Investigation of thermoluminescence characteristics of Li2B4O7:Mn (TLD-800)

Thermochimica Acta 575 (2014) 300–304

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Investigation of thermoluminescence characteristics of Li2 B4 O7 : Mn (TLD-800) V. Emir Kafadar ∗ , R. Güler Yildirim, H. Zebari, D. Zebari University of Gaziantep, Department of Engineering Physics, 27310 Gaziantep, Turkey

a r t i c l e

i n f o

Article history: Received 20 September 2013 Received in revised form 18 November 2013 Accepted 19 November 2013 Available online 26 November 2013 Keywords: Kinetic parameters Thermoluminescence Heating rate Li2 B4 O7 :Mn

a b s t r a c t Li2 B4 O7 :Mn is a well-known TLD material with an excellent human tissue equivalence. It is used in practice for personnel dose monitoring in variable branches. In the given study, the additive dose (AD), variable heating rate (VHR), peak shape (PS), three points method (TPM), and computerized glow deconvolution (CGCD) methods were used to determine the kinetic parameters of the main glow peak (P3) of Li2 B4 O7 :Mn (TLD-800). In addition, the fading characteristics and the effect of the heating rate on the dose dependence of TL glow curves have been investigated. The results of the analysis have shown that TLD-800 has a general order main glow peak with an activation energy of Ea = 0.58 eV at 170 ◦ C at a linear heating rate of 1 ◦ C/s. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Thermoluminescence is the emission of light from an insulator or semiconductor, which takes place during the heating of a solid, following an earlier absorption of energy from radiation [1]. The borates are relatively stable chemical compounds and respond without serious problems for attempts to dope them with TL sensitizers such as the rare earths, copper or manganese ions [2]. Thermoluminescence dosimetry with borates has been of interest for some years and has been stimulated by a number of factors. Materials such as the manganese activated lithium borate have drawn an extensive attention because of their low atomic number, near to tissue equivalence, simple glow curve and low cost. The effective atomic number of the lithium tetraborate (Li2 B4 O7 ) is (Zef = 7.3) which is very close to human tissue (Zef = 7.4) and it is therefore an appropriate material for (TL) dosimetry in support of human protection. The first TL material based on lithium borate which was introduced in dosimetry was Li2 B4 O7 :Mn phosphor with low TL sensitivity caused partly by the emission in the 600 nm region of the spectra, far from the response region of most photomultipliers [3–5]. Lithium tetraborate is a supporting material for nonlinear optics and piezoelectric devices [6]. However, this material is also crucial for an older application: when doped with impurities such as Cu, Mn, or Eu, it is known to be useful in producing thermoluminescent detectors (TLD) of ionizing radiation.

∗ Corresponding author. Tel.: +90 3423172241. E-mail address: [email protected] (V.E. Kafadar). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.11.017

A typical glow curve for Li2 B4 O7 :Mn consists of a single peak centered at 185 ◦ C when heated at 10 ◦ C/s, but the glow peak position has been reported to move to the (Mn) impurity level [7]. The emission spectrum of thermoluminescence dosimeter (TLD-800) has been demonstrated to be a case where (Mn) acts as a recombination center, the emission spectrum show orange emission [8]. Some researchers have made studies on synthesis and dosimetry properties of Mn doped Li2 B4 O7 [9–12]. A reliable dosimetric study of a thermoluminescent material should be based on a good knowledge of its kinetic parameters, which quantitatively describes the trapping-emitting centers principally responsible for the thermoluminescence emission. There are various methods to evaluate the trapping parameters from TL glow curves. If a glow peak is highly isolated from the others, the experimental techniques such as initial rise (IR), variable heating rates (VHR) and peak shape (PS) methods are suitable methods to determine them. The initial rise method of analysis stems from the recognition by Garlick and Gibson [13] that the initial rise part of a thermoluminescence curve is exponentially dependent on temperature according to I(T) = Cexp(− E/kT). Plotting ln(I) against 1/T a linear plot is obtained with slope equal to –E/k. Hence it is possible to evaluate E without any knowledge of the frequency factor s by means of equation E = − kd(ln(I))/d(1/T). When the IR method is applied to a continuous distribution [14], the obtained activation energy corresponds to “effective” activation energy, Eeff , depending on the actual energy distribution of active traps [15]. Instead of employing simple formulae to derive values for the parameters E, s, and b, methods of computerized curve fitting have been used on several occasions with apparent success.

