The following is a list of some of our recently published papers in the fields of Thermoluminescence (TL) and Optically stimulated luminescence (OSL).
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Please send all Comments to vpagonis@mcdaniel.edu
A theoretical model for a newdating protocol for quartz based on thermally transferred OSL (TT-OSL) (Pagonis et al., 2008)
Computerized curve deconvolution analysis for LM-OSL (Kitis and Pagonis, 2008)
Duplicitous thermoluminescence peak associated with a thermal release of electrons and holes from trapping states (Chen et al., 2007)
Dependence of the anomalous fading of the TL and blue-OSL of 0uorapatite on the occupancy of the tunnelling recombination sites (Tsirliganis et al., 2007)
A quantitative kinetic model for Al2O3:C: TL response to UV-illumination (Pagonis et al., 2007)
Simulations of the effect of pulse annealing on optically-stimulated luminescence of quartz (Pagonis et al., 2008)
Thermoluminescence glow-peak shape methods based on mixed order kinetics (Kitis et al., 2008)
A unified presentation of thermoluminescence (TL), phosphorescence and linear-modulated optically stimulated luminescence (LM-OSL) (Chen and Pagonis, 2008)
Peak shape methods for general order TL glow-peaks: A reappraisal (Kitis et al., 2007)
Modelling thermal transfer in optically stimulated luminescence of quartz (Pagonis et al., 2007)
Thermoluminescence response and apparent anomalous fading factor of Durango fluorapatite as a function of the heating rate
The effects of annealing and irradiation on the sensitivity and superlinearity properties of the 110 0C thermoluminescence peak of quartz
Thermoluminescence under exponential heating function I: Theory
Thermoluminescence under exponential heating function II: Deconvolution of experimental glow-curves
A comprehensive study of the predose effect for three quartz crystals of different origin.
Theoretical modeling of experimental diagnostic procedures employed during predose dosimetry of quartz
Non-monotonic dose dependence of OSL intensity due to competition during irradiation and read-out
Comparison of experimentla and modelled quartz thermal-activation curves (Kitis et al, 2007)
A quantitative kinetic model for Al2O3:C: TL response to ionizing radiation (Pagonis et al, 2007)
Absorbed dose measurements of a handheld 50kVp X-ray source in water with TL dosemeters (Soares et al, 2006)
The nonmonotonic dose dependence of optically stimulated luminescence in Al2O3:C: Analytical and numerical simulation results (Chen and Pagonis, 2006)
Evaluation of activation energies in the semi-localized transition model of thermoluminescence (Pagonis, 2005)
A model for non-monotonic dose dependence of thermolluminescence (Lawless et al, 2005)
Modelling thermal activation characteristics of the sensitization of TL in quartz (Chen and Pagonis, 2004)
Simulation of the experimental predose technqiue for retrospective dosimetry in quartz (Pagonis and Carty, 2004)
Applicability of the Zimmerman predose model in the TL of predoses and annealed synsthetic quartz samples (Pagonis et al, 2003)
Cooling rate effects on the TL glow curves of Arkansas quartz (Kitis et al, 2003)
Search for common characteristics in the glow curves of quartz of various origins (Pagonis et al, 2002)
Detailed kinetic study of the TL glow curve of synthetic quartz (Pagonis et al, 2002)
An improved experimental procedure for separating a composite TL glow curve into its components (Pagonis and Shannon, 2000)
Fit of second order TL peaks using the Logistic ditribution function (Pagonis and Kitis, 2001)
Fit of first order TL glow peaks using the Weibull distribution function (Pagonis et al, 2001)
A theoretical model for a newdating protocol for quartz based on thermally transferred OSL (TT-OSL)
V. Pagonis, A.G.Wintle, R. Chen, X.L.Wang
Radiat. Meas. (2008), doi: 10.1016/j.radmeas.2008.01.025
Abstract
Recently a new dating procedure has been suggested, which is based on the thermally transferred optically stimulated luminescence (TT-OSL) signal that is measured after irradiated quartz is optically bleached and then preheated. Experimentally the TT-OSL signal was measured after a high temperature preheat (260 0C for 10 s) following an optical bleach at 125 0C for 270 s to deplete the fast and medium OSL components. The TT-OSL signal was measured for 90 s at 125 0C in order to avoid the effect of re-trapping of electrons in the 110 0C trap of quartz. The luminescence sensitivity changes were monitored by the OSL response to a test dose. In this paper, we use a modi0ed version of a comprehensive model to simulate the complete experimental sequence of the new protocol, and to 0t the experimental dose–response graphs of the OSL, TTOSL and basic-TT-OSL signals for doses up to 4000 Gy. Two possible mechanisms for the production of the TT-OSL signals in this quartz are discussed, namely the double transfer mechanism suggested for the recuperation effect, and a single transfer mechanism in which the TT-OSL signal is due to the thermal transfer of charge from a “source trap” into the fast OSL trap of quartz. The results of the simulation indicate that the latter mechanism is more likely to be responsible for the observed TT-OSL dose growth in 0ne-grained quartz extracted from Chinese loess.
