TL Signal Research Articles

Gamma dose rate effects in luminescence signals of various artificial, well established dosimetric phosphors

The present work attempts to study tentative gamma dose rate effects for the stable TL and OSL signals of specific, well established artficial luminescence dosimeters. Two different dose rates of gamma radiation (2 and 31 Gy/h) were used. The study has been performed to the stable TL peaks of 5 different TL phosphors (LiB4O7:Cu,In; MgB4O7:Dy,Na; LiF:Mg,Ti; CaF2:Dy; BeO) as well as to both OSL components of BeO. Tentative dose rate effects were studied for both dosimetric dose response features as well as kinetic parameters after deconvolution for both signals. Two different groups of materials were revealed. The first one includes materials such as LiF: Mg,Ti, BeO and MgB4O7: Dy,Na; for these, both TL dose response features and TL kinetic parameters are independent on the dose rate. This is also the case for both OSL components of BeO besides the linearity index of the dose response of the fast OSL component; the latter seems to mildly depend on the dose rate. At the same time, experimental arguments based on both dosimetric as well as deconvolution features suggest the presence of tentative dose rate effects for the cases of LiB4O7:Cu, In and CaF2:Dy. Moreover, the effective use of these aforementioned features as probes for identification of dose rate effects is also discussed. The results of the present study suggest that the basic postulate of most phenomenological TL models that traps pre-exist and they are simply filled during irradiation has to be reconsidered.

Thermoluminescence properties of two natural colorful fluorite samples of Anatolian origin for dosimetric applications

Two different fluorite samples of Turkish origin were studied with respect to their thermoluminescence properties suitable for dosimetric applications. The fluorites are colorful and naturally occurring. Both samples indicate sensitivity comparable to the sensitivity of CaF2:N material, and PXRD and XRF measurements could help to understand the sensitivity differences between these two samples. Deconvolution and fractional glow techniques indicated useful convergent information regarding the trapping parameters of the corresponding peaks. The dose response features were studied only for the appropriate for dosimetry thermoluminescence peaks, after deconvolution, for two different cases: (a) promptly after irradiation and (b) for the residual signal after 100 s of bleaching. The results showed that specific thermoluminescence peaks of both materials could be effectively used for dosimetric applications within the dose range of 10−3–50 Gy. Finally, a specific peak with delocalization temperature 338 °C is promising for dosimetric applications within the same dose range even using the residual TL signal after 100 s illumination.

Investigation of dosimetric properties of newly-developed Li2B4O7:Ag+,La3+ using thermoluminescence (TL) technique

The aim of this study was to investigate the dosimetric properties of newly developed Li2B4O7 doped with Ag+ and La3+ using TL technique. In this study, the concentrations of Ag1%wt and La0.1-3%wt dopants were introduced in Li2B4O7 by Solution Combustion Synthesis (SCS) method. The most TL sensitivity of the synthesized phosphors was recorded at 1%wt for each concentration of Ag+ and La3+. The structural characterizations were investigated using X-ray Diffraction (XRD), Scanning Electron Microscope (SEM). Besides, X-ray Luminescence (XL) spectrum was studied. The main dosimetric TL Peak of the Li2B4O7:Ag1%,La1% exhibited a simple TL Peak located at 220 °C with a heating rate of 2 oCs−1. Some dosimetric properties of this TL Peak, such as dose-response, reusability, fading in dark, were investigated under beta radiation. The TL signal of the phosphor exhibited a linear ranging from 0.1 to 50 Gy. The good reusability, by 4% standard deviation, was in the experiment cycle of 10. Fading was studied up to 4 weeks. The kinetic parameters using whole Glow curve, Initial rise (IR), Fraction Glow Curve (FGT) and CGCD were obtained and discussed.

STUDY OF EFFECT OF CONSECUTIVE HEATING ON THERMOLUMINESCENCE GLOW CURVES OF MULTI-ELEMENT TL DOSEMETER IN HOT GAS-BASED READER SYSTEM.

The objective of this paper is to study the effect of consecutive heating of TL elements of a thermoluminescence dosemeter (TLD) card in hot N2 gas-based TLD badge reader. The effect is studied by theoretical simulations of clamped heating profiles of the discs and resulting TL glow curves. The simulated temperature profile accounts for heat transfer to disc from hot gas as well as radiative and convective heat exchanges between the disc and the surrounding. The glow curves are simulated using 10 component glow peak model for CaSO4:Dy using the simulated temperature profile. The shape of the simulated glow curves and trend in total TL signal of the three discs were observed to match closely with the experimental observations when elevated surrounding temperature was considered for simulation. It is concluded that the readout (heating) of adjacent TLD disc affects the surrounding temperature leading to the changes in temperature profile of the next disc.