V.E. Kafadar et al. / Thermochimica Acta 575 (2014) 300–304

The procedure is to establish the approximate positions of the most prominent peaks in the glow-curve and to estimate initial values of E, s, and b by using one of the analytical methods. The procedure continues by sequentially changing the E, s, and b values until a minimum value of the RMS deviation is obtained. This method has the advantage over experimental methods in that they can be used in largely overlapping-peak glow curves without resorting to heat treatment [16]. In this study, the additive dose (AD), variable heating rate, peak shape, three point methods (TPM) along with the deconvolution method have been utilized to evaluate trapping parameters of the main dosimetric peak, called P3, of Mn-doped Li2 B4 O7 (TLD-800). We have also investigated the fading characteristics and the effect of heating rate on the integrated TL peak area of glow curves of lithium tetraborate (Li2 B4 O7 ) crystals at different linear heating rates between 1 ◦ C/s and 20 ◦ C/s.

photomultiplier tube. This filter allows the light whose wavelength is between ≈250 nm and ≈1000 nm to pass through the filter and thereby eliminates unwanted infrared light emitted from heater. At each experimental measurement, four chips were read out. Each chip was read out twice and the second readout is considered to be the background of the reader plus chip; this was subtracted from the first one and all of the analyses have been carried out after the subtraction. Glow curve readout was carried out on a platinum planchet at a linear heating rate of 1 ◦ C/s up to 400 ◦ C, except for the heating rate experiments. In the variable heating rate (VHR) method, the heating rates were changed between 1 ◦ C/s and 20 ◦ C/s.

3. Results In order to form an opinion about the number of glow peaks and kinetic orders (b) of all individual glow peaks in the glow curve structure of Mn-doped Li2 B4 O7 , the additive dose method was firstly utilized in the current study. The samples were irradiated at several doses between ≈2.4 Gy and ≈7 kGy and some of the selected glow curves after variable dose levels can be seen in Fig. 1. The results of the additive dose experiments were also utilized to calculate the trapping parameters, namely, activation energy (Ea ), kinetic order (b) and the frequency factor (s), by using the peak shape method of Chen and McKeever [6]. The kinetic parameters have also identified by using Gartia et al. peak shape method [17] in addition to Chen’s peak shape method. The results of the methods are summarized in Table 1. The trapping parameters of TLD-800 crystal were also calculated by the Rasheedy’s [18] three-points method (TPM). In the present study, the trapping parameters evaluated from the TPM is the results of calculations from the maximum intensity at temperature ≈170 ◦ C (P3) up to 15% of the maximum intensity. The average values of the trapping parameters associated with peak 3 are given as b = 1.55, Ea = 0.54 eV and ln(s) = 13 s−1 .

2. Experimental procedure The sample used in this study was lithium tetraborate (Li2 B4 O7 , TLD-800) from Thermo Electron Corporation. The dimensions of the sample are (3.2 mm × 3.2 mm × 0.89 mm) with a mean mass of ≈0.230 g. The samples were firstly annealed at 300 ± 1 ◦ C for 30 min prior to irradiation with a specially designed microprocessor controlled electrical oven then cooled rapidly in the air at (75 ◦ C/min) to the room temperature. The samples were irradiated at room temperature with a newly calibrated 90 Sr-90 Y beta source. The activity of ␤-source is about 100 mCi. The typical strength of a 100 mCi Sr-90 ␤-source installed in a 9010 Optical Dating System is 2.64 Gy/min = 0.0438 Gy/s for fine grains on aluminum, or 3.3 Gy/min = 0.055 Gy/s for 100 m quartz on stainless still. The irradiated samples were read out in an N2 atmosphere with a Harshaw QS 3500 manual type reader which has an S-11 response photomultiplier tube. The reader is interfaced to a PC where the TL signals were studied and analyzed. A standard clean glass filter was always installed in the reader between the sample and

