Journal of Luminescence 139 (2013) 84–90

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Journal of Luminescence

0022-23

http://d

n Corr

E-m

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

Synthesis, thermoluminescence and dosimetric properties of La-dopedzinc borates

Nil Kucuk a,n, Ilker Kucuk a, Merve Cakir a, Sule Kaya Keles b

a Physics Department, Faculty of Arts and Sciences, Uludag University, Gorukle Campus, 16059 Bursa, Turkeyb Institute of Nuclear Science, Ankara University, Tandogan Campus, 06550 Ankara, Turkey

a r t i c l e i n f o

Article history:

Received 11 November 2012

Received in revised form

6 February 2013

Accepted 12 February 2013Available online 20 February 2013

Keywords:

Thermoluminescence

Dosimetry

Dose response

Zinc borate

Lantanium

Synthesis

13/$ - see front matter & 2013 Elsevier B.V. A

x.doi.org/10.1016/j.jlumin.2013.02.027

esponding author. Tel.: þ90 2242941705; fa

ail address: nilkoc@uludag.edu.tr (N. Kucuk).

a b s t r a c t

Dependence on the dose of beta radiation of undoped and La-doped ZnB2O4 powder samples at 1%, 2%,

3%, 4%, 5% and 10% (by weight) was investigated by thermoluminescence technique. Powder samples

were synthesized by the nitric acid method using the starting oxides [zinc oxide (ZnO), boric acid

(H3BO3) and doped element oxide (La2O3)]. The samples were characterized by X-ray diffraction (XRD).

The thermoluminescence (TL) properties of the powder samples were measured with Risø TL/OSL DA-

20 reader. TL glow curves were obtained with heating at a constant heating rate of 5 1C/s up to 450 1C.

The dose response curves of the powder samples exposed to 90Sr beta radiation (40 mCi) were obtained

in the dose range from 143 mGy to 60 Gy. Dose responses and minimum detectable dose (MDD) values

for increasing radiation doses of the powder samples were determined. The dose responses of all the

samples have shown a quite linear response to beta radiation. MDD value of 10% La-doped ZnB2O4

powder sample which shows a high temperature peak at 207 1C was determined as 4 mGy. MDD values

for 1%, 2%, 3%, 4% and 5% La-doped ZnB2O4 powder samples were also determined as 10 mGy, 50 mGy,

10 mGy, 10 mGy and 30 mGy, respectively. Luminescence intensities of the powder samples were

shown to be likely to be used for low radiation doses.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

The increasing use of thermoluminescence dosimeter (TLD)with application in areas such as clinical, personal and environ-mental monitoring of ionizing radiation has motivated researchon new materials with adequate dosimetric properties [1].Borates are attractive candidates in TLD for the quantitativemeasurement of radiation dose. The thermoluminescence (TL) ofdoped borates has been extensively investigated because of theirnear tissue equivalent absorption coefficient [2–6], very low cost,lower synthesis temperature, good thermal stability and mechan-ical properties, higher sensitivity and relatively easy preparation[7–12]. Therefore they are worth considering for personal dosi-metry. The sensitivity and thermal stability of borates vary widelyand depend not only on the starting materials, but also on thepreparation method. This situation led to investigations into themechanisms involved in thermoluminescence [13,14], and intohow they depend on the synthesis methods [15–17].

Zinc borate (ZnB2O4) is a boron-based inorganic materialwidely used as flame retardant, antibacterial and additive toprotect wood products above ground from insect and fungal

ll rights reserved.

x: þ90 224 2941899.

attacks [18]. It is a white crystalline or amorphous powderinsoluble in water. Its toxicity is low. Its melting point is 980 1C.Zinc borate can be isolated as crystalline material in various formshaving different chemical compositions and structures [19].

Some researchers have made studies on photoluminescence,thermoluminescence and dosimetry properties of doped zincborate [20–22]. To our knowledge, the luminescence and dosi-metric properties of La-doped ZnB2O4 powder samples by betairradiation have not been reported in the literature so far. In thispaper, we first report the thermoluminescence and dosimetricproperties by beta irradiation of undoped and various La-dopedZnB2O4 powder samples (ZnB2O4, Zn0.99La0.01B2O4, Zn0.98La0.02

B2O4, Zn0.97La0.03B2O4, Zn0.96La0.04B2O4, Zn0.95La0.05B2O4 and Zn0.9La0.1

B2O4) by the thermoluminescence technique.

