Recombination pathways in a BeO yielding two main dosimetric TL peaks

Apart from the commercially used Thermalox, BeOR is a new type of Beryllium Oxide fabricated by a different company in Turkey. As it is shown by other groups, this dosimeter yields two overlapping main dosimetric TL peaks in the temperature region of 100–300 ◦C and one peak at 350 ◦C.

Radiation Measurements 151 (2022) 106716 Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas Recombination pathways in a BeO yielding two main dosimetric TL peaks P.G Konstantinidis a, *, E Tsoutsoumanos b, c, G.S Polymeris b, G Kitis a a Aristotle University of Thessaloniki, Physics Department, Nuclear Physics and Elementary Particles Physics Section, GR, 54124, Thessaloniki, Greece Institute of Nanoscience and Nanotechnology, NCSR Demokritos, GR, 15310, Ag Paraskevi, Athens, Greece c Condensed Matter Physics Laboratory, Physics Department, University of Thessaly, GR, 35100, Lamia, Greece b A R T I C L E I N F O A B S T R A C T Keywords: BeOR Activation energy de-localized and localized pathways Isothermal decay Peak shape methods Fractional glow technique Apart from the commercially used Thermalox, BeOR is a new type of Beryllium Oxide fabricated by a different company in Turkey As it is shown by other groups, this dosimeter yields two overlapping main dosimetric TL peaks in the temperature region of 100–300 ◦ C and one peak at 350 ◦ C The aim of the present study is the calculation of the activation energies of these two peaks with the use of specific protocols, including the Frac­ tional Glow Technique (FGT), an Isothermal Decay signal (PID) and the Peak Shape Methods (PSM) Also, these experimental procedures were conducted in two different readers, namely the Harshaw and the Risø reader, for better repeatability and in order to minimize the error of the calculations Moreover, another goal is to correlate the activation energy values with the recombination pathways that are present in the BeOR and its corresponding peaks Specifically, both peaks have activation energies around 1–1.3 eV in most cases, although there are some kinds of indications for a difference in the pathways, as the high-temperature region TL main peak seems to follow the localized recombination via tunneling, in contrast with the low-temperature region TL main peak which sticks to the delocalized model with the passing via the conduction band Introduction Beryllium oxide (BeO) is a well-established dosimeter and is highly used as medical, environmental, and personal dosimeter, due to its unique characteristics (Nieto, 2016; Ogorodnikov et al., 2016; Tochilin et al., 1969) First of all, the tissue equivalence (Zeff = 7.14, almost identical to that of Zeff = 7.42 of the biological tissues) of BeO is considered ideal for an excessive number of devices in a variety of medical fields, which use radiation for diagnostics or therapy (Watanabe et al., 2010) Even if it is described as an insulator, it possess a thermal conductivity which even exceeds that of some metals This fact ensures a swift and unfluctuating transfer of heat throughout the stage of the readout of the TL signal, and a TL sensitivity close to that of the commonly used TLD-100 (Becker et al., 1970) Additionally, its annealing is easier and simpler as it does not have peaks at the low temperature region This tough ceramic material is insoluble in water, insensitive to most chemical agents, has a high resistance to the influ­ ence of mechanical forces, and can be formed at high temperature en­ vironments into rods or discs (Kiiko et al., 1999), having ready availability and low cost While being solid, free of dust, BeO is nearly innocuous and the possible health risks accrued from it being machined, squashed, or converted to its reactive dispersed form are minimal (Stockinger, 1966) For the thermoluminescence and dosimetry uses, the Ceramic BeO (product name; Thermalox@ 995, fabricated by Brush Beryllium Co.) is the most common of the alternative commercially used BeO Ceramics, as it offers quality assurance, an almost stable response regarding the longterm uses and is mostly free of noise signals (Crase and Gammage, 1975) Gammage and Garrison (1974) pointed out Al2+ substitutional for Be as the TL trapping entity in Thermalox@ 995, while its elemental composition (ppm) as it is given by the manufacturing company is mainly Si, Mg and Fe, with Ca, Al, Ti, and Cu at lower concentrations Its commercial name is Thermalox 995 BeO (BeOT), and yields one main