Thermally stimulated luminescence of new ZnO–CdSO4

exposed to beta radiation

C. Cruz-Vázquez a,⁎, V.R. Orante-Barrón a, H. Grijalva-Monteverde a,V.M. Castaño b, R. Bernal c

a Departamento de Investigación en Polímeros y Materiales de la Universidad de Sonora, Apartado Postal 130, Hermosillo, Sonora 83000, Méxicob Departamento de Física Aplicada y Tecnología Avanzada, Instituto de Física de la Universidad Nacional Autónoma de México, Apartado Postal 1-1010,

Querétaro, Querétaro 76000, Méxicoc Departamento de Investigación en Física de la Universidad de Sonora, Apartado Postal 5-088, Hermosillo, Sonora 83190, México

Received 17 February 2006; accepted 22 June 2006Available online 10 July 2006

Abstract

Novel microcrystalline ZnO–CdSO4 thermoluminescent phosphors were synthesized by thermal annealing of chemically-modified ZnSpowders, obtained by precipitation. Pellets exposed to beta radiation showed that the thermoluminescence response increases linearly as theradiation doses increased in the 25–300 Gy range. The glow curve exhibited two maxima centered at 112 and 216 °C. The latter displays aremarkable stability when the phosphor is stored at room temperature. The results indicate that these new ZnO–CdSO4 phosphors are promisingdetectors and dosimeters for beta radiation.© 2006 Elsevier B.V. All rights reserved.

Keywords: Thermoluminescence; ZnO; CdSO4; Dosimetry; Beta radiation

1. Introduction

Zinc oxide (ZnO) is a direct-band gap semiconductor withmany attractive properties. Its 3.4 eV band gap energy at roomtemperature may be modified by introducing impurities. Forinstance, it diminishes by Cd doping and increases when dopedwith Mg. The most common crystalline structure in ZnO ishexagonal (zincite). Among other defects existing in ZnO, thereare the interstitial Zn ions, oxygen vacancies and hydrogen [1].

ZnO exhibits striking features useful for developingcomponents for optoelectronic applications. Accordingly, thephysical properties of ZnO have been intensively investigated,focused mainly on the characterization of its optical andelectrical properties. Examples of applications include thin filmgas sensors, varistors, ultraviolet and visible lasers and solarcells components [1–4].

The thermally stimulated luminescence technique, commonlyknown as ThermoLuminescence (TL), is widely accepted as auseful and reliable technique to study defects in insulators andsemiconducting materials, but the more widely spread andsuccessful application of the TL is in the field of radiationdosimetry [5,6].

ZnO exhibits TL under irradiation by different sources andstriking radiation robustness. Moreover, ZnO is inert to environ-mental conditions, non-toxic and insoluble in water. In spite ofthese characteristics, there is not much information related to thepotential application ofZnO inTLdosimetry. The lack of interest ofZnO as dosimetric material is due perhaps to its other importantapplications, for instance in optoelectronics, and also to the lowefficiency of the TL emission reported for the samples previouslystudied [7–10].

Many thermoluminescent phosphor materials, synthetic aswell as natural, have been characterized in order to evaluate itsfeasibility as thermoluminescence dosimeters (TLD), applicablein several dosimetry areas, from X-ray medical diagnose andradiotherapy to radiological protection. A particular material may

Materials Letters 61 (2007) 1097–1100www.elsevier.com/locate/matlet

⁎ Corresponding author.E-mail address: cathy@correom.uson.mx (C. Cruz-Vázquez).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.06.055

or may not be useful for radiation dosimetry depending on thekind of radiation to be used and on the dose range values to bemeasured. Nowadays, much research work is focused on thedevelopment of new dosimetric materials applicable to thedetection and measurement of the several radiation fields.

It is well known that the properties of amaterial depend upon theprocedure followed to produce it. To growZnO is currently amajoractivity in materials chemistry. Among the methods to synthesizeZnO and other semiconductor compounds thin films on distinctsubstrates, one can mention the chemical bath deposition method(CBD), which is now established as a versatile and non-expensiveoption. When a CBD method is used, a quantity of the materialprecipitates on the bottom of the solution during the growth of thinfilms, which can be then collected as powder.

We have recently reported the synthesis and thermolumines-cence properties of new pellet-shaped ZnO nanophosphors [11],obtained by thermal annealing of ZnS powder precipitated by aCBD method [12]. They exhibited good thermoluminescenceproperties and may be considered as a promising material to beused in thermoluminescence dosimetry. The whole thermogramsof ZnO exhibit a complex structure in the form of a broad curverevealing the overlap of several TL glow peaks. It was also shownthat the TL of the chemically-synthesized ZnO phosphors is overten times greater than that of ZnO obtained from commercially-available ZnS subjected to the same thermal annealing. Previousto this work, there was no information related to the potentialapplicability of ZnO to thermoluminescence dosimetry.

Aiming to improve the TL features of the ZnO phosphorspreviously reported, we have carried out the chemical modi-fication of ZnS with a Cd salt prior to the thermal annealingprocess. In this way, we have obtained a new mixed com-position thermoluminescent phosphor, ZnO–CdSO4. Some ofthe samples were exposed to beta radiation in order to inves-tigate their thermoluminescence response.

