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.

Available online 1 February 2022

1350-4487/© 2022 Elsevier Ltd All rights reserved

Recombination pathways in a BeO yielding two main dosimetric TL peaks

aAristotle University of Thessaloniki, Physics Department, Nuclear Physics and Elementary Particles Physics Section, GR, 54124, Thessaloniki, Greece

bInstitute of Nanoscience and Nanotechnology, NCSR Demokritos, GR, 15310, Ag Paraskevi, Athens, Greece

cCondensed Matter Physics Laboratory, Physics Department, University of Thessaly, GR, 35100, Lamia, Greece

A R T I C L E I N F O

Keywords:

BeO R

Activation energy

de-localized and localized pathways

Isothermal decay

Peak shape methods

Fractional glow technique

A B S T R A C T 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

1 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 long- term 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 Saraç, 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)

Contents lists available at ScienceDirect Radiation Measurements

journal homepage: www.elsevier.com/locate/radmeas

https://doi.org/10.1016/j.radmeas.2022.106716

Received 19 November 2021; Received in revised form 23 January 2022; Accepted 29 January 2022

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

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

2 Apparatus and experimental procedure

The inorganic compounds of BeOR are colorless under normal con-ditions and are created to have dimensions of 4 × 4 × 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 2 ◦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) TLD- Reader, according to the following protocol:

1 Test dose and TL up to Tmax

Fig 1 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 3

and 4 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

2 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

Fig 2 Residual TL glow curves according to step 4 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 3 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

3 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

4 TL readout up to Tmax (400 ◦C or 500 ◦C, depending on the reader) to

require the remnant Residual TL signal (RTL) from step 3

5 Repeat steps 2–4 for Ti: 110–240 ◦C with a step of 5 ◦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 4 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:

1 Test Dose (TD) of 0.38 Gy and record TL glow curve up to Ti =40 C

2 Record TL glow curve up to Ti C, varying from 50 ◦C to 500 ◦C in

steps of 10 ◦C

3 Method of analysis

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

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:

I(T)∝n0se

(

−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 2

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):

Fig 4 Example of deconvolution of an Isothermal decay at 150◦C for the

Harshaw un preheated case Two components are used for a better fit

Fig 5 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

Once again the s(s− 1) is the frequency factor, E(eV) is the activation

energy, and the T i is the temperature that is used in the step 3 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):

I(t) = I0

[

1 + (b − 1) t

τ

]−b b− 1

(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

parameters Nevertheless, the FOM value is given by the following equation:

i

[⃒⃒Y exp− Y fit

⃒

A

]

(4)

In this case Yexp regards the experimental data, Y fit is the theoretical

data resulting from the whole procedure and A is the area of the fitted

curve of the signal that is analyzed

3.3 Peak shape methods (PSM)

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 T m and the high and low

tem-peratures (T 1 and T 2 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

4 Results and discussion

Fig 2 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

Fig 6 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)

Table 1

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

un preheated 1.26 ±

0.02 1.02 ±0.04 1.27 ±0.03 1.08 ±

0.01 preaheated 1.27 ±

0.01 1.07 ±0.01 1.26 ±0.03 1.12 ±

0.03

Chen 1.00 ±

0.19 1.19 ±0.19 1.06 ±0.20 1.05 ±0.12 1.14 ±

0.15

1.11 ± 0.12 Kitis 1.03 ±

0.2 1.03 ±0.3 1.03 ±0.4 1.18 ±0.08 1.18 ±

0.09

1.18 ± 0.10

165 C1 165- 215 140- C2

210

C1 140-

165

C1 165-

215

C2 140-

210

un preheated 0.96 0.05 0.56 0.96 0.06 0.68 preheated 0.96 0.14 0.59 – 0.1 0.44

(eV)

130–280 1.25 ±

0.04 320–390 1.36 ±

0.04 440–490 1.90 ±

0.09

rate of 2 ◦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 3 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 4 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 3 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 1 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

5 Conclusions

In the present study, the methodology proposed by Polymeris et al

(2020) was used in order to generate specific experimental data for the

case of a newly developed BeO dosimeter with two main dosimetric TL

peaks In the framework of this methodology, the activation energy of

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

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