PUBLICATIONS  

The following is a list of some of our recently published papers in the field of thermoluminescence.

You can click on the links to view and DOWNLOAD the PDF files.

Please send all Comments, Suggestions and Corrections to vpagonis@mcdaniel.edu

The nonmonotonic dose dependence of optically stimulated luminescence in Al2O3 :C: Analytical and numerical simulation results

R. Chen, V. Pagonis and J. L. Lawless

JOURNAL OF APPLIED PHYSICS 99, 033511,2006

Abstract  

Nonmonotonic dose dependence of optically stimulated luminescence _OSL_ has been reported in a number of materials including Al2O3 :C which is one of the main dosimetric materials. In a recent work, the nonmonotonic effect has been shown to result, under certain circumstances, from the competition either during excitation or during readout between trapping states or recombination centers. In the present work, we report on a study of the effect in a more concrete framework of two trapping states and two kinds of recombination centers involved in the luminescence processes in Al2O3 :C. Using sets of trapping parameters, based on available experimental data, previously utilized to explain the nonmonotonic dose dependence of thermoluminescence including nonzero initial occupancies of recombination centers _F+ centers_, the OSL along with the occupancies of the relevant traps and centers are simulated numerically. The connection between these different resulting quantities is discussed, giving a better insight as to the ranges of the increase and decrease of the integral OSL as a function of dose, as well as the constant equilibrium value occurring at high doses.

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Evaluation of activation energies in the semi-localized transition model of thermoluminescence

V Pagonis

J. Phys. D: Appl. Phys. 38 (2005) 2179–2186

Abstract  

Recently a semi-localized transition (SLT) kinetic model was developed for thermoluminescence (TL), which is believed to be applicable to important dosimetric materials like LiF : Mg,Ti. This model contains characteristics of both a localized transition model and a single trap model and is characterized by two distinct activation energy levels. This paper describes the simulation of several standard methods of analysis for the TL peaks calculated using the SLT model in an effort to extract the two activation energy parameters of the model. The methods of analysis are applied to both possible types of transitions within the model, namely the direct recombination of the hole–electron pairs as well as the delocalized transitions involving the conduction band. In the former case of direct recombination, the methods of  analysis give consistent results for the activation energy E. In the latter case of transitions involving the conduction band, it was found that extra caution must be exercised when applying standard methods of analysis to the SLT model because of the possibility of strongly overlapping TL peaks. Specifically the peak shape methods consistently fail to yield the correct value of E, while careful application of the fractional glow, thermal cleaning and variable heating rate methods can yield the correct energy values when no retrapping is present within the localized energy levels. A possible explanation is given for the previously reported failure of the peak shape methods to yield the correct activation energies within the SLT model. The heating rate methods of analysis consistently yield the correct activation energies E with an accuracy of a few per cent.

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A model for non-monotonic dose dependence of thermoluminescence

J L Lawless, R Chen, D Lo and V Pagonis

J. Phys.: Condens. Matter 17 (2005) 737–753

Abstract  

In the applications of thermoluminescence (TL) in dosimetry and archaeological and geological dating, a desirable dose dependence of TL intensity is a monotonically increasing function, preferably linear. It is well known that in many dosimetric materials, nonlinear dependence is observed. This may include a superlinear dependence at low doses and/or sublinear dose dependence at higher doses, where the TL intensity approaches saturation. In quite a number of materials, non-monotonic dose dependence has been observed, namely, the TL intensity reached a maximum value at a certain dose and decreased at higher doses. This effect is sometimes ascribed to ‘radiation damage’ in the literature. In the present work we show, both quasi-analytically and by using numerical simulation, that such dose dependence may result from a simple energy level scheme of at least one kind of trapping state and two kinds of recombination centers. One does not necessarily have to assume a destruction of trapping states or recombination centers at high doses. Instead, the main concept involved is that of competition which takes place both at the excitation stage and the readout stage during the heating of the sample. This may explain the fact that the phenomenon in question, although very often ignored, is rather common. Cases are identified in which competition during excitation dominates, and others in which competition during read-out dominate

 

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SIMULATION OF THE EXPERIMENTAL PRE-DOSE TECHNIQUE FOR RETROSPECTIVE DOSIMETRY IN QUARTZ

