Fading study and readout optimization for routinely use of LiF:Mg,Ti thermoluminescent detectors for personal dosimetry

doi:10.1016/j.radmeas.2020.106342

Radiation Measurements

Volume 135, July 2020, 106342

https://doi.org/10.1016/j.radmeas.2020.106342 Get rights and content

Highlights

•
The influence of fading in the routines of the individual monitoring services were investigated at Seibersdorf Labor GmbH.
•
Several Harshaw TLD-100 dosimeters have been irradiated and fully read out after different periods of storage.
•
A following fading study with optimizing the standard procedure (different Time-Temperature-Profiles) was performed.

Abstract

LiF:Mg,Ti thermoluminescent dosimeters (TLDs) are frequently used for personal dosimetry by individual monitoring services (IMS) all over the world. This study focused on the investigation and accurate description of thermoluminescent signal fading in these dosimeters. The routine readout procedure was performed with the controlled heating of the detector elements according to defined time-temperature profiles (TTPs). The output of this procedure was a temperature dependent light signal, called glow curve. Additionally, the readout optimization was achieved with the simulation and validation of glow curves with different fast readout TTPs. The TLDs were annealed, irradiated and read out with different storing times at an ambient temperature of 25 °C. Six batches of each 100 TLDs were irradiated and read out with different TTPs and compared afterwards. A fading study with several Harshaw TLD-100 dosimeters was performed, in which the results were responses to describe the pre-irradiation fading and the post-irradiation fading with mathematical functions that give a correction factor for the dose calculation.

Introduction

During and after irradiation, TLDs store charge carriers in metastable states, which can be released by controlled heating. When released, the charge carries undergo recombination and the stored energy is release in the form of light. The amount of light can be related with quantities of dosimetric interest. This simple system allows an affordable, fast and full determination of several dose quantities. However an accurate readout system is needed. The way the readout system heats the crystal is performed with a controlled heating according to a defined time-temperature profile (TTP). The output of the readout procedure is a time and temperature dependent light signal, called glow curve (GC). This study concentrates on the thermoluminescent material LiF:Mg,Ti, e.g. Harshaw TLD-100. One of the properties of this material is the appearance of reactions between defects in the crystal, which leads to fading (Horowitz and Moscovitch, 2012). This phenomena result in sensitivity changes in the crystal, called the sensitivity loss, and charge transfer/releases, which is commonly termed as signal loss. The pre-irradiation fading is characterized by the loss of sensitivity and the post-irradiation fading by the loss of the signal. Pre-irradiation fading occurs before the exposure with ionizing radiation and the post-irradiation fading after the exposure. Doremus and Higgins (1994) described these separated fading behaviors for TLD-600 and TLD-700 with experimental times up to 80 weeks for neutron and photon irradiation fields (Doremus and Higgins, 1994). In an extensive study (up to 500 days), the behavior of fading in LiF:Mg,Ti is described in terms of the sensitivity loss and the signal loss for fast readout TTPs at different storage temperatures (Luo, 2008).

The aim of this work was to find an accurate way to handle fading within the routines of an individual monitoring service (IMS) and optimize the readout procedure with a fading algorithm and an optimal fast readout TTP. One motivation for the described fading study was the need of more data for shorter times (<100 days). Two main aspects were especially targeted to improve:

•
Fading: The experiments in the study led to a physical accurate description of the pre- and post-irradiation fading for fast readout TTPs.
•
Improvement of the quality of the glow curves: This led to an optimization of the TTP.

Section snippets

Setup of the fading study

The commercially available TLD-cards have four detector positions. The TLDs have a size of 3.2 mm × 3.2 mm and a thickness of 0.38 mm. The cards for personal dosimetry have detectors on the second and third chip-positions. In order to study the fading under routine conditions, the dosimeters were stored under a controlled room temperature of 25 °C between annealing and irradiation, and between irradiation and readout. A fast readout TTP was used (TTP A from Table 1).

In the routine, TLDs are

Fading study

The standard deviations of all response values for constant pre-irradiation time ( $t_{1} = c o n s t$ ) with respect to a variable pre-irradiation time ( $t_{2} = v a r i a b l e$ ) were analysed. For the “direct readout”, relative deviations were at maximum 5.5%, while for the “preheat readout” the deviations were at a maximum of 2.2%. The preheating process could not fully stabilize the signal. The influences of the pre-irradiation fading were measured, e.g. $t_{1} = v a i r a b l e$ and $t_{2} = c o n s t$ : For the “direct readout”, the

Conclusion

A fading study under routine conditions with a fast readout TTP was performed at the IMS in Seibersdorf Laboratories. The readout optimization led to a more accurate measurement of the TL-signal and a faster readout without the need of an external furnace for preheating. Tables of relative responses for each pre- and post-irradiation time combination were calculated. Furthermore, functions with and without any preheat of the TLDs were introduced.

