Phototransferred Thermoluminescence Signal Research Articles

A Photo-transfer Thermoluminescence (PTTL) Study of TLD-100 over a Wide Dose Range

Photo-transferred thermoluminescence (PTTL) is defined as the transfer of electrons from deep traps into shallow traps via optical stimulation. The importance of PTTL is that it allows for a second measurement of dose assessments for accuracy in cases such as an erroneous dose evaluation. In this study, the PTTL signal of TLD-100 was investigated in detail for a wide dose range from mGy to Gy. The investigation of PTTL signals in the order of Gy is the main innovation of this study. Based on the results of the low dose measurement (mGy), the PTTL dose-response curve has a significant sublinear characteristic in the order of mGy for the total area condition. Additionally, PTTL signals could not be distinguished from the background signal up to 5mGy. Therefore, the PTTL method can be used by taking into account the sublinear function obtained after 5mGy for the total area. On the other hand, it can be applied to TLD-100 between 0.5mGy and 50mGy using ROI. Based on the high dose measurement results (Gy), the PTTL method can be applied up to 10Gy regardless of the total area and ROI. Therefore, the dose reassessment can be performed with PTTL signal in high dose measurements (Gy) such as in the radiotherapy field. Furthermore, in future studies, heating the dosimeters during UV exposure, predose effect, or subjecting the dosimeters to fast cooling following the annealing process may provide important outputs to obtaining higher PTTL intensity, thus, it may allow measuring lower radiation doses.

PTTL characteristics of glass samples from mobile phones

Phototransferred thermoluminescence (PTTL) of mobile phone display glass (category A) is systematically investigated to develop a robust measurement protocol for its emergency dosimeter usage after an incident with ionizing radiation. First, optimal readout parameters were defined by varying preheat temperature and holding time preceding violet exposure (405 nm). Next, the detection window of the PTTL measurement was adjusted to optimize the ratio between radiation-induced (RIS) and non-radiation-induced signals (nRIS) of the PTTL. Finally, the developed protocol determines PTTL after preheating to 400 °C and holding for 10 s was tested using the detection window centered at 340 nm. Dosimetric properties such as the PTTL reproducibility and dose response were investigated. PTTL signal stability tests showed that the violet PTTL signal originates from deeper and consequently more thermally stable traps. A signal loss of less than 10% after 10.6 days (254 h) storage was observed. Additionally, TL and PTTL spectra were recorded to investigate the luminescence emissions after beta and gamma irradiation and different UV exposures. PTTL emissions are similar to TL emission. Varying UV energy a qualitative comparison demonstrate that the RIS and nRIS PTTL signal intensities increase significantly with decreasing UV stimulation wavelength. In general, the developed PTTL protocol indicates better signal stability and thus some advantages over other techniques, however, further research is needed to test the potential of a new method for physical retrospective dosimetry.

Studies of blue light induced phototransferred thermoluminescence in CaSO4:Mg

In this work, phototransferred thermoluminescence (PTTL) was observed in polycrystalline powder samples of Mg-doped CaSO4 (CaSO4:Mg) which were produced via the precipitation technique. The thermoluminescence (TL) glow curve showed the two separate main peaks. Composition of the peaks was at 90, 145, 180 and 235 °C and on the high temperature side, at 355, 410 and 475 °C with a 5 °C/s heating rate after delivering 100 Gy beta dose. The PTTL induced in CaSO4:Mg using blue light (470 nm) was reported and some PTTL characteristics were investigated. The PTTL intensity as a function of duration and temperature of illumination was recorded. As a function of increasing beta dose, a linear dose–response was obtained for the sample at 100 °C within the dose range from 1 to 150 Gy. The decaying of the PTTL signals of the previously irradiated sample was examined through successive blue light stimulation cycles. PTTL was used to determine dark fading of the material which was found stable with 2% loss of charge in the deep traps accessed by the PTTL measurement procedure. The thermal activation energy of the source traps involved in the PTTL was evaluated as 1.90 eV using the pulse annealing technique. Although these results showed that CaSO4:Mg presented a good PTTL performance and the blue light-induced PTTL signal can be used for dosimetry, it needs further study to understand the physical mechanisms associated with the transfer of charge from the deep traps probed by the PTTL measurements.