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Computerized curve deconvolution analysis for LM-OSL
G. Kitis, V. Pagonis
Radiat. Meas. (2008), doi: 10.1016/j.radmeas.2007.12.055
The computerized curve deconvolution analysis (CCDA) technique is well known in the case of thermoluminescence (TL). In the present work we investigate the application of CCDA to the linear modulated optically stimulated luminescence curves (LM-OSL). We derive single LM-OSL peak equations which are based on variables which can be extracted directly from the experimental OSL curve, for both 0rst order and general order LM-OSL peaks. The similarities and differences between TL and OSL CCDA analysis are discussed. The resolution of the technique is also examined in the cases of synthetic curves consisting of two or four constituent components. Finally a new experimental procedure is suggested which can be used to separate composite LM-OSL curves into their constituent components.
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Duplicitous thermoluminescence peak associated with a thermal release of electrons and holes from trapping states
R. Chen, V. Pagonis, J.L. Lawless
Radiat. Meas. (2007), doi: 10.1016/j.radmeas.2007.09.021
A thermoluminescence (TL) glow peak may result from a transition of electrons from traps into the conduction band, followed by a recombination with holes trapped in a luminescence center. Another possibility is that holes trapped in a hole trap are thermally released into the valence band and recombine with electrons in an electron recombination center. A series of glow peaks emitted from a given sample may include peaks of both kinds. In some cases, peaks may be identi0ed as being of one kind or the other, say, by using thermally stimulated electron emission (TSEE), which can take place when the free carriers are electrons. In the present work, we demonstrate by the use of simulation that two peaks may result from one electron and one hole trapping states and a single hole recombination center. The 0rst TL peak
is observed when thermally stimulated electrons recombine with holes in the center. The TL peak is terminated when the holes in the center are exhausted. At higher temperatures, holes from a hole trap are released into the valence band and then captured by the hole center, thus this center is replenished. More electrons from the electron trap are thermally released now and recombine with the newly arrived holes in centers. A second TL peak may be observed which carries some information concerning the hole trap. It is thus demonstrated that some of the usual methods for distinguishing between electron and hole traps can lead to incorrect conclusions. It is possible for a hole trap, for example, to induce an increase in electron recombination in such a way that it produces a peak that looks nearly identical to TL from an electron trap. This simulation may bring about a new look at TL peaks occurring in materials used in TL dosimetry and dating. A new interpretation may also be given to “Auger” TSEE associated with the thermal release of electrons from the surface of a material, which indirectly results from the thermal release of holes from traps. The performance of some methods for evaluating the activation energies and the signi0cance of the results in the present situation are discussed.
