Indian Journal of Pure & Applied Physi cs Vol. 41 , August 2003, pp . 621 -626

Kinetics of thermoluminescent calcite V Ponnusamy, V Ramasamy & M Dheenathayalu*

Department o f Physics, Annamalai Uni versity, Annamalainagar 608 002

Received 28 August 2002; revised I 0 Fehru nry 2003; accepted 2 1 April 2003

The thermo-stimulated luminescence (TSL) glow curve characteristics of five, blue coloured calcite samples of southern Tamilnadu are put to analysis. NTS L glow peaks are obtained for these samples . The glow curves of annealed (600 °C) and un-annealed samples, irradiated with gamma dose of 500 Gy show three peaks at 145, 255 and 345°C, respecti vely. recorded with linear heating rate of I 0 °C/sec. The kinet ic parameters such as act ivation energy (I:_l , frequency fac tor (5) and order of kinetics (b) are evaluated, using partial heating, peak shape, isothermal and initi al rise methods. The investigations show that, the trapping centres are no t affected by the annealing procedure and it follows second-order kinetics. The activation energies have been determined .

[Key words: Calcites, Thermo-stimul ated luminescence, Kinetic parameters]

1 Introduction

Thermo-stimulated luminescence is one of the active field s of research and its application in radiation dosimetry and archaeological dating continues to attract a wide attention of researchers. The thermo-stimulated luminescence (TSL) characteristics of any material is customarily labelled by a few parameters such as: (i) the order of kinetics obeying the process of TSL; (ii) the activation energy (or) trap depth ; and (iii) the frequency fac tor. A method for calculating activation energies (£) by TSL glow curve was given by Urbach ', for the first time. The TSL properties of synthet ic phosphor depend largely on the dopant concentration. But, in natural samples, the TSL properties rely on the several factors such as their geneses, chemical composition, incorporation of impurity and geological history2

•3

.

The first theoretical treatment of an isolated TSL peak was given by Randall & Wilkins4

• Further development and convenient method for investigating the TSL kinetic parameters was proposed by Chen & Kirsh 5

•

The TSL glow curves and TSL spectral characteri stics of calcites have been studied by many <.JU thorsr·- '". Very few studies on kinetics have been attempted. In the present investigation, an attempt was made to study the TSL kinetic

*Phone: 04 144-2207X4; e-mail: dheena@ r~diffmail. com

parameters of five calcite samples collected from prominent locations (Thazhayuthu mines, Tamil Nadu) of calcite deposits of southern Tamilnadu reg10n, India, where number of cement manufacturing industries came up . It has promising applications in geological and archaeological dating and in the determination of radiation doses. In fact, the order of kinetics of TSL peak is an important trapping parameter because, the accuracy of TSL dating largely depends on whether the TSL peak follows first order kinetics or not 1

1.12. The TSL glow curves of un-annealed and annealed samples are analysed and the ir kinetic parameters such as , activation energy (E), frequency factor (S) and order of kinetics (b) are evaluated, using various methods.

2 Materials and Methods

The five , natura l-blue coloured calcite samples were collected from geologically important locations (Thazhayuthu mines). They were ground, using agate mortar and sieved to the grain size of 125-250 f..l. . The samples were washed for 2 min with 1 % HCI sol uti on and di stilled water to remove organic materials present, if any. These dried samples were used to carry out the NTSL and A TSL measurements. All the samples were annealed in air atmosphere, in the temperature range from 200-700 °C with an interval of 50 °C, for I hr durat ion , in a muffle fu rnace and the TS L measurements (ATSL) were carried out. The TSL sensit ivity was found to be maximum at 600 oc_ At 600 °C, the

622 INDIAN J PURE & APPL PHYS, VOL 41 , AUGUST 2003

2000 ~----------------------------------------------------------~------------,

1800

1600

1400

~ 1200 ·v; c ~ 1000 -_, ~ 800

600

400

200

0 50 100 150 200 250 300 350 400 450 500

Temperature°C

Fig. I -Thermo-stimulated luminescence of natural blue colour calcite, (a) natural thermoluminescnce (NTL); (b) NTL+Dosde (500 Gy); (c) annealed at 600 oc for I hr (ATSL)

450 -

400

350 -

fS 300

E 250 ,_

200

150

100

50 100 150 200 250 300 350 400

T stop ( oC)

Fig. 2- Partial heating of (Tm-T""P curve) natural blue coloured calcite sampl e' s glow peaks at 140, 255 and 345 °C

annealed samples were irradiated, by using r•1Co­gamma source and the TSL measurements were taken by using Nucleonix TLD-96 readout with a heating rate of 10 °C/s . The TSL glow curves were recorded immediately after irradiation to avoid fading. The data was collected from computer after subtrac ti on of the black body radiation .

