LET and dose dependence of TLD-100 glow curve after exposure to intermediate-energy ions

We present results from measurements performed with low fluences (10(5)-10(6) cm(-2)) of 15, 25 and 40 MeV u(-1) carbon, 25 MeV u(-1) oxygen and 40 MeV u(-1) neon ions incident on TLD-100 chips. Dosemeters were arranged individually or in stacks in front of the beam, allowing the study of various linear energy transfer (LET) values simultaneously. The thermoluminescence (TL) total signal is observed to be a linear function of deposited energy. To assess the contribution to the glow curve from the high-temperature peaks, two methods were studied: ratios of peak heights (peak 7 with respect to peak 5), and ratios of areas of the deconvoluted high-temperature peaks with respect to peak 5. The ratios were evaluated as a function of dose, showing in both methods a dependence on LET and ion identity. Some of the studied ions show these ratios to be independent of dose, up to 500 mGy, while for other ions, departures from linearity up to 4.5% +/- 2.5% per 100 mGy are observed at 500 mGy. These results show that, in general, the incident radiation LET is not a parameter that can be deduced from the glow curve.     

Investigation of the ionisation density dependence of the glow curve characteristics of LIF:MG,TI (TLD-100)

The dependence of the shape of the glow curve of LiF:Mg,Ti (TLD-100) on ionisation density was investigated using irradiation with (90)Sr/(90)Y beta rays, 60 and 250 kVp X rays, various heavy-charged particles and 0.2 and 14 MeV neutrons. Special attention is focused on the properties of high-temperature thermoluminescence; specifically, the behaviour of the high-temperature ratio (HTR) of Peaks 7 and 8 as a function of batch and annealing protocol. The correlation of Peaks 7 and 8 with average linear-energy-transfer (LET) is also investigated. The HTR of Peak 7 is found to be independent of LET for values of LET approximately >30 keV microm(-1). The behaviour of the HTR of Peak 8 with LET is observed to be erratic, which suggests that applications using the HTR should separate the contributions of Peaks 7 and 8 using computerised glow curve deconvolution. The behaviour of the HTR following neutron irradiation is complex and not fully understood. The shape of composite Peak 5 is observed to be broader following high ionisation alpha particle irradiation, suggesting that the combined use of the HTR and the shape of Peak 5 could lead to improved ionisation density discrimination for particles of high LET.     

Glow curve analysis of composite peak 5 in LiF:Mg,Ti (TLD-100) using optical bleaching,thermal annealing and computerised glow curve deconvolution

The relative intensity of glow peak 5a in the composite glow peak 5 of LiF:Mg,Ti (TLD-100) is very weak following gamma irradiation, and has been estimated at approximately 0.1 of the intensity of peak 5. Typical glow curve analysis using computerised glow curve deconvolution with unconstrained variation of the peak shape parameters, yields values of the relative intensity of glow peak 5a varying from 0 to 15%. Due to the potential of peak 5a to fulfil the criteria of a quasi-tissue-equivalent nanodosemeter which estimates quality factor, considerable efforts have been invested in ancilliary techniques to improve the reliability of the estimation of the intensity of peak 5a. Optical bleaching and thermal annealing techniques were used to obtain single-peak glow curves consisting of peak 4 only and peak 5 only. A multi-stage CGCD protocol was then constructed using these peak shape parameters for peaks 4 and 5, which allows more accurate estimation of the relative intensity of peak 5a. Following 60Co irradiation of ten chips to a dose level of 1 Gy, the technique yields a relative intensity of 0.08 +/- 0.008 (1 SD).     

A study on the behaviour of TLD-100 glow peaks at extreme ambient temperatures in Riyadh, Saudi Arabia

In this study, the temperature-induced variations in the TLD-100 response and the modifications in its glow peaks are investigated in real environmental exposure conditions in Riyadh, Saudi Arabia, where ambient temperatures during summer reach >45 degrees C and with relative humidity of <10%. Three groups of 12 TLD-100 cards in Harshaw type 8814 TLD cardholders were deployed as environmental dosemeters for a period of approximately 1 month for 12 consecutive months. One group was irradiated to 5 mGy 137Cs prior to deployment; another was irradiated to the same dose after deployment, while the last group was left unirradiated. Analysis of glow curves was done using commercially available glow curve deconvolution software (CGCD). Monthly variations in peak 3, 4 and 5 areas relative to the corresponding peak areas of a prompt glow curve are presented. Results of this study show good TL signal compensation between peaks 4 and 5 at all ambient temperatures encountered in this experiment, despite the observed individual variations experienced by each of these peaks. The sum of peak 4 and 5 areas is constant to within approximately 10%, for both pre- and post-irradiated dosemeters, during this 12-month cycle.     