A stable and reproducible glow curve is the most

Acknowledgments

This work arose from a Master's thesis, which was performed at Seibersdorf Laboratories in cooperation with Graz University of Technology. The presentation of the corresponding poster was supported by the Austrian Radiation Protection Association (ÖVS).

References (7)

H. Balian et al.
Figure of Merit (FOM), an improved criterion over the normalized chi-squared test for assessing the goodness-of-fit of gamma ray spectral peaks
Nucl. Instrum. Methods
(1977)
L.Z. Luo
Extensive fade study of Harshaw LiF TLD materials
Radiat. Meas.
(2008)
H. Stadtmann et al.
Glow curve deconvolution for the routine readout of LiF:Mg,Ti thermoluminescent detectors
Radiat. Meas.
(2017)

There are more references available in the full text version of this article.

Cited by (8)

Correlation between figure of merit function and deviation in the assessment of kinetic parameters in realistic TLD-100 behavior using fully-connected neural networks
2023, Radiation Physics and Chemistry
In this study, a realistic model was employed to research the LiF:Mg:Ti behavior (commercially known as TLD-100), a widely employed as thermoluminescent (TL) material in dosimetry services. The research was focused on exploring the association between the Figure of Merit (FOM) function and deviations from the true kinetic parameter values, that were obtained through fitting experimental measures. FOM function is the most used magnitude to measure the quality of a fit achieved between a mathematical model and experimental data. Indeed, FOM function is particularly relevant when the TL glow curve deconvolution method is implemented to obtain the kinetic parameters: Activation Energy E (eV), Frequency Factor s (s⁻¹), value of kinetic order b, Temperature at which the intensity maximum T_Max(^oC), Intensity Maximum I_Max (a.u.) and Distribution Width σ (eV) of every peak. FOM function is subject to degeneracy under certain conditions, this means that different sets of kinetic parameters can lead to similar FOM values. Furthermore, as the obtained kinetic parameters are deemed acceptable when the FOM value is below 5%, then the physical degeneracy is greater than the mathematical ones. This contribution provides a rigorous analysis of how the FOM values can be impacted by deviations between experimental and estimated data of kinetic parameters from the analysis process. This studio was carried out using a mathematical method based on Artificial Intelligence with a high capability to manage a large volume of data. Another advantage of this method is that the analysis to be extended to a wide range of experimental data.
The quest for new thermoluminescence and optically stimulated luminescence materials: Needs, strategies and pitfalls
2022, Radiation Measurements
Citation Excerpt :
Variation in the sensitivity with time is called “aging”, whereas the variation in the signal after irradiation is called “fading”. As an example, such effects have been observed in LiF:Mg,Ti (Ptaszkiewicz, 2007; Luo, 2008; Sorger et al., 2020). The TL signal stability is typically associated with the temperature of the respective TL peaks, the stability increasing with the temperature of the peak in a first approximation.
The quest for new materials for thermoluminescence (TL) and optically stimulated luminescence (OSL) dosimetry continues to be a central line of research in luminescence dosimetry, occupying many groups and investigators, and is the topic of many publications. Nevertheless, it has also been a research area with many pitfalls, slow advances in our understanding of the luminescence processes, and rare improvements over existing materials. Therefore, this paper reviews the status of the field with the goal of addressing some fundamental questions: Is there a need for new luminescence materials for TL/OSL dosimetry? Can these materials be designed and, if so, are there strategies or rules that can be followed? What are the common pitfalls and how can they be avoided? By discussing these questions, we hope to contribute to a more guided approach to the development of new luminescent materials for dosimetry applications.
Electronic and optical responses, Compton spectroscopy and manifestation of trapping centres of LiF:Mg,Ti
2022, Physica B: Condensed Matter
Citation Excerpt :
The behaviour of various TL dosimeters (in the temperature range 50–230 °C) was investigated by Konstantinidis et al. [21]. Moreover, fading study, readout optimisation, and optical response are also available in the literature [22,23]. From above review, it is seen that TL response and dosimetric aspects for occupancies like Mg, Cu, Ti, Si, Dy ions in LiF are available in the literature.