2.1 Methods of kinetic analysis

An attempt was made to analyse the TSL glow peaks assuming the mono-energetic trap mode l. Several methods for the analysis of the glow curves have been proposed to determine acti vation energy (E), and they are rev iewed by Shalgaonkar & Narlikar 13

• The order of kinetics and activation

energies are determjned by partial heating method4,

peak shape methodS, isothermal decay method 14 and initial rise method 14

• The experimental values of E and b were used to fi nd out the frequency factor (S). The general mathematical expression relating the parameters E, band Sis:

[3£ [ - E l ( 2KT ) --2

= S exp -- l +(b - 1)--111

KT,, KT,, E

where ~ is the linear heating rate, T"' the glow peak temperature, K the Boltzman constant , E the act ivat ion energy, S the frequency fact or and h is the order of kinetics.

PONNUSAMY et al.:THERMOLUMINESCENT CALCITE 623

Partial heating method is used to identify the number of peaks, if the corresponding traps are mono-energetic or not, and also to make a preliminary evaluation of the order of kinetics. It requires the detection of changes in peak position while the population of trapped carriers is thermally decreased .

For this investigation, a set of samples were simultaneously irradiated to carry out the partial heating measurement with the heating rate of 2 °C/s. After attaining a particular stop temperature (T,..,r) lower than that of the peak temperature (T,11), then cooled rapidly to room temperature and then re­heated at the same heating rate in order to record all of the remaining glow curves . In thi s manner, the whole process was repeated on each sample and its residual TL signal was recorded. A plot of Tm as a function ofT""" could be made.

In the peak shape method, it is assumed that, the TSL glow curves are asymmetric in nature around a max imum and the asymmetry depends on the order of kinetics in the light emission process. In TSL, this method is used to investigate the order of kinetics as well as the activation energies of the trapping centres following the above assumption. The trap depth can be calculated in three ways viz., total half width (ro), lower temperature side half

80

·f ..EI ...:I 60 ~

I 40 C> z;

20

0

width (1) and higher temperature s ide half width (8). A better energy determination can be obtained by choosing any one of these parameters ro, 1 and- 8.

The general formula proposed by Chen5 for the peak shape method is given by:

where a represents any of 1, 8 and ro, the c" and ba values depend on the symmetry factor ~g and this is equal to 8/ro. If ~g is 0.42, the TSL glow peak is due to first-order kinetics and if it is 0.52, then the peak is due to second-order kinetics' .

In isothermal decay method, the TSL material is kept at a constant temperature and the light emission, which decays as a function of time (t) , is monitored. In first-order kinetics, a plot of In (///.,) against t will result in a straight line of slope m = S exp (-EIK1) . If the decay is monitored at different temperatures, the slope is obtained. A plot of In (m) against liT will give a straight line of slope ElK from which E can be calculated.

In the case of general-order kinetics , a plot of (///.,)(1 ·"11

" versus t should be a straight line and the activation energy (E) can be directly obtained from Boltzman plot of the slopes . This method gives estimation of the order of kinetics also and is perhaps the only method unaffected by temperature

0 10 20 30 40 50 60 70 80 90 Annealin~ Time (Min)

Fi g. 3 - Isothermal decay of 255 oc glow peak of calcite after an exposure of 500 Gy at temperature (a) 160 °C( • ), (b) 180 °C(+ )

624 INDIAN J PURE & APPL PHYS , VOL 41 , AUGUST 2003

2.6

~ --::-o 2.4 ":::! e..

2.2 .:..· -~

2 ~ ~ en E-- 1. 8

] ] 1.6

~ 1 . 4

1.2

1

0 10 20 30 40 50 60 70 80 90

A nne a ling Time (min)

0 10 20 30 40 50 60 70 80 90

Annealing Tune (min)

Fig. 4- Plot of (///.,)<1· hl/h versus anneal ing Lime for 255 °C glow peak of calcite for ( I). b= 1. 1 (• ), (2). h = 1.5 {'' )and

(3).b = 2.0 (~ )decay recorded at, (a) 160 oc and (b) 180 oc:

dependent fac tors such as frequency fac tor and quantum effi c iency.