2. Experimental

A controlled chemical deposition reaction was carried out tosynthesize ZnS powder as follows: 80 ml of a 0.1 M CS(NH2)2

(thiourea) (Aldrich, 99%) solution was added to 250 ml of an8 mM Zn(en)3SO4 solution with stirring. Then, 50 ml of a 4 MNaOH (Merck, 99%) solution was added. The reaction wasallowed to proceed at 60 °C by stirring for 10 h. White-grayishZnS powder was collected by filtration. It was washed withdeionized water and dried in vacuum. The zinc (II) complexused, Zn(en)3SO4, was synthesized by adding an aqueousethylenediamine (Aldrich, 99%) solution to an aqueousZnSO4·7H2O (J. T. Baker, 99.81%) solution in a mole ratio3:1. The colorless zinc (II) complex obtained was re-crystallizedfrom water.

Chemical modification of the ZnS powder was carried out asfollows: 0.06 g of the synthesized ZnS powder were weightedand added into 50 ml of a 1.848 mM Cd(NO3)2·4H2O (Aldrich,98%) aqueous solution and this reaction proceeded at 50 °C for24 h. Next, the product was filtered in vacuum, and washed with10 ml of deionized water and dried in vacuum during 3 h.Finally, the powder was placed in a 7 mm diameter cylindricalmold and compressed at 0.5 ton during 3 min using a hydraulicpress. With this procedure, 0.8 mm thickness pellets wereobtained. Afterwards, the pellets just obtained were subjected toa sintering process at 700 °C during 24 h under air atmosphereusing a Thermolyne 4000 furnace.

A Risø TL/OSL model TL/OSL-DA-15 unit equipped with a90Sr beta source was used to perform beta irradiations and theTL measurements. All irradiations were accomplished using a5 Gy/min dose rate at room temperature (ca. 22 °C). The TLmeasurements were carried out from room temperature (22 °C)up to 450 °C under N2 atmosphere using a heating rate of 5 °C/s.The X-ray diffraction (XRD) patterns were obtained in a RigakuGeigerflex diffractometer equipped with a graphite monochro-mator by using Cu-Kα radiation (λ=1.542 Å). Scanningelectron microscope (SEM) images and the samples

Fig. 1. Scanning electron microscopy (SEM) of a pellet-shaped sample afterbeing sintered at 700 °C during 24 h under air atmosphere. Two phases aredistinguished.

Fig. 2. X-ray diffraction patterns of ZnS (a) and ZnS:Cd (b) pellets, subjected tothermal annealing at 700 °C during 24 h under air atmosphere. The symbol ^ isused here to mark the peaks corresponding to zinc oxide (zincite, JCPDS # 36-1451), and an X indicates those peaks corresponding to cadmium sulfate(JCPDS # 14-352).

1098 C. Cruz-Vázquez et al. / Materials Letters 61 (2007) 1097–1100

composition were obtained using a JEOL JSM-5410LVapparatus equipped with an Oxford EDS analyzer, operatingat 15 keV.

3. Results and discussion

The ZnS powder color changed from white-grayish to bright yellowduring the chemical modification reaction, and to beige after thethermal annealing of the pellets. Fig. 1 shows the scanning electronmicroscopy image of a pellet-shaped sample, obtained after thermalannealing at 700 °C for 24 h under air atmosphere. To obtain themicrograph, the sample was coated with gold. Two microcrystallinesize phases can be observed in the image.

Energy-dispersive X-ray spectrometry (EDS) after the chemicalmodification and before being subjected to thermal annealing, revealeda 30:20:50 weight percent relation for Zn:S:Cd. These results suggestthat Cd is incorporated in ZnS during the ionic interchange reaction ofZnS powder with the Cd salt. After the thermal annealing, elementaryanalyses revealed a ratio of 46:5:49 wt.% for Zn:S:Cd. It can beobserved that the Cd relative weight does not meaningfully changeafter the sintering process. On the other hand, the S percent diminished

by the thermal treatment under air atmosphere, which is a consequenceof an oxidation process during the sintering.

Fig. 2 shows the X-ray (XRD) diffraction patterns of ZnS (a) and aZnS:Cd (b) pellets, both after the thermal annealing at 700 °Cduring 24 h.ZnS:Cd denotes the ZnS treated with Cd salt. As can be seen, the numberof peaks increases for the Cd treated samples. The diffraction peaks inFig. 2(a) coincides with those of ZnO,whereas the observed peaks in Fig.2(b) correspond to ZnO and CdSO4. The symbol ^ is used here to markthe peaks corresponding to zinc oxide (zincite, JCPDS # 36-1451), and anX indicates those peaks corresponding to cadmium sulfate (JCPDS # 14-352). Therefore, we have obtained a newZnO–CdSO4 phosphor after thesintering of ZnS:Cd obtained by chemical synthesis.

Fig. 3 shows the thermoluminescence glow curves of ZnO–CdSO4

samples after exposure to beta radiation in the dose range from 50 to300 Gy. Two main maxima are observed, at 112 and 216 °C,respectively. The peak at 216 °C is closer to that previously reported forZnO [11] at similar irradiation doses. A third less intense TL band isobserved between 300 and 350 °C. The shape of the whole curve isquite different to that of ZnO; ZnO exhibited a very complex glowcurve composed of several overlapped peaks, which makes verydifficult the analysis of the dosimetric behavior of the single peaks. Inthis new phosphor, on the other hand, we can identify two sets of peaks,

Fig. 3. Thermoluminescence glow curves of ZnO–CdSO4 samples after beingexposed to beta radiation in the dose range from 50 to 300 Gy.

Fig. 4. Integrated thermoluminescence as a function of the irradiation dose, inthe range from 25 to 150 Gy of beta irradiated ZnO–CdSO4 pellet-shapedsamples.