2. Experimental details

2.1. Procedure for the synthesis of ZnB2O4

Powder samples of undoped and La-doped ZnB2O4 at 1%, 2%,3%, 4%, 5% and 10% (by weight) were prepared by the nitricacid method. This method is relatively easy and cheap. ZnB2O4

powder samples were synthesized using zinc oxide (ZnO, with aminimum purity 99.99%) and boric acid (H3BO3, with a purity of

Fig. 1. The X-ray powder diffraction patterns of (a) ZnB2O4, (b) Zn0.99La0.01B2O4,

(c) Zn0.98La0.02B2O4, (d) Zn0.97La0.03B2O4, (e) Zn0.96La0.04B2O4, (f) Zn0.95La0.05B2O4

and (g) Zn0.9La0.1B2O4 sintered at 900 1C; ( ) H3BO3, ( ) B2O3. The standard XRD

pattern of ZnB2O4 is taken from JCPDS card no. 39-1126.

N. Kucuk et al. / Journal of Luminescence 139 (2013) 84–90 85

99.99%). Appropriate amounts of ZnO and H3BO3 powders wereseparately weighted to prepare zinc borate. Starting materialswere mixed, while heating at 80 1C, in 1 M nitric acid solution(HNO3, standard solution) by using magnetic stirrer. All theexperiments were carried out in a glass beaker of 250 ml volume.For 5 g of initial powders, 80 ml acid was used. During thisprocess, all oxides and boric acid were converted into the metallicnitrates [i.e. Zn(NO3)2, B(NO3)3 and La(NO3)3] through the reac-tions below. La-doped ZnB2O4 powder samples were also pre-pared in a similar manner by taking the starting material instoichiometric ratio and adding La2O3 to the mixture.

ZnOþ2HNO3-ZnðNO3Þ2þH2O ð1Þ

H3BO3þ3HNO3-BðNO3Þ3þ3H2O ð2Þ

12 La2O3þ3HNO3-LaðNO3Þ3þ

32H2O ð3Þ

Mixing was continued until dry precursor was obtained. Theprecursor was ground in an agata mortar for about 15 min. Then,it was calcined at 450 1C for 5 h to remove possible organiccompounds. Gasses such as NO and NO2 were also released upto this temperature and metallic nitrates were converted into theoxides again [23]. Finally, the precursor was pelletized under thepressure of 3 ton before annealing at temperatures from 700 1C to900 1C for 2 h which leads to the formation of zinc borates. Afterannealing, the powder samples were cooled to room temperatureand triturated in an agata mortar.

2.2. Measurements

The structure analysis of all the samples and the effect ofdoping on the structure of undoped ZnB2O4 powder sample werestudied by X-ray powder diffraction. The X-ray diffraction (XRD)measurements were taken at the interval of Bragg angle 2y from(101o2yo901), using a Rigaku Ultima IV X-ray diffractometer at40 kV at 3 deg. min�1 and 30 mA with Cu-Ka (l¼1.5405 A)radiation.

The TL glow curves of undoped and various La-doped ZnB2O4

powder samples were recorded with Risø TL/OSL DA-20 readerusing Corning 7/59 and Schott BG/39 optical filters in nitrogenatmosphere. The measurements were carried out on 10 mgsamples. During measurements of all the powder samples, pre-heating process up to 140 1C for the heating rate of 2 1C/s and thereading process up to 450 1C for the heating rate of 5 1C/s wereused. TL intensity for each glow peak was calculated by taking thearea under dosimetric peak. The dose responses of all the powdersamples exposed to 90Sr beta radiation (40 mCi) were obtained inthe dose range from 143 mGy to 60 Gy.

3. Results and discussion

3.1. X-ray powder diffraction

In this work the ZnB2O4 phase formation condition has beeninvestigated by XRD. Fig. 1 shows the XRD patterns of ZnB2O4,Zn0.99La0.01B2O4, Zn0.98La0.02B2O4, Zn0.97La0.03B2O4, Zn0.96La0.04