The composite structure of peak 5 in the glow curve of LiF:Mg,Ti (TLD-100): confirmation of peak 5a arising from a locally trapped electron-hole configuration

The hypothesis that glow peak 5a arises from localised e-h capture is confirmed by the following experimental observations: (i) The high conversion efficiency (CE) (CE5a-->4 = 3 +/- 0.5) of peak 5a to peak 4 (a hole-only trap) deduced from detailed Im-Tstop optical bleaching studies at 310 nm compared to the much lower CE of peak 5 (an electron-only trap) (CE5-->4 = 0.0026+/-0.012). (ii) The lack of an increase in the sensitivity of glow peak 5a following 2.6 MeV and 6.8 MeV He ion irradiation in 'sensitised' material compared to the factor two increase in the sensitivity of peak 5; (S/S0)5a = 0.86+/-0.12, compared to (S/S0)5 = 2.0+/-0.2. (iii) The late entry into saturation of the 2.6 MeV and 6.8 MeV He ion TL-fluence response curves for peak 5a compared to peak 5 in sensitised and normal material resulting in the following values for the track radial saturation parameter: (r50)5a = 100+/-20) Angstroms compared to (r50)5 = 380+/-30 Angstroms. (iv) The low value of 0.1 for the 'track-escape' parameter of peak 5a deduced from the Extended Track Interaction Model analysis of He ion TL fluence response compared to order of magnitude greater values for peaks 5 and 5b.     

Thermoluminescent parameters from a Brazilian calcite glow curve

A thermoluminescent glow curve of gamma-irradiated Brazilian calcite consists of three peaks at 150, 250 and 350°C when recorded with a linear heating rate of 2.7°C/s. Symmetry factors, obtained using peak-shape analysis, agree with second-order kinetics for the three peaks. The partial-heating method shows that all peaks are related with monoenergetic traps. Activation energies determined for the quoted glow peaks are 1.3±0.2, 1.5±0.2 and 1.7±0.2 eV, respectively.     

Advanced multistage deconvolution applied to composite glow peak 5 in LiF:Mg,Ti (TLD-100)

The effects of 'slow-cooling' on the structure of composite peak 5 following low-ionisation density beta/gamma irradiation are described and analysed in both 'slow-cooled' and 'normally-cooled' samples. Computerised glow curve deconvolution is employed with constrained 'peak-shape' parameters deduced from anciliary studies using 4 eV and 5 eV optical excitation.     

Experimental determination of relative light conversion factors of TLD-100 for protons with energies from 2.0 to 9.0 MeV

The efficiency of thermoluminescent (TL) detectors to heavy charged particles is described by the so-called light conversion factor η. Relative light conversion factors for protons, alphas and heavier recoils are needed for the calculation of the neutron sensitivity of TL detectors. Such light conversion factors can be determined experimentally. In this paper a method is presented for the experimental determination of relative light conversion factors. Using the experimental arrangement described, relative light conversion factors for LiF material (TLD-100) for protons were determined. In LiF the relative main peak (peak V) efficiency is always lower than 1. It increases with increasing proton energy whereas the relative efficiency of the high temperature peak (peak VI) shows an opposite dependence on the proton energy. Relative light conversion factors for peak VI clearly exceed 1.     

On the reassessment of thermal neutron doses in TLD-100 by measuring the residual dose

By employing second readouts and the Phototransferred thermoluminescence (PTTL) method, high doses may be reassessed on the basis of residual dose information. It was shown in the past that for TLD-100, gamma doses can be reassessed by using a simple and efficient method, which consists of expanding the heating time to 30 s. In the present study, the 'extended time' method and the PTTL residual dose evaluations are used for reassessing thermal neutron doses when using TLD-100 crystals. Reassessment characteristics are presented for relatively low thermal neutron doses, in the range between approximately 1 and 18 mSv gamma dose equivalent.     

Performance of Harshaw TLD-100H two-element Dosemeter

One of the advantages of LiF based thermoluminescent (TL) materials is its tissue-equivalent property. The Harshaw TLD-100H (LiF:Mg,Cu,P) material has demonstrated that it has a near-flat photon energy response and high sensitivity. With the optimized dosemeter filters built into the holder, the Harshaw TLD-100H two-element dosemeter can be used as a whole body personnel dosemeter for gamma, X ray and beta monitoring without the use of an algorithm or correction factor. This paper presents the dose performance of the Harshaw TLD-100H two-element dosemeter against the ANSI N13.11-2001 standard and the results of tests that are required in IEC 1066 International Standard.     

On the possibility of using commercial software packages for thermoluminescence glow curve deconvolution analysis

This paper explores the possibility of using commercial software for thermoluminescence glow curve deconvolution (GCD) analysis. The program PEAKFIT has been used to perform GCD analysis of complex glow curves of quartz and dosimetric materials. First-order TL peaks were represented successfully using the Weibull distribution function. Second-order and general-order TL peaks were represented accurately by using the Logistic asymmetric functions with varying symmetry parameters. Analytical expressions were derived for determining the energy E from the parameters of the Logistic asymmetric functions. The accuracy of these analytical expressions for E was tested for a wide variety of kinetic parameters and was found to be comparable to the commonly used expressions in the TL literature. The effectiveness of fit of the analytical functions used here was tested using the figure of merit (FOM) and was found to be comparable to the accuracy of recently published GCD expressions for first- and general-order kinetics.     