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Dependence of the anomalous fading of the TL and blue-OSL of 0uorapatite on the occupancy of the tunnelling recombination sites
N.C. Tsirliganis, G.S. Polymeris, G. Kitis, V. Pagonis
Journal of Luminescence 126 (2007) 303–308
The anomalous fading (AF) of thermoluminescence (TL) and optically stimulated luminescence (OSL) signals in Durango apatite is attributed to tunnelling effects. Electrons from the TL and OSL traps in this material are transferred, via a tunnelling effect, to the recombination sites. The availability of recombination sites for tunnelled electrons is of major importance for the degree of AF rate observed in this material. It is expected that a variation of the number of the electron recombination sites will be re0ected in the experimentally measured AF rate. In the present work an investigation of the recombination sites for the tunnelled electrons is attempted by studying the AF effect using a special technique, in which the anomalously faded TL (OSL) is replaced by an equal amount of TL (OSL) induced by a beta dose.
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A quantitative kinetic model for Al2O3:C: TL response to UV-illumination
Pagonis, V., Chen, R. and Lawless, J.L.
Radiation Measurements (2007), doi:10.1016/j.radmeas.2007.10.046
This paper presents a quantitative kinetic model applicable to the important dosimetric material Al2O3:C. The model describes successfully the thermoluminescence (TL) response of the material to UV illumination (206 nm). The energy levels in this model consist of the main dosimetric trap, two competing deep hole and deep electron traps, and the luminescence center. The model also describes successfully the experimental variation of the optical absorption coefficient K with UV fluence. The values of the kinetic parameters are not arbitrary, but are obtained either from published experimental data, or by using reasonable physical assumptions. A correction factor is applied to the calculated UV-fluence to account for the fact that the samples used in the experiments were several optical lengths thick. By using this correction factor, the experimental data can be shown on the same graph as the calculated curves of TL vs. UV-fluence and K vs. UV-fluence, with the UV fluences given in photons/cm2 and not in some arbitrary units.
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Simulations of the effect of pulse annealing on optically-stimulated luminescence of quartz
V. Pagonis, A.G.Wintle, R. Chen
Radiation Measurements 42 (2007) 1587 – 1599
Pulse annealing techniques are commonly used in OSL studies of quartz to obtain information on the kinetic parameters of OSL traps and hole reservoirs. In this paper, simulations of pulse annealing experiments are carried out using the comprehensive model for quartz developed by Bailey [2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiat. Meas. 33, 17–45] for both natural and laboratory irradiated aliquots. The results of the simulations are in qualitative agreement with, and reproduce, several unusual features of the experimental data of Wintle and Murray [1998. Towards the development of a preheat procedure for OSL dating of quartz. Radiat. Meas. 29, 81–94]. The simulations are also carried out using different heating rates, and show that pulse annealing experiments can be used to recover appropriate kinetic parameters for both the OSL traps and the hole reservoirs known to exist in quartz. The results of the
simulations show the importance of these hole reservoirs in determining how the OSL signal depends upon the preheat temperature.
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Thermoluminescence glow-peak shape methods based on mixed order kinetics
George Kitis, Reuven Chen, and Vasilis Pagonis
phys. stat. sol. (a), 1– 9 (2008) / DOI 10.1002/pssa.200723470
The peak shape methods used in thermoluminescence (TL) theory to evaluate the activation energy E, are based on first and second order kinetics equations. For the intermediate kinetic orders the peak shape methods are based on the empirical theory of general order kinetics. In the present work we derive peak shape methods based on the physically meaningful mixed order kinetics model. The derived equations are tested for their accuracy and are compared with other peak shape methods existing in the TL literature.
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A unified presentation of thermoluminescence (TL), phosphorescence and linear-modulated optically stimulated luminescence (LM-OSL)
Reuven Chen and Vasilis Pagonis
J. Phys. D: Appl. Phys. 41 (2008) 035102
It has been shown in the past that a thermoluminescence (TL) peak has a relatively simple shape if the heating function is hyperbolic. Also, two different ways have been given for the transformation of the featureless decaying phosphorescence and optically stimulated luminescence (OSL) functions into peak-shaped curves. In the former case, one plots L · t versus ln(t), where L is the phosphorescence intensity and t the time. In the latter, linear-modulated OSL (LM-OSL) is reached by changing the stimulated light intensity linearly with time, and pseudo-LM-OSL by manipulating the regular CW-OSL curve. In this work, a modified presentation of LM-OSL is suggested and a unified presentation of these phenomena is offered, which may help in analysing the results and elucidating the details of the relevant kinetics process. The implications on the use of LM-OSL for dosimetry are also discussed.
