TL Response Research Articles

Study of the TL response of LiF:Mg,Ti to 3 and 7.5 MeV helium ions: Measurements and interpretation in terms of the track interaction model

We have measured the LiF:Mg,Ti (TLD-100) thermoluminescent response, efficiency and supralinearity, to 3 and 7.5 MeV helium ions in the fluence interval between 6×107 to 1.5×1011 cm−2. Ion irradiations were performed using a flux of elastically scattered ions from a thin gold target. The measured efficiencies with respect to 60Co γ-rays are equal to 0.074 and 0.174 at 3 and 7.5 MeV, respectively, for the response of peak 5. The supralinearity functions for peaks 4, 5, 6a, 6b, 7, 8, 9 and the total TL signal indicate that supralinearity is an increasing function of energy and that it increases with peak temperature. The data for peaks 5, 8 and 9 have been successfully described by Monte Carlo simulations of the track interaction model. The fitted parameters show that peak 5 displays a special behaviour which requires the use of a much larger effective track radius than the higher temperature peaks.

A Comparative Study of the Thermal Stability of LiF:Mg.Ti and LiF:Mg,Cu,P Detectors for Environmental Monitoring

The results obtained in an investigation on the thermal stability of LiF:Mg,Cu,P (GR-200) compared to the more traditional LiF:Mg,Ti (TLD-100) at 40(C and 70(C are presented. Samples of both varieties of dosemeters were stored irradiated or unirradiated in order to evaluate the relative importance of the temperature/storage induced effects on either traps or trapped charges. The measured glow curves were analysed using the deconvolution programs developed at CIEMAT. These techniques allow the detailed characterisation of the evolution pattern followed by each individual peak during the experiment. The results obtained confirm that, for both varieties of LiF phosphor, the process affecting their respective main dosimetric peaks is not fading, understood as the spontaneous release of trapped charges. On the contrary, the observed variations in the TL response should be addressed to the modifications experienced by the trap system during storage. However, some slight differences between the evolution of TLD-100 and GR-200 have been found in this comparison and are analysed.

Thermoluminescence Characteristics of a New Production of Chemical Vapour Deposition Diamond

The dosimetric properties are presented of a recent production of chemical vapour deposition diamond growth. Experimental data concerning the TL response as a function of dose, the energy response and fading behaviour are reported. Very preliminary results suggest that diamond can be used in TL mode as well as an activation detector.

Proton Irradiation of 7LiF:Mg,Ti Thermoluminescent Detectors: Influence of Dopant Concentration on Dose Response and LET Dependence of TL Efficiency

Variation of Ti and Mg concentration is known to modify substantially the dosimetric characteristics of the standard LiF:Mg,Ti detector commonly used in radiotherapy. Preliminary studies conducted by the Institute of Nuclear Physics at Krakow suggested that the Ti concentration could be optimised with respect to low supralinearity and high TL efficiency in heavy particle beams. These results led to further investigation of new TL detectors appropriate to proton therapy. In particular, the influence of both Mg and Ti dopant concentrations on TL response in proton beams was studied.(LiF)-Li-7:Mg,Ti pellets (diameter 4.5 mm, thickness 0.5 mm) doped with 30 different combinations of Ti and Mg (five Ti concentrations: 4, 13, 39, 130, 260 ppm; six Mg concentrations: 30, 60, 120, 240, 480, 960 ppm) were irradiated in a 65 MeV proton beam at two different positions in a Perspex phantom corresponding to different depths in the Bragg curve. From the analysis of dose response and TL efficiency and its dependence on LET, recommendations on optimal dopant concentration are drawn for the clinical use of the dosemeter in proton therapy.