B2O4, Zn0.95La0.05B2O4 and Zn0.9La0.1B2O4 powder samples sinteredat 900 1C. These XRD patterns were found to be consistent with thatreported in JCPDS card no. 39-1126. The XRD data shown in Fig. 1indicated that the La-doped ZnB2O4 powder samples were almostpure phase with high crystallinity. It is known that the ionic radii (r)of Zn2þ and B3þ are 0.60 A and 0.21 A [24], respectively. As theionic radius of B3þ is too small, it is difficult for these elements toreplace B3þ in ZnB2O4. Hence, in this study, it is believed that theZn2þ sites are substituted by La3þ in the lattice. The decrease in

crystallinity of La-doped ZnB2O4 with increasing La dopant contentwas observed from the XRD patterns, which can be attributed to thevariation of charges of dopants that results in the defect formationin the lattice. Little impurity phases, such as H3BO3 and B2O3 wereobserved at 2y¼16.301, 21.541, 28.381, 31.181 and 35.661. The X-raydiffraction patterns of Zn1�xLaxB2O4 (x¼0 and x¼0.01) powderssintered at 450 1C for 5 h and 700, 800, 900 1C for 2 h in air are alsoshown in Fig. 2a and b, respectively. It is clearly seen that thefraction of the zinc borate phase is considerably higher in theLa-doped samples. At 900 1C, for example, ZnB2O4 is the major phasein the 1% La-doped sample while a fraction of impurity phasesappears in the undoped specimen.

3.2. Thermoluminescence of undoped and La-doped ZnB2O4

Specification of thermoluminescence characteristics of a can-didate for TL dosimetry is an important step in the production of aTL. Shape and structure of the glow curves, linear dose response,minimum detectable dose, sensitivity, the occurrence tempera-ture and intensity of TL peaks, fading, sunlight susceptibility, andvulnerability against humidity should be studied [25]. In thisstudy the TL properties of undoped and various La-doped ZnB2O4

powder samples, namely, glow curves, linear dose response andminimum detectable dose have been determined.

3.2.1. TL glow curves

The glow curves are particularly important since they are themain indicators of whether a material can be used for TLdosimetry purposes or not. Generally it is desired that the glowcurve gives a simple, if possible single, peak at around 200 1C [25].Glow curve shapes and peak temperatures are also affected by thesynthesis and doping methods [26].

In this study it has been observed that the main TL peak is ataround 200 1C on the glow curves of all the powder samples. Fig. 3shows a set of the selected TL glow curves which were obtainedfrom ZnB2O4 powder samples of undoped and La-doped withdifferent amounts (1%, 4% and 10%) sintered at 900 1C for143 mGy, 715 mGy, 1.43 Gy and 15 Gy beta radiation doses,respectively.

The glow curves of undoped and La-doped ZnB2O4 at 1%, 4% and10% powder samples exhibit a very well defined main peak havingthe maximum temperature at around 191 1C (Fig. 3a), 201 1C with

Fig. 2. (a) The X-ray diffraction patterns of ZnB2O4 powders prepared at 450, 700, 800 and 900 1C. (b) The X-ray diffraction patterns of 1% La-doped zinc borate powders

synthesized at 450, 700, 800 and 900 1C. Peaks corresponding to impurity phases are also marked.

N. Kucuk et al. / Journal of Luminescence 139 (2013) 84–9086

low intensity shoulders at about 325 1C (Fig. 3b), 200 1C (Fig. 3c)and 207 1C with low intensity shoulders at about 350 1C (Fig. 3d).

The glow peak intensity of the main peak for undoped andLa-doped ZnB2O4 powder samples depends on the applied dose.It increases with increasing dose levels. The glow peak intensitiesof undoped and 1%, 4%, 10% La-doped ZnB2O4 powder samples forthe 15 Gy beta dose increased by approximately 12.5, 20, 23 and46 times according to the 1.43 Gy beta dose.

Fig. 4 shows the TL glow curves of undoped and variousLa-doped ZnB2O4 powder samples exposed to the same dose(1.43 Gy). It can be seen that the shape of the TL peaks in theglow curve structures of all produced samples remain almostconstant, showing a concentration-independent character. Theglow peak intensity of the main peak for La-doped ZnB2O4

powder samples depends on the dope amount. Their intensitiesand peak temperatures change with the amount of dopantconcentration of La (see the inset in Fig. 4). On the other hand,the peak temperature of main glow peak was slightly shifted withincreasing the level of La-concentration. From Fig. 4, it is alsoobvious that the TL intensity of 3% La-doped ZnB2O4 powdersample is much stronger than that of the other samples. Fig. 5shows the glow cures of the 3% La-substituted samples synthe-sized at 450, 800 and 900 1C for 715 mGy, 1.43 Gy and 15 Gybeta radiation doses, respectively. The maximum TL intensity

appeared for 715 mGy in the 3% La-substituted sample sintered at800 1C. On the contrary, the maximum TL intensity was observedfor 1.43 Gy and 15 Gy in the sample sintered at 900 1C. This isattributed to the appearance of the small fractions of impurityphases in the samples sintered at 800 1C as seen in Fig. 6 whichshows the XRD patterns of the 3% La-doped ZnB2O4 samplesprepared by oxidation in nitric acid method and produced at 450,800 and 900 1C.