Fig. 5. Integrated TL as a function of the time interval elapsed between theirradiation and the corresponding TL readout.

Fig. 6. Thermoluminescence glow curves of 300 Gy beta irradiated ZnO–CdSO4 samples, obtained after different elapsed time intervals betweenirradiation and the corresponding thermoluminescence readout.

1099C. Cruz-Vázquez et al. / Materials Letters 61 (2007) 1097–1100

one at low temperature, with a maximum at 112 °C, a shoulder at140 °C, and the other maximum centered at 216 °C. Up to theinvestigated dose, no saturation was observed in the behavior of the TLcurves, but rather their TL intensity increases with increasing doses, asdisplayed in Fig. 3. The new ZnO–CdSO4 phosphor is 1.8 times moresensitive than the previously reported ZnO behavior. The glow curvepeaks show no shifting as irradiation dose changes, suggesting a firstorder kinetic process.

As it is pointed out in the literature, an important consideration in thechoice of a thermoluminescence detector is how stable the signal is in theenvironment in which the dosimeter is to be operated. For dosimetrypurposes, it is desirable for the detector material to be characterized by aglow curve with a peak between 200 and 250 °C, since this temperaturerange usually ensures that the trap depth is large enough for no appreciabletrap emptying to have taken place, but also it is low enough such that theinterference from the black-body background signal is negligible [13].

Fig. 4 shows the integrated thermoluminescence as a function of theirradiation dose, in the range from 25 to 150 Gy of beta irradiatedZnO–CdSO4 samples. Pre-irradiation thermal annealing was applied tothe samples previous to each irradiation: at 300 °C for 30 min, followedfor a second annealing at 200 °C during 30 min. The ZnO–CdSO4

pellets exhibit a remarkable linear dosimetric response in this doserange, in which there are no indications of response saturation. Fordosimetry applications, a linear dependence on the dose is highlydesirable [5].

Fig. 5 shows the integrated TL as a function of the time intervalelapsed between the irradiation and the corresponding TL readout. Inall cases, samples were exposed to 300 Gy of beta radiation. Theintegrated TL fades down to 70% of its value just after irradiation for2 h after which it tends to remain constant. In the same figure, the216 °C peak height is also displayed, an evidence that the initial decayof the integrated TL is primary due to the vanishing of lowertemperature peaks, rather than to the vanishing of the 216 °C peak. Thedecay curve of the integrated TL can be fitted to a simple exponentialfunction, ITL=ITLe−t /τ, where τ being the time constant, or mean timespent by a charge carrier in a localized trapping state [14]. A curvefitting using Origin 7.5 software gives τ=96.73 min. This is a verysimplified approximation for estimating the mean time correspondingto the trapping state that gives rise to the lowest temperature peak, but itis in agreement with the results displayed in Fig. 6. The figure showsthe thermoluminescence glow curves from which the data displayed inFig. 5 was obtained. Here it is observed that after 2 h (more than90 min) the peak at 112 °C completely vanishes. The apparently smalldecrease of the 216 °C TL peak could be really due to the decrease ofthe overlap with the tail of the lower temperature peaks rather than to itsown decrease.

4. Conclusions

In this work, we report the chemical synthesis of a new ZnO–CdSO4 thermoluminescent phosphor. The experimental evidencepresented shows that these novel materials exhibit very goodthermoluminescence properties, as to be considered for thedevelopment of detectors and dosimeters of beta radiation. Thesephosphors display both, a highly linear dependence of TL as afunction of irradiation dose, and a very stable peak at 216 °C.

Acknowledgements

The authors gratefully acknowledge the financial support forthis work from CONACYT (México), under Grant J35222-E,from SESIC-SEP (México) Grant PROMEP-UNISON-PTC-01-01, and from Universidad de Sonora.

References

[1] D.P. Norton, Y.W. Heo, M.P. Ivill, K. Ip, S.J. Pearton, M.F. Chisholm,T. Steiner, Materials Today 7 (2004) 34.

[2] K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1.[3] N.J. Dayan, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, Thin Solid Films 325

(1998) 254.[4] G. Kiriakidis, N. Katsarakis, J. Phys., Condens. Matter 16 (2004) S3757.[5] R. Chen, S.W.S. McKeever, Theory of Thermoluminescence and Related

Phenomena, World Scientific, Singapore, 1997.[6] S.W.S. McKeever, M. Moscovitch, P.D. Townsend, Thermoluminescence

Dosimetry Materials: Properties and Uses, Nuclear Technology Publishing,Ashford, 1995.

[7] D. De Muer, W. Maenhout-van der Vorst, Physica 39 (1) (1968) 123.[8] D. Diwan, S. Bhushan, S.P. Kathuria, Cryst. Res. Technol. 19 (9) (1984)

1265.[9] S. Bhushan, D. Diwan, S.P. Kathuria, Phys. Status Solidi, A Appl. Res. 83

(2) (1984) 605.[10] C. Coskun, D.C. Look, G.C. Farlow, J.R. Sizelove, Semicond. Sci.

Technol. 19 (2004) 752.[11] C. Cruz-Vázquez, R. Bernal, S.E. Burruel-Ibarra, H. Grijalva-Monteverde,

M. Barboza-Flores, Opt. Mater. 27 (2005) 1235.[12] C. Cruz-Vázquez, F. Rocha-Alonzo, S.E. Burruel-Ibarra, M. Barboza-Flores,

R. Bernal, M. Inoue, Appl. Phys., A Mater. Sci. Process. A79 (8) (2004)1941.

[13] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge UniversityPress, Cambridge, 1985.