Electronic structure to judge trapping centres and activation energies in Mg-doped (2–8%) and Mg,Ti co-doped (2–3% each) LiF phosphors are presented using full-potential linearized APW method. Compton measurement for LiF is performed for appropriate choice of exchange-correlation potentials used in electronic properties. It is seen that 2–6% of Mg activator leads to formation of electronic traps with activation energy between 0.79 and 0.49 eV. At high concentration (>6%) of Mg, the formation of trap centres vanishes. Co-doping of 2% Mg,Ti permits the formation of trap centres, which is in contrast to higher concentrations. Mg-3s states play key role in the building of electronic trap centres. Higher concentration of Ti and Mg activators in LiF leads to the widening of 3d and 3s states, respectively. Analysis of optical response (frequency-dependent dielectric constants, trapping centres, and absorption coefficients) of 2% Mg-doped and 2% Mg,Ti co-doped LiF suggest their dosimetric applications and in ultra-violet detection.
Photoluminescence, scintillation, and dosimetric properties of Ce-codoped MgF<inf>2</inf>:Tb ceramics
2021, Journal of Luminescence
Citation Excerpt :
These phenomena are called optically- and thermally-stimulated luminescence (OSL [10] and TSL [11]), and particularly used in radiation dosimetry. As examples of OSL and TSL dosimeter, Al2O3:C [1,12] [–] [14] and LiF:Mg, Ti [15–18] are currently in practical use, respectively. Dosimetric materials have been usually used in personal dosimetry.
Photoluminescence (PL), scintillation, and dosimetric properties of the Ce-codoped MgF₂:Tb ceramics were evaluated. The Ce-codoped MgF₂:Tb ceramics showed some PL emission peaks ascribed to the 4f–4f transitions of the Tb³⁺ ion and the 5d–4f transitions of the Ce³⁺ ion. It was confirmed that the emission wavelength of Ce³⁺ overlapped with the excitation wavelength of Tb³⁺, and the quantum yield was improved by codoping of Tb and Ce. In addition, the thermally-stimulated luminescence (TSL) glow curves mainly had the glow peak at 245 °C, and TSL spectra showed emission peaks due to the Tb³⁺ ion. TSL dose response functions were also evaluated, and the dynamic range with linear responses was from 0.01 to 1000 mGy.
Towards a new promising dosimetric material from formation of thulium-yttria nanoparticles with EPR response
2021, Materials Chemistry and Physics
Citation Excerpt :
Even though dysprosium doped calcium sulfate (CaSO4:Dy) presents high effective atomic number (15.3), it is widely used as thermoluminescent dosimeter for diverse types of radiation such as, beta [5], gamma [6], X-ray [7], clinical electron [8], clinical photon [9], UV [10], and laser [11], due to its great reproducibility, high sensitiveness, excellent optical absorbance (OA) properties, and Electron Paramagnetic Resonance (EPR) response, excellent reproducibility and high sensitivity (40 times higher than LiF:Mg,Ti) [12]. Lithium fluoride (LiF) is a standard thermoluminescent (TL) dosimeter, which is routinely used in radiation protection dosimetry [13–16]. Mrozik et al. [17] proposed that dopants (Mg, P, Cu, Ti) may have a significant influence on radio-photoluminescent (RPL) effect on LiF, which indicates that RPL is not entirely driven by intrinsic electron defects as F2 and F3+ centers.
Advances toward new materials for dosimetry application is essential to enhance quality assurance and quality improvement practices based on radiation protection concept. Face to this challenge the present work reports an approach to produce thulium-yttria nanoparticles with electron paramagntic resonance response by an alternative hydrothermal synthesis based on a relative low temperature and pressure. Distinct compositions of thulium-yttria nanoparticles with up to 2 at.%Tm (at.%, atomic percentage) were prepared and characterized by XRD, SEM, PCS, and EPR. The proposed synthesis method followed by thermal treatment of the precursor powder at 1100 °C for 2 h provided thulium-yttria nanoparticles with rounded shape, cubic C-type structure, and mean particle size (d₅₀) less than 160 nm. Among all compositions formed, thulium-yttria nanoparticles prepared with 0.1%Tm presented the most remarkable EPR response. The production of fine thulium-yttria nanoparticles with EPR response supply meaningful parameters to advance in the formation of new dosimetry materials based on rare earths.
Correlation between Figure of Merit Function and Deviation in the Assessment of Kinetic Parameters in Realistic Tld-100 Behavior Using Fully-Connected Neural Networks
2023, SSRN

View all citing articles on Scopus

View full text