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Peak shape methods for general order TL glow-peaks: A reappraisal
G. Kitis and V Pagonis
Nuclear Instruments and Methods in Physics Research B 262 (2007) 313–322s
This paper presents a reappraisal of the well known peak shape expressions for calculating the activation energy E in a thermoluminescence (TL) glow-peak. This study leads to new insights as to the meaning of the coefficients used in the original equations. The reappraisal leads to new equations for the coefficients of the peak shape expressions which contain the general order parameter b, instead of the experimentally determined geometrical shape factor which is used in the original equations. Previously only the coefficients for first and second order kinetics were determined on the basis of existing theory and the coefficients for intermediate kinetics order were determined empirically using a linear interpolation–extrapolation method. In the present work the improved peak shape coefficients are evaluated in analytical form as a function of the kinetic order b, by using the general order kinetics expression for the TL intensity. The intrinsic errors in the newly derived expressions for E are evaluated and their relevance to experimental work is discussed in detail. A method for a further improvement of the accuracy of the peak shape methods is suggested.
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Modelling thermal transfer in optically stimulated luminescence of quartz
V Pagonis, R Chen and AG Wintle
J. Phys. D: Appl. Phys. 40 (2007) 998–1006
A previously published kinetic model for the production of luminescence signals in quartz is used to investigate the production of thermally
transferred optically stimulated luminescence (TT-OSL) signals. This paper provides a mathematical description of the thermal transfer mechanism for two different phenomena that have been observed in previously published experiments (Aitken and Smith 1988 Quat. Sci. Rev. 7 387–93). The starting point is the model proposed by Bailey (2001 Radiat. Meas. 33,17–45). The numerical values of some of the parameters are varied so that they match the experimental data. The effect caused by varying these values is investigated.
The first of these phenomena takes place after storing optically bleached samples at room temperature; this involves the traps responsible for the
110 0C thermoluminescence (TL) peak of quartz acting as a refuge trap. The second phenomenon concerns OSL signals that are induced by heating the samples after the bleaching of the OSL signal and involves a putative TL peak at ~230 0C associated with the refuge trap; specifically, the paper presents a simulation of the temperature dependence of the OSL signal measured by successively heating the quartz samples to higher temperatures up to ~400 0C.
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Thermoluminescence response and apparent anomalous fading factor of Durango fluorapatite as a function of the heating rate
G. Kitis, G. Polymeris, V. Pagonis, and N. Tsirliganis
phys. stat. sol. (a) 203,No. 15, 3816-3823 (2006)
The anomalous fading (AF) of the Thermoluminescence (TL) signal has been observed in Durango apatite, and is believed to be caused by electrons transferred to the recombination sites via a tunnelling effect. The AF factor is a property unique for each glow-peak of a glow-curve and shows a remarkable stability, even when extreme experimental conditions are applied to this material. In the present work the AF is studied as a function of the rate at which the material is heated in order to obtain its TL. In this experimental study it is found that the TL response of Durango apatite is drastically increased as the rate of heating increases, contrary to what is expected from standard kinetic models of TL. An important consequence of this result is that, since the AF rate is monitored through the TL signal, the variations of TL as a function of the heating rate will be passed on to the evaluation of the AF rate. The result is an apparent decrease of the AF with an increasing heating rate, although the actual AF effect has no relation with the TL readout process. A possible explanation is proposed for the increase of the TL response and for the apparent decrease of the AF rate with the rate of heating.
