A N Singh, S N Singh & L L Singh

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Singh & L L Singh Indian Journal of Physics ISSN 0973-1458 Volume 89

1 Analysis of thermoluminescence of Li 2 B 4 O 7 :Cu, Ag, P phosphor by simplified General one Trap differential equation A N Singh, S N Singh & L L Singh Indian Journal of Physics ISSN Volume 89 Number 1 Indian J Phys (2015) 89:41-44 DOI /s z 1 23

2 Your article is protected by copyright and all rights are held exclusively by Indian Association for the Cultivation of Science. This e-offprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com. 1 23

Indian J Phys (January 2015) 89(1):41 44 DOI 10.

3 Indian J Phys (January 2015) 89(1):41 44 DOI /s z ORIGINAL PAPER Analysis of thermoluminescence of Li 2 B 4 O 7 :Cu, Ag, P phosphor by simplified General one Trap differential equation A N Singh 1 *, S N Singh 2 and L L Singh 3 1 Department of Physics, Thoubal College, Thoubal , Manipur, India 2 Department of Physics, Churchandpur College, Churchandpur , Manipur, India 3 Department of Physics, Manipur University, Canchipur , Manipur, India Received: 24 January 2014 / Accepted: 03 June 2014 / Published online: 26 June 2014 Abstract: Lithium tetraborate based phosphor (Li 2 B 4 O 7 :Cu, Ag, P) is a suitable material for thermoluminescence dosimetery. It is prepared at Thermoluminescence Dosimetry Laboratory, Thoubal College, Manipur and has been analyzed using recently formulated simplified General one Trap equation to determine the trapping parameters. This new simplified equation helps in finding the key trapping parameters E, s, N, c and a, which is not possible with the well known kinetic order formalism. From the analysis, it is found that activation energy of the phosphor is in the range ev and frequency factor is * s -1. Keywords: PACS No.: Trapping parameters; Activation energy; Lifetime; Frequency factor Kn 1. Introduction Lithium tetraborate based phosphor (Li 2 B 4 O 7 :Cu, Ag, P) is a suitable material for thermoluminescence (TL) dosimetry because of its effective atomic number (Z eff = 7.39), which is very close to that of human tissue (Z eff = 7.42). This property has attracted attention of researcher in lithium tetraborate based TL dosimeters [1] and have been basic foundation in research and applications for several decades [2 4]. The first TL material based lithium tetraborate which has been introduced in radiation dosimetry is Li 2 B 4 O 7 :Mn phosphor [5]. However, it gives low TL sensitivity, caused partly by the emission in the 600 nm region of the spectra, which is far from the ideal wavelength region for most commercial photomultipliers (*400 nm). Afterward different methods of preparation of these TL materials have subsequently been developed, but different activators give rather different TL characteristics i.e., glow curves, TL sensitivity, linearity etc. [6 9]. Sintered Li 2 B 4 O 7 co-doped with several dopants have been introduced in order to *Corresponding author, ancsingh@yahoo.co.in produce TLD materials with better dosimetric characteristics [10, 11]. Li 2 B 4 O 7 :Cu, Ag, P phosphor possess other basic dosimetric properties such as TL sensitivity, dose response, low fading, low detection limit, reproducibility etc. [1, 12, 13]. However, dosimetric characteristic of a material depends on lifetime of the traps present in the material, which again depends on trapping parameters associated with traps. Lifetime in general order kinetics has been formulated by Singh and Gartia [14]. Recently, Singh and Gartia [15] have formulated a simplified form of the one Trap one Recombination (OTOR) differential equation for routine analysis and they have also formulated the lifetime equation for evaluating the lifetime based on the simplified OTOR differential equation. The simplified General one Trap (GOT) differential equation removes the empirical nature involved in the general order kinetics and spans the region from first order kinetics to second order kinetics. The key feature of the simplified GOT differential equation is the ability to extract not only the trapping parameters namely activation energy (E) and frequency factor (s) but also the other three basic trapping parameters viz. N (number of traps present), a (ratio of the retrapping probability to the recombination probability) and c (=N/n o, n o is the number of electrons in the traps). Ó 2014 IACS