[14] C. Furetta, Handbook of Thermoluminescence, World Scientific, Singapore,2003.

1100 C. Cruz-Vázquez et al. / Materials Letters 61 (2007) 1097–1100

Thermally stimulated luminescence of new ZnO prepared by a chemical

method

V. R. Orante-Barrón 1, R. Bernal 2, *, C. Cruz-Vázquez 1, F. Brown 1, H. Grijalva-Monteverde 1, V. M. Castaño 3

1 Departamento de Investigación en Polímeros y Materiales, Universidad de

Sonora, Apartado Postal 130, Hermosillo, Sonora 83000 México 2 Departamento de Investigación en Física, Universidad de Sonora, Apartado

Postal 5-088, Hermosillo, Sonora 83190 México. Tel.: +52 (662) 2592156 Ext. 135, Fax: +52 (662) 2126649.

E-mail: rbernal@cajeme.cifus.uson.mx 3 Departamento de Física Aplicada y Tecnología Avanzada, Instituto de Física

de la Universidad Nacional Autónoma de México, Apartado Postal 1-1010, Querétaro, Querétaro 76000 México

*Corresponding author.

Abstract

Novel pellet-shaped ZnO phosphors were synthesized by a chemical route.

X-ray diffraction analysis of samples annealed at 973 K during 24 h provided

evidence of the hexagonal ZnO phase. Samples exposed to beta irradiation

showed a thermoluminescence response that increases linearly as dose

increased in the 5 – 2560 Gy interval. There is no evidence of saturation in the

investigated dose range. Characteristic glow curves display

thermoluminescence emission from below 373 K up to above 673 K, with

maximum located at 523 K, a very suitable position for dosimetry applications.

The results here reported show that the novel ZnO exhibit striking properties

as detector and dosimeter of ionising radiation.

Keywords: ZnO; Thermoluminescence; Dosimetry; Beta radiation.

Biographical Notes about the authors: Rodolfo Bernal received his PhD

degree in 1999 from Centro de Investigación Científica y de Educación

Superior de Ensenada (CICESE) in the field of Materials Physics. Since 1999,

he joined to Universidad de Sonora as a researcher involved in the

development of new dosimetric phosphors. He is also a professor for materials

science subjects to graduate students.

Víctor Ramón Orante Barrón received his Ms degree in 1995 from

Universidad de Sonora in the field of Materials Science. He is a research

assistant and graduate student developing his PhD Thesis in the synthesis of

novel thermoluminescent ZnO phosphors.

Catalina Cruz Vázquez received her PhD degree in 1998 from Universidad de

Sonora, in the field of Materials Science. She is a researcher at Universidad de

Sonora in chemical synthesis and characterization of novel semiconductor and

insulator materials. She is also a professor for materials science subjects to

graduate students.

Heriberto Grijalva Monteverde received his PhD degree in 1999 from

Universidad de Sonora in the field of Materials Science. He is a researcher at

Universidad de Sonora in synthesis and characterization of novel inorganic

semiconductor materials.

Victor Manuel Castaño Meneses received his PhD degree in 1985 from

Universidad Nacional Autónoma de México (UNAM). He is CEO of Applied

Physics and Advanced Technology Department of UNAM, author of more

than four hundred articles in scientific journals, and winner of a number of

awards for contributions in science and technology. Biography included in

Who’s who in plastics?.

Francisco Brown received his PhD degree in 2002 from Universidad de

Sonora in the field of Materials Science. He is a researcher at Universidad de

Sonora in solid state chemistry. He is also a professor for materials science

subjects to graduate students.

1 Introduction

Zinc oxide (ZnO) is a direct gap semiconductor with many attractive features

for optoelectronic applications (Norton et al., 2004). On the other hand,

thermally stimulated luminescence, commonly termed thermoluminescence

(TL), it is widely accepted as an useful and reliable technique to study defects

in insulators and semiconducting materials, but its more widely spread

successful application is in the field of radiation dosimetry (McKeever, 1985;

Chen and McKeever, 1997; Furetta, 2003).

ZnO exhibits TL under irradiation by different sources and striking

radiation robustness. Moreover, ZnO is inert to environmental conditions, non-

toxic, and insoluble in water. In spite of these features, there are only few

reports concerning its potential application in radiation dosimetry. The lack of

interest in ZnO as a dosimetric phosphor is due perhaps to the other important

applications of this semiconductor, and to the low TL emission efficiency

reported for samples previously studied (De Muer and Maenhout-van der

Vorst, 1968; Diwan et al., 1984; Bhushan et al., 1984; Nikitenko et al., 2001;

Coskun et al., 2004).

Nowadays, because the increasing number of applications in science

and technology involving the use of radiations, it is necessary to carry out

research work focused to the development of new dosimetric materials tailored

for distinct fields of radiation and dose intervals. The number of materials for

high dose dosimetry (more than 100 Gy) is limited, since at such doses many

materials suffer from severe superlinearity, which can lead to an

overestimation of the delivered dose (Chen and McKeever, 1997).

The properties of a given material strongly depend upon the procedure

followed to produce it. We have recently reported the synthesis and

thermoluminescence characterization of new pellet-shaped ZnO

nanophosphors, obtained by thermal annealing of ZnS powder precipitated

during the deposit of ZnS thin films using a chemical bath deposition method.