Dose-rate dependence of natural TL signals from feldspars extracted from bedrock samples

The influence of dose rate on feldspar thermoluminescence shape is considered for a range of natural dose rates. The geologic dose rate experienced by bedrock samples is observed to control the position of the low-temperature edge of the bulk TL signal (the T1/2 parameter), with higher dose rates producing natural TL signals that begin to emit at lower measurement temperatures. This behavior can be explained in terms of the natural equilibrium between electron trapping and detrapping rates, as the trapping rate depends directly on the dose rate. The role of anomalous fading is more subtle. While a wide range of T1/2 values is found at low fading rates, only high T1/2 values are found when the fading rate is greater. This suggests that high fading rates may also influence the natural T1/2 value by making low-temperature regions of the TL curve unstable. Our results illustrate the need to consider dose-rate and fading-rate variations between bedrock samples before interpreting the minimum stability of each natural TL signal, a consequential result for future low-temperature thermochronology applications.

Ceramic resistors as optically stimulated luminescent retrospective dosimeters

Application of optically stimulated luminescence (OSL) as a retrospective dosimetric technique is studied using ceramic resistors available in commonly used instruments like personal computers, printers, household electronic appliances etc. In order to understand the OSL process the composition of the ceramic resistors was analysed using XRD, FTIR and ED-XRF techniques and it is found to have hydrated calcium aluminium silicate as the major constituent along with trace impurities for the origin of radiation induced OSL. The important retrospective dosimetric characteristics like its OSL sensitivity, reproducibility, OSL dose response and stability of the signal upon storage after exposure have been studied. The dose response was found to be linear from 100 mGy to 5 Gy. It is also observed that the OSL signal decreases by 30% in the first 3 days and thereafter there is negligible fading up to 30 days. The correlation between TL and OSL signals in the material was also studied. Using Single Aliquot Regenerative (SAR) technique, the dose was recovered with very good accuracy. These dosimetric studies showed that carbon film ceramic resistors have potential use as emergency dosimeter.

A correlation study on the TL, OSL and ESR signals in commercial BeO dosimeters yielding intense transfer effects

BeO ceramics have been used for quite a long time as radiation dosimeters besides being used in many other areas. BeO dosimeters can be stimulated both thermally and optically, giving rise to TL and OSL signals. BeO has two main dosimetric peaks, 2 and 3, by ignoring peak 1 that corresponds to a shallow trap, while the OSL curve is composed of two components, termed as C1 and C2. Irradiated BeO dosimeters showed ESR spectra at room temperature consisting of three centers I, II and III. These centers were assigned to Al+2 and O- centres; two different typical oxygen centre behaviours were monitored, one showing anisotropic and one isotropic properties. The present study attempted to perform correlation between the TL peaks 2 and 3, the CW-OSL components C1 and C2 and ESR centers I, II and III. The bleaching and decay features of the ESR centers are presented for the first time in the literature. The OSL component C1 is correlated with the TL peak 2; both luminescence entities are associated with the ESR center I, attributed to the Al+2 center. Finally, thermal stability of center II suggests that it can be also tentatively correlated to both TL peak 2 and OSL component C1. Moreover, the present study provides strong experimental arguments towards a correlation between TL peak 3, CW-OSL component C2 and ESR center III, namely oxygen ion (isotropic). Intense transfer was monitored either (a) from deeper traps to both traps responsible for TL peaks 2 and 3, as well as (b) from the trap of TL peak 3 to that of TL peak 2. Nevertheless, reconstruction as well as indirect TA-OSL signal monitoring indicates that the transfer is more intense to the trap corresponding to TL trap 3. For this reason, even after depleting both TL peaks, intense OSL curves were still measured.

Luminescent properties of Tb3+ doped ZrO2 films

In the present work luminescence properties of terbium trivalent (Tb3+) doped ZrO2 were studied. Ultrasonic spray pyrolysis (USP) technique was employed to deposit the films. PL emission of the ZrO2:Tb3+ films exhibits emission peaks around 489, 548, 588, and 620 nm corresponding to transitions 5D4→7F6, 5D4→7F5, 5D4 →7F4, and 5D4→7D3 was seen when excited by excitation wavelength 288 nm. TL glow curves for ZrO2:Tb3+ exhibit a glow curve with a well-defined peak centered in around 170°C. TL signal as a function of dopant concentration showed a maximum at 10 at%. The best intensity of TL response as a function of UV radiation wavelength was in 230 nm. TL response fading and reusability were also analyzed. The ZrO2:Tb3+ films has been found to have simple and sharp glow peak which should be considered for dosimetric applications.