Thermoluminescence (TL) dosimetric peak at the dosimetric region between the temperatures of 100–250 ◦ C as well as one peak at 350 ◦ C This type of BeO was investigated, for its OSL (Bulur and Goksu, 1998), LM-OSL (Bulur and Yeltik, 2010) and TR-OSL (Bulur and Saraỗ, 2013; Bulur, 2014) properties, as well as for the phototransfer and the prop­ erties of the TL signals (Bulur, 2007) On the same note, Yukihara (2011, 2019a, 2019b) investigated the TR-OSL and TT-OSL signals of BeOT and proposed a new protocol for the usage of BeO chips with the help of automated OSL readers, such as Risø TL/OSL (Yukihara et al., 2016) * Corresponding author E-mail address: pavkonst@physics.auth.gr (P.G Konstantinidis) https://doi.org/10.1016/j.radmeas.2022.106716 Received 19 November 2021; Received in revised form 23 January 2022; Accepted 29 January 2022 Available online February 2022 1350-4487/© 2022 Elsevier Ltd All rights reserved P.G Konstantinidis et al Radiation Measurements 151 (2022) 106716 private company (Radkor), with laboratory code name BeOR yields two separate TL peaks at the dosimetric temperature region of 100–300 ◦ C and one peak at 350 ◦ C (Fig 1a) BeOR was studied extensively by As¸lar et al (2021) with a plethora of techniques, which besides TL and opti­ cally stimulated luminescence (OSL) include electron spin resonance (ESR), scanning electron microscopy (SEM) coupled with energy dispersive x-ray spectroscopy (EDX) and x-ray diffraction analysis (XRD) Moreover, the same team conducted some experiments on a variety of the luminescence dosimetric properties of BeOR, such as dose-response, minimum detectable dose threshold, thermal stability and quenching, reproducibility, and bleaching These authors concluded that the difference in the TL glow curve between the two types of do­ simeters could arise from different sintering and/or calcination appli­ cations during the manufacturing process or from the increased content of Cr, Mg and Sn On the other hand, Polymeris et al (2020) concentrated on the recombination pathways during the readout stage regarding four different known dosimeters, including BeOT It is known that these pathways can be either delocalized, via the conduction band, or local­ ized, involving tunneling through an excited state of the trapped elec­ trons These authors have proposed a series of experiments in order to compare the value of the activation energy of each peak and to distin­ guish its recombination pathway, by including the fractional glow technique (FGT), an isothermal decay signal (PID) and the peak shape methods (PSM) The main aim of the present study is to use the methodology that is proposed by Polymeris et al (2020) in order to generate experimental data for the case of both main dosimetric peaks of BeOR Specifically, it would be beneficial to exploit this methodology to the case of one single material that yields two different and overlapping TL glow peaks with different recombination pathways By conducting the three series of experiments, it is possible to calculate the activation energy of both main dosimetric peaks at different scenarios and thus being able to compare them The second aim of this study, as the protocol suggests, is to distinguish the recombination pathways that are available in the BeOR dosimeter, after discriminating them Apparatus and experimental procedure The inorganic compounds of BeOR are colorless under normal con­ ditions and are created to have dimensions of × × 1mm The do­ simeters that are used in the experiments are either un-preheated or preheated at 160 ◦ C All of the experiments were conducted in two different readers, namely a Harshaw-3500 and a Risø TLD-reader The Harshaw-3500 TLD-reader includes only neutral density filters, taking advantage of a nitrogen atmosphere and a low constant heating rate of ◦ C/s in order to minimize the thermal lag that may occur On the other hand, the Risø TLD-reader (Model DA-20) is equipped with an EMI 9235QB PM Tube, and the filter used in front of the PMT is a 7.5 mm Hoya U-340 filter (270–380 nm, FHWM 80 nm) The irradiations were accomplished via a90Sr/90Y beta source with a dose rate of 0.53 Gy/min One of the dif­ ferences of the two readers lies in the thermal contact, of which the Harshaw reader is superior This may lead to a temperature lag in the case of the Risø measurements resulting in a shift of the TL glow curves Fig 1a shows that the BeO fabricated by Radkor indeed has a double TL main dosimetric peak The activation energies of each