They exhibited good TL properties to be considered as promising phosphors to

be used in TL dosimetry. For comparison, commercially available ZnS was

subjected to the same thermal treatment than synthesized ZnS. It was not

possible to fabricate compressed pellets from the commercial ZnS under the

same conditions used in the sintering process, and a very poor TL emission

compared with that of our ZnO samples was recorded. Previous to this work,

there was no information related to the potential application applicability of

ZnO to TL dosimetry (C. Cruz-Vázquez et al., 2004; C. Cruz-Vázquez et al.,

2005).

Motivated by the previous outstanding results, in this work we report

the thermoluminescence properties of novel ZnO phosphors synthesized by a

chemical route. X-ray diffraction patterns of samples annealed at 973 K during

24 h provide experimental evidence of hexagonal ZnO. Pellet-shaped samples

were studied in order to investigate their dosimetric capabilities under beta

particles exposure, and the results reveals that these new phosphors are very

suitable as detectors and TL dosimeters.

2 Experimental

A controlled chemical reaction was carried out to synthesize ZnO powder as

follows: a solution containing 100 ml of 0.16 M ZnSO4.7H2O, 100 ml of

0.16 M urea [CO(NH2)2], and 40 ml of 0.1 M NaOH aqueous solution , was

allowed to proceed at 333 K (60 °C). The experimental variables were the

NaOH concentration and the time of the reaction. The white powder obtained

after each reaction was collected by filtration. It was washed with deionized

water and dried in vacuum. Then, 0.06 g of the synthesized ZnO powder were

weighted, and next placed into a 7 mm diameter cylindrical mold, and

comprised at 0.5 ton during 3 min using a hydraulic press to make each pellet-

shaped sample. With this procedure, ≈ 0.8 mm thickness pellets were

obtained, which were subjected to thermal annealing at 973 K during 24 h

under air atmosphere using a Thermolyne 4000 furnace.

A Risø TL/OSL model TL/OSL-DA-15 unit equipped with a 90Sr beta

radiation source was used to perform beta irradiations and the TL

measurements. All irradiations were accomplished using a 5 Gy/min dose rate

at room temperature ( ≈ 295 K (22 °C)). The TL readouts were carried out

under N2 atmosphere using a heating rate of 5 K/s. The X-ray diffraction

(XRD) patterns were collected with a Rigaku Geigerflex diffractometer

equipped with a graphite monochromator by using Cu-Kα radiation (λ = 1.542

Å). The fluorescence spectra were obtained with a Perkin Elmer LS50 B

luminescence spectrometer.

3 Results and discussion

Figure 1 shows the XRD pattern of a ZnO pellet obtained as described in the

previous section, together with the reference lines of wurtzite (JCPDS # 36-

1451). It can be seen that ZnO pattern coincides with that of the cited

reference, except for the weaker peaks at angles 2θ lower than 30 degrees,

which could be associated with traces of organic impurities in the material

incorporated during the chemical reaction.

The fluorescence spectrum exhibits emission peaks at 422, 485, and

530 nm when excited with 343 nm wavelength light, as shown in figure 2. The

530 nm emission is related with oxygen vacancies (Wu et al., 2005). It is

necesary for dosimetric phosphors to exhibit emission in the visible range of

the electromagnetic spectrum, since detectors of commercially available TL

reader systems are sensitive to visible light.

Figure 3 shows some TL glow curves of samples exposed to beta

particle irradiation, in the dose range from 5 to 2560 Gy. A two steps pre-

irradiation treatment consisting of an annealing at 573 K during 30 minutes

followed for a second annealing at 473 K for 30 minutes was applied. The

absolute maximum of the TL emission is located at 523 K. One important

characteristic taken into account for a phosphor being considered suitable for

dosimetry purposes is how stable its TL is in the environment in which it is to

be operated. It is considered that a peak located between 473 and 523 K

assures such stability in normal conditions.

No saturation was observed in the behavior of the TL curves. Instead of

that, their intensities increased as exposure doses increased, as displayed in

Figure 3. No shift is observed for the TL maximum as dose increases,

suggesting that first order kinetics processes predominate. For the ZnO

obtained by annealing of ZnS previously reported, the TL maximum shifted to

higher temperatures when dose increases, and by computerized glow curve

deconvolution second order kinetics was found (Cruz-Vázquez et al., 2005;

Cruz-Vázquez et al., 2007). So, processes involved in the new ZnO seem to be

quite different to that of already reported phosphors.

Figure 4 shows the integrated TL as a function of the irradiation dose,

in the range from 5 to 2560 Gy of beta particles exposure.

Thermoluminescence as a function of dose exhibits a sublinear response in this

dose interval, in which there are no indications of response saturation. The

linear range is limited to doses below 200 Gy. For dosimetry applications, it is

a desirable feature that TL emission to be linearly proportional to irradiation

dose, but superlinear or sublinear dependences may also be useful if suitable

corrections to the estimated doses is done. The TL as a function of dose

exhibits a linear dependence for doses below 200 Gy. Medical applications are

included in this dose interval.

Figure 5 shows the integrated TL as a function of the time interval

elapsed between the irradiation and the corresponding TL readout. In all cases,

samples were exposed to 300 Gy of beta radiation. Past 2 h just after

irradiation, the integrated TL fades down to 60 % of its initial value and next it

tends to remain constant. The TL fading curve (figure 5) can be fitted to a sum

of at least two exponential decaying functions, which indicate that not only

one localized level trap is contributing to the fast initial TL decay.