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The effects of annealing and irradiation on the sensitivity and superlinearity
properties of the 110 0C thermoluminescence peak of quartz
G. Polymeris, G. Kitis, V. Pagonis
Radiation Measurements 41 (2006) 554 – 564
Quartz samples which undergo heating and irradiation treatments exhibit changes in their sensitivity to irradiation, as well as in their TL dose response. These changes of thermoluminescence (TL) sensitivity and superlinearity of the 110 0C TL peak of quartz have been the subject of several experimental and theoretical studies, because they form the basis of the predose technique for dating ceramics and for accident dosimetry. In an effort to separate experimentally the effects of irradiation and annealing on the predose effect, three quartz samples of different origin were prepared under three different conditions: unannealed samples, samples annealed at 500 0C, and samples annealed at 900 0C.
Complete TL versus dose and sensitivity S versus predose curves were obtained for the dose range of 0.1<D<400 Gy. Additional complete sets of data were obtained for samples that underwent a combined predose irradiation and a heat treatment to 500 0C. Although the TL versus dose curves and the sensitivity versus predose data showed very different behaviors, preannealing the samples at 900 0C removed the observed differences that are due to the thermal or irradiation history of the quartz samples. The experimental data is consistent with the assumption that high-temperature anneals and/or high dose irradiation of the samples reduces the concentration of available competitor sites. The concentration of these competitor sites, as described by the Zimmerman model of quartz, is identified as the most important factor in causing the observed differences in predose behavior between quartz samples of different origin. Strong evidence in support of this competitor theory is provided by the estimated equivalent doses (EDs) for the three quartz samples.
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Thermoluminescence under exponential heating function I: Theory
G. Kitis, R. Chen, V. Pagonis, E. Carinou and V. Kamenopoulou
J. Phys. D: Appl. Phys. 39, 1500-1507 (2006).
Constant temperature hot gas readers are widely employed in thermoluminescence dosimetry. In such readers the sample is heated according to an exponential heating function. The single glow-peak shape derived under this heating condition is not described by the TL kinetics equation corresponding to a linear heating rate. In the present work TL kinetics expressions, for first and general order kinetics, describing single glow-peak shapes under an exponential heating function are derived. All expressions were modified from their original form of I (n0,E, s, b, T ) into I (Im,E, Tm, b, T ) in order to become more efficient for glow-curve deconvolution analysis. The efficiency of all algorithms was extensively tested using synthetic glow-peaks.
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Thermoluminescence under exponential heating function II: Deconvolution of experimental glow-curves
G. Kitis, R. Chen, V. Pagonis, E. Carinou, P. Ascounis and V. Kamenopoulou
J. Phys. D: Appl. Phys. 39, 1508-1514 (2006).
Thermoluminescence (TL) glow-curves measured using an exponential heating function (EHF) in constant temperature hot gas readers, cannot be analysed using the existing single TL glow-peak equations derived assuming a linear heating rate. In the present work single TL glow-peak equations, which were recently derived assuming an EHF, are used to perform a computerized glow-curve deconvolution analysis of experimental glow-curves measured using a stable temperature hot gas reader. Glow-curves of the most commonly used dosimetric material LiF:Mg,Ti were analysed using the first order kinetics glow-peak equations. The glow-curves were analysed for samples that were pre-irradiation annealed at 400 0C for 1 h and 100 0C for 2 h, with and without a post-irradiation annealing at 80 0C for 1 h. TL glow-peak equations of the general order kinetics were used to analyse experimental glow-curves of the dosimetric material Li2B4O7 : Mn,Si. The results showed that the recently derived TL equations are very efficient for analysing glow-curves measured using stable temperature hot gas readers.
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A comprehensive study of the predose effect for three quartz crystals of different origin.
G. Kitis, V. Pagonis, R. Chen and G. Polymeris
Radiation Protection Dosimetry (2006), Vol. 119, No. 1–4, pp. 438–441
The study of the thermoluminiscence (TL) sensitivity of quartz due to heat and irradiation treatments is of importance in dating and retrospective dosimetry. A comprehensive comparative study of the predose effect was carried out for three types of quartz of different origin. Complete TL vs. dose and sensitivity S vs. predose curves were obtained for the dose range of 0.1<D<400 Gy. Additional complete sensitivity vs. predose curves were obtained for samples which underwent a combined predose irradiation and a subsequent heat treatment to 500C. Although the TL vs. dose curves showed very different behaviours, the sensitivity vs. predose curves showed several common characteristics. The sensitivity vs. predose curves showed abrupt changes at ~10 Gy. The sensitivity after a combined predose irradiation and heat treatment to 500C showed a very gradual change in the whole dose range studied. These results are explained qualitatively by using the modified Zimmerman model for quartz.