Thermoluminescence characteristics and dose response of electron beam and gamma rays irradiated alumino-phosphate glasses doped with Gd2O3 and Tb2O3

Phosphate glass samples doped with Gd2O3 and Tb2O3 oxides were synthesized and studied towards potential application in high dose radiation measurements. Glass/polytetrafluoroethylene composite detectors were prepared and used to study dosimetric signal dependencies, and thermoluminescence dosimetry (TLD) technique was used as a main investigation tool to study detectors response. Influence of dopants and polymer on TL signal were investigated. The dosimetric properties, like dose response, linearity and signal stability of the composite dosimeters were investigated. Incorporation of lanthanide oxides into glass matrix changes substantially the thermoluminescent behavior of phosphate glass. The highest luminescence yield exhibits glass doped with terbium oxide. Negligible influence of polytetrafluoroethylene agglutinator on dosimetric signal was observed. Dependence of the integrated thermoluminescent signals for all samples vs. absorbed dose can be considered as linear. The new phosphate glass-polytetrafluoroethylene composites show good material and thermoluminescent properties for the high gamma and electron beam dose measurement applications.

Thermoluminescence response of X-ray irradiated commercial chalk

Present research concerns the TL signal stored in chalk of the variety commercially available for writing on blackboards. Samples of this have been subjected to x-ray irradiation, the key dosimetric parameters investigated including dose and energy response, sensitivity, fading and glow curve analysis. Three types of chalk have been investigated, each in five different colours. The samples were annealed at 323 K prior to irradiation. For all three chalk types and all five colours, the dose response has been found linear over the investigated dose range, 0–9 Gy. Regardless of type or colour, photoelectric energy dependency is apparent at the low energy end down to the lowest investigated accelerating potential of 30 kV. Crayola (Yellow) has shown the greatest TL sensitivity, thus selection has been made to limit further analysis to this medium alone, specifically in respect of glow curve and fading study. In addition, elemental compositional and structural change characterizations were made for the same medium, utilizing Energy Dispersive X-Ray (EDX) and Raman spectroscopy, respectively.

Thermoluminescence dosimetry of mixed neutron – gamma fields

The method of measurement dosimetry in neutron – gamma field by using CaSo4 : Dy (PTFE) disc which has a diameter of 1.3mm and thickness of 0.2mm and using hydrogenated material as a converters of neutron to recoil protons (n-p) reaction, the discs were irradiated by neutron source (241Am-Be) with flux of 4.5?105 n/cm2s for different time to obtain different dose. The TL signals, which we have been obtained by using the converters, are increases to 71%. So we can resolve the neutron and gamma in mixed field.

Thermoluminescence dosimetry of mixed neutron – gamma fields

The method of measurement dosimetry in neutron – gamma field by using CaSo4 : Dy (PTFE) disc which has a diameter of 1.3mm and thickness of 0.2mm and using hydrogenated material as a converters of neutron to recoil protons (n-p) reaction, the discs were irradiated by neutron source (241Am-Be) with flux of 4.5?105 n/cm2s for different time to obtain different dose. The TL signals, which we have been obtained by using the converters, are increases to 71%. So we can resolve the neutron and gamma in mixed field.

Phototransferred TL properties of alumina substrates

Alumina substrates, such as those found as surface-mount resistors in mobile phones, are currently the strongest candidate as a surrogate dosimeter material in emergency radiological scenarios using luminescence techniques. However, the rate of fading of the luminescence signal (TL or OSL) imposes a limitation on their longer term use, and also increases the uncertainty in dose assessment. The potential of phototransferred thermoluminescence (PTTL) techniques to access deep traps in alumina substrate samples is reported here. A measurement procedure employing blue (470 nm) illumination was found to produce a PTTL signal with a detection limit of ca 100 mGy, but with a supralinear dose response below 10 Gy. By using a UV source with emission between 307 and 575 nm a linear dose response was obtained within this dose range, although the detection limit was higher (ca 200 mGy), partly arising from the presence of a non-radiation-induced photostimulated TL signal. Pulse annealing experiments indicate that deep traps providing a reservoir of charge are thermally accessible above 500 °C and require annealing to ca 700 °C to thermally clean them. Significantly, using blue illumination, storage experiments performed under dark conditions at room temperature indicate that the loss of charge in the deep traps accessed by the PTTL measurement procedure was less than 30% for storage periods of up to 224 days. Although the physical mechanisms associated with the transfer of charge from the deep traps probed by the PTTL measurements require further clarification, the possibility of significantly reducing the fading observed in conventional TL or OSL measurements introduces a potentially valuable tool in the use of this material for both short and long term dosimetry.