4 Conclusions

In this article, the chemical synthesis of new pellet-shaped ZnO

thermoluminescent phosphors is reported. According to the glow curve

evolution as a function of dose, first order kinetic processes predominates in

the TL emission. We present experimental evidence on good features of the

synthesized ZnO to be considered useful in the manufacture of

thermoluminescence dosimeters of ionizing radiation: the temperature at which

occurs the maximum of TL (523 K) is very suitable for dosimetry purposes,

and the TL response is linear for doses below 200 Gy without saturation

clouds in the investigated dose range. More studies are required to establish

either or not this new phosphor may be used as practical thermoluminescence

dosimeter.

Acknowledgements

The authors gratefully acknowledge the financial support for this work from

CONACYT (México), under Grant J35222-E, from SESIC-SEP (México)

Grant PROMEP-UNISON-PTC-01-01, and from Universidad de Sonora.

References

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Chen, R., McKeever, S. W. S. (1997) Theory of thermoluminescence and

related phenomena, Singapore: World Scientific, p. 283.

Coskun, C., Look, D. C., Farlow, G. C., Sizelove, J. R. (2004) Semicond. Sci.

Technol., Vol. 19, p. 752.

Cruz-Vázquez, C., Rocha-Alonzo, F., Burruel-Ibarra, S. E., Barboza-Flores,

M., Bernal, R., Inoue, M. (2004) Appl. Phys. A-Mater. Sci. Process.,

Vol. 79, p. 1941.

Cruz-Vázquez, C., Bernal, R., Burruel-Ibarra, S. E., Grijalva-Monteverde, H.,

Barboza-Flores, M. (2005) Opt. Mater., Vol. 27, p. 1235.

Cruz-Vázquez, C., Burruel-Ibarra, S. E., Grijalva-Monteverde, H., Chernov,

V., Bernal, R. (2007) Radiat. Eff. Defects S., Vol. 162, p. 737.

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p. 1265.

Furetta, C. (2003) Handbook of thermoluminescence, Singapore: World

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Nikitenko, V. A., Tarkpea, K. E., Pykanov, I. V., Stoyukhin, S. G. (2001)

Journal of Applied Spectroscopy, Vol. 68, p. 502.

Norton, D. P., Heo, Y. W. , Ivill, M. P., Ip, K., Pearton, S. J., Chisholm, M. F.,

Steiner, T. (2004) Materials Today, Vol. 7, p. 34.

McKeever, S. W. S. (1985) Thermoluminescence of solids, Cambridge:

Cambridge University Press, p. 11.

Wu, L., Wu, Y., Lu, W. (2005) Physica E, Vol. 28, p. 76.

Figure Captions:

Figure 1. X-ray diffraction pattern of a ZnO pellet-shaped sample. The vertical

lines correspond to zinc oxide (zincite, JCPDS No. 36-1451).

Figure 2. Photoluminescence emission spectra using an excitation wavelength

of 343 nm and a filter of 390 nm of a ZnO pellet-shaped sample.

Figure 3. Thermoluminescence glow curves of ZnO pellet-shaped samples

after being exposed to beta radiation in the dose range from 5 to 2560 Gy.

Figure 4. Integrated thermoluminescence as a function of the irradiation dose,

in the range from 5 to 2560 Gy of beta irradiated ZnO pellet-shaped samples.

Figure 5. Integrated TL as a function of the time interval elapsed between the

irradiation and the corresponding TL readout.

Orante-Barrón et al.

10 20 30 40 50 60 70

2θ (degrees)

Figure 1

Orante-Barrón et al.

400 500 600 700 800 9000

1x107

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Figure 5

SYNTHESIS AND THERMOLUMINESCENCE OF NEW ZnO PHOSPHORS

C. Cruz-Vázquez1, H. A. Borbón-Núñez1, V. R. Orante-Barrón1, V. M. Castaño2 and R.

Bernal3, *

1Departamento de Investigación en Polímeros y Materiales, Universidad de Sonora,

Apartado Postal 130, Hermosillo, Sonora 83000 México

2Centro de Física Aplicada y Tecnología Avanzada, Instituto de Física de la

Universidad Nacional Autónoma de México, Apartado Postal 1-1010, Querétaro,

Querétaro 76000 México

3Departamento de Investigación en Física, Universidad de Sonora, Apartado Postal 5-

088, Hermosillo, Sonora 83190 México

*Corresponding autor: rbernal@gimmunison.com

ABSTRACT

In this work, the synthesis and thermoluminescence properties of new ZnO phosphors

obtained by a chemical method is presented. Some samples were exposed to beta

particle irradiation for doses in range from 50 up to 12800 Gy. The thermoluminescence

response is linear for doses below 1 kGy, with saturation clouds appearing for doses

greater than 6400 kGy. TL maximum is located above 200 °C, which suggest thermal

stability, and it shifts to lower temperatures as dose increases, indicating second order

kinetics processes. We conclude that the phosphors under study are promising to

develop dosimeters for high dose radiation dosimetry.

Keywords: ZnO, dosimetry, beta particle irradiation.

INTRODUCTION

Zinc oxide (ZnO) is a direct gap semiconductor with many attractive features for

optoelectronic applications(1). On the other hand, thermally stimulated luminescence,

commonly termed thermoluminescence (TL), it is widely accepted as an useful and

reliable technique to study defects in insulators and semiconducting materials, but its

more widely spread successful application is in the field of radiation dosimetry(2-4).

ZnO exhibits TL under irradiation by different sources and striking radiation

robustness. Moreover, ZnO is inert to environmental conditions, non-toxic, and

insoluble in water. In spite of these features, there are only few reports concerning its

potential application in radiation dosimetry. The lack of interest in ZnO as a dosimetric

phosphor is due perhaps to the other important applications of this semiconductor, and

to the low TL emission efficiency reported for samples previously studied(5-8).