For the investigation by Init ia l rise method, the quantity of materi al is d iv ided in to two parts after the irrad iati on; one part can be used to record the full TSL glow peak under evaluati on, the remaining part can be heated repeatedly , many number of times, in such a way that, each heating is terminated at a temperature, where the TSL intensity reached is hardl y I 0% of the peak intensity recorded in the first experiment. Seri es of partia l readings are made fo r each treated sample, with a rapid cooling a fter each reading. A study of hi gh te mperature peaks was performed after a thermal treatment of the

sample, whi ch empti es the traps corresponding to the first peak. The reby, a pl ot of In/ versus 1/T produces a straight line whose s lope gives ElK, and E is evaluated.

By the above methods, the presence of glow peaks, the act ivation energy, order of kinetics and frequency fac tor are studied .

3 Results and Discussion

3.1 Glow curve characteristics

T he TSL g low curves of f ive, blue co loured, natural calci te samples we re recorded and the g low curve of a representati ve samp le is illustrated in Fig. I. There are we ll-defined natura l thermo-

PONNUSAMY et a/.:THERMOL UMINESCENT CALCITE 625

stimul ated luminescence (NTSL), high temperature peaks at 260 and 345 °C with high intensity, which are shown in Fig. I (a) . After thermal treatment for I hr at 600 °C and exposed to 6°Co gamma source in the dose level of 500 Gy, shows a new peak at 145 °C, in additi on to the already ex ist ing higher temperature peak (260 °C) which is now slightl y shifted to lower temperature side at 255 °C and also the 345 °C peak is found to be not shifted but reduced in intensity which are shown in F ig. I (b). The un-annealed sample is exposed to the dose level of 500 Gy and shows peaks at 140 °C and 255 °C which are shown in F~g. I (c) . In a similar way, the experiment was performed to observe the results of other samples. These observed results are very much simil ar to those obtained by Engin & Guven15 for natural calc ite samples.

0~--~--~--~--~--~~--~ 2.1 2.15 !.2 2.25 2.3 2.35 2.4 2.45

1/T (10~ 1<'1)

Fi g. 5 - Ini tial rise plot or gamma irradiated natural blue colou r calcite or a glow peak at 255 °C

3.2 Partial heating method

Fig. 2 shows the glow peak temperature ( 7~11), as a function of stopping temperature (T, .. ,r) of partia l heating and the resoluti on of the TSL peaks. As in the case of previous method, the total number of peaks are observed to be three in thi s case al so, and these three TSL peaks of laboratory-i rradiated samples corresponds to mono-energetic traps. It can also be seen that, the TSL process re lated to

these three peaks do not obey the first-order kinetics, because the temperature of the peak maximum intensity ( 7~,) increases at higher va lues of T,.,r (T,.,r> T"'). Similar results are also observed fo r the remaining samples.

3.3 Peak shape method

According to thi s method, the symmetry fac tor (IJ.g) , for the samples , are calcul ated from the TSL glow peaks of un-annealed (u) and annealed (a) samples. They are found to be nearl y IJ.,=0 .52, wi th in the experimenta l errors. These value~ agree very well with the second-order kinetics. The determination of average acti vati on energy va lues of the fi ve samples studied by thi s method using e ither total width (E,.) or half width (£6 and Er) obtained from three flow peaks ( 145, 255 and 345 °C) shows 0 .885±0.002, 1.671 ±0.005 , 1.962±0.004 for un­annealed samples and 0 .874±0.002, 1.589±0.005, 1.933±0.004 fo r annealed samples. On compari son with earlier work 15 on natural calci te, the trend observed in the present study is di fferent from the order of kinetics. Thi s may be due to impuri ties present in the samples. Therefore, it is a di fficu lt task to compare the behavi our of glow curve with literature data, since the impuri ty concentrati on of natural calcite samples is unknown and varies fro m place to place.

