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SYNTHESIS of LANTHANUM ALUMINATE DOPED with CARBON (Laalo3: C) by SOLID STATE REACTION for APPLICATION in THERMOLUMINESCENT DOSIMETRY

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SYNTHESIS of LANTHANUM ALUMINATE DOPED with CARBON (Laalo3: C) by SOLID STATE REACTION for APPLICATION in THERMOLUMINESCENT DOSIMETRY 2013 International Nuclear Atlantic Conference - INAC 2013 Recife, PE, Brazil, November 24-29, 2013 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-05-2 SYNTHESIS OF LANTHANUM ALUMINATE DOPED WITH CARBON (LaAlO3: C) BY SOLID STATE REACTION FOR APPLICATION IN THERMOLUMINESCENT DOSIMETRY Alves, N.1; Ferraz, W. B.1; Faria, L. O.1 1 Centro de Desenvolvimento da Tecnologia Nuclear (CDTN / CNEN - MG) Av. Antônio Carlos 6627, 941 30270-901. Belo Horizonte, MG [email protected]; [email protected]; [email protected] ABSTRACT One of the best TL materials ever discovered is α-Al2O3 single crystals doped with carbon atoms. Recently, LaAlO3:Ce,Dy crystals have been proposed to be used as a TL dosimeter for UV fields. Its crystalline structure is very similar to the Al2O3 ones. Thus, in this work, we explore the idea of introducing carbon atoms in the polycrystalline lattice of LaAlO3, in order to investigate its TL properties for X and gamma fields. In this study, the lanthanum aluminate was synthesized using the solid state reaction method by mixing aluminum and lanthanum oxide in a 1:1 molar ratio. The mixture with different concentrations of graphite was then first calcinated in the air and then sintered under reducing atmosphere at high temperature. The resulting powder was characterized by X-ray diffraction, confirming the attainment of the desired phase (LaAlO3). Among the different doping levels investigated all showed thermoluminescence, including the undoped LaAlO3. The best TL responses were obtained for pristine compositions and those doped with 0.5 at.% of carbon. An additional evaluation evidenced the presence of OSL peaks for samples doped with 2.0 at.% of carbon. The synthesis method has revealed to be effective for forming the lanthanum aluminate, with significant TL sensitivity. Because of these results, we believe that this new TL material has great potential for applications in radiation dosimetry. 1. INTRODUCTION Thermoluminescence (TL) is one of thermally stimulated phenomena of condensed matter. It can be induced by ionizing radiation and materials that exhibit these properties find large applications in radiation dosimetry. Normally the thermoluminescence is obtained by doping the crystal lattice of ionic crystals with impure atoms that in turn behave as electronic traps that, under exposure to ionizing radiation, capture some free electrons, releasing them under constant heating. One analogous phenomenon is the optically stimulated luminescence (OSL), obtained by using low energy photons instead of heat. The main goal of dosimetric devices is to determine the quantity of energy per unit mass of material (dose) that has been absorbed during the irradiation, thereby leading to applications such as personal and environmental dosimetry, diagnostic imaging and computed radiography [1]. Lanthanum aluminate is a ceramic compound that has a perovskite like crystalline structure. Materials with these properties are utilized for several technological applications due to their special electrical and magnetic properties. These include insulators and superconductors, ferroelectric crystals and the use as a substrate for superconductors of high critical temperature, among others [2; 3]. The perovskite-type ceramics (ABO3) have received much attention because this unique crystalline structure can accommodate a large distribution range of cation sizes in their sublattices. Oxygen vacancies can be generated to compensate the charge of substituting ions, enabling the aliovalent cations to be distributed in both A and B site cation sublattices. The Oxygen vacancies concentration are directly related to the enhancement of TL sensibility [4; 5]. The main applications of lanthanum aluminate crystals are mostly related to their electrical properties. However, the first investigation concerned to the TL properties of LaAlO3 applied to radiation dosimetry has been recently reported [6]. The investigation revealed that LaAlO3:Ce,Dy crystals present high TL output for UV radiation fields, comparable to the TL output of the best dosimeters ever reported in literature, i.e. Al2O3:C and ZrO2 crystals. We remark that the doping of alumina with carbon atoms has resulted in excellent TL and OSL output properties for UV, X and gamma radiation fields and that the crystalline structure of both LaAlO3 and Al2O3 are very similar. Thus, we think that the doping of LaAlO3 with carbon atoms could result in the aggregation of good TL and/or OSL properties to this material, for X and gamma field[6]. In the present study we investigate the TL and OSL properties of polycrystalline LaAlO3 doped with different concentrations of carbon atoms. The samples were synthesized by the solid state reaction method and sintered under high reducing atmosphere. 