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Fig. 1. The glow curve of Mn-doped Li2 B4 O7 (TLD-800) measured after different radiation exposed dose levels (ˇ = 1 ◦ C/s).

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2 Fig. 2. (a) Some of the selected glow curves of Mn-doped Li2 B4 O7 (TLD-800) measured at different heating rates for (1, 3, 6, 10 and 20 ◦ C/s). (b) Plot of ln(Tm /ˇ) vs 1/Tm at 72 Gy.

Table 1 The values of the trapping parameters of TL peaks of Li2 B4 O7 :Mn determined by Chen’s PS, Mazumdar PS, TPM, VHR and CGCD methods. Mazumdar P.S.

E (eV) b ln s (s−1 )

Chen P.S.

1/2 ratio

2/3 ratio

4/5 ratio

E

Eı

Eω

0.57 1.6 12

0.54 1.6 12

0.58 1.6 12

0.54 1.6 12

0.6 1.6 12

0.57 1.6 12

Another method that was used to determine the kinetic parameters in this study is the variable heating rate (VHR) method. This method is based on the shift position of the temperature (Tm ) at the maximum point of intensity (Im ) to higher temperatures when 2 /ˇ) against 1/(kT ) the heating rate is increased. A plot of ln(Tm m could offer a straight line of rise Ea /k and intercept ln(sk/Ea ). The major advantage of this method is that the required data is to be taken at a peak maximum (Im , Tm ) which, in the case of a large peak surrounded by smaller satellites, can be reasonably accurately determined from the glow curve. The important point that has to be taken into consideration to avoid large errors in the kinetic parameters determined by variable heating rate method is the temperature lag (TLA) between the heating element and the thermoluminescent sample during the TL readout in readers using contact heating. To avoid this problem, a simple method has been recently proposed by Kitis et al. [19] to correct the TLA and to determine the exact peak temperatures after different heating rates by using the following equation:

 j Tm

=

i Tm

− C ln j

ˇi ˇj

TPM

VHR

0.55 1.6 12

0.54 1.55 13

0.63 1.6 12.5

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i are the maximum temperatures of a glow peak where Tm and Tm with heating rates ˇj and ˇi , respectively, and C is a constant, which is initially evaluated by using two very low heating rates where TLA can be considered as negligible. In preference, the low heating rates should be chosen below 1 ◦ C/s to calculate the constant C. For the VHR method, the five heating rates of 1 ◦ C/s, 3 ◦ C/s, 6 ◦ C/s, ◦ 10 C/s and 20 ◦ C/s were applied. The measured and then normal2 /ˇ) ized glow curves after these five heating rates and a plot of ln(Tm against 1/(kTm ) are shown in Fig. 2. As seen from Fig. 2, the temperature of a TL peak shifts to higher values as the heating rate increases. This shift in temperature was held responsible for the increase in the contribution of non-radiative transitions. In addition to that, there is a small decrease in each of the intensities of the

CGCD

glow peaks. The decreasing luminescence intensity of glow peaks of Li2 B4 O7 :Mn phosphor as a function of the increasing heating rate is an event frequently observed in the practice of TSL. It has been suggested that it is because the effect of thermal quenching which reduces the effectiveness of the luminescence when increases temperature because of the increased non-radiative transition probability [11,12]. The trapping parameters of P3 calculated by various methods are given in Table 1. The glow curve of this sample was also analyzed by the CGCD method. The CGCD is a powerful technique in the study of TL and it is frequently used to determine the trapping parameters and in the study of thermoluminescent dosimeters. Instead of employing simple formulae to derive values for the parameters E, s, and b, methods of computerized curve fitting have been used on several