Vasilis Pagonis and Hezekiah Carty

Radiation Protection Dosimetry (2004), Vol. 109, No. 3, pp. 225±234

Abstract  

The pre-dose technique of thermoluminescence for quartz has been used extensively for retrospective dosimetry of quartz and other natural materials. A recently published model that is a modification of the well-known Zimmerman theory is used here to simulate the complete sequence of experimental steps taken during the additive dose version of the pre-dose technique. The results of simulation show how the method can reproduce accurately the accumulated dose or paleodose received by the sample. The solution of the kinetic differential equations elucidates the various electron and hole processes taking place during the experimental pre-dose procedure and shows clearly the mechanism of hole transfer from the reservoir to the luminescence centre caused by heating to the activation temperature. The numerical results show that the pre-dose technique can reproduce the paleodose with an accuracy of _1±5%, even when the paleodose is varied over more than an order of magnitude. New quantitative results are presented for the effect of the test dose and of the calibration beta dose, b, on the accuracy of the pre-dose technique. The conclusions drawn from the simple model for quartz can be used to make improvements to more general quartz models.

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Applicability of the Zimmerman predose model in the thermoluminescence of predosed and annealed synthetic quartz samples

Vasilis Pagonis, George Kitis, Reuven Chen

Radiation Measurements, 37, (2003), 267-274.

Abstract  

The "110 oC" thermoluminescence (TL) peak of unfired synthetic quartz is known to exhibit a highly superlinear growth with absorbed dose. In this paper, it is shown that the well-known Zimmerman predose model can explain recent experimental results on the superlinearity of annealed synthetic quartz, as well as experimental results on the superlinearity of heavily predosed samples at room temperature. In the case of the predosed samples, the simulation solves the kinetic rate equations for the various stages in the experimental TL predose process. The results of the simulation explain the behavior of the TL versus dose curves at different predoses, as well as the detailed behavior of the superlinearity coefficient k as a function of the predose amount. In the case of the annealed samples, the simulation solves the kinetic equations for different values of the initial concentration of holes in the recombination center. The results of the simulation explain the behavior of the TL versus dose curves at different annealing temperatures, as well as the detailed behavior of the superlinearity coefficient k in each of the two distinct superlinearity regions. The simulation also produces the correct order of magnitude for the large sensitivity changes of the TL intensity observed in both sets of experiments.

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Cooling rate effects on the thermoluminescence glow curves of Arkansas quartz

G. Kitis, V. Pagonis, and C. Drupieski

Phys. stat. sol. (a) 198, No. 2, 312– 321 (2003)

Abstract

Samples of quartz annealed at high temperatures are known to exhibit thermoluminescence (TL) properties which depend on the rate of cooling of the samples to room temperature. Powder samples of Arkansas quartz were annealed in air at temperatures between 500 and 900 °C and were cooled to room temperature at different cooling rates. The TL of both slowly and fast cooled samples was measured at various doses of beta radiation; a fast cooling rate leads to significant enhancements of the TL intensity for the "110 °C" TL peak, as well as a change in the ratio of the relative intensities of the main TL peaks. The well-known Tm – Tstop method of analysis resulted in several well-defined plateaus at different temperatures for the fast cool and slow cool samples, while the thermal quenching parameters C and W and kinetic parameters of the TL peaks were found to be independent of the cooling rate. This new result provides further evidence for the cooling rate effects being due to slow ionic processes, rather than the much faster electronic processes involved in thermal quenching.

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Modelling thermal activation characteristics of the sensitization of thermoluminescence in quartz

Reuven Chen and Vasilis Pagonis

J. Phys. D: Appl. Phys. 36 (2003) 1–6 

Abstract

The sensitization of the «110°C thermoluminescence peak in quartz, also termed the ‘pre-dose’ effect, was previously explained using an energy level model including two electron trapping states and two hole centers. The experimental procedure includes a stage of high temperature activation following a relatively large irradiation of the sample. The response to a small test-dose was found to depend on this activation temperature. With different quartz samples, different behaviors of the thermal activation characteristics (TACs) were found. In typical TACs, the sensitivity reached a maximum at 500°C, followed by a rather sharp decline in some samples; in others a maximum was reached at 350°C followed by a slight decline towards a plateau level. In this work, we show that these behaviors can rather easily be explained within the framework of the two traps–two centers model. This is done by numerical solution of the relevant sets of differential equations governing the different stages of the experimental procedure. The different kinds of dependence were simulated with different sets of trapping parameters. A better insight into the processes taking place is reached, which may have implications in the application of pre-dose dating of archaeological quartz samples and in retrospective dosimetry.