one between these two overlapping TL traps were calculated using Fractional Glow and Initial Rise Techniques, Peak Shape Methods, and analysis of Isothermal TL The prompt isothermal decay (PID) experiments were conducted in both a Harshaw-3500 (Tmax = 400 ◦ C) and a Risø (Tmax = 500 ◦ C) TLDReader, according to the following protocol: Fig a) TL signals of BeOR measured using the (1) Risø-reader and (2) Harshaw-Reader BeOR has two different overlapping peaks at the main dosi­ metric region At the same figure, glow curve (3) represents BeO Thermalox (b) Examples of Isothermal and Residual TL signals of BeOR according to steps and of the corresponding protocol In all curves, the arithmetic corresponds to the temperature at which Isothermal TL was measured, while the index rep­ resents the corresponding step of the protocol For the case of the Isothermal signals the x axis corresponds to the temperature up to the maximum intensity (170 ◦ C and 215 ◦ C), while the decaying part is plotted versus the stimulation time As for the Residual TL curves, the x axis corresponds only to the temperature Recently, Altunal et al (2018), synthesized four different nano-powders of BeO with the help of the sol-gel method and studied their OSL properties, including the thermal stability, the minimum dose that can be detected and the fading of the signals The same team found out that the BeO pellet sintered at 1100 ◦ C has a TL peak around 200 ◦ C In another study of Altunal et al (2019), some BeO pellets were syn­ thesized using the precipitation method at three different sintering temperatures It was found out that the higher the sintering temperature, the higher the OSL signal was Finally, Altunal et al (2020), who investigated the effects of the different impurities in Beryllium oxide ceramics, found that a BeO dosimeter with Dysprosium or Erbium im­ purities in its Bravais lattice form, yields a double peak at the dosimetric region However, a new type of BeO ceramic, originating from a Turkish Test dose and TL up to Tmax P.G Konstantinidis et al Radiation Measurements 151 (2022) 106716 Fig Residual TL glow curves according to step corresponding to (a) Risø equipment for un preheated sample, (b) Risø equipment for preheated sample, (c) Harshaw reader for un preheated sample, (d) Harshaw reader for preheated sample In all cases the residual curves show a slight shift towards higher temperatures with increasing stimulation temperature (illustrated as arrows) Fig Normalized Isothermal decay signals for the case of (a) Harshaw reader for un preheated sample, (b) Harshaw reader for preheated sample, (c) Risø equipment for un preheated sample, (d) Risø equip­ ment for preheated sample Isothermal decay curves fall clearly within two distinguished groups The first one includes curves with different shape according to the Isothermal temperature (illustrated as arrows); this group is prominent in figures a (where the cor­ responding decay curves are highlighted within ellipsoidal for temperature region 110–165 ◦ C) and c The second group consists of decay curves that, despite their different stimulation temperature, indi­ cate shape that coincides This phenomenon is prominent in all four plots and specifically in plot a is highlighted as a rectangle within the temperature region of 165–220 ◦ C Dose of 0.275 Gy for Harhsaw and 0.5 Gy for Risø The experiment was performed two times in different chips, namely with and without a preheat with a constant heating rate up to 160 ◦ C in order to remove the unstable low temperature TL peaks and to ensure that the only remaining peak from the two overlapping ones in the dosimetric region will be the higher temperature one P.G Konstantinidis et al Radiation Measurements 151 (2022) 106716 Fig Example of deconvolution of an Isothermal decay at 150◦ C for the Harshaw un preheated case Two components are used for a better fit Temperature increase up to Ti with a constant heating rate, in which the isothermal decay will take place for 110 s, in order to record the PID signal TL readout up to Tmax (400 ◦ C or 500 ◦ C, depending on the reader) to require the remnant Residual TL signal (RTL) from step Repeat steps 2–4 for Ti: 110–240 ◦ C with a step of ◦ C for the Harshaw reader and 10 ◦ C for the Risø one One example of the measurement sequence of the aforementioned protocol is Fig 1b, in which the first part of the signal, up to the maximum intensity, is used for the Initial Rise method, while the second decaying part can be used for the