R. K. Gartia

Evidence of continuous trap distribution in the glow curve of a Brown microcline

The formidable task of determining a mathematical form of the trap distribution in a solid has been solved for the case of an unusually broad thermoluminescence (TL) glow peak of a Brown microcline (feldspar, triclinic form of KAlSi3O8). An exponential distribution of traps gives the best fit to the experimental glow peak. Five TL glow peaks of the Brown microcline which was previously irradiated with γ-rays and then subjected to fading at room temperature are numerically analyzed using the exponential distribution model of traps. The dependence of the best-fit trap parameters are found: (1) the width and the characteristic depth of the trap distribution; (2) the rates of recombination and retrapping of the released electrons from the traps; and (3) the concentration of thermally disconnected traps on the fading time. These trap parameters show a clear dependence on the time of fading. The procedure of the TL analysis using a continuous trap distribution is also reported in detail.

Method of computerized glow curve deconvolution for analysing thermoluminescence

The conventional worldwide accepted method of computerized glow curve deconvolution based on the general order kinetics formalism has two fatal defects in systems where the trapping levels (two or more) have non-zero retrapping probability. The first one is ignoring the thermal connectivity between thermoluminescence (TL) peaks. This arises from the fact that under such a situation electrons trapped at one trapping level, once activated, can be retrapped in another thermally connected level via the conduction band during the recording of the glow curve. The other is the impossibility of obtaining a global minimum, in fitting the experimental TL with the theoretical one with existing techniques.This paper aims to provide answers to these defects. The first one can be overcome by resorting to rigorous analysis using appropriate mathematical rate equations describing the flow of charge carriers. Though the second defect cannot be overcome completely, one can obtain a reasonable fit, which may not be unique. The algorithm is tested for synthetic as well as experimental glow curves.

On the determination of activation energy in thermoluminescence by the initial rise method

Expressions are derived which estimate the systematic error involved in the determination of the activation energy by the initial rise method. It has been shown that the general belief that the initial rise method does not depend on the order of kinetics is rather a misconception if one takes the value of E thus determined in a quantitative sense. The applicability of these expressions has not only been discussed but also tested for some experimental peaks.

The determination of activation energy from the shape of a thermoluminescence peak

A new set of expressions have been derived to evaluate the thermal activation energy, E, of a thermoluminescence (TL) peak. The order of kinetics, b, of the peak may be first order (b=1), second order (b=2) or general order for which the authors have taken b=1.5 as an example. The derived expressions involve temperatures at which the intensity of the peak is 1/2, 2/3, 4/5 of the maximum and/or the peak temperature (Tm). The selection of these points is based on the fact that the upper half of the peak is free from interference of the satellite peaks. The applicability of these expressions has been checked by applying them to some typical experimental TL peaks. The value of E determined by using these new sets of expressions is in good agreement with that found by curve fitting.

A critical appraisal of methods of various heating rates for the determination of the activation energy of a thermoluminescence peak

An attempt has been made to find out precisely the errors involved in the determination of the activation energy (E) of a thermoluminescence (TL) peak by using the different variants of method of various heating rates, namely, those due to Hoogenstraaten (1958), Chen and Winer (1970), and Dussel and Bube (1967). It has been found that for all practical purposes the Chen-Winer and Hoogenstraaten methods can be considered to be independent of the order of kinetics of the TL process. Finally, the applicability of the findings has been tested experimentally by considering a non-first-order TL peak. The results suggest that the theoretical errors in all the cases are less than the experimental ones and hence these methods can be safely used for all types of TL peaks irrespective of their order of kinetics.

Origin of the exponential distribution of traps in glass

A recent rigorous analysis of the broad thermoluminescence (TL) peak of a Brown microcline has shown it to be due to an exponential distribution of traps. Incidentally, from the statistical point of view one expects the traps to follow a Gaussian distribution. In order to elucidate the origin of the exponential trap distribution we have analyzed a set of glow peaks of a light green glass recorded under varying conditions of trap filling. The sample upon γ-rays irradiation (dose=7×103 Gy) exhibits a broad TL peak around 191 °C. The broad peak becomes narrower once a thermal cleaning technique is applied to the irradiated specimen. Application of computerized glow curve deconvolution reveals that the broad glow curve is due to the presence of a Gaussian distribution of traps. However, in thermally cleaned glow curves the distribution of traps is found to be exponential. This shows that the exponential distribution of traps results from the deeper tail portion of the Gaussian distribution.

The determination of intrinsic trapping parameters of a thermoluminescence peak of BeO

It has been shown that a thermoluminescence (TL) peak of BeO cannot be fitted with a numerically generated glow peak involving only three trapping parameters, namely trap depth, frequency factor and order of kinetics. The peak can be fitted with a TL peak numerically generated by the exact solutions of the basic differential equations. This enables one to determine the five important intrinsic trapping parameters: trap depth, frequency factor, retrapping probability, recombination probability and the concentration of a disconnected trap, an impossible feat under the kinetics formalism.

Computerized glow curve deconvolution: the case of LiF TLD-100

It has been accepted by a large number of workers that the glow curve of LiF TLD-100 can be described by thermoluminescence (TL) peaks following the Randall-Wilkins (RW) equation, even though the model fails to explain a number of experimental facts. A further simplification of the model is the Podgorsak-Moran-Cameron (PMC) approximation which is also in use. This paper points out the limitation of the PMC approximation in deconvoluting glow curves of LiF TLD-100.

The determination of the trapping parameters of a thermoluminescence peak by using the Kirsh method

In this paper we analyse the suitability of a method proposed by Kirsh for the determination of the trapping parameters, namely the order of kinetics, activation energy and frequency factor, of a thermoluminescence (TL) peak by applying it both to numerically generated TL peaks and to an experimental TL peak of NaCl:I irradiated with 2.04 kGy of -rays. It is found that the method can be used irrespective of the order of kinetics. It is also shown that the method proposed by Rasheedy ( 8 (1996) 1291 and 29 (1996) 1340) is a special case of the Kirsh method.

On the determination of the activation energy of a thermoluminescence peak by the two-heating-rates method

Precise estimation of the systematic error involved in the determination of the activation energy of a non-first-order thermoluminescence (TL) peak by using the two-heating-rates method of Booth (which is strictly valid for a first-order peak) has been made. A new method analogous to the Booth (1954) method is proposed, which involves both the peak temperature and peak intensity. The systematic errors involved in both these methods are found to be within the experimental error which one generally encounters in the analysis of TL. The applicability of these findings has been tested by considering a second-order TL peak of limestone.