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Theoretical modeling of experimental diagnostic procedures employed during predose dosimetry of quartz
V. Pagonis, R. Chen and G. Kitis
Radiation Protection Dosimetry (2006), Vol. 119, No. 1–4, pp. 111–114
The pre-dose technique in thermoluminescence (TL) is used for dating archaeological ceramics and for accident dosimetry. During routine applications of this technique, the sensitisation of the quartz samples is measured as a function of the annealing temperature, yielding the so-called thermal activation characteristic (TAC). The measurement of multiple TACs and the study of the effect of UV-radiation on the TL sensitivity of quartz are important analytical and diagnostic tools. In this paper, it is shown that a modified Zimmerman model for quartz can successfully model the experimental steps undertaken during a measurement of multiple TACs.
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Non-monotonic dose dependence of OSL intensity due to competition during irradiation and read-out
V. Pagonis, R. Chen and J.L. Lawless
Radiation Measurements 41 (2006) 903– 909
The nonmonotonic dose dependence of thermoluminescence has been observed in several materials; a recent publication (Lawless, J.L., Chen, R., Lo, D., Pagonis, V., 2005. A model for non-monotonic dose dependence of thermoluminescence (TL). J. Phys. Condens. Matter 17,737–753.) gave a theoretical account based on competition between trapping states or recombination centers during the excitation and/or readout stages. A similar effect has been observed in the optically stimulated luminescence (OSL) of some materials such as quartz and Al2O3:C(e.g., Yukihara, E.G., Whitley, V.H., McKeever, S.W.S., Akselrod, A.E., Akselrod, M.S., 2004a. Effect of high-dose irradiation on the optically stimulated luminescence of Al2O3:C. Radiat. Meas. 38, 317–330; Yukihara, E.G., Gaza, R., McKeever, S.W.S., Soares, C.G., 2004b. Optically stimulated luminescence and thermoluminescence efficiencies for high-energy heavy charged particle irradiation in Al2O3:C. Radiat. Meas. 38, 59–70.). The model of competition has now been developed to explain the nonmonotonic dose dependence of OSL. A distinction is made between two cases. In one, the competition during excitation causes the filling of the relevant radiative center to be nonmonotonic with the dose, and as a result, the OSL intensity behaves in a similar way. This can take place with a “minimal” model including one trapping state and two kinds of recombination centers, one radiative and the other nonradiative. In the other case, all the relevant concentrations increase monotonically with the dose, and the nonmonotonic dose dependence of the OSL signal is mainly due to competition in the readout stage. In this case, it appears that the requirement for nonmonotonic dose dependence is a system with two trapping states and two kinds of recombination centers.
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Comparison of experimental and modelled quartz thrmal-activation curves obatined using multiple-and single-aliquot procedures
G. Kitis, V. Pagonis and R. Chen
Radiation Measurements 41 (2006) 910 – 916
This paper presents the results of a broad study of the thermal-activation characteristics of three quartz samples of different origin. The thermal-activation characteristics of synthetic, sedimentary and Arkansas quartz are measured as a function of several experimental parameters and over the temperature range 20.500 0C using both multiple-aliquot and single-aliquot techniques. The studies are carried out also as a function of the thermal preparation of the samples, for “as-is” samples, for samples fired at temperatures of 500 and 900 0C. Additional experimental parameters varied in this study include the test dose used in measuring the thermal activation curves and the temperature interval between successive heating of the samples. The results of this experimental study for all three quartz samples are interpreted by using a simple modified Zimmerman model for quartz consisting of 2 electron traps, a luminescence center and three hole reservoirs. The results are also discussed within the recently published complex theoretical quartz model by Adamiec (2005. Investigation of a numerical model of the pre-dose mechanism in quartz. Radiat. Meas. 39, 175–189), which is based on a luminescent center, three electron traps and three reservoir hole traps.
