Thermoluminescence induced by UV radiation in mixed systems of ZrO2-La2O3 AND ZrO2 -CeO2

Rodolfo F. Estrada Guerrero¹, D. Mendoza-Anaya², P. González Martínez², R. Pérez-Hernández², P. Salas Castillo³.

1. Dpto Física y Matemáticas. Universidad Iberoamericana, Prolongación Paseo de la Reforma 880, Lomas de Santa Fe, México D.F. 01210, México. rodolfo.estrada@uia.mx .

2. Instituto Nacional de Investigaciones Nucleares, Km 36.5 Carretera México-Toluca, Ocoyoacac Estado de México, México. dma@nuclear.inin.mx .

3.  Instituto mexicano del petróleo México D.F.

Abstract

    In the study of the properties of the ceramic materials has been observed that some of them, such as TiO2, Al2O3, ZrO2, La2O3 and CeO2, are highly sensible to the ultraviolet radiation (UV); this characteristic can be taken as an advantage to detect this kind of radiation. The present work presents preliminary results obtained in the UV-induced thrmoluminescence (TL) response in mixed systems of ZrO2-La2O3 and ZrO2-CeO2 which were obtained by the Sol-Gel method. The originality of this contribution is that this mixed systems has not been synthesized and studied on its properties presented here. The first system presents high sensibility to the UV radiation with a TL curve formed mainly by two peaks localized at 70 °C and 140 °C; a third less intense peak at 255 °C was obtained too. On the other hand, the mixed system of ZrO2-CeO2 shows less sensibility with a TL curve formed by two peaks localized at 60 °C and 140 °C. With respect to the stability signal at environmental temperature, it can be observed, in both materials the signal vanishes for the peak at low temperature during the first minutes after the irradiation time, while the second peak have higher stability. According to the sensibility and stability of the signal associated to the ZrO2-La2O3 indicate that this system would be a promising for dosimetric applications in UV radiation fields.

Key words: UV radiation, Thermoluminescence, mixed oxides, ZrO 2 , La 2 O 3 , CeO 2.

Resumen

    En el estudio de las características y propiedades de materiales cerámicos se ha observado que algunos de estos materiales, tales como TiO 2 , Al 2 O 3 , ZrO 2 La 2 O 3 y CeO 2 , son altamente sensibles a las radiación ultravioleta (UV); característica que puede ser aprovechada para detectar este tipo de radiación. Este trabajo es una contribución importante que trata sobre los resultados obtenidos para la respuesta termoluminiscente (TL) inducida por la radiación UV en sistemas mixtos de ZrO 2 -La2O 3 y ZrO 2 -CeO 2 obtenidos por el método sol-gel. La originalidad de esta contribuciòn es que estos sistemas mixtos no han sido sintetizados ni estudiados en las propiedades que se presentan en este trabajo. El primer sistema posee alta sensibilidad a la radiación UV con una curva TL compