intensity.
Fig. 4. Batch to batch homogeneity of synthesized Li2B4O7:Cu (doped with 2500 ppm Cu) nanophosphors exposed to 1 103 Gy γ-radiation.
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Doping with Cu introduces predominantly deeper trapping levels in Li2B4O7 and also the enhancement in TL intensity. The change in trap distributions may be due to the lattice perturbation caused by incorporation of Cu in Li2B4O7 [13]. Cu on its introduction as an impurity in Li2B4O7 may easily occupy one of the interstitial positions. When in close proximity to alkali-associated trapping species, copper ions may augment the trapping profile. The copper ions may form positively charged complexes with oxygen vacancies and other structural defects. As an impurity, copper is known to exist both in Cu+ and Cu2+ states [9]. Therefore it is reasonable to assume that the copper ion (preferably in Cu2+ state) could slip quite easily into various locations in the matrix and in the domain of intrinsic lattice defects. There could be two possible situations which lead to the excited state of copper ions and the peaks shown in glow curves of Li2B4O7:Cu phosphors are related to the relaxation of these excited copper ions [14]. So, we may conclude that the concentration of doped Cu ions can greatly affect the distribution of traps produced by γ-exposure. The results of maximum TL response of Li2B4O7:Cu (1000 ppm and 2500 ppm Cu) lead us to select these samples for further study of TL properties.
3.3.3. Effect of dose on glow curves The effect of gamma dose in the range 1 101–1 102 Gy on TL glow curves of synthesized Li2B4O7:Cu (doped with 1000 ppm Cu) nanophosphor is shown in Fig. 5. The glow curves are found to have prominent peak at 447 K and small humps at 386 K and 496 K. It is inferred from Fig. 5 that with increase in dose from 1 101 Gy to 1 102 Gy, the peak at 447 K becomes prominent, the TL intensity keeps on increasing and the humps at 386 K and 496 K keep on diminishing. Further Fig. 6 shows the effect of gamma dose in the range 5 102–3 104 Gy on TL glow curves of synthesized Li2B4O7:Cu (doped with 2500 ppm Cu) nanophosphor. The prominent peak at 457 K and a small hump at 395 K is observed. With increase in dose, the TL intensity of prominent peak keeps on increasing and small hump is observed to be diminished. It is known that the cation disorder and nonstoichiometry of Li2B4O7 provides a large number of lattice defects which may serve as trapping centers. During irradiation an electron hole pair is created. The hole is trapped at the V center, cation vacancy is stabilized by the surrounding oxygen ions and the electrons are trapped to form F+ center. During heating the hole from V center is released and recombines with the electron trapped in F+ center. This recombination energy is transferred to nearby Cu+ ions and Cu+ ions get excited and on de-excitation emit its characteristic emission [15]. The occurrence of various peaks and changes in their intensities for the synthesized nanocrystalline material are explained using the model shown in previous work [16]. Further the comparison of TL glow curves of synthesized nanocrystalline Li2B4O7:Cu doped with Cu of concentrations 1000 ppm and 2500 ppm, exposed to gamma dose of 1 103 Gy from 60Co source are shown in Fig. 7. It is noticed that with change in dopant concentration only peak
Table 1 Variation of TL response with dopant concentration of Li2B4O7:Cu samples at different gamma doses having range 1 100–5 103 Gy. TL response (a.u.) Dose (Gy) Concentration (ppm)
1 100
1 101
5 101
1 102
5 102
1 103
5 103
500 1000 1500 2000 2500 3000
54.67 157.2 128.2 112.0 104.9 59.4
514.4 1440.6 727.9 790.1 1170.8 564.3
3968.6 6137.8 4460.0 2841.2 6156.8 3507.2
6763.4 12,413.3 8735.4 10,184.3 12,241.4 9229.4
39,971.2 60,928.0 54,206.0 44,182.5 67,572.5 51,034.9
108,162.9 127,444.1 140,907.7 150,174.8 160,591.7 145,775.3
311,057.7 547,034.2 500,630.7 452,109.9 826,631.2 530,569.4
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Fig. 7. Comparison of TL glow curves of Li2B4O7:Cu nanophosphors doped with 2500 ppm and 1000 ppm Cu and exposed with a dose of 1 103 Gy γ-radiation. Fig. 5. Effect of gamma dose on TL glow curves of synthesized nanocrystalline Li2B4O7:Cu (doped with 1000 ppm Cu) having dose ranging 1 101–1 102 Gy.
Fig. 6. Effect of gamma dose on TL glow curves of synthesized nanocrystalline Li2B4O7:Cu (doped with 2500 ppm Cu) powder.
intensity and peak temperature shifts occur. The peak intensity for 1000 ppm doped samples is found to be less than that of 2500 ppm doped samples and there is no change in the shape of glow curve with change in concentration. 3.3.4. TL response TL response (area under the curve covering the full range of temperature) of Li2B4O7:Cu doped with Cu (1000 ppm and 2500 ppm) for lower doses in the range of 2 10−4–1 100 Gy does not show a linear pattern. Hence Li2B4O7:Cu nanophosphor cannot be used as TLD material within this range. Further TL responses of samples exposed for higher doses in the range of 1 100–5 104 Gy are shown in Fig. 8(a and b). It is clearly observed from Fig. 8(a and b) that Li2B4O7:Cu doped with 1000 ppm and 2500 ppm Cu exhibit a linear response from 1 100 Gy to 5 103 Gy. On further increasing the dose from 5 103 Gy to 1 104 Gy, the response becomes supralinear, then finally resulting in saturation over 1.5 104 Gy. This linearity over a wide range of dose is found within an experimental error of 710%. Therefore synthesized Li2B4O7:Cu nanophosphor doped with Cu (1000 ppm and 2500 ppm) could be used for estimation of very high doses (in the range 1 100 Gy to 5 103 Gy) of gamma radiations. 3.3.5. Fading In order to determine the fading characteristics of Li2B4O7:Cu doped with Cu (2500 ppm and 1000 ppm), several samples were
Fig. 8. TL response of nanocrystalline Li2B4O7:Cu doped with (a) 1000 ppm and (b) 2500 ppm Cu for wide range of γ- doses.
irradiated to a γ dose of 1 103 Gy and were stored in dark conditions at room temperature. The results of fading of synthesized nanocrystalline Li2B4O7 doped with Cu are presented in Fig. 9. It reveals that in case of 1000 ppm Cu doped samples, the fading on third day is 7%, on 7th day it is 8% and on 15th day it is 9%, whereas in 1 month the total fading recorded is 10%. The fading results are found to have an experimental error of 75%. The maximum fading recorded in our system is seen in first week after exposure of the samples. The fading recorded in the samples doped with Cu of concentration 2500 ppm is explained in previous reported paper and is found to be 7% in a period of 1 month [1].
V. Chopra et al. / Nuclear Instruments and Methods in Physics Research A 717 (2013) 63–68
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Table 2 Values of trap depth (E) and frequency factor (s) for isolated peaks calculated by GCD glow fit method. S. no
Peak
Tm (K)
E (eV)
s (s−1)
1. 2. 3. 4.
1 2 3 4
392.7 446.5 468.5 499.7
0.86 1.08 1.10 1.20
8.61 1010 1.24 1012 4.12 1011 7.87 1011
with Cu (2500 ppm and 1000 ppm) are shown in Fig. 10. It was found that the TL response reduced to almost 8% in 10th cycle for Li2B4O7:Cu doped with Cu (1000 ppm), while the TL response reduced only 2% even after 10 cycles for synthesized Li2B4O7:Cu doped with Cu (2500 ppm). So, the synthesized Li2B4O7:Cu doped with Cu (2500 ppm) has better reproducibility than that of 1000 ppm Cu doped Li2B4O7:Cu sample. Fig. 9. Fading of synthesized nanocrystalline Li2B4O7:Cu doped with 1000 ppm and 2500 ppm Copper.
Fig. 10. Reproducibility of synthesized nanocrystalline Li2B4O7:Cu doped with 1000 ppm and 2500 ppm Copper.
3.4. Calculations of trapping parameters The glow curve is related to the trap levels that lie at different depths in the band gap between the conduction and the valence bands of a solid. These trap levels are characterized by different trapping parameters. Hence, several peaks are found to appear in the TL glow curve at characterized temperatures. The trapping parameters associated with the various TL bands, are evaluated after deconvolution of composite glow curves. In the present work, the trapping parameters associated with the glow peaks were calculated using the glow curve deconvolution (GCD) glow fit method. The aim of the analysis is to calculate the electron depths for traps and their frequency factors so as to understand the complete mechanism of TL glow curve. The recorded composite glow curves for gamma dose of 5 103 Gy were first isolated by a computer program, GlowFit [17] and are shown in Fig. 11. It was found that the composite glow curve is composed of four peaks. The parameter describing the quality of fitting, called Figure of Merit (FOM) was calculated. Glow curves with FOM values in excess of 5% are subjected to further investigation to determine the reasons for the poor fit. Hence minimum four peaks are required to get the best fit. These all isolated four peaks show first-order kinetics, i. e. the probability of electron re-trapping during the thermoluminescence process was negligible. The values of trapping parameters of all the four isolated peaks for the glow curve were calculated by the GCD glow fit method and are shown in Table 2. FOM was found to be 2.71%. It is observed that the energy levels (activation energy) of various traps (corresponding to various peaks) are very much different. Therefore it is clear that there are some deep and shallow traps. The competition among these traps might be giving various releasing probabilities, which might have resulted in different frequency factors [18]. The dosimetry information stored in the materials after irradiation strongly depends on the position of trapping levels within the forbidden gap, i. e. trap depth or activation energy.
4. Conclusions Fig. 11. Deconvulation of TL glow curve of Li2B4O7:Cu (doped with 2500 ppm Cu) exposed to 5 103 Gy of γ-dose.
3.3.6. Reproducibility In order to assess the reproducibility of the dose measurements of synthesized samples, a large set of repeated post read-out annealings at 300 1C for 30 min were carried out at 1 103 Gy dose level, everytime using 60Co γ-ray source. The results of assessment of reproducibility of synthesized Li2B4O7:Cu nanophosphor doped
The main interesting feature reported in this work is concerning the TL response in a very wide range (2 10−4–5 104 Gy) of gamma rays for nanocrystalline Li2B4O7:Cu phosphors, doped with different concentrations of Cu. It is observed that Li2B4O7:Cu doped with Cu (1000 ppm and 2500 ppm) exhibit a linear response from 1 100 Gy to 5 103 Gy of gamma dose. So it can be concluded that good sensitivity, simple glow curve structure, linear response over a wide range of exposure, low fading and excellent reproducibility are some of the required characteristics of Li2B4O7:Cu doped with Cu of
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concentrations 1000 ppm and 2500 ppm, making them suitable for radiation dosimetry. Acknowledgments We are grateful to Dr Sundeep Chopra of Inter University Accelerator Centre, New Delhi for providing necessary facilities to carry out this work. We are thankful to Dr. S.K. Sharma and Dr A. Choubey, from Indian School of Mines, Dhanbad for valuable discussions. References [1] L. Singh, V. Chopra, S.P. Lochab, Journal of Luminescence 131 (2011) 1177. [2] M. Takenega, O Yamamoto, T. Yamashita, Nuclear Instruments and Methods 175 (1980) 77. [3] M. Prokic, Radiation Measurements 33 (2001) 393. [4] A.C. Fernandes, M. Osvay, J.P. Santos, V. Holovey, M. Ignatovych, Radiation Measurements 43 (2008) 476.
[5] N.Q. Thanh, V.X. Quang, N.T. Khoi, N.D. Dien, VNU Journal of Science Mathematics Physics 24 (2008) 97. [6] B.T. Huy, B.M. Ly, V.X. Quang, H.K. Hanh, D.P.T. Tien, V. Hao, T. Ngoc, Journal of Physics: Conference Series 187 (2009) 012039. [7] L.H. Jiang, Y.L. Zhang, C.Y. Li, J.Q. Hao, Q. Su, Journal of Alloys and Compounds 482 (2009) 313. [8] A.T. Ege, E. Ekdal, T. Karali, N. Can, Radiation Measurements 42 (2007) 1280. [9] J.K. Srivastava, S.J. Supe, Journal of Physics D: Applied Physics 22 (1989) 1537. [10] N.A. El-Faramawy, S.U. El-Kameesy, A.I. Abd El-Hafez, M.A. Hussein, G.E. Metwally, Egyptian Journal of Solids 23 (1) (2000) 103. [11] R. Chen, S.W.S. Mckveer, World Scientific Press, Singapore, 1997, pp. 1–559. [12] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, London, 1959. [13] V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, R Kumar, S.P. Lochab, V. Mishra, N. Singh, Optical Materials 32 (2009) 164. [14] B.T. Huy, V.X. Quang, H.T.B. Chau, Journal of Luminescence 128 (2008) 1601. [15] E.A. Raja, B. Dhabekar, S. Menon, S.P. More, T.K. Rao, R.K. Kher, International Journal Pure and Applied Physics 47 (2009) 420. [16] N. Salah, P.D. Sahare, A. Rupasov, Journal of Luminescence 124 (2006) 357. [17] M. Puchalska, P. Bilski, Radiation Measurements 41 (2006) 659. [18] N. Salah, P.D. Sahare, S. Nawaz, S.P. Lochab, Radiation Effects and Defects 159 (2004) 321.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Thermoluminescence characteristics of gamma irradiated Li2B4O7:Cu nanophosphor Vibha Chopra a, Lakhwant Singh a,n, S.P. Lochab b a b
Department of Physics, Guru Nanak Dev University, Amritsar, Punjab 143005, India Inter University Accelerator Centre, Aruna Asaf Ali Marg, PostBox 10502, New Delhi 110067, India
art ic l e i nf o
a b s t r a c t
Article history: Received 28 May 2012 Received in revised form 7 February 2013 Accepted 6 March 2013 Available online 30 March 2013
Nanocrystals of the Li2B4O7:Cu were synthesized by the combustion method using different concentrations of Cu. X-ray diffraction (XRD) spectra and transmission electron microscopy (TEM) image confirm the nanometric size of synthesized material. The thermal stability of phosphor was obtained by thermogravimetric analysis (TGA). TL characteristics of the synthesized Li2B4O7:Cu material doped with Cu of concentrations 1000 ppm and 2500 ppm were studied. It is observed that Li2B4O7:Cu doped with Cu (both 1000 ppm and 2500 ppm) exhibit a linear response in the range 1 100–5 103 Gy of gamma radiations. Finally the trapping parameters associated with the glow peaks were calculated using the glow curve deconvolution (GCD) glow fit method. Fading and reproducibility of phosphors were also studied and it was found that the Li2B4O7:Cu is quite suitable for radiation dosimetry. & 2013 Elsevier B.V. All rights reserved.
Keywords: Li2B4O7:Cu Thermoluminescence Tissue equivalent Trapping parameters
1. Introduction Nanocrystalline lithium tetra borate activated by copper (Li2B4O7: Cu) is one of the known thermoluminescent phosphor for high dose measurement of gamma radiations [1]. Li2B4O7 activated with copper, first developed by Takenaga et al. in 1980, has an exceptional TL characteristics close to that of an ideal TL dosimeter [2]. Later Prokic worked on Li2B4O7:Cu sintered pallets and found that their TL sensitivity is similar to that of TLD-100 [3]. But at the same time this material was found to have disadvantage of light induced fading and a degradation of dosimetric properties under high humid atmosphere [4]. Later on the TL properties of microcrystalline Li2B4O7:Cu were studied by different authors [5,6] for the improvement of these disadvantages. It is a well known fact that the sensitivity of material varies widely depending not only on the starting materials but also on the preparation method and particle size. So recently the combustion method was used for the first time by our group to synthesize nanocrystalline Li2B4O7:Cu [1]. The dosimetric characterstics of TL materials mainly depend upon the trapping parameters quantitatively describing the trapping centers responsible for TL emission [7]. There are numerous methods given in the literature [8–11] to find out the trapping parameters namely activation energy (E), order of kinetics (b) and frequency factor (s), with their own advantages and disadvantages. To the best of our knowledge, trapping parameters of nanocrystalline Li2B4O7:Cu have never been reported. The present study is
n
Corresponding author. Tel.: +91 9915893609. E-mail address: [email protected] (L. Singh).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.03.015
the extension of our previously reported work [1] in which Li2B4O7 nanophosphor was synthesized by using the combustion method. Presently, Li2B4O7:Cu samples have been synthesized using different concentrations of Cu and their TL characteristics have been studied. Further the glow curve deconvolution (GCD) glow fit method has been used to analyze the TL glow curves and hence calculating the trapping parameters for Li2B4O7 doped with Cu. These results may be helpful in the development of tissue equivalent TL nanocrystalline detectors best suited for wide high range of radiation exposures. 2. Experimental 2.1. Synthesis Li2B4O7:Cu nanophosphor was synthesized by the combustion method [1]. Lithium nitrate, boric acid, ammonium nitrate, urea and copper nitrate were the starting materials. Out of these, urea works as fuel, ammonium nitrate as an oxidizer and copper acts as an activator. The starting mixture with a molar ratio of Li(NO3): H3BO3:NH4NO3:NH2CONH2 ¼2.0:3.2:10.2:10.2 was put in a large quartz crucible and introduced in a muffle furnace preheated to temperature of 580 1C. An appropriate amount of Cu(NO3)2 3H2O was added in starting mixture to get lithium borate doped with Copper. The mixture undergoes smoldering combustion to produce the corresponding phosphor. The phosphor thus obtained was then crushed to get the powder form. Finally the nanocrystalline powder was annealed at 300 1C for 10 min in the crucible in the presence of air and was quenched by taking the crucible out of
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V. Chopra et al. / Nuclear Instruments and Methods in Physics Research A 717 (2013) 63–68
the furnace and placing it on a metal block. The synthesized samples were then used to study their TL properties. 2.2. Characterization The formation of the compound was confirmed by X-ray diffraction pattern taken at room temperature by using Cu-target (Cu-Kα1 line, λ ¼1.54056 Å) on a Bruker AXS-D8 diffractometer at a scan step of 0.011. The results obtained were matched with the standard data available (JCPDS card no. 84-2191). The particle size and shape of the concerned phosphor was analyzed by using a transmission electron microscopy (TEM) Hitachi (H-7500) operated at 100 kV. The thermal stability of the concerned phosphor was obtained by thermogravimetric analysis (TGA) using a Mettler-(4000) thermal analyzer coupled to a DSC-30 S cell, in the air atmosphere at a heating rate of 10 1C/min. To study thermoluminescence (TL) properties, the annealed samples were then irradiated with γ-rays using calibrated 137Cs source (from BARC, Mumbai, India) for low dose ranging 2 10−4–1 100 Gy and calibrated 60Co source(from BARC, Mumbai, India) for dose range 5 101–5 104 Gy at room temperature. TL glow curves were recorded using a Harshaw TLD reader (Model 3500) fitted with a 931B photo multiplier tube (PMT). The amount (5 mg) of the sample and a heating rate (5 K s−1) was kept constant for each TL recording.
Fig. 2. TEM image showing the formation of synthesized nanocrystalline compound.
3. Results and discussion 3.1. Particle shape and size X-ray diffraction (XRD) pattern (shown in Fig. 1) confirms the formation of synthesized nanocrystalline compound. The well known Debye–Scherrer's relation [12] was used to estimate the particle size for synthesized Li2B4O7:Cu. The average grain size was calculated to be approximately 26 nm. When the data was fitted with the powder X-ray Data Analysis System, it was revealed that the Li2B4O7:Cu compound exhibits tetragonal structure having lattice parameters a¼b¼ 9.566 Å, c¼10.445 Å and α¼ β¼γ¼901. Further the transmission electron microscopy (TEM) has been used to determine the shape and size of the particles of the concerned phosphor. The TEM photograph of Li2B4O7:Cu shown in Fig. 2 reveals that the particles are of uniform rod shape with their average diameter approximately 28 nm, which are found to be well consistent with XRD results. 3.2. Thermal stability The thermal behavior of sample was studied using thermogravimetric analysis (TGA). Fig. 3 shows the change in the weight percent
Fig. 3. TGA thermogram of synthesized nanocrystalline Li2B4O7:Cu compound.
of material with temperature. TGA curve reveals that the weight of Li2B4O7:Cu sample at 35 1C was 100%. When the sample was heated to 250 1C, it shows a weight loss of 1.2%, this weight loss was most probably due to the trapped moisture in the samples. As the temperature was increased further to 450 1C, the total weight loss of 2.3% was observed and finally when the temperature reached 900 1C (close to its melting point) in case of Li2B4O7:Cu, the total weight loss was 2.5% only. At low temperatures, most of the bonds have energy less than the threshold energy that is required to decompose the bond. However with increase in temperature the energy of bonds becomes equal to or greater than threshold value, hence the bond dissociates that may lead to weight loss of the sample. Since the weight loss is only 2.5% before the phosphor reached its melting point, the synthesized Li2B4O7:Cu is found to be thermally stable phosphor, that is the essential requirement for being a good thermoluminescence dosimeter (TLD).
3.3. Thermoluminescence (TL) studies
Fig. 1. XRD spectra showing the formation of synthesized nanocrystalline compound.
The TL studies of synthesized Li2B4O7:Cu phosphors includes the effect of concentration on TL sensitivity, effect of dose on TL glow curve, TL response, fading, reproducibility and calculations of trapping parameters.
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3.3.1. Batch homogeneity Five batches of Li2B4O7:Cu samples were synthesized under the same conditions of preparation. The synthesized samples were then irradiated with gamma radiations of dose 1 103 kGy and their TL glow curves were noticed (shown in Fig. 4). It was observed that all the five batches of synthesized samples shows homogeneity with an experimental error of 77%. 3.3.2. Effect of concentration on TL response TL glow curves were recorded for each sample having different dopant concentrations in host Li2B4O7. Since the background is 28 nC (TL response), standard deviation is around 84 nC, so corresponding to this the lowest detectable dose for Li2B4O7:Cu (doped with 1000 ppm Cu) is 2 10−4 Gy and that for Li2B4O7:Cu (doped with 2500 ppm Cu) is 1 100 Gy. The variation of TL response (area under the curve covering the full range of temperature) with dopant concentration of samples, irradiated by gamma radiations of different doses (1 100–5 103 Gy) are shown in Table 1. TL response of Li2B4O7:Cu doped with Cu of concentration 1000 ppm is found to be maximum when irradiated with gamma dose in the range 2 10−4–1 101 Gy. Further with increase in radiation dose, the Li2B4O7:Cu doped with Cu of concentration 2500 ppm also starts giving better TL response results (as shown in Table 1). It is found that for the doses of 5 101 Gy and 1 102 Gy, the samples doped with Cu of concentrations 1000 ppm and 2500 ppm give same TL response. With further increase in dose upto 5 103 Gy, Li2B4O7:Cu doped with Cu (2500 ppm) gives the better response than that of 1000 ppm doped samples. The better response for only 1000 ppm and 2500 ppm Cu doped samples may be due to the ordering of atoms that may occur at this particular concentration of dopant, which may leads to the enhancement of TL intensity.
Fig. 4. Batch to batch homogeneity of synthesized Li2B4O7:Cu (doped with 2500 ppm Cu) nanophosphors exposed to 1 103 Gy γ-radiation.
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Doping with Cu introduces predominantly deeper trapping levels in Li2B4O7 and also the enhancement in TL intensity. The change in trap distributions may be due to the lattice perturbation caused by incorporation of Cu in Li2B4O7 [13]. Cu on its introduction as an impurity in Li2B4O7 may easily occupy one of the interstitial positions. When in close proximity to alkali-associated trapping species, copper ions may augment the trapping profile. The copper ions may form positively charged complexes with oxygen vacancies and other structural defects. As an impurity, copper is known to exist both in Cu+ and Cu2+ states [9]. Therefore it is reasonable to assume that the copper ion (preferably in Cu2+ state) could slip quite easily into various locations in the matrix and in the domain of intrinsic lattice defects. There could be two possible situations which lead to the excited state of copper ions and the peaks shown in glow curves of Li2B4O7:Cu phosphors are related to the relaxation of these excited copper ions [14]. So, we may conclude that the concentration of doped Cu ions can greatly affect the distribution of traps produced by γ-exposure. The results of maximum TL response of Li2B4O7:Cu (1000 ppm and 2500 ppm Cu) lead us to select these samples for further study of TL properties.
3.3.3. Effect of dose on glow curves The effect of gamma dose in the range 1 101–1 102 Gy on TL glow curves of synthesized Li2B4O7:Cu (doped with 1000 ppm Cu) nanophosphor is shown in Fig. 5. The glow curves are found to have prominent peak at 447 K and small humps at 386 K and 496 K. It is inferred from Fig. 5 that with increase in dose from 1 101 Gy to 1 102 Gy, the peak at 447 K becomes prominent, the TL intensity keeps on increasing and the humps at 386 K and 496 K keep on diminishing. Further Fig. 6 shows the effect of gamma dose in the range 5 102–3 104 Gy on TL glow curves of synthesized Li2B4O7:Cu (doped with 2500 ppm Cu) nanophosphor. The prominent peak at 457 K and a small hump at 395 K is observed. With increase in dose, the TL intensity of prominent peak keeps on increasing and small hump is observed to be diminished. It is known that the cation disorder and nonstoichiometry of Li2B4O7 provides a large number of lattice defects which may serve as trapping centers. During