The increase in the intensities of the glow curves withincreasing dose can be understood by the fact that more andmore traps responsible for these glow peaks are getting filled withincreasing irradiation dose and subsequently these traps releasetheir charge carriers on thermal stimulation to finally recombinewith their counterparts, thus giving rise to different glow peaks[27]. Generally the undoped sample, whose emission can berelated to intrinsic defects or to some unwanted impurity in thestarting powder, exhibits a poor TL efficiency than the dopedsamples. In such cases, TL response is found to be insufficient forreliable studies in the low-dose-range measurements. On theother hand, as expected, the TL signal steadily increased afterincorporation of rear earth metal impurities such as La impurities.Moreover, the glow curve shapes of undoped samples can bechanged after the doping of impurities. If the glow curve structureof undoped sample is changed after the doping, it indicates that

Fig. 3. Some of the selected TL glow curves of (a) ZnB2O4, (b) 1% La-doped ZnB2O4, (c) 4% La-doped ZnB2O4 and (d) 10% La-doped ZnB2O4 sintered at 900 1C after

different doses.

Fig. 4. TL glow curves of undoped and various La-doped ZnB2O4 powder samples exposed to the same dose (1.43 Gy).

N. Kucuk et al. / Journal of Luminescence 139 (2013) 84–90 87

there are interactions between intrinsic defects and doped impu-rities. In our results, the glow curve shape of ZnB2O4 is highlychanged after the doping. This result indicates that there arestrong interactions between intrinsic defects and the dopant suchas La ion.

The magnetic dipole transition was permitted according to theJudd–Ofelt theory. However, the electric dipole transition waspermitted only when the Lanthanum ion occupied a site withoutan inversion center. The intensity was significantly affected by thesymmetry in local environments around La3þ ion, as described for

Eu3þ ion in Ref. [22]. If the La3þ ions occupied an inversionsymmetry site in the crystal lattice, the magnetic dipole transitionwas the dominant transition. On the contrary, the La3þ ionsoccupied without an inversion center, the electric dipole transi-tion was the dominant transition. From these considerations,the electric dipole transition/the magnetic dipole transitionratio gave a measure of distortion from the inversion symmetryof the La3þ ion local environment. The results showed that theTL intensity of La3þ was greatly enhanced when it was added tozinc borate.

Fig. 5. TL glow curves of the 3% La-substituted samples synthesized at different sintering temperatures for (a) 715 mGy, (b) 1.43 Gy, and (c) 15 Gy.

Fig. 6. XRD patterns of the 3% La-doped ZnB2O4 samples prepared by oxidation in nitric acid method and produced at 450, 800 and 900 1C. Peaks corresponding to

impurity phases are also marked.

N. Kucuk et al. / Journal of Luminescence 139 (2013) 84–9088

3.2.2. Linear dose response

Linear dose response shows that the response displayed by thematerial is directly proportional to the dose of radiation that it isexposed to [25]. An important property of a TL dosimeter materialis that it exhibits a linear relation between TL intensity andabsorbed dose.

It can be assumed that the TL dose responses follow anequation of the form: TL¼a(Dose)b. Taking logarithms of bothsides of this equation, [log(TL)¼ log (a)þb log(Dose)] can beobtained. The slope (b¼ linearity factor) will be found with alinear fit of log(TL) versus log(Dose). The dose response is linearfor b¼1. It is also sublinear for bo1 and superlinear for b41. In

N. Kucuk et al. / Journal of Luminescence 139 (2013) 84–90 89

this study the dose response curves of all the powder sampleswere obtained using beta radiation in the dose range from143 mGy to 60 Gy. Fig. 7 shows a set of the logarithmic integratedTL as a function of the logarithmic dose response of 2%, 3% and 5%La-doped ZnB2O4 powder samples. As can be seen from thefigures almost all the samples have shown a quite linear responseto beta radiation. Table 1 also lists integral area ranges, Tmax,background counts and the values of b with its standard error(Db) corresponding to each peak. We obtained excellent line andvalues of b around 1 which means that the dose response is linear.

Fig. 7. The dose response curves of (a1)–(a2) 2% La-doped ZnB2O4, (b1)–(b2) 3% La-dope

dose range from 143 mGy to 60 Gy.