2. EXPERIMENTAL PROCEDURE 2.1. Lanthanum aluminate Polycrystalline LaAlO3 was synthesized by solid state reaction method by mixing equimolar ratios of aluminum oxide (Vetec, 99.99%) and lanthanum oxide (Alfa, 99.98%). The powder was weighed and then manually grinded in an agate mortar with isopropyl alcohol. The mixed powders were dried at 100 °C for 24 h and then sieved through an 80-mesh sieve. The powder mixture was calcinated at 1600 °C for 2 h in the air. The resulting powder was reground using agate mortar. Stoichiometric amounts of carbon were added by homogenization in lanthanum aluminate using agate mortar, giving an appropriate molar ratio. The doping percentages were in range of 0.1 – 5.0 at.%. The powder was then pressed into circular disks of 11 mm diameter and 3 mm thickness using a uniaxial isostatic pressing under a pressure of 600 MPa. The samples were then sintered at 1770 °C for 2 h in hydrogen atmosphere. 2.2 X-ray diffraction XRD was conducted on raw materials to confirm if were without an additional phase and crystalline structure of the powders. In order to examine the nucleation of LaAlO3 phase, the crystal structure of the doped and undoped samples were analyzed using Regaku D/Max ÚLTIMA. The CuKα radiation was used and the scanning rate was set at 0,02 °/s at a 2θ range between 4 and 80°. 2.3 Absorption spectra INAC 2013, Recife, PE, Brazil. The absorption spectra of Carbon doped LaAlO3 and the undoped one were measured via a UV – 2401pc type ultraviolet visible recording SHIMADZU spectrophotometer, in the 190-900 nm wavelength range. All measurements were performed at room temperature. The measurement of optically stimulated luminescence was performed by exciting the samples with a 405nm source LED and collecting the PL emission with a USB2000 mini spectrometer manufactured by Ocean Optics. 2.4 Irradiation procedure The gamma irradiation was performed using a 137Cs irradiator, manufactured by Steuerungstechnik & Strahlenschutz GmbH, STS Irradiators, type OB85/3. The UV irradiation was performed using the light source of the OLIS Cary- 14 spectrophotometer adjusted to 230 nm, in order to investigate the TL sensitivity of LaAlO3 at this wavelength. 2.5 Thermoluminescent Analysis The TL glow curves of LaAlO3 were taken using a Harshaw TLD-4500 TL reader. The readings were performed with heating rate of 15 °C/s, maximum temperature of 265°C and acquisition time of 20 seconds. 3. RESULTS AND DISCUSSION After mixing the La2O3 and Al2O3 powders in equal proportions, they were calcinated in air for 2 hs at 1600 ºC. The formation of the LaAlO3 phase was obtained after this first heat treatment. Data diffraction analysis revealed that all observed diffraction peaks belong to the rhombohedral LaAlO3 phase (JCPDS 31-0022), as seen in Fig.1. The lattice parameter for this phase was found to be a = 5.364 Å and c = 13.11Å. However very additional weak diffraction peaks were assigned to the second phase, belonging to LaAl11O18 (JCPDS 33-0699), which have persisted even after sintering. LaAlO 3 T LaAl O 11 18 Intensity (a.u) Intensity T T T T T T T T T T T 20 30 40 50 60 70 80 2 (deg.) Fig. 1. XRD pattern of LaAlO3 powders calcined for 2 hours at 1600 °C in an air atmosphere. INAC 2013, Recife, PE, Brazil. In order to enhance oxygen vacancy production, the polycrystalline lanthanum aluminate was doped with hyperpure carbon by mechanic mixture in various percentages. Amounts of LaAlO3 were weight and compacted in an isostatic press and then sintered in hydrogen atmosphere at 1770 ºC. The purpose of growing a crystalline material in reducer atmosphere is to induce a large concentration of oxygen vacancies inside samples. When these vacancies are occupied by strange atoms in the crystalline structure, some of them may create electronic trappers. At this point, they are directly related with thermoluminescence sensitivity [4]. It is well known that oxygen vacancies may form F+-centers in Al2O3 by doping with carbon atoms during the crystal growth, in highly reducing atmosphere. This is due to the charge compensation of divalent carbon ion substituting trivalente Al3+-ion [7]. Occupancy of an oxygen vacancy by two electrons gives rise to a neutral F-center, whereas occupancy by one electron forms a positively charged, with respect to the lattice (F+-center). F and F+-centers are known to play a key role in the high luminescent + output of Al2O3. It was found that an increase in the concentration of F -centers in Al2O3:C causes significant increase in OSL and TL sensitivity [7] . In Fig. 2(a), the absorption spectra of our undoped and C- doped LaAlO3 polycrystals are plotted. An Absorption band centered at 206 nm has been observed in all samples. However, five new absorption peaks at 198, 237, 268, 304 and 316 nm have appeared in the carbon doped samples. Similar absorption peaks were reported in literature to be related to undoped and Ce-LaAlO3 single crystals, grown by Czochralski process [8]. It is known that LaAlO3 crystal have strong band-to-band absorption at 190-220 nm, which is assigned to F–Center formation due to high concentrations of oxygen vacancies, just as it has been described for the Al2O3:C crystals [7].
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