Thermoluminescence Properties of Novel Thin Clear Fused Quartz Dosemeters

The thermoluminescence properties of novel thin (100 μm) dosemeters of clear fused quartz (CFQ) have been investigated as part of a progranime to optimise the measurement of the spatial beta dose distribution around hot particles and to provide benchmark measurements for the parallel validation of Monte Carlo code calculations. Using the isothermal annealing method seven TL peaks (P1-P7) have been identified for CFQ in the temperature range 122-310°C. The isothermal method has been used to determine the energy depth E and frequency factor s for peaks P2, P3 and P4. The heating rates method has been used to determine the values of E and s for peaks P1 and P6. The results have been compared with'those of computerised glow curve deconvolution. There were large disparities between the results of different methods. These are probably due to artefacts in the methods related to the large number of overlapping peaks. The reproducibility of individual CFQ on the basis of TL integration up to 400°C is good (±5%). On the other hand, the individual sensitivity is different among all the dosemeters (40) from the same batch and the difference in the TL signals sometimes reaches a factor of 5. An analysis of the TL dose response in terms of low and high temperature regions (PL, 50-150°C; P H , 250-350°C) showed a linear response up to ∼60 Gy for PL and a slightly sub-linear response up to only -2 Gy for PH. The sensitivity of CFQ has been found to be -15% of that of LiF TLD 100 and the minimum detectable dose (2σ above background) is ∼580 μGy. The TL response of CFQ was found to be sensitive to natural light exposure. The TL yield was reduced to typically 20% of is original value after only 10 h exposure to natural light at room temperature. After this period of time both peak P1 and peak P2 have been completely removed. Samples kept shielded from light suffered only a 12% decrease of total TL intensity after 48 h at room temperature, A feasibility study has been carried out successfully using CFQ to measure directly the dose distribution around a test 60 Co hot particle source using an imaging photon detector (IPD) technique. CFQ/IPD measurements have a much greater sensitivity than extrapolation chamber and radiochromic dye film techniques, which have previously been used for this application.

Thermoluminescence of LiF Thin Films

The TL dosimetric properties of the LiF thin films produced on aluminium and stainless steel substrates has been investigated. The influence of some deposition parameters and thermal annealing on the film TL response was studied. The films were irradiated with a 60 Co source with doses from 0,2 to 100 Gy. The films' TL glow curves grown on both substrates are similar. The maximum TL intensity is obtained for 300°C deposition on aluminium and for 150°C on stainless steel substrates. Thin NaF layers were added to distribute the stress inside the LiF films. The multilayer films present the same glow peak structure as the LiF film, but with lower intensity. The stress field seems to change the TL film response.

Response of TLD-300 to Heavy Charged Particles

The TL response of CaF 2 :Tm to 0.7 MeV protons, 5.3 MeV α particles and 120 MeV 12 C ions has been investigated. Maximum fluences were typically 10 10 - 10 11 cm -2 . Deconvolution techniques were applied to separate the glow curve into its component peaks. In all cases, the area of peak 3 showed a linear response as a function of fluence, for all the fluences studied, while peaks 5 and 6 followed a linear-supralinear behaviour. The departure from linearity occurs at high doses, above 10 2 Gy. Concerning the dependence of the glow curve on type and energy of the incident radiation, the sensitivities of peaks 3 and 5 were studied as compared to the response to equal doses of gamma rays. The results, as a function of LET, follow the same general systematics observed previously by other authprs. However, the quantitative comparison suggests that the linear energy transfer may not be the parameter which unifies all the observations.

Superlinearity of Synthetic Quartz: Dependence on Pre-Dose

The highly superlinear 110°C glow peak of synthetic quartz is used in studying the recombination stage model of superlinearity. In the present study the concentration of the competing traps is controlled, so that the dependence of the degree of superlinearity on the number of competitqrs is established. The resulting thermoluminescence dose-response curves are fitted by analytical functions and the behaviour of the obtained analytical TL responses have been analysed through the superlinearity index g(D). The results show the experimental dependence of the superlinearity index on the concentration of competitors.