3.4 Isothermal decay method

Fig. 3 shows the isothermal decay curve of 255 oc TSL glow peak of annealed natural calc ite sample with the exposure of 500 Gy at 160 and 180 °C. The values of (1/f.,)<'·hl/h are plotted against decay time with the different va lues of b in the interva l of 0. 1 (Fig. 4). However, for the sake of c larity , only three plots with h= 1.1, 1.5 and 2 .0 are shown in Fig. 4 . The averaged out acti vation energies (£ ,,,) determined by the slope of the stra ight lines fo r the decay recorded at 160 and 180 oc shows 0.873±0.002. 1.664±0.005, 1.973±0.004 fo r un-annealed sampl es and 0.866±0.002, 1.663±0.005, 1.963±0.004 for annealed samples. These results are in good agreement with the va lues calcu lated by previous method (peak-shape method).

It shows c learl y, there is not much difference between the annealed and un-annealed samples, in the acti vation energies. Thi s findin g shows that, the annealing procedure of the sample does not affect

626 INDIAN 1 PURE & APPL PHYS, VOL 41 , AUGUST 2003

the nature of the trapping centers. By comparing the results obtained by En gin & Guven 15 for other calcite samples, the present investigations agree very well with their findin gs . By using the calculated values of activation energies from peak shape and isothermal methods, the averaged out frequency factors of the five samples obtained from the peaks ( 145, 255 and 345 oq show 3.209x I 0 10

,

3. 112x l0 1\ 2.100xi0 1'J, l.877xl0 111

, 3.616x l0 1\

2. 183x I 0 19 for unannealed samples and 3.220x 10 10,

6.472x l0 1 ~, 2.047x l0 19, 2.122xl0 10

, 2.934xl0 1\

2.141 x I 0 19 for annealed samples, respectively.

3.5 Initial rise method

Fig. 5 shows a plot of In f verses 1/T for the second glow peak (255 °C) of a representative sampl e, which gives a straight line. Simi lar results are obtained for other two peaks of all other samples. The average activation energies (£;,),

obtained by thi s method, shows 0.874±0.002, 1.665±0.005, 1.974±0.004 for un-annealed samples and 0.855±0.002, 1.678±0.005, 1.980±0.004, for annealed samples. These results are very much simil ar to those calculated by the previous methods.

4 Conclusion

Thermo-stimulated luminescence studies of blue co loured, natural calcites collected from southern Tamilnadu (India) irradiated with gamma rays after a convenient thermal treatment show three peaks at 145, 255 and 345°C when recorded with linear heating rate of I 0 °C/s, from room temperature to 500 oc. The conclusive remarks are as follows: ( I) Annealing procedure does not affect the trapping center; (2) The TSL parameters of the glow peaks corresponds to mono-energetic traps and showed a second-order kinetics .

Acknowledgement

The authors are indebted to Dr A R Lakshmanan, Head, Radiation Dosimetry Section and Mr M T Jose, Health and Safety Di vision, IGCAR, Kalpakkam, for the ir generous help and cooperation in taking measurements and useful discussion during the progress of the work.

References

Urbach F, Wiener Berlla , 139 ( 1930) 363.

2 Nambi K VS, Th ennochemica Aela. 276 ( 197l)) 6 1.

3 Takatohi U E, Master thesis, lnsituto, de Fisica da Univeridade de Sao Paulo ( 1988).

4 Randall 1 1 & Wilkins M H, Proc Ror Soc A, 184 ( 1945) 366.

5 Chen R & Kirsh Y, Analvsis of thermallv stimulated process, (Pergamon Press, Oxford), 198 1.

6 Medlin W L, 1 Chem Phys, 30 ( 1959) 451.

7 Calderon T, Aguilar M, 1aque F & Coy-yll R, 1 Phys C, 17 ( 1984) 2027.

8 Down 1 S, Flower R, Strain J A & Townsend P D, Nucl Tracks & Radial Mats , I 0 ( 1985) 58 1.

9 Khanl ary M R & Townsend P D. Nuci Tracks & Radial Meas, 18( 199 1) 29.

I 0 Townsend P D, Luff 8 J & Wood R A, Radial Meas, 23 ( 1994) 433 .

II Gartia R K & Singh S D. Phys Status Solidi (a), 132 ( 1992) 197.

12 Gart ia R K, Singh S D & Mazumdar P S, Phys Status Solidi (a), 138 ( 1993) 3 19.

i3 Shalgaonkar C S & Narlikar A V. 1 .Mater Sci. 7(1972) 1465.

14 McKeever S W S, Thamo/uminescence o( solids, (Cambridge University Press). 1985 .

15 Engin Biro) & Guven Olgun , Radial Meas, 32 (2000) 253 .