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occasions with apparent success. It is apparent that if the number of data points used in the analysis increases, the potential for accurate determination of the kinetic parameters gets better [17,18]. The program used, CGCD which depended upon the procedure of least square minimization, was developed at the Reactor Institute at Delft, The Netherlands. An IRI-CIMAT Report gave the detailed results of these models [20]. The TL glow curves were analyzed by using the following approximate solution of the general-order kinetics of the  TL; I(T ) = n0 s exp



 1+

−

−

E kT

(b − 1)s kT 2 E ˇ

exp(−

E kT

  ×

(0.9920 − 1.620

kT Ea

b/b−1

where n0 (m−3 ) is the concentration of trapped electrons at t = 0, s (s−1 ) is the frequency factor for first-order and the pre-exponential factor for the general-order, E (eV) the activation energy, T (K) the absolute temperature, k (eV/K) Boltzmann’s constant, ˇ (C/s) heating rate and b the kinetic order.

A careful investigation of Fig. 3 indicates that glow curve structure of Li2 B4 O7 :Mn is described by a linear combination of at least three glow peaks between RT and 350 ◦ C. The parameters were finally so selected that the values yielded the best overall fit to the designated high priority features of the TL results. Fig. 3 shows one of the CGCD analyzed glow curve and results of fitting on the assumption of three peaks. Table 1 summarizes the Ea and s values of peak 3. In the given study we have also studied the storage time effect on the intensity of the glow peaks of Li2 B4 O7 :Mn. All lithiumborate TLDs are hygroscopic, including TLD-800. Fading in TLD-800 is highly dependent on humidity and other environmental factors. It is reported that in Li2 B4 O7 :Mn 10% fading at 0% relative humidity increasing to 40% fading at 95% relative humidity for 90 days storage at 20 ◦ C. [21]. For the storage time experiment, the material was annealed at 300 ± 1 ◦ C for 30 min and irradiated up to 36 Gy. The storage time experiments were performed for different time

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periods. Fig. 4a shows the measured glow curves at the end of the scheduled storage periods. Also as shown from Fig. 4b the main glow peak (P3) of TLD-800 at the end of the scheduled storage times typically reduces 50% of its original value. The behavior of the glow peak temperature and the total peak area of glow curves were also investigated at different linear heating rates between 1 ◦ C/s and 20 ◦ C/s. It is seen from Fig. 5, the peak temperature of the main glow peak (P3) increases and the total peak area of the glow curves decreases as the heating rate increases. The total area of the glow curves was normalized at the lower heating rate (1 ◦ C/s) and it can be seen that a reduction in the integrated area of the peaks by about 65%.

thermal quenching. It is well known that the dosimetric characteristics of many TL materials are influenced by changes in location, size and shape of the glow curves due to changes in the heating rate. Thermal quenching was understood to be due to the increased probability of non-radiative transitions competing with the radiative transitions. The glow peaks occurring at high temperatures must exhibit high thermal quenching than those occurring at lower temperatures in any TLD material. Apart from the increased probability of non-radiative transitions at higher temperatures, the observed effects have also been assigned to the effects of heating rate on the migration of charge carriers released during the TL readout.