Presence of Effector CD8+ T Cells in Hepatitis C Virus-Exposed Healthy Seronegative Donors

CTL responses against multiple hepatitis C virus (HCV) epitopes were detected in 7 of 29 (24.1%) healthy family members (HFM) persistently exposed to chronically HCV-infected patients (HCV-HFM). These precursor CTL were at very low or undetectable frequencies, as determined by limiting dilution analysis. However, when HCV-specific effector CD8+ T cells, freshly isolated from PBMC of HCV-HFM, were assessed by a sensitive enzyme-linked immunospot assay, their frequencies were severalfold higher than those of precursor CTL. These results indicate that the two assays detect two functionally distinct T cell populations and that the effector cells are not assayed by the 51Cr-release assay. Furthermore, the combination of cell depletion and enzyme-linked immunospot analyses showed that the effector cells were confined into a CD8+ CD45RO+ CD28- population. The persistence of effector CD8+ T cells specific for both the structural and nonstructural viral proteins in uninfected HCV-HFM, suggest that: 1) an immunological memory is established upon a subclinical infection without any evidence of hepatitis, in a large cohort of HCV-exposed individuals; 2) because these cells required neither restimulation nor the addition of particular cytokines in vitro for differentiating in effectors, they should be capable of prompt HCV-specific effector function in vivo, possibly providing antiviral protection; and 3) the maintenance of effector T cell responses may be sustained by persisting low-level stimulation induced by inapparent infections.

TL and ESR signals of mollusc shells—correlations and suitability for the detection of irradiated foods

Measurements are described which show the suitability of TL and ESR measurements of mollusc shells for the detection of irradiation treatment. Experiments on 18 samples of molluscs belonging to 10 different species were carried out. Significant TL signals of the irradiated samples were obtained from 3 of the 10 species investigated, while the others did not show a higher signal than the unirradiated ones. When looking at the X-ray diffraction spectra, it became obvious that all the radiation-sensitive samples contained calcite and in most cases, also aragonite, while the unsensitive samples contained only aragonite. The TL intensities of the sensitive samples did not correlate to the contents of calcite estimated by X-ray diffraction. Analyses of ESR spectra showed that there is a correlation between relative content of Mn 2+ ions and TL intensities of these samples. In contrast to TL, the ESR spectra of all samples revealed high radiation-specific signals being very stable and dose-dependent. Experiments were carried out to characterize signal stability under various commercially used storage conditions and dose response of both TL and ESR and to examine the emission bands of TL. In the case of mollusc shells, ESR seems to be the more sensitive and widely applicable method than TL for detection of irradiation.

Isothermal decay of optically stimulated luminescence in quartz

Isothermal decay curves are presented for the optically stimulated luminescence from an Australian sedimentary quartz, measured for 0.1 s at 125°C, using a broad band 420–550 nm light source (∼26 mW cm −2). Natural and laboratory irradiated samples were held at 160, 180, 200, 220, 240, 260 and 280°C for up to 22 h. Laboratory irradiated samples showed a marked increase in OSL prior to the decrease expected as a result of thermal detrapping of charge. This increase was caused by thermally induced sensitivity changes, which were monitored using the response to a test dose of the 110°C TL peak after each OSL measurement. After dividing the OSL data by this TL response to remove the effects of these sensitivity changes, the corrected OSL decay curves showed that, in this sample, the natural signal was dominated by a single decay component, whereas those from laboratory irradiated sub-samples were dominated by two clearly separate components. Kinetic analysis suggests that the less thermally stable signal has a half-life of <1000 yr at 20°C, and would be removed by the preheats commonly used in dating applications. The more stable signal, present in both the natural and laboratory irradiated samples, has a lifetime of about 1×10 8 yr at 20°C, confirming its suitability as for dating.

A Method for Correcting the Calibration Factor Used in the TLD Dose Calculation Algorithm

The method is described for estimating calibration factors used in the TLD neutron dose calculation algorithm in order to assess the personal neutron dose equivalent to radiation workers in a nuclear power plant in accordance with ICRP 60 recommendations. Neutron spectra were measured at several locations inside the reactor containment building of Youngkwang Unit 4 in Korea by using a Bonner multisphere spectrometer (BMS) system. Based on the fractional distribution of measured neutron fluence, four locations were selected for in situ TLI) calibration. TL responses for the four selected locations were calculated from the measured spectra and the reported fit response function of TLD-600. TL responses were also measured with Harshaw type 8806 albedo dosemeters mounted on the water phantom, and compared with the calculated TL responses. From the responses measured with Harshaw 8806 TLDs, thermal neutron fluence was evaluated, and used to adjust the neutron spectrum obtained with BMS. TL responses calculated for the adjusted neutron spectra showed an excellent consistency with the measured TL responses within 15% difference. Neutron calibration factors were calculated for the measured neutron spectra and the D 2 O-moderated 252 Cf spectrum, and used to calculate correction factors which ranged from 2.38 to 11.18 The correction actor estimated in this way for the known neutron spectrum at an area can he conveniently used to calculate the personal dose equivalent at the area from the calibration factor obtained for a calibration neutron spectrum.