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SEARCH FOR COMMON CHARACTERISTICS IN THE GLOW CURVES OF QUARTZ OF VARIOUS ORIGINS

 V. Pagonis, E. Tatsis, G. Kitis and C. Drupieski

Radiation Protection Dosimetry (2002), Vol. 100, Nos. 1-4, pp. 373-376

Abstract  

The thermoluminescence glow curves of quartz of various origins were measurd under two different conditions, (1) unannealed samples and (2) samples annealed at 500 oC and 900 oC. The different glow curves were analyzed using first order kinetics and glow curve deconvolution analysis (GCD). The comparison of the glow curves obtained was mainly concentrated in studying the sensitivities of the glow peaks as a function of the annealing temperature, and in obtaining the kinetic parameters of the glow peak at 110 oC. Furthermore, in four samples the detailed comparison was extended to the trapping parameters of all existing glow peaks. It was found that despite their different origin and the different shapes of the glow curves, there are several basic characteristics which are common to all samples studied.

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DETAILED KINETIC STUDY OF THE THERMOLUMINESCENCE GLOW CURVE OF SYNTHETIC QUARTZ

G. Kitis, V. Pagonis, H. Carty and E. Tatsis

Radiation Protection Dosimetry (2002), Vol. 100, Nos. 1-4, pp. 225-228

Abstract  

A detailed kinetic analysis has been performed of the thermoluminescence glow curve of high purity synthetic quartz. The kinetic parameters of the glow peak at 110 oC were evaluated for doses ranging from 0.1 Gy to 100 Gy using glow curve deconvolution (GCD) methods, initial rise, variable heating rate and phosphorescence decay methods. All the methods gave results that agree within the experimental errors.

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An improved experimental procedure of separating a composite thermoluminescence glow curve into its components

by V. Pagonis and C. Shannon

Radiation Measurements, 32, 805-812 (2000).

Abstract

We present an improved experimental procedure of separating a composite thermoluminescence glow curve into its components. Careful monitoring of the isothermal cleaning process using the initial rise method ensures the complete thermal removal of TL peaks. Digital subtraction of two experimental TL glow curves yields individual experimental TL glow peaks. Several standard methods (initial rise and whole glow curve) are used to obtain the energy values and frequency factors of the traps. The method has been used successfully to analyze the well-known composite TL glow curve of the dosimetric material LiF (TLD-100). The limitations of the method are illustrated by analyzing the highly complex TL glow curve of a UV irradiated synthetic calcite consisting of at least 6 TL peaks. Although the method works best for TL glow curves described by first order kinetics, it should also be applicable to more general kinetics.

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FIT OF SECOND ORDER THERMOLUMINESCENCE GLOW PEAKS USING THE LOGISTIC DISTRIBUTION FUNCTIOn

by V. Pagonis and G. Kitis

Radiation Protection Dosimetry, 93, No.3, 225-229 (2001).

Abstract

A new Thermoluminescence glow-curve deconvolution (GCD) function is introduced that accurately describes second order thermoluminescence (TL) curves. The logistic asymmetric (LA) statistical probability function is used with the function variables being the maximum peak intensity, the temperature at the maximum peak intensity and the LA width parameter a2. An analytical expression is derived from which the activation energy E can be calculated a sa function of Tm and the LA width parameter a2 with an accuracy of 2% or better. The accuracy of the fit was tested for values of E between 0.7 and 2.5 eV, for s values between 105 and 1025 s-1, and for trap occupation numbers no/N between 1 and 106. The goodness of fit of the LA function is described by the Figure of Merit (FOM) which is found to be of the order of 10-2.

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FIT OF FIRST ORDER THERMOLUMINESCENCE GLOW PEAKS USING THE WEIBULL DISTRIBUTION FUNCTIOn

by V. Pagonis, S.M. Mian and G. Kitis

Radiation Protection Dosimetry, 93, 11-17 (2001).

Abstract

A new Thermoluminescence glow-curve deconvolution (GCD) function is introduced that accurately describes first order thermoluminescence (TL) curves. The GCD functions are found to be accurate for first order TL peaks with a wide variety of the values of the TL kinetic parameters E and s. The 3-parameter Weibull probability function is used with the function variables being the maximum peak intensity (Im), the temperature of the maximum peak intensity (Tm) and the Weibull width parameter b. An analytical expression is derived from which the activation energy E can be calculated as a function of Tm and the Weibull width parameter b. The accuracy of the Weibull fit was tested using the ten reference glow-curves of the GLOCANIN intercomparison program and the Weibull distribution was found to be highly effective in describing both single and complex TL glow curves. The goodness of fit of the Weibull function is described by the Figure of Merit (FOM) which is found to be of comparable accuracy to the best FOM values of the GLOCANIN program. The FOM values are also comparable to the FOM values obtained using the recently published GCD functions of Kitis et al [3]. It is found that the TL kinetic analysis of complex first-order TL glow curves can be performed with high accuracy and speed by using commercially available statistical packages.

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