4. Conclusion

References

The results of this paper indicates that the glow curve of Mndoped Li2 B4 O7 in the temperature range from room temperature to 400 ◦ C is the superposition of at least three general order glow peaks. Different techniques such as Chen’s and Mazumdar peak shape methods, Rasheedy’s three points method (TPM), variable heating rate (VHR) and computer glow curve deconvoluation methods have been applied to evaluate the kinetic parameters of Ea , b, and s. Table 1 summarizes the value of the kinetic parameters calculated by all of the methods. As seen from this table the values are in agreement with each other. The storage time experiments were performed for different time periods from 1 to 4 weeks for dark fading. The obtained glow curves of Li2 B4 O7 at the end of the different storage periods are shown in Fig. 4a and b. As seen from these figures, the main glow peak (P3) of TLD-800 at the end of the planned storage times reduced typically 50% of its original value. The results of the heating rate experiments have showed that, the peak temperatures of all peaks are shifted higher temperatures and the integrated peak area of the curves decreases as the heating rate increases as expected in theory. It is seen from Fig. 5, the total peak area of the glow curves decreases as the heating rate increases. The total area of the glow curves were normalized at the lower heating rate (1 ◦ C/s) and it can be seen that the decrease in the integrated area of the peaks by about 65%. The effect of heating rate on the glow curves can be best described by the effect of

[1] S.W.S. McKeever, Thermoluminescence of Solids, first ed., Cambridge University Press, Cambridge, 1985 (Chapter 2). [2] A.S. Pradhan, R.C. Bhatt, Appl. Radiat. Isot. 31 (1980) 671. [3] Z.Y. Xiong, C.X. Zhang, Q. Tang, Chin. Sci. Bull. 52 (2007) 1776. [4] J.H. Schulman, R.D. Kirk, E.J. West, Proceedings International Conference on Luminescence Dosimetry, Stanford, USA, 1965, p. 113. [5] M. Prokic, Radiat. Res. 33 (2001) 393. [6] R. Chen, S.W.S. McKeever, Theory of Thermoluminescence and Related Phenomena, first ed., World Scientific, Singapore, 1997 (Chapter 2). [7] R. Lakshmanan, B. Chandra, R.C. Bhatt, Radiat. Prot. Dosimetry 3 (1981) 191. [8] S.W.S. McKeever, Thermoluminescence Dosimetry Materials: Properties and Uses, Nuclear Technology Publishing, Ashford, 1995, pp. 185. [9] V.S. Gorelik, A.V. Vdovin, V.N. Moiseenko, J. Russ. Laser Res. 24 (2003) 553. [10] A.Yu. Kuznetsov, A.V. Kruzhalov, I.N. Ogorodnikov, A.B. Sobolev, L.I. Isaenko, Phys. Solid State 41 (1999) 48. [11] M.M. Islam, V.V. Maslyuk, T. Bredow, C. Minot, J. Phys. Chem. B 109 (2005) 135974. [12] A. Ratasa, M. Danilkina, M. Kerikmäea, A. Lusta, H. Mändarb, V. Seemanb, G. Slavinc, Mater. Sci. 61 (2012) 279. [13] G.F.J. Garlick, A.F. Gibson, Proc. Phys. Soc. 60 (1948) 574. [14] J.M. Gomez-Ros, V. Correcher, J. Garcia-Guinea, A. Delgado, Radiat. Prot. Dosimetry 119 (2006) 339. [15] V. Correcher, J.M. Gomez-Ros, J. Garcia-Guinea, M. Lis, L. Sanches-Munoz, Radiat. Meas. 43 (2008) 269. [16] A.N. Yazici, M. Bedir, A.S. Sökücü, Nucl. Instrum. Methods Phys. Res. B 259 (2007) 955. [17] R.K. Gartia, S.J. Singh, P.S. Mazumdar, Phys. Stat. Sol. (a) 114 (1989) 407. [18] M.S. Rasheedy, Thermochim. Acta 429 (2005) 143. [19] G. Kitis, J.M. Gomez-Ros, J.W.N. Tuyn, J. Appl. Phys. 31 (1998) 2636. [20] A.J.J. Bos, J.M. Piters, J.M. Gomez-Ros, A. Delgado, IRI-CIEMAT Report 131-93005, IRI, Delft, 1993. [21] Y.S. Horowitz, Thermoluminescence and Thermoluminescent Dosimetry, vol. 1, CRC Press, Inc., Boca Raton, FL, 1984, pp. 126.