Dose re-evaluation in personal dosimetry using the phototransferred thermoluminescence method of LiF:Mg,Cu,Si TLD

Phototransferred thermoluminescence (PTTL) characteristics of LiF:Mg,Cu,Si, developed by the Korea Atomic Energy Research Institute, are presented as part of a simple and reliable method for dose re-evaluation that is compatible with routine personal dosimetry services. A 4.5 mm diameter 0.8 mm thick pellet-type thermoluminescence dosimeter (TLD) was used after dual-step thermal annealing at 300 °C for 10 min, and then at 260 °C for 10 min, in order to obtain thermal stability. For optimal UV illumination, samples were placed under a 254 nm ultraviolet (UV) lamp for 90 min. The PTTL signal was only lower than 2% that of the TL because of low residual signals in high-temperature deep traps. The difference of the PTTL signals between samples appeared up to two times, indicating high variation in the charge density of the deep traps between samples. Various thermal treatments were performed after each PTTL measurement to remove the residual PTTL signal. UV exposure at 250 °C for 20 min fully reset the residual PTTL signals to the same level as that after dual-step annealing. No degradation of the TL peak from repeated thermal treatments was observed until after the eighth treatment when a decrease was noticeable. The measured dose response of PTTL in the range of 1.14–51.3 mGy showed good linearity. The zero dose and minimal detectable dose of the PTTL were 1.36 ± 0.51 mGy and 1.53 mGy, respectively. The attenuation of the PTTL signal resulting from successive TL measurements was recorded to provide a correction factor for quarterly recorded TL measurements applicable to the dose re-evaluation of TLDs in routine personal dosimetry services.

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.

Phototransferred thermoluminescence (PTTL) dosimetry using Gorilla® glass from mobile phones

Samples of Gorilla® glass (Corning®) from several touchscreen mobile phones were prepared and analyzed using phototransferred thermoluminescence (PTTL) in an attempt to retrospectively assess radiation doses absorbed by the glass for potential applications in emergency dosimetry. Samples of glass were prepared by cleaning and cutting into small pieces. Glow curves were recorded after irradiation to different doses, pre-heating to a given temperature, and exposure to ultraviolet (UV) light (365 nm) for various times. The dependencies of the thermoluminescence signals so-obtained on the pre-heat temperature and the duration of UV light exposure were studied. The stability of the PTTL signals was also studied for samples stored under ambient conditions. The PTTL signals were quite stable with little change after storage over several days. Dose response curves (PTTL versus initial applied dose) were linear up to ∼20 Gy. Values of minimum detectable dose (MDD) evaluated using PTTL were determined for samples from different phone brands. A non-radiation-induced, UV-light-stimulated TL signal was also observed as a result of the UV exposure. Variability in the strength of this signal and in the PTTL sensitivity of the various glasses leads to a variation in, and places limits on, the obtained values for the MDD. Typical MDD values were found to range between 0.1 Gy and 5 Gy. A method for accounting for the UV-induced signal is suggested involving pre-heating to high temperatures (550 °C). An exploratory test of a suggested protocol for evaluating dose from smartphones using PTTL is described and the results are compared with those obtained using optically stimulated luminescence (OSL) and thermoluminescence (TL) methods from other phone components.

Photo-transferred thermoluminescence of shallow traps in β-irradiated BeO ceramics

Photo-transferred thermoluminescence signals from beryllium oxide (BeO) ceramics were measured in the low temperature range of 10–300K. Samples irradiated at room temperature using a Sr90/Y90 beta source were cooled down to 10K and trapped charges were photo-transferred at this low temperature using the light from a high power blue LED emitting at ~470nm (2.6eV). Thermoluminescence glow curve recorded at 0.2K/s heating rate exhibited three peaks around 90, 160 and 185K. The analyses of the TL peaks of the glow curve were accomplished using curve fitting, differential and peak shape methods. Results obtained from these techniques are in good agreement about the presence of three trapping centers in BeO ceramics with activation energies of 0.24, 0.48 and 0.56eV. Moreover, the analyses indicated that first-order kinetics (slow retrapping) is the dominant mechanism in the luminescence process. Heating rate dependence of the glow curves was also investigated between 0.2 and 0.8K/s rates.

Preliminary results on the photo-transferred thermoluminescence from Ge-doped SiO 2 optical fiber

Abstract A study is made of photo-transferred thermoluminescence (PTTL), the TL being induced by transferring charge carriers from deeper to more superficial traps through energetic light exposure. Potential applications include dose reassessment in radiation dosimetry and also as a useful tool for dating. With incomplete emptying of deep traps following first readout, subsequent UV exposure is shown to lead to charge transfer to more shallow traps. Using Ge-doped SiO2 optical fibers exposed to 60Co gamma rays, the PTTL from the medium has been characterized in terms of the stimulation provided by exposure to a UV lamp and duration of exposure, maximum read-out temperature and pre-gamma irradiation dose. Ge-doped SiO2 optical fibers of flat cross-sectional shape have been used in this study. The efficiency of dose reassessment was compared to that of the highly popular phosphor-based TL detector TLD-100. Results show the maximum temperature of readout to have no measurable effect on the PTTL signal. For doses from 20 to 500 cGy, the method is shown to be effective using a UV lamp of wavelength 254 nm, also being indicative of potential application for doses on either side of the range currently investigated. A study was also made of the effect of UV exposure time on PTTL, seeking to determine the greatest accessible sensitivity and lowest measurable dose.