Isothermal Decay Method Step is very important, as the Residual Curves can be used for the Initial Rise method as well as for the Peak Shape Methods (PSM) for the preheated versions The Fractional Glow Technique (FGT) protocol is conducted in Risø equipment according to the following: Fig lnτ for the dominant component of the Isothermal Decay signal for a) the entire temperature region, in which the activation energies are calculated with the help of the corresponding slopes and b) a close up at higher temperatures Test Dose (TD) of 0.38 Gy and record TL glow curve up to Ti = 40 C Record TL glow curve up to Ti oC, varying from 50 ◦ C to 500 ◦ C in steps of 10 ◦ C ) ( Method of analysis I(T)∝n0 se As it was mentioned, the activation energies of the two TL glow peaks of the main dosimetric region of BeOR are calculated with different methods, which are proposed by Polymeris et al (2020), in two different readers in order to accumulate more data and thus better results − E/ kT (1) Where s(s− 1) is the frequency factor, E(eV) is the activation energy and T (K) is the temperature According to equation (1), the plot of ln(T) versus 1/kT represents a straight line, the slope of which is the activation energy -E In the present study the Initial rise method was applied to the first region of Fig 1b up to the maximum intensity and to the RTL data of Fig 3.1 Initial Rise method (IR) The Initial Rise method (IR) was firstly given by Garlick and Gibson (1948), simulated by Kitis et al (2017) and is the one of the most important techniques for the calculation of the activation energy of a trap It was generalized, by recording the luminescence during both the heating and cooling stages, and accepted as the Fractional Glow tech­ nique (FGT) by Gobrecht and Hofmann (1966) and Tale (1981) In the present study, a consecutive Initial Rise method in order to increase the temperature step wise (or the heating stage of the FGT) is applied, ac­ cording to which the TL intensity in the region of negligible disturbance of trap population is given by the following equation: 3.2 Isothermal decay method (PID) The isothermal decay method can be described by three identical models as (1) delocalized recombination model (Kitis and Vlachos, 2013), (2) localized recombination model (Kitis and Pagonis, 2018) and (3) localized tunneling recombination model (Kitis and Pagonis, 2013) In all aforementioned models the decay constant λ can be given by the following equation (Chithambo and Niyonzima, 2014): λ = se − E kTi (2) P.G Konstantinidis et al Radiation Measurements 151 (2022) 106716 Table Activation energies of the two main dosimetric traps, resulting from four different analyses for both readers N corresponds to the total number of the curves that were used in the whole analysis For each equipment the first arithmetic indicates the total number of un preheated curves, while the second the total number of preheated curves Activation Energy (eV) Risø (N ¼ 22, 20) Harshaw (N ¼ 23, 10) IR un preheated RTL 1.26 ± 0.02 IR 1.02 ± 0.04 RTL 1.27 ± 0.03 preaheated 1.27 ± 0.01 1.07 ± 0.01 1.26 ± 0.03 PSM Chen Еω 1.00 ± 0.19 Eδ 1.19 ± 0.19 Eτ 1.06 ± 0.20 Еω 1.05 ± 0.12 Kitis 1.03 ± 0.2 1.03 ± 0.3 1.03 ± 0.4 1.18 ± 0.08 PID C1 140165 C1 165215 un preheated preheated 0.96 0.96 0.05 0.14 C2 140210 0.56 0.59 C1 140165 0.96 – FGT (N ¼ 43) T Energy (eV) 1.25 ± 0.04 1.36 ± 0.04 1.90 ± 0.09 130–280 320–390 440–490 IR 1.08 ± 0.01 1.12 ± 0.03 Eδ 1.14 ± 0.15 1.18 ± 0.09 Eτ 1.11 ± 0.12 C1 165215 0.06 0.1 C2 140210 0.68 0.44 1.18 ± 0.10 parameters Nevertheless, the FOM value is given by the following equation: ⃒ ⃒ ∑[⃒Yexp − Yfit ⃒] (4) FOM(100%) = 100 × A i In this case Yexp regards the experimental data, Yfit is the theoretical data resulting from the whole procedure and A is the area of the fitted curve of the signal that is analyzed Fig a) Results of the Fractional glow technique; three different plateaus become prominent for the entire glow curve b) Activation energies calculated via all different experiments of Initial Rise for all samples, based on the first half of the PID signal (IR) and the Residual TL curves (RTL) 3.3 Peak shape methods (PSM) Once again the s(s− 1) is the frequency factor, E(eV) is the activation energy, and the Ti is the temperature that is used in the step of the aforementioned PID protocol The deconvolution of the PID signals can be accomplished with the general order formula, introduced by Kirsh and Chen (1991): − b [ t ]b− I(t) = I0 + (b − 1) τ The peak shape methods