Nowadays, because the increasing number of applications in science and

technology involving the use of radiations, it is necessary to carry out research work

focused to the development of new dosimetric materials tailored for distinct fields of

radiation and dose intervals. The number of materials for high dose dosimetry (more

than 100 Gy) is limited, since at such doses many materials suffer from severe

superlinearity, which can lead to an overestimation of the delivered dose(3).

The properties of a given material strongly depend upon the procedure followed

to produce it. We have recently reported the synthesis and thermoluminescence

characterization of new pellet-shaped ZnO nanophosphors, obtained by thermal

annealing of ZnS powder precipitated during the deposit of ZnS thin films using a

chemical bath deposition method. They exhibited good TL properties to be considered

as promising phosphors to be used in TL dosimetry. For comparison, commercially

available ZnS was subjected to the same thermal treatment than synthesized ZnS. It was

not possible to fabricate compressed pellets from the commercial ZnS under the same

conditions used in the sintering process, and a very poor TL emission compared with

that of our ZnO samples was recorded. Previous to this work, there was no information

related to the potential application applicability of ZnO to TL dosimetry(9,10).

Motivated by the previous outstanding results, but taking into account the fact

that the chemical bath deposition method is designed for the deposition of thin films,

and so is not efficient to obtain powder, in this work we report the thermoluminescence

properties of novel ZnO phosphors synthesized by an alternative chemical route. X-ray

diffraction patterns of samples annealed at 700 °C during 24 h provide experimental

evidence of hexagonal ZnO. Pellet-shaped samples were studied in order to investigate

their dosimetric capabilities under beta particles exposure, and the results reveals that

these new phosphors are promising materials to be used as detectors and TL dosimeters.

EXPERIMENTAL

A controlled chemical reaction was carried out to synthesize ZnO powder as follows: a

solution containing 250 ml of 16 mM ZnCl2, 80 ml of 0.1 M thiourea [CS(NH2)2], and

40 ml of 1 M NaOH aqueous solution, was allowed to proceed at 20 °C during 5.5 h

with constant stirring. The pale yellow powder obtained was collected by filtration. It

was washed with deionized water and dried in vacuum. Then, 0.06 g of the synthesized

ZnO powder were weighted, and next placed into a 7 mm diameter cylindrical mold,

and comprised at 0.5 ton during 3 min using a hydraulic press to make each pellet-

shaped sample. With this procedure, ≈ 0.8 mm thickness pellets were obtained, which

were subjected to thermal annealing at 850 °C during 24 h under air atmosphere using a

Thermolyne 4000 furnace.

A Risø TL/OSL model TL/OSL-DA-20 unit equipped with a 90Sr beta radiation

source having an activity of 0.04 Ci was used to perform beta irradiations and the TL

measurements. All irradiations were accomplished using a 5 Gy/min dose rate at room

temperature ( 22 °C). The TL readouts were carried out under N2 atmosphere using a

heating rate of 5 °C/s. The X-ray diffraction (XRD) patterns were collected with a

Rigaku Geigerflex diffractometer equipped with a graphite monochromator by using

Cu-Kα radiation (λ = 1.542 Å). Scanning electron microscope (SEM) images and the

samples composition were obtained using a JEOL JSM-5140LV scanning electron

microscope equipped with an Oxford EDS analyzer operating at 15 keV.

≈

RESULTS AND DISCUSSION Figure 1 shows the scanning electron microscopy image of a pellet-shaped ZnO sample

obtained as described in the previous section, i. e., subjecting to a sintering process

consisting of thermal annealing at 850 °C during 24 h in air atmosphere pellet made

from the chemically synthesized powder. It can be seen flake – like morphology

particles, whose dimensions varies from nano to micro dimensions. Just one phase can

be distinguished in the surface image of the sample. Energy-dispersive X-ray

Spectrometry elementary analysis revealed the ZnO composition of the samples.

Figure 2 shows the X-ray diffraction (XRD) pattern of a ZnO sample. As is

displayed, diffraction peaks coincides with that of the ZnO zincite (ICDD # 36-1451)

that are show for comparison as vertical lines. No traces of any other compounds

appears in the XRD data. The followed procedure lead to good degree of cristallinity,

which is known to be very important for the thermoluminescence (TL) phenomenom.

Figure 3 shows the TL glow curves of pellet-shaped ZnO samples after being

exposed to beta particle irradiation in the dose range from 50 up to 6400 Gy. A TL

Maximum located at temperature higher than 200 °C is observed. Normally, it is

expected that TL emission above 200 °C is stable if an irradiated sample is stored under

standard environmental conditions, since the localized trapping states are depth enough

to avoid the thermal releasing of trapped charges. As irradiation dose increases, the

intensity of the whole glow curve grows, with no clouds of saturation for the doses

displayed in Figure 3. It can be seen a shift of the TL maximum to lower temperatures

when irradiation doses are increased, which indicate that second order kinetics

processes are involved in the TL emission. That second order kinetics is present in the

TL of ZnO obtained by other chemical routes is documented(11,12).

Figure 4 shows the integrated TL (ITL) as a function of the irradiation dose, in

the range from 25 up to 150 Gy of beta irradiated ZnO samples. No pre-irradition

thermal annealing was applied to the samples before each irradiation. As can be seen,

the synthesized ZnO phosphors exhibit a TL response that increases as a function of

dose, in which there are no indications of response saturation. It should be noted from

Figure 5, that a linear response is observed for doses up to 1 kGy, and so the linear

response range covers the doses delivered in radiotheraphy, for instance. On the other

hand, since no saturation of the TL response as function of dose occurs for doses up to 6

kGy, the new ZnO here reported may be useful – with the suitable calibration – for high

dose radiation dosimetry.