OSL and photo-transferred TL of quartz single crystals sensitized by high-dose of gamma-radiation and moderate heat-treatments

This study investigates the optically stimulated luminescence (OSL) and the photo-transferred thermoluminescence (PTTL) signals in quartz single crystals showing a strong TL peak near 300°C after being sensitized by irradiation with 25kGy of gamma rays and heating at 400°C. Natural and sensitized samples were prepared from two crystals with different sensitivity levels in the 300°C TL region. Continuous-wave (CW) and linearly-modulated (LM) OSL signals were stimulated with blue light-emitting diodes during 40 and 1000s, respectively. Two components were isolated from the CW-OSL signals of sensitized samples. These components were clearly seen in LM-OSL measurements together with two long-term components. LM-OSL showed that the sensitization process considered in this study sensitized an ultrafast OSL component of these crystals. The similar behavior found for the thermal stabilities of OSL and TL signals and the dependence of these signals with sample origins suggested that the trapping site related to the ultrafast component is also related to the TL process of the sensitized peak. The PTTL signal induced by blue LEDs increased the intensity of the sensitized glow peak. On the other hand, a remarkable reduction in the intensity of this peak as a result of the accumulated effect of blue light exposure was clearly seen in both kinds of crystals. These results were explained by a mechanism of competition between optically unstable deep traps and trap levels responsible to the sensitized TL peak.

Photo-transferred thermoluminescence from deep traps in quartz

Abstract Photo-transferred thermoluminescence (PTTL) measurements were performed on analytical grade quartz with the aim to study the dependence of the PTTL signal on preheat temperature and light-exposure time. The results proved that, in addition to the well-known contribution due to the electron traps related to the 325 °C thermoluminescence peak, a non-negligible role was played by deep thermally disconnected traps. Moreover, the shape of the time-response curve for the PTTL signal of quartz samples preheated at high temperature values suggested that recombination processes of optically stimulated electrons from deep traps involved also a non-radiative recombination centre.

Phototransferred thermoluminescence

We describe an analysis of phototransferred thermoluminescence (PTTL) using simple energy band models and considering the dynamics of charge transfer between localized trapping states during irradiation, illumination and heating. The various behaviours which are possible are highlighted and described. Particular attention is focused upon the dependences of the PTTL signal upon the illumination time and upon the wavelength of excitation. Some comparisons with experimental PTTL signals from crystalline quartz are discussed.

Phototransferred Thermoluminescence in Alpha-Al2O3:C

As part of an effort to understand the mechanisms of light-induced effects in α-Al 2 O 3 :C the phototransferred thermoluminescence (PTTL) peaks at ∼260 K and at ∼450 K have been monitored as a function of the time of illumination, the temperature to which the sample is heated between irradiation and illumination, the wavelength of the illuminating light and the dose of irradiation. At the excitation powers and wavelengths used in this study, the PTTL signal is linearly proportional to illumination time in the range of 10 s to 3000 s. The most effective wavelengths are in the short wavelength visible to UV range. The PTTL signal is proportional to the applied dose in the dose range from 6 mGy to 40 Gy. In addition, the position of the 450 K PTTL peak is observed to depend upon the dose of irradiation and the preheat temperature. The phototransfer processes are complex and appear to involve both electrons and holes.

Phototransferred thermoluminescence (PTTL) in LiF (Mg, Cu, P) (GR-200)

The phototransferred thermoluminescence (PTTL) technique is used to reassess doses in various materials. In this work, PTTL properties of LiF (Mg, Cu, P) have been studied. The optimum PTTL signal occurred at an ultraviolet (UV) exposure of ∼ 1770 J m -2 and corresponded to (1.0 ± 0.3)% of the primary TL signal for gamma doses in the range 0.05–4.0 Gy. Whether the ultraviolet irradiation was performed at room temperature, 0°C, or liquid-nitrogen temperature did not make any difference to the PTTL signal. A sensitization technique involving a ‘pre-dose’ followed by annealing was then used in an attempt to increase PTTL sensitivity. The PTTL sensitivity showed a big increase as the sensitization dose increased beyond 100 Gy.