are used in order to calculate the activation energy of a TL peak with the condition of it being isolated Thus, these methods will be applied only in the cases of the preheated samples, in order to analyze one TL glow peak at a time The peak shape methods take advantage of the geometrical characteristics of the TL glow curve, mainly the maximum peak temperature Tm and the high and low tem­ peratures (T1 and T2 accordingly) sides of the glow curve in the half maximum intensity Chen (1969) and Kitis and Pagonis (2007) have independently created peak shaped equations that give similar but no identical values for the activation energies based on the general order kinetics model Thus, the presenting calculations in this work are based on both groups of PSM (3) In this equation b∕ =1 is the kinetic order, I(t) is the luminescence intensity and τ = 1/λ is the mean lifetime Thus, the decay constant λ can be calculated by the fitting of Fig 4, with the help of the spreadsheets of Afouxenidis et al (2011) By replacing λ with τ in equation (2) and with the use of the natural algorithm, the plot of ln(τ) versus 1/kT is a straight line with a slope that represents the activation energy E of the trap However, in order to ensure a good fit for the signals, it is essential that the Figure Of Merit (FOM %) -introduced by Balian and Eddy (1977)should be as low as possible These authors concluded that a FOM of 3% or lower is highly desirable, while a FOM of 10% and above is highly unrecommended as an acceptable value For values between 3% and 10% the whole fit should be revised with some alterations in the initial Results and discussion Fig shows all the residuals according to the Isothermal decay protocol and can be used for two experimental analyses All of the peaks in the case of Risø have a higher Tm temperature (a deviation of 40–50 ◦ C) and are clearly wider than those of the Harshaw cases, even though the experiments were conducted in both readers with a heating P.G Konstantinidis et al Radiation Measurements 151 (2022) 106716 rate of ◦ C/s The aforementioned phenomenon can be ascribed to the thermal gradient between the sample and the heater plates (Kon­ stantinidis et al., 2020; Bilski et al., 2014) When a Harshaw reader is being used, there is a direct heating of the sample, as it is placed on the corresponding planchette (Bilski et al., 2014) On the other hand, in the case of a Risø reader, the sample is firstly placed inside the stainless-steel cup, which is then placed on the heating planchette of the Risø reader This geometry leads to an indirect heating of the used dosimeter or sample Having in mind the previous analysis, it can be assumed that there is smaller thermal gradient in the Harshaw readers and thus, the values of Tm and FWHM acquired by this reader are closer to the actual form of the TL glow curve/peaks As for the comparison of the signals between the two readers, it is worth mentioning that the TL curves of the experiments of the Risø system are more intense than those of the Harshaw 3500 reader, despite the fact that the Risø system has a Hoya U-340 filter and the Harshaw has none, as there is a difference within the same applied dose For the Initial Rise method all four figures can be utilized, while only the preheated versions are used in order to acquire the necessary data for the Peak Shape Methods Also, the clearly visible temperature shift of the second glow peak can be a first indicator of a possible localized recombination pathway without the passing via the conduction band Fig shows the four different versions of the isothermal decay sig­ nals, which follow the same trend The difference in the starting tem­ perature between the two readers (10–20 ◦ C) is due to the thermal lag, as mentioned and discussed before Also, the preheated samples are used mainly for the analysis of the second peak of the main dosimetric region Moreover, these figures are the second indicator of the different path­ ways that are used in the two overlapping peaks of the BeOR dosimeters Specifically, the de-localized model pathway can be found up to 165 ◦ C, while the localized one is at temperatures higher than 165 ◦ C, as can be seen from the example of Fig 3a Fig presents an example of the deconvolution analysis of these PID signals, with the use of the spreadsheets of Afouxenidis et al (2011) and equations (2) and (3) As BeOR exhibits two overlapping peaks, the use of two different compo­ nents, namely C1 and C2, is essential for the experimental