For practical use, it is very important to know how stable (or unstable) the TL

signal is in the environmental in which the dosimeter is to be operated. In Figure 6 is

shown the ITL of samples exposed to 800 Gy of beta particle irradiation, as a function

of the time interval elapsed between irradiation and the corresponding TL readout.

There is not a meaningful vanishing of the TL for the longer time tested, 48 h.

CONCLUSIONS

In this work, we report on a method to obtain new pellet-shaped ZnO phosphors, as well

as their thermoluminescence properties after being exposed to beta particle irradiation in

the dose range from 50 to 6400 Gy. A maximum that shifts to lower temperatures when

dose increases is observed above to 200 °C, position considered suitable for radiation

dosimetry applications. The maximum’s shift is indicative of second order kinetics TL

processes. The linear range of the TL as a function of dose is around 1 kGy, with no

saturation for doses below 6 kGy. The new ZnO phosphor is a promising phosphor as

detector and thermoluminescence dosimeter.

REFERENCES

1. Norton, D. P., Heo, Y. W., Ivill, M. P., Ip, K., Pearton, S. J., Chisholm, M. F. and

Steiner, T. ZnO: growth, doping & processing. Materials Today 7 (6), 34-40 (2004)

2. McKeever, S. W. S. Thermoluminescence of solids. (Cambridge: Cambridge

University Press) (1985) ISBN: 0-521-36811-1.

3. Chen, R. and McKeever, S. W. S. Theory of thermoluminescence and related

phenomena. (Singapore: World Scientific) (1997) ISBN: 981-02-2295-5.

4. Furetta, C. Handbook of thermoluminescence. (Singapore: World Scientific) (2003)

ISBN: 981-238-240-2.

5. De Muer, D. and Maenhout-van der Vorst, W. Thermoluminescence of ZnO

powder. Physica 39, 123-132 (1968).

6. Diwan, D., Bhushan, S. and Khathuria, S. P. Cryst. Res. Technol. 19, 1265 (1984)

7. Nikitenko, V. A., Tarkpea, K. E., Pykanov, I. V. and Stoyukhin, S. G. EPR and

thermoluminescence in ZnO single crystals with anionic vacancies. Journal of

Applied Spectroscopy 68, 502-507 (2001).

8. Coskun, C., Look, D. C., Farlow, G. C. and Sizelove, J. R. Radiation hardness of

ZnO at low temperatures. Semicond. Sci. Technol. 19, 752-754 (2004).

9. Cruz-Vázquez, C., Rocha-Alonzo, F., Burruel-Ibarra, S. E., Barboza-Flores, M.,

Bernal, R. and Inoue, M. A new chemical bath deposition method for fabricating

ZnS, Zn(OH)2, and ZnO thin films, and the optical and structural characterization

of these materials. Appl. Phys. A – Mater. Sci. Process. 79, 1941-1945 (2004).

10. Cruz-Vázquez, C., Bernal, R., Burruel-Ibarra, S. E., Grijalva-Monteverde, H. and

Barboza-Flores, M. Thermoluminescence properties of new ZnO nanophosphors

exposed to beta irradiation. Opt. Mater. 27, 1235-1239 (2005).

11. Pal, U., Meléndrez, R., Chernov, V. and Barboza-Flores, M. Thermoluminescence

properties of ZnO and ZnO:Yb nanophosphors. Appl. Phys. Lett. 89, 183118-

1,183118-3 (2006).

12. Cruz-Vázquez, C., Burruel-Ibarra, S. E., Grijalva-Monteverde, H., Chernov, V. and

Bernal, R. Thermally and optically stimulated luminescence of new ZnO

nanophosphors exposed to beta particle irradiation. Radiat. Eff. Defect. S. 162,

737-743 (2007).

FIGURE CAPTIONS:

Figure 1. Scanning electron microscopy image of the surface of a pellet-shaped ZnO

sample.

Figure 2. X-ray diffraction pattern of the ZnO pellet-shaped samples investigated in this

work. Vertical lines corresponding to ZnO zincite (ICDD # 36-1451) are including for

comparison.

Figure 3. Thermoluminescence glow curves of ZnO pellet-shaped samples after being

exposed to beta particle irradiation in the dose range from 50 up to 6400 Gy.

Figure 4. Integrated thermoluminescence (ITL) as a function of dose, of Zno pellet-

shaped samples, for doses up to 12.8 kGy of beta particle irradiation.

Figure 5. Integrated thermoluminescence (ITL) as a function of dose, of ZnO pellet-

shaped samples, for doses from 50 up to 800 Gy of beta particle irradiation.

Figure 6. Integrated thermoluminescence (ITL) of pellet-shaped ZnO phosphors, as a

function of the time interval elapsed between irradiation and the corresponding

thermoluminescence readout. I all cases, the delivered dose was 800 Gy of beta particle

irradiation.

Figure 1. Cruz-Vázquez et al.

5 10 15 20 25 30 35 40 45 50 55 60 65 70

2θ (grados)

Figure 2. Cruz-Vázquez et al.

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Figure 3. Cruz-Vázquez et al.

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Figure 4. Cruz-Vázquez et al.

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Figure 5. Cruz-Vázquez et al.

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