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A quantitative kinetic model for Al2O3:C: TL response to ionization radiation
V. Pagonis, R. Chen and J.L. Lawless
Radiation Measurements (In press)
This paper presents a quantitative kinetic model for the important dosimetric material Al2O3:C. The model consists of two traps and two centers, and reproduces the experimental thermoluminescence (TL) vs. dose behavior, as well as the experimental variation of the optical absorption coefficient K with beta dose. Initial estimates of the kinetic parameters in the model are obtained either from published experimental data, or by using reasonable physical assumptions. Good agreement between the experimental data and calculations from the model are obtained for three different types of samples of alumina. This is achieved by keeping the trapping and recombination probabilities constant for all three samples, while the concentrations of the carriers are varied. The kinetic model provides also a quantitative description of the experimentally observed nonmonotonic behavior of the TL dose–response curves for all three samples.
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ABSORBED DOSE MEASUREMENTS OF A HANDHELD 50 kVP X-RAY SOURCE IN WATER WITH THERMOLUMINESCENCE DOSEMETERS
Christopher Soares, Chris Drupieski, Brian Wingert, Garey Pritchett, Vasilis Pagonis, Michelle O’Brien, Alan Sliski, Pawel Bilski and Pawel Olko
RADIATION PROTECTION DOSIMETRY (2006), Vol. 120, No. 1–4, pp. 78–82
Absorbed dose rate measurements of a 50 kV p handheld X-ray probe source in a water phantom are described. The X-ray generator is capable of currents of up to 40 lA, and is designed for cranial brachytherapy and intraoperative applications with applicators. The measurements were performed in a computer-controlled water phantom in which both the source and the detectors are mounted. Two different LiF thermoluminescence dosemeter (TLD) phosphors were employed for the measurements, MTS-N LiF:Mg,Ti) and MCP-N (LiF:Mg,Cu,P). Two small ionisation chambers (0.02 and 0.0053 cm3) were also employed. The TLDs and chambers were positioned in watertight mounts made of water-equivalent plastic. The chambers were calibrated in terms of air-kerma rate, and conventional protocols were used to convert the measurements to absorbed dose rate. The TLDs were calibrated at National Institute of Standards and Technology (NIST) in terms of absorbed dose rate using a 60Co teletherapy beam and narrow-spectrum X-ray beams. For the latter, absorbed dose was inferred from air-kerma rate using calculated air-kerma-to-dose conversion factors. The reference points of the various detectors were taken as the center of the TLD volumes and the entrance windows of the ionisation chambers. Measurements were made at distances of 3–45 mm from the detector reference point to the source center. In addition, energy dependence of response measurements of the TLDs used was made using NIST reference narrow spectrum X-ray beams. Measurement results showed reasonable agreement in absorbed dose rate determined from the energy dependence corrected TLD readings and from the ionisation chambers. Volume averaging effects of the TLDs at very close distances to the source were also evident.
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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
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) 2179-2183
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
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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Modelling thermal activation characteristics of the sensitization of thermoluminescence in quartz
Reuven Chen and Vasilis Pagonis
J. Phys. D: Appl. Phys. 37 (2004) 159-164
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 500oC, followed by a rather sharp decline in some samples; in others a maximum was reached at 350oC 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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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
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.
The "110oC" 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)
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 900oC 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 oC" 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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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
The thermoluminescence glow curves of quartz of various origins were measurd under two different conditions, (1) unannealed samples and (2) samples annealed at 500oC and 900oC. 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
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 110oC 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
V. Pagonis and C. Shannon
Radiation Measurements, 32, 805-812 (2000).
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
V. Pagonis and G. Kitis
Radiation Protection Dosimetry, 93, No.3, 225-229 (2001).
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
V. Pagonis, S.M. Mian and G. Kitis
Radiation Protection Dosimetry, 93, 11-17 (2001).
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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