and the theoretical signals to be in agreement with a FOM value lower than 3% It must be noted that the study focuses on the dominant C1, as C2 rep­ resents a slower component to the deconvolution process From the Isothermal Decay signal (PID) study, the activation en­ ergies of the first component in Fig 5a in the region of 130–165 ◦ C seems to be near 0.96 eV, regarding the first peak of the BeOR in the dosimetric region From the data of Fig 5b, it is concluded that at temperatures beyond 165 ◦ C the analysis does not give a satisfactory result, indicating once again the localized pathway of the second peak Moreover, Fig 6a shows the results of the Fractional Glow Technique through the tem­ perature range of 50–500 ◦ C There are different plateaus, namely at 130–280 ◦ C, 320–390 ◦ C and 440–490 ◦ C The main dosimetric peaks are located within the first plateau, both having activation energies of 1.25 eV From Fig 6b one can see the activation energies that accrues from all the experimental procedures of the Initial Rise that were con­ ducted in the present study, for the un preheated and the preheated samples The same results can be viewed in Table and are in agreement with the findings of As¸lar et al (2021) From the Initial Rise method of the residuals both versions seem to have an activation energy of 1.26eV, while the IR method from the isothermal signals are set to be at 1.08eV Lastly, the peak shape methods of the residuals of the preheated samples have visible differences between the two readers For the case of Risø, the activation energy is at 1.03eV while for the Harshaw it is at 1.18eV each one between these two peaks was calculated using Isothermal Decay, Fractional Glow Technique (or Initial Rise) and Peak Shape Methods According to the analysis of the present study, BeOR yields two main dosimetric peaks with two different recombination pathways For the case of the main TL trap corresponding to the low-temperature re­ gion, all techniques yield similar values of activation energy, namely around 1–1.30 eV, providing strong experimental indications towards a recombination pathway via the conduction band On the contrary, regarding the main TL trap corresponding to the high-temperature re­ gion, Isothermal TL provides values of activation energies below 0.2 eV, while the other two techniques around 1.10–1.30 eV In a nutshell, the effective use of the aforementioned methodology was successfully exploited even for the case of one single material that yields two different and overlapping TL glow peaks with different recombination pathways Discrimination could be achieved using solely the Isothermal TL and neither Initial Rise technique nor the Peak Shape Methods could identify localized recombination pathways Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper References Afouxenidis, D., Polymeris, G.S., Tsirliganis, N.C., Kitis, G., 2011 Computerized curve deconvolution of TL/OSL curves using a popular spreadsheet program Radiat Protect Dosim 149, 363–370 Altunal, V., Yegingil, Z., Tuken, T., Depci, T., Ozdemir, A., Guckan, V., Nur, N., Kurt, K., Bulur, E., 2018 Optically stimulated luminescence characteristics of BeO nanoparticles synthesized by sol-gel method Radiat Meas 118, 54–66 Altunal, V., Guckan, V., Ozdemir, A., Sotelo, A., Yegingil, Z., 2019 Effect of sintering temperature on dosimetric properties of BeO ceramic pellets synthesized using precipitation method 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G.S., Kitis, G., 2020 Thermoluminescence response of various dosimeters as a function of irradiation temperature Radiat Phys Chem 177, 109156 Nieto, Azorin J., 2016 Present status and future trends in the development of thermoluminescent materials Appl Radiat Isot 117, 135–142 Ogorodnikov, I.N., Petrenko, M.D., Ivanov, V Yu, 2016 Low-temperature luminescence and thermoluminescence from BeO: Zn single crystals Opt Mater 62, 219–226 ... According to the analysis of the present study, BeOR yields two main dosimetric peaks with two different recombination pathways For the case of the main TL trap corresponding to the low-temperature... may lead to a temperature lag in the case of the Risø measurements resulting in a shift of the TL glow curves Fig 1a shows that the BeO fabricated by Radkor indeed has a double TL main dosimetric. .. overlapping TL glow peaks with different recombination pathways By conducting the three series of experiments, it is possible to calculate the activation energy of both main dosimetric peaks at