On energy storage of Lu2O3:Tb,M (M=Hf, Ti, Nb) sintered ceramics: Glow curves, dose-response dependence, radiation hardness and self-dose effect

Thermoluminescent properties and energy storage characteristics of Lu2O3:Tb,M (M = Hf, Ti, Nb) sintered ceramics induced by ionizing radiation are presented and discussed. Dose-response dependence, radiation hardness and fading are studied. A linearity of the former exceeding seven orders of magnitude is confirmed for Lu2O3:Tb,Hf and Lu2O3:Tb,Nb ceramics. Lu2O3:Tb,Hf shows the best TL performance and also its fading is the lowest reaching 15% over 7 h and shows tendency to saturate. During the same period of time the Lu2O3:Tb,Ti, despite having TL at higher temperatures, losses about 25% of the stored energy and the TL signal of Lu2O3:Tb,Nb fades by almost 40% over 7 h. First order TL kinetics is confirmed for all three compositions. A self-dose effect in Lu2O3:Tb,Hf due to a natural content of the radioactive isotope (2.6%) is proved to be important for long-time reading of low doses.

Simultaneous microbeam IBA and beam-induced luminescence analysis of strained doped silica fibre radiation dosimeters

We demonstrate that the simultaneous combination of ion beam analysis (IBA) and ion beam induced luminescence (IL) can reveal valuable information concerning the performance of strained doped silica fibre thermoluminescence microdosimeters. The micron scale spatial resolution and low detection limits of IBA allow the lateral distribution of dopant elements to be mapped and then correlated with the distribution of prompt radioluminescence. Measurement of the decay of the IL signal with dose provide information concerning the saturation of the subsequent TL signal at high doses. MeV ion beams can deposit relatively high energy in localized, well-quantified small volumes and so this method is valuable for studying high dose effects in TL dosimeters. We describe a simple modification of the target chamber microscope which enables sensitive low background light detection in two wavelength bands and present preliminary results from three types of germanium doped silica fibre dosimeter.

QA measurement of gamma-ray dose and neutron activation using TLD-400 for BNCT beam

TLD-400 (CaF2:Mn) chips were applied for the gamma-ray dose measurement in a PMMA phantom exposed to a BNCT beam because of their very low neutron sensitivity. Since TLD-400 chips possess an adequate amount of Mn activator they have been employed in this work simultaneously for neuron activation measurement. The self-irradiation TL signals owing to the decay of the neutron induced 56Mn activity have been applied for a calibration of the TLD-400 chip in situ, where the activities were measured by an HPGe detector system and the energy deposition per disintegration of 56Mn was calculated by applying a Monte Carlo code. It was accidentally found that the irradiated TLD-400 chips were capable of emitting prominent scintillation lights owing to the induced 56Mn activity, which can easily be recorded by the TLD reader without heating and after a calibration can be used to determine the 56Mn activity.

A detailed investigation of the TL and OSL trap properties and signal stability of commercial pharmaceutical glass containers towards their use as post-sterilization dosimeters of liquid drugs

Drug sterilization with ionizing radiation is a well-established technology, which allows the adequate sterilization of heat-sensitive pharmaceutical preparations. However, the effects of irradiated drugs on the consumer when they are introduced in the human body are still unknown. Moreover, appropriate methods and protocols for the detection of irradiated drugs have not been established yet.In two previous studies, the possibility to identify irradiated liquid-state drugs by means of TL and OSL measurements on their glass containers was explored with very promising findings. The present work complements the previous works, conducting a detailed investigation of the TL and OSL trap properties and signal stability on two types of glass containers of liquid pharmaceutical preparations.Both the TL and OSL signals are composed by several components with linear dose response up to several kGy, while their stability with time is good, since an appreciable signal remains unaffected even 6 months post irradiation in both containers.The new findings are very supportive and point towards the efficient use of the glass containers of commercial liquid drugs as probes for the post-sterilization dosimetry and accidental dosimetry of these drugs.