irradiation an electron hole pair is created. The hole is trapped at the V center, cation vacancy is stabilized by the surrounding oxygen ions and the electrons are trapped to form F+ center. During heating the hole from V center is released and recombines with the electron trapped in F+ center. This recombination energy is transferred to nearby Cu+ ions and Cu+ ions get excited and on de-excitation emit its characteristic emission [15]. The occurrence of various peaks and changes in their intensities for the synthesized nanocrystalline material are explained using the model shown in previous work [16]. Further the comparison of TL glow curves of synthesized nanocrystalline Li2B4O7:Cu doped with Cu of concentrations 1000 ppm and 2500 ppm, exposed to gamma dose of 1 103 Gy from 60Co source are shown in Fig. 7. It is noticed that with change in dopant concentration only peak
Table 1 Variation of TL response with dopant concentration of Li2B4O7:Cu samples at different gamma doses having range 1 100–5 103 Gy. TL response (a.u.) Dose (Gy) Concentration (ppm)
1 100
1 101
5 101
1 102
5 102
1 103
5 103
500 1000 1500 2000 2500 3000
54.67 157.2 128.2 112.0 104.9 59.4
514.4 1440.6 727.9 790.1 1170.8 564.3
3968.6 6137.8 4460.0 2841.2 6156.8 3507.2
6763.4 12,413.3 8735.4 10,184.3 12,241.4 9229.4
39,971.2 60,928.0 54,206.0 44,182.5 67,572.5 51,034.9
108,162.9 127,444.1 140,907.7 150,174.8 160,591.7 145,775.3
311,057.7 547,034.2 500,630.7 452,109.9 826,631.2 530,569.4
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Fig. 7. Comparison of TL glow curves of Li2B4O7:Cu nanophosphors doped with 2500 ppm and 1000 ppm Cu and exposed with a dose of 1 103 Gy γ-radiation. Fig. 5. Effect of gamma dose on TL glow curves of synthesized nanocrystalline Li2B4O7:Cu (doped with 1000 ppm Cu) having dose ranging 1 101–1 102 Gy.
Fig. 6. Effect of gamma dose on TL glow curves of synthesized nanocrystalline Li2B4O7:Cu (doped with 2500 ppm Cu) powder.
intensity and peak temperature shifts occur. The peak intensity for 1000 ppm doped samples is found to be less than that of 2500 ppm doped samples and there is no change in the shape of glow curve with change in concentration. 3.3.4. TL response TL response (area under the curve covering the full range of temperature) of Li2B4O7:Cu doped with Cu (1000 ppm and 2500 ppm) for lower doses in the range of 2 10−4–1 100 Gy does not show a linear pattern. Hence Li2B4O7:Cu nanophosphor cannot be used as TLD material within this range. Further TL responses of samples exposed for higher doses in the range of 1 100–5 104 Gy are shown in Fig. 8(a and b). It is clearly observed from Fig. 8(a and b) that Li2B4O7:Cu doped with 1000 ppm and 2500 ppm Cu exhibit a linear response from 1 100 Gy to 5 103 Gy. On further increasing the dose from 5 103 Gy to 1 104 Gy, the response becomes supralinear, then finally resulting in saturation over 1.5 104 Gy. This linearity over a wide range of dose is found within an experimental error of 710%. Therefore synthesized Li2B4O7:Cu nanophosphor doped with Cu (1000 ppm and 2500 ppm) could be used for estimation of very high doses (in the range 1 100 Gy to 5 103 Gy) of gamma radiations. 3.3.5. Fading In order to determine the fading characteristics of Li2B4O7:Cu doped with Cu (2500 ppm and 1000 ppm), several samples were
Fig. 8. TL response of nanocrystalline Li2B4O7:Cu doped with (a) 1000 ppm and (b) 2500 ppm Cu for wide range of γ- doses.
irradiated to a γ dose of 1 103 Gy and were stored in dark conditions at room temperature. The results of fading of synthesized nanocrystalline Li2B4O7 doped with Cu are presented in Fig. 9. It reveals that in case of 1000 ppm Cu doped samples, the fading on third day is 7%, on 7th day it is 8% and on 15th day it is 9%, whereas in 1 month the total fading recorded is 10%. The fading results are found to have an experimental error of 75%. The maximum fading recorded in our system is seen in first week after exposure of the samples. The fading recorded in the samples doped with Cu of concentration 2500 ppm is explained in previous reported paper and is found to be 7% in a period of 1 month [1].
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Table 2 Values of trap depth (E) and frequency factor (s) for isolated peaks calculated by GCD glow fit method. S. no
Peak
Tm (K)
E (eV)
s (s−1)
1. 2. 3. 4.