3.2.3. Minimum detectable dose (MDD)

MDD values of all the powder samples were determined as doseof 3 times the corresponding value of background count. Theseresults are given in Table 1. As seen from Table 1, MDD value of 10%La-doped ZnB2O4 powder sample which shows a high temperaturepeak at 207 1C was determined as 4 mGy. MDD values for 1%, 2%,3%, 4% and 5% La-doped ZnB2O4 powder samples were alsodetermined as 10 mGy, 50 mGy, 10 mGy, 10 mGy and 30 mGy,respectively. Hence luminescence intensities of the samples wereshown to be likely to be used for low radiation doses.

d ZnB2O4 and (c1)–(c2) 5% La-doped ZnB2O4 after exposure to beta radiation in the

Table 1Dose responses and MDD values of undoped and La-doped ZnB2O4 powder samples.

Sample Integral area range (1C) Tmax (1C) Background counts b7Db I60/I30 MDD (Gy)

ZnB2O4 Peak 1 (100–260) 191 5630 1.15970.038 2.6 0.13

Zn0.99La0.01B2O4 Peak 1 (100–250) 201 2118 1.32670.014 2.2 0.01

Peak 2 (260–280) 325 5699 1.08770.006 2.2 0.76

Zn0.98La0.02B2O4 Peak 1 (100–240) 196 5661 1.33270.031 2.3 0.05

Peak 2 (246–400) 310 8509 1.09070.036 2.2 1.02

Zn0.97La0.03B2O4 Peak 1 (100–250) 210 3655 1.25870.015 2.0 0.01

Peak 2 (260–380) 310 12435 1.09970.009 2.3 0.64

Zn0.96La0.04B2O4 Peak 1a (100–300) 200 2463 1.35970.021 2.5 0.01

Zn0.95La0.05B2O4 Peak 1a (100–250) 200 1369 1.28570.019 2.3 0.03

Peak 2 (280–400) 340 12706 0.97270.031 2.1 3.90

Zn0.9La0.1B2O4 Peak 1 (100–280) 207 1659 1.51670.039 2.4 0.004

Peak 2 (285–450) 350 1659 0.90470.061 2.1 5.02

a The total area of nested hills has been taken into account.

N. Kucuk et al. / Journal of Luminescence 139 (2013) 84–9090

Linearity of the samples to high radiation doses were alsoobtained from the division of integrated TL intensities observedfor 60 Gy to the integrated TL intensities observed for 30 Gy (i.e.I60/I30). As seen from Table 1, this value should be close to 2. It canbe said that it is supralinear for greater values than 2, andsublinear for lesser values than 2.

4. Conclusions

Undoped and La-doped ZnB2O4 powder samples at 1%, 2%, 3%,4%, 5% and 10% (by weight) were prepared by the nitric acidmethod. TL properties and dependence on the dose of betaradiation of all the powder samples have been investigated bythe thermoluminescence technique. In this research it has beenfound that the sharp main peak is around 200 1C in TL glow curvesfor all the powder samples. It has been found that the TL intensityof ZnB2O4: 0.03La powder sample is much stronger than that ofthe other samples. It has been observed that the shapes of theglow curves are dose dependent. The dose responses of all thepowder samples to beta radiation in the dose range from143 mGy to 60 Gy showed good linearity. MDD value ofZn0.90La0.10B2O4 sample was determined as 4 mGy. MDD valuesfor Zn0.99La0.01B2O4, Zn0.98La0.02B2O4, Zn0.97La0.03B2O4, Zn0.96

La0.04B2O4 and Zn0.95La0.05B2O4 powder samples were also deter-mined as 10 mGy, 50 mGy, 10 mGy, 10 mGy and 30 mGy, respec-tively. Luminescence intensities and MDD values of the sampleswere shown to be likely to be used for low radiation doses.Therefore, ZnB2O4: La sample can be a promising TLD material fordosimetric applications. However, the present studies are notenough to propose any concrete and meaningful TL mechanism inthese samples. Hence, further studies to understand the TLmechanism for these phosphors are under progress. Investigationstudies of other luminescence properties of the samples are alsoin progress.

Acknowledgments

This work was supported by the Commission of ScientificResearch Projects of Uludag University, Project number UAP(F)-

2010/28. We would like to express our sincere gratitude to Prof.Dr. H.Y. Goksu for guidance. We are thankful to the Middle EastTechnical University Central Laboratory (ODTU) for the XRDanalysis.

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