Charge carrier traps in tris-(8-hydroxyquinoline) aluminum

Thermoluminescent measurements between 40 and 300C revealed two traps in the irradiated Alq3 powder synthetized from Al(OH)3, at 0.8 and 1.1eV below the conduction band. These two traps have been assigned with the Alq3 facial and meridional isomers and the trapping process stand for a long time after the switching off the irradiation source. The spectra recorded an hour after switching recover 60% from the photoluminescence and also decreasing the TL signals. A theoretical model was proposed in order to explain the trapping and de-trapping processes in both meridional and facial Alq3 isomers based on the XPS measurements for the HOMO, LUMO, ionization potential and optical bandgap, completed with the DFT calculated parameters and similar experimental results concerning the evidences of the triplet states in Alq3 compound.

(Invited) Investigation of Thermal Quenching Process for 5d-4f and 3d-3d Luminescence

White light LEDs (light-emitting diodes) are rapidly replacing incandescent lamps and (compact) fluorescent tubes in all lighting markets (consumer, automotive, and professional). The most widely used type of white LEDs (w-LEDs) is a phosphor-converted w-LED (PC-LED), which is composed of an InGaN-based blue LED and visible light emitting inorganic phosphors, such as oxides and nitrides doped with Ce3+, Eu2+ , Mn4+ as an active center. These phosphors are required to have excellent thermal quenching behavior because the LED chip reaches temperatures up to ∼200 °C in recent high-power w-LED. In these phosphors, two processes of thermal quenching are considered generally. One is the thermally activated cross over from the excited state to the next lower state. The other is the thermal ionization from the excited state to the bottom of the conduction band (CB). In this study, we investigated the thermal quenching mechanism for the 5d-4f and 3d-3d luminescence in w-LED phosphors doped with Ce3+ or Mn4+. Ce3+-doped Y3Al5O12 garnet (yttrium aluminum garnet,YAG) is the most prominent phosphor in w-LEDs. The excellent thermal quenching behavior of the Ce3+ luminescence in YAG:Ce3+ is well established, but the mechanism for thermal quenching remains unclear. To elucidate the mechanism of thermal quenching of YAG:Ce3+, thermoluminescence excitation (TLE) spectra were recorded at room temperature and 300 °C. At room temperature, the lowest 5d1 band at 450 nm does not contribute to the charging process for TL while at 300 °C, a temperature corresponding to the onset of thermal quenching of the Ce3+ luminescence, the excitation in 5d1 band gives rise to a TL signal. This result indicates that thermal ionization is responsible for thermal quenching of the Ce3+luminescence. We also investigated the thermal quenching process for calcium aluminum magnetoplumbite, CaAl12O19, doped with Mn4+ (CAO:Mn4+), which is narrow red luminescence phosphor. We tried to understand the quenching mechanism of Mn4+ compared with the luminescence quenching properties of Mn2+. From the temperature dependence of Mn4+: 2E-4A2 red luminescence and Mn2+:4T1- 6A1 green luminescence intensities in CaAl12O19, the quenching temperatures of Mn4+ and Mn2+ are 330K and 800K, respectively. There is a big difference between the quenching temperatures of Mn4+ and Mn2+ in spite of the same host material and the similar luminescence transtion energy. The Mn4+:2E-4A2 red luminescence is quenched by the thermally activated crossover through the 4T2 excited state whose potential curve possesses a large configurational offset in a configurational coordinate diagram. Mn2+ green luminescence is also quenched by the thermally activated crossover, but the activation energy for non-radiative process is much larger than that for Mn4+ because there are no other excited states related with the quenching process.

Thermoluminescence of electronic components from mobile phones for determination of accident doses

The dosimetric properties of resistor substrates extracted from the circuit board of mobile phones were studied in detail using Thermoluminescence for the purpose of dose reconstruction following a radiological accident or attack. Many studies have shown the usefulness of this material using Optically Stimulated Luminescence but TL studies are scarce. The typical glow curve of a set of irradiated resistors has a peak at about 170 °C. Unexposed resistors showed a strong zero dose signal in the higher temperature region, peaking between 300 and 350 °C, which also led to a minor zero dose being detected in the chosen integration window of 100–200 °C. Exposure to white light can significantly increase the zero dose, presumably due to phototransfer of charge carriers from deep traps into the dosimetric trap. The dose response of the TL signal was linear up to several Gy but an irreversible sensitivity change was observed in the first thermal readout. A correction factor was therefore deduced from a number of dose recovery tests. Similar to the OSL signal, the TL signal is not stable but fades with time since irradiation. Overall, a slightly smaller fading rate up to 60 days was observed for the TL signal, as compared to the OSL signal. A protocol is proposed for TL based on the measured properties and validated in irradiation trials.