1 2 3 4
392.7 446.5 468.5 499.7
0.86 1.08 1.10 1.20
8.61 1010 1.24 1012 4.12 1011 7.87 1011
with Cu (2500 ppm and 1000 ppm) are shown in Fig. 10. It was found that the TL response reduced to almost 8% in 10th cycle for Li2B4O7:Cu doped with Cu (1000 ppm), while the TL response reduced only 2% even after 10 cycles for synthesized Li2B4O7:Cu doped with Cu (2500 ppm). So, the synthesized Li2B4O7:Cu doped with Cu (2500 ppm) has better reproducibility than that of 1000 ppm Cu doped Li2B4O7:Cu sample. Fig. 9. Fading of synthesized nanocrystalline Li2B4O7:Cu doped with 1000 ppm and 2500 ppm Copper.
Fig. 10. Reproducibility of synthesized nanocrystalline Li2B4O7:Cu doped with 1000 ppm and 2500 ppm Copper.
3.4. Calculations of trapping parameters The glow curve is related to the trap levels that lie at different depths in the band gap between the conduction and the valence bands of a solid. These trap levels are characterized by different trapping parameters. Hence, several peaks are found to appear in the TL glow curve at characterized temperatures. The trapping parameters associated with the various TL bands, are evaluated after deconvolution of composite glow curves. In the present work, the trapping parameters associated with the glow peaks were calculated using the glow curve deconvolution (GCD) glow fit method. The aim of the analysis is to calculate the electron depths for traps and their frequency factors so as to understand the complete mechanism of TL glow curve. The recorded composite glow curves for gamma dose of 5 103 Gy were first isolated by a computer program, GlowFit [17] and are shown in Fig. 11. It was found that the composite glow curve is composed of four peaks. The parameter describing the quality of fitting, called Figure of Merit (FOM) was calculated. Glow curves with FOM values in excess of 5% are subjected to further investigation to determine the reasons for the poor fit. Hence minimum four peaks are required to get the best fit. These all isolated four peaks show first-order kinetics, i. e. the probability of electron re-trapping during the thermoluminescence process was negligible. The values of trapping parameters of all the four isolated peaks for the glow curve were calculated by the GCD glow fit method and are shown in Table 2. FOM was found to be 2.71%. It is observed that the energy levels (activation energy) of various traps (corresponding to various peaks) are very much different. Therefore it is clear that there are some deep and shallow traps. The competition among these traps might be giving various releasing probabilities, which might have resulted in different frequency factors [18]. The dosimetry information stored in the materials after irradiation strongly depends on the position of trapping levels within the forbidden gap, i. e. trap depth or activation energy.
4. Conclusions Fig. 11. Deconvulation of TL glow curve of Li2B4O7:Cu (doped with 2500 ppm Cu) exposed to 5 103 Gy of γ-dose.
3.3.6. Reproducibility In order to assess the reproducibility of the dose measurements of synthesized samples, a large set of repeated post read-out annealings at 300 1C for 30 min were carried out at 1 103 Gy dose level, everytime using 60Co γ-ray source. The results of assessment of reproducibility of synthesized Li2B4O7:Cu nanophosphor doped
The main interesting feature reported in this work is concerning the TL response in a very wide range (2 10−4–5 104 Gy) of gamma rays for nanocrystalline Li2B4O7:Cu phosphors, doped with different concentrations of Cu. It is observed that Li2B4O7:Cu doped with Cu (1000 ppm and 2500 ppm) exhibit a linear response from 1 100 Gy to 5 103 Gy of gamma dose. So it can be concluded that good sensitivity, simple glow curve structure, linear response over a wide range of exposure, low fading and excellent reproducibility are some of the required characteristics of Li2B4O7:Cu doped with Cu of
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concentrations 1000 ppm and 2500 ppm, making them suitable for radiation dosimetry. Acknowledgments We are grateful to Dr Sundeep Chopra of Inter University Accelerator Centre, New Delhi for providing necessary facilities to carry out this work. We are thankful to Dr. S.K. Sharma and Dr A. Choubey, from Indian School of Mines, Dhanbad for valuable discussions. References [1] L. Singh, V. Chopra, S.P. Lochab, Journal of Luminescence 131 (2011) 1177. [2] M. Takenega, O Yamamoto, T. Yamashita, Nuclear Instruments and Methods 175 (1980) 77. [3] M. Prokic, Radiation Measurements 33 (2001) 393. [4] A.C. Fernandes, M. Osvay, J.P. Santos, V. Holovey, M. Ignatovych, Radiation Measurements 43 (2008) 476.
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