A New Hypersensitive Thermoluminophor Based on the CaF2

A new hypersensitive thermoluminophor called CaF 2 :Cu, prepared at the Moscow State University, has been studied with respect to chemical composition, crystal structure, luminescence and dosimetric properties. The vitreous detector pellets contain holmium (Ho) in a concentration that indicates the dopant character of this element. The dose characteristic shows a linear shape in the range between 10 -6 and 10 Gy and the TL response is ∼160 times higher than that of LiF-TLD-100. This high sensitivity permits precise low dose measurements after short irradiation periods. Furthermore, the high thermal, chemical and mechanical stability enables multiple use of the detectors. The investigations on glow curve and luminescence emissions permit the proposal of an energy level scheme for this new thermoluminophor.

The Study on Pre-Dose Effect of Quartz

The pre-dose technique is based on the increase in sensitivity observed in the 110°C TL peak of quartz that has received a prior dose of radiation followed by heating to 400°C. The sensitivity of the pre-dose sample is dependent upon the thermal activation temperature. The pre-dose effect of quartz has been investigated. The optimum conditions of thermal activation were chosen: the temperature is about 400°C, activation time is about 30 min. Over a wide range, there is a linear relationship between TL response in the 110°C peak and accumulated dose in samples.

CTL Effector Function Within the Central Nervous System Requires CD4+ T Cells

CTL responses induced during most viral infections are independent of help derived from the CD4+ T cell population. However, clearance of virus from the central nervous system (CNS) during infection with the neurotropic JHM strain of mouse hepatitis virus is inhibited in the absence of CD4+ T cells. Adoptive transfer of activated CD8+ T cells with virus-specific cytolytic activity into CD4+ T cell-depleted hosts demonstrated that CD4+ T cells were one component of the host response required for expression of CTL effector function(s) within the CNS. Analysis of mice infected with the JHM strain of mouse hepatitis virus demonstrated that, in contrast to CD8+ T cells, few CD4+ T cells entered the brain parenchyma. Although fewer CD8+ T cells entered the brain parenchyma in mice depleted of CD4+ T cells, access of CTL was not inhibited in the absence of CD4+ T cells. The number of apoptotic lymphocytes in the CNS increased in the absence of CD4+ T cells, suggesting that CTL enter the CNS during viral infection in a CD4-independent manner. However, these cells rapidly undergo apoptosis, indicating that expression of CTL effector function with the parenchyma of the CNS is CD4 dependent. These data raise the possibility that programmed cell death of CD8+ T cells within the CNS is due to the increased Ag present in the CNS of infected CD4 depleted mice or that autocrine cytokines, which maintain CTL activity within peripheral tissues, are inhibited in the microenvironment of the CNS.

Automated TLD system for tumor dose estimation from exit dose measurements in exteranl beam radiotherapy