Localised and delocalised optically induced conversion of composite glow peak 5 in LiF:Mg,Ti (TLD-100) to glow peak 4 as a function of postirradiation annealing temperature

The composite structure of glow peak 5 in LiF:Mg,Ti (TLD-100) has been investigated using optical bleaching by 310 nm (4 eV) light. The glow peak conversion efficiency of peak 5a (Tm = 187 degrees C) to peak 4 traps is very high at a value of 3+/-0.5 (1 SD) whereas the glow peak conversion efficiency of peak 5 (Tm = 205 degrees C) to peak 4 traps is 0.0026+/-0.0012 (1 SD). The high conversion efficiency of peak 5a to peak 4 arises from direct optical ionisation of the electron in the electron-hole pair. leaving behind a singly-trapped hole (peak 4), a direct mechanism, relatively free of competitive mechanisms. Optical ionisation of the 'singly-trapped' electron (peak 5), however, can lead to peak 4 only via multi-stage mechanisms involving charge carrier transport in the valence and conduction bands, a mechanism subject to competitive processes. The conduction/valence band competitive processes lead to the factor of one thousand decrease in the conversion efficiency of peak 5 compared to peak 5a.     

Phenomenological study of the ionisation density-dependence of TLD-100 peak 5a

Horowitz and collaborators have reported evidence on the structure of TLD-100 peak 5. A satellite peak, called 5a, has been singled out as arising from localised electron-hole recombination in a trap/luminescent centre, its emission mechanism would be geminate recombination and, therefore, its population would depend on incident radiation ionisation density. We report a phenomenological study of peak 4, 5a and 5 strengths for glow curves previously measured at UNAM for gammas, electrons and low-energy ions. The deconvolution procedure has followed strict rules to assure that the glow curve, where the presence of peak 5a is not visually noticeable, is decomposed in a consistent fashion, maintaining fixed widths and relative temperature difference between all the peaks. We find no improvement in the quality of the fit after inclusion of peak 5a. The relative contribution of peak 5a with respect to peak 5 does not seem to correlate with the radiation linear energy transfer.     

Discrimination of photon from proton irradiation using glow curve feature extraction and vector analysis

Two types of thermoluminescence dosemeters (TLDs), the Harshaw LiF:Mg,Ti (TLD-100) and CaF(2):Tm (TLD-300) were investigated for their glow curve response to separate photon and proton irradiations. The TLDs were exposed to gamma irradiation from a (137)Cs source and proton irradiation using a positive ion accelerator. The glow curve peak structure for each individual TLD exposure was deconvolved to obtain peak height, width, and position. Simulated mixed-field glow curves were obtained by superposition of the experimentally obtained single field exposures. Feature vectors were composed of two kinds of features: those from deconvolution and those taken in the neighbourhood of several glow curve peaks. The inner product of the feature vectors was used to discriminate among the pure photon, pure proton and simulated mixed-field irradiations. In the pure cases, identification of radiation types is both straightforward and effective. Mixed-field discrimination did not succeed using deconvolution features, but the peak-neighbourhood features proved to discriminate reliably.     

Pre- and post-irradiation fading of 6LiF:Mg,Ti (TLD-600) exposed to thermal neutrons

The results of (6)LiF:Mg,Ti (TLD-600) glow peaks fading after irradiation by thermal neutrons, as a function of pre- and post-irradiation times up to 3 months, are presented. The measured glow curves were analysed using the computerised glow curve deconvolution and region of integration methods, to study the decay characteristics of each individual peak and the sum of selected peak areas in the glow curve. The results confirm a high stability for the sum of peaks 4 + 5 and 6 + 7, whereas peaks 2 and 3 have a dominant contribution to the fading effect.     

Thermoluminescence and storage stability of TLD-100 dosimeter irradiated with 60Co-gamma rays

Thermoluminescence glow curves of TLD-100 revealed three peaks at 373, 460 and 518 K for all samples irradiated with gamma ray doses of 0.5 to 700 Gy. The total thermoluminescence response and the height of the main peak at 460 K showed similar characteristics to radiation dose. On the other hand, the total area under the glow curve increases continuously with radiation dose up to 1000 Gy. All irradiated samples investigated showed no significant fading over 28 d. Activation energy, E, and escape frequency factor, s, for the main glow peak were calculated by the modified empirical equation, as well as by methods depending on the shape of the glow peak. It was found that E has a value of 1.33 to 1.83 eV and s falls between 5.8×10¹³ and 3.06×10¹⁹ sec^–1, depending on the method used.