4 42 A N Singh et al. In this paper Li 2 B 4 O 7 doped with Cu, Ag and P in powder form has been analyzed in the simplified GOT formalism. This analysis provides details of the key trapping parameters associated with traps present in the material and the lifetime or mean lifetime of traps. It also deals with possible use of the traps present in dating and dosimetry. 2. Materials and methods Li 2 B 4 O 7 :Cu, Ag, P phosphors were prepared at Thermoluminescence Dosimetry Laboratory, Thoubal College, Manipur, by sintering method [16] with the addition of CuCl 2, AgNO 3 and H 3 PO 3 (0.02 wt% each) to Li 2 B 4 O 7 powder sample from MERC company. These mixtures were mixed with acetone and then homogenized by stirring for 30 min using a magnetic stirrer with hot plate. Afterwards, acetone was allowed to evaporate at ambient temperature in the hot plate. Drying was completed in a laboratory oven at 100 C for 15 h. The samples were exposed to heat treatment for 3 h at 800 C in a laboratory furnace and cooled down at room temperature. After grounding it was annealed at 525 C for 1 h and c- irradiated from a 60 Co source up to 5 Gy. TL glow curves were recorded using commercial TL Reader Model 900I (Neocleonix Systems Pvt. Lt., Hyderabad, India) with different heating rates of 0.5, 1.0, 2.0 and 5.0 C/s. For TL readout 20 mg each of the powder sample were used. A second readout was performed to record the background radiation which included the black body radiation. The data presented were all with background subtraction. Glow curves of low heating rates namely 0.5 and 1.0 C/s were used for suitable correction of thermal lag. The goodness of fit of the measured glow curve was again tested using standard statistical test i.e., v 2 test of normality, which measured the goodness of fit in terms of normalcy of error distribution. Figure of Merit (FOM) was also calculated as a cross check. 3. Methods of analysis Theoretical derivation of the present work has been described in details in a recent work of Singh and Gartia [17]. Only two equations of importance [i.e., Eqs 22(a) and 22(b)] of Singh and Gartia [17] are presented below: Nas expð E=kTÞ I TL ¼ ð1þ b 2 xðxðtþþð1 þ xðxðtþþ where N (cm -3 ) the concentration of traps of the kind responsible for the peak being considered, a the ratio of the retrapping probabilities to the recombination probabilities, s (s -1 ) is the frequency factor, E (ev) the activation energy, b ( C/s) the heating rate, x the Wright omega function (detail given in the appendix of Singh and Gartia [17]) and Z s T E ca xðtþ ¼ exp dt þ b kt T o ð2þ ca þ log where n o (cm -3 ) is the concentration of electron traps and c = N/n o. The above equations are valid for 0 \ a \ 1. It is found that the equation tends to the first order kinetics equation when a B n o /100 N and the second order kinetics equation when a = The proposed model for analyzing these TL glow curves assumes a set of discrete electron traps and a set of hole traps (recombination center). All the glow curves of different heating rates are subjected to computerized glow curve deconvolution (CGCD) using simplified GOT differential equations. Computing and fitting of the glow peaks following the simplified GOT differential equations have been done from the program developed [18]. Lifetime of the trap electrons are calculated from the Eq. 19 of Singh and Gartia [15] i.e., s ¼ Na ðn o nþþlog n o 1 n 1 þ n Pn o Na n Na n o ð3þ Substituting n = 1 in the above expression we have s ¼ Na ðn o 1Þþlogðn o Þ 1 1 þ 1 Pn o Na Na n o ð4þ where s gives the expression for time required for a nonsaturated trap concentration to be reduced to n = 1. The goodness of fit of the measured TL glow curve has been again tested using v 2 test of normality [19 21], which measures the goodness of fit in terms of normality of error distribution (i.e., difference between the observed and calculated intensity). As a cross check, Figure of Merit (FOM) [22, 23] has been also calculated. In order to obtain trapping parameters for higher rate of heating, evaluation of the real glow peak temperatures (T m ) is necessary [24] and is calculated using the relation Tm j ¼ Ti m c ln b i ð5þ b j i j where T m and T m are the maximum temperatures of a glow peak with heating rates b i and b j respectively and c is a constant, which is usually evaluated by using two very low heating rates (preferably below 1 C/s), where the thermal lag (TLA) can be considered as negligible [25]. Effective heating rate (b eff ) between the heating element and

5 Analysis of thermoluminescence TL Intensity (a.u.) TL Intensity (a.u.) thermoluminescent sample during TL readout in reader (using contact heating) has been taken into consideration to avoid errors in determining the trapping parameters by glow curve deconvolution. A simple method of heating rate correction [24] has been used to avoid this problem and determine the exact effective heating rate of the TL sample by using the equation: b eff ¼ T g T 0 DT ð b ¼ T m T 0 Þ b ð6þ T g T 0 T g T 0 where DT = T g - T m, T g is observed peak temperature (K) and T m is real peak temperature (i.e., with thermal lag correction); T o is room the temperature (25 C). 4. Results and discussion Temperature ( C) Fig. 1 Glow curves of Li 2 B 4 O 7 :Cu, Ag, P phosphor with different heating rates (0.5, 1.0, 2.0 and 5 C/s) Figure 1 shows TL glow curves of the prepared Li 2 B 4 O 7 :Cu, Ag, P for different heating rates namely 0.5, 1.0, 2.0 and Temperature ( C) Fig. 2 CGCD of Li 2 B 4 O 7 :Cu, Ag, P phosphor, heating rate = 2 C/s. Serial ring experimental curve. Dashed line numerically generated curve. Solid line sum of the numerically generated best curves 5.0 C/s. The glow curves show shifting of peak positions from lower to higher temperature region with increase in heating rates. All the curves are broad with a single looking peak. These glow curves are subjected to glow curve deconvolution using simplified GOT model [17] after thermal correction [24]. From the analysis, we have observed that the glow curve of Li 2 B 4 O 7 :Cu, Ag, P consists of three constituent peaks. The fittings are subjected to statistical analysis (FOM and v 2 test) and we have found that FOM is less than 1 % and v 2 test passed at 5 % level of probability. The trapping parameters from this analysis are presented in Table 1 and some of the typical fittings are presented in Figs. 2 and 3. The analyses shows that key trapping parameters namely E and s are in the physically realistic range. From Table 1, we have found that the trap of the glow peak having activation energy * 0.9 ev has the highest number of trap concentration and is highly active and prone to Table 1 Trapping parameters as obtained using glow curve deconvolution with different heating rates 0.5, 1, 2 and 5 C/s Heating rate ( C/s) T m ( C) E (ev) s (s -1 ) a N c s FOM (%) v 2 (df) , (1) , , , (4) , , , (3) , , , (days) (4) , (days) , years Calculated v 2 values are accepted at 5 % level of probability