An automated TLD facility has been commissioned and calibrated, and techniques have been developed for the measurement of exit doses in external beam radiotherapy, to enable the routine estimation of delivered tumor doses. An automated TLD system, originally intended for use in diagnostic radiology and radiation protection, has been evaluated and configured for the measurement of exit doses in radiotherapy. Linearity, optimum heating cycles and calibration procedures have been determined. At the photon energies used, encapsulated lithium fluoride chips provide insufficient buildup to ensure electronic equilibrium, necessitating calibration to allow for oblique exit surfaces. Expressions are derived to allow the calculation of delivered tumor doses. Under the calibration conditions described, the uncertainty in a single TLD measurement is approximately +/-2% (+/-1 standard deviation). Over the dose range 0.4-1.5 Gy, TL response is linear. The total heating cycle time, including annealing, is 75 s. Measurements of R(exit) (the ratio of exit dose with and without full backscatter), used in the estimation of tumor doses, decreases with field size for small fields and varies only slightly for field sizes greater than 7 x 7 cm. Lack of electronic equilibrium leads to a decrease in R(exit) with increasing exit surface obliquity for all energies considered. Application of the technique to a simulated treatment showed good agreement between estimated and applied tumor doses, when surface obliquity was taken into account. This work describes the commissioning and calibration of an automated TLD facility and demonstrates that exit surface measurements using TLD chips used under conditions where electronic equilibrium was not established, have the potential for identifying discrepancies in delivered tumor doses.

Thermoluminescent characteristics of KBr:Eu and KBr:Eu,Mg irradiated with UV light

The TL response of KBr:Eu (0.1% Eu) and KBr:Eu,Mg (0.1% Eu, 0.01 and 0.001% Mg) to 254 nm UV light has been studied. Crystals were subjected to different thermal treatments. The Mg concentration and the thermal treatment that produced the best glow curve reproducibility and sensitivity increase were identified. The sensitivity to UV radiation increased by about three times after the optimum thermal treatment. The addition of Mg (0.001%) was responsible for improvements in the reproducibility of the glow curve characteristics for different samples. The glow curves showed two high-temperature peaks above , similar to those observed after exposure to rays, and the TL response was linear for fluences between 15 and .

Dose Measurements in Mixed (n,gamma) Radiation Fields in Aircraft with TLDs Under Consideration of the High Temperature Ratio

For dose measurements in unknown mixed (n, y) fields in aircraft the neutron energy dependence of the TL response was measured using TLDs embedded in polyethylene spheres (diameter 25 cm). For this purpose the pair method of 6 Li (TLD-600) and 7 Li (TLD-700) was utilised using irradiation with several neutron sources (thermal, Pu-Be, Am-Be, 15 MeV neutrons). For optimising the neutron shielding of TLD-700 a special arrangement of the TLDs inside the sphere was set up. It is shown that the peak height ratio of the neutron glow curves indicates the influence of several effects with different LET. The calibrated Bonner sphere and additional TLD packets with different absorber thicknesses were used for determination of the equivalent dose and the neutron flux in aircraft. The dependence of the results on the absorbing layer is given.

Determination of Patient Surface Doses from Computerised Tomography Examination of the Head

The radiation doses absorbed by various organs of patients (gonads, chest, thyroid and the lens of the eye) were measured at the body surface of patients during computerised tomography (CT) examinations of the head. According to our earlier energy response studies with X ray irradiations, the TL responses of LiF TLDs have the lowest energy dependence and hence LiF:Mg,Ti TLD discs (produced in Poland) and TLD-700 (Harshaw) were used for measurements of the dose. Approximately 600 TLD discs and 200 TLD-700 were used in this study for surface organ dose measurements on 54 patients of different sex and age. The highest dose measured in this study (71.10 mGy) delivered to the eye lens during a CT examination of the head.

Applicability in Dosimetry of Thermoluminescent Li2B407 Doped with Cu or Eu Impurities

Thermoluminescence properties of Li2B4O7 polycrystalline powder undoped and doped with Cu or Eu impurities are investigated. Glow curves of ß irradiated samples are recorded and the dependences of their shapes and of the peak intensities on the dopant concentrations are determined. The results obtained for Li2B4O7:Cu show, besides a very good agreement with previously reported data for slightly doped samples, the appearance of extra TL peaks with increasing Cu concentration. In the case of Li2B4O7:Eu, a high temperature thermoluminescence peak caused by the impurity is observed at any dopant concentration. Comparison with data concerning undoped Li2B4O7 allows for identification of the contributions of impurities to the glow curves. As an important dosimetric feature, the linearity of the TL response of all dosemeters is determined: a satisfactory behaviour is found for appreciably doped Li2B4O7:Cu, whereas samples doped with Eu or with low concentrations of Cu exhibit supralinearity and sub-linearity phenomena.