6 44 A N Singh et al. TL Intensity (a.u.) Temperature ( C ) Fig. 3 CGCD of Li 2 B 4 O 7 :Cu, Ag, P phosphor, heating rate = 5 C/s. Serial ring experimental curve. Dashed line numerically generated curve. Solid line sum of the numerically generated best curves irradiation with a lifetime of 285 days which would be useful for dosimetry. 5. Conclusions Glow curves of as prepared Li 2 B 4 O 7 :Cu, Ag, P phosphor has been analyzed using simplified GOT model. Activation energy obtained by the deconvolution of the glow curves are in the range of ev and frequency factor * s -1. Main peak with activation energy *0.9 ev has electron lifetime *285 days, which infers that this phosphor could be a potential candidate for routine use as clinical dosimeter as this phosphor possesses other basic dosimetric properties. Acknowledgments The authors are thankful for the facility provided by the TL Dosimetry Laboratory,Thoubal College, Thoubal,Manipur (India) sponsored by AERB/CSRP, Mumbai. References [1] Z Y Xiong, C X Zhang and Q Tang Chinese Sci. Bulletin (2007) [2] M Takenaga, O Yamamoto and T Yamashita Nucl. Instru. Methods (1980) [3] G Kitis, C Furetta, M Prokic and V Prokic J. Phys. D: Appl. Phys (2000); P M Bhujbal and S J Dhoble Indian J. Phys (2012) [4] C Furetta, M Prokic, R Salomon, V Prokic and G Kitis Nucl. Instru. Methods A (2001) [5] J H Schulman, R D Kirk and E J West Proc Internat. Conf. Lumin. Dosimetry (USA: Stanford) p 113 (1967) [6] Y Kutomi, A Tomita and N Takeuchi Radiat. Prot. Dosim (1986) [7] M Martini, F Meinardi, L Kovacs and K Polgar Radiat. Prot. Dosim (1993) [8] M Prokic Radiat. Meas (2001) [9] N Can, T Karali, P D Townsend and F Yıldız J. Phys. D: Appl. Phys (2006) [10] Y S Horowitz and D Yossian Radiat Prot. Dosim (1995) [11] A Ege, E Ekdal, T Karali, N Can and M Prokic Meas. Sci. Technol (2007) [12] M Prokic Radiat Prot. Dosim (2002) [13] S Miljanic, M Ranogajee-Komor, Z Knezevic, M Stubec and M Prokic Radiat Prot. Dosim (2006) [14] L L Singh and R K Gartia Radiat. Effect. Defects Solids (2011) [15] L L Singh and R K Gartia Nucl. Inst. Methods B (2013) [16] M Takenaga, O Yamamoto and T Yamashita Nucl. Instru. Methods (1980) [17] L L Singh and R K Gartia Radiat. Meas (2013) [18] L L Singh and R K Gartia Nucl. Inst. Methods B (2014) [19] B Siegmund Data Analysis (New York: Springer-Verlag) p. 235 (1999) [20] B A Sharma, T B Singh and R K Gartia Indian J. Pure Appl. Phys (2004) [21] S N Singh, B A Sharma, R K Gartia and A N Singh Indian J. Pure Appl. Phys (2004) [22] H G Balian and N W Eddy Nucl. Instru. Methods (1997) [23] S K Mishra and N W Eddy Nucl. Instru. Methods (1979) [24] G Kitis and J W N Tuyn J. Phys. D: Appl. Phys (1998) [25] A N Yazici and M J Topaksu J. Phys. D: Appl. Phys (2003)

A COMPUTER PROGRAM FOR THE DECONVOLUTION OF THERMOLUMINESCENCE GLOW CURVES K. S. Chung 1,, H. S. Choe 1, J. I. Lee 2, J. L. Kim 2 and S. Y.

Radiation Protection Dosimetry (200), Vol. 11, No. 1, pp. 3 39 doi:10.1093/rpd/nci073 A COMPUTER PROGRAM FOR THE DECONVOLUTION OF THERMOLUMINESCENCE GLOW CURVES K. S. Chung 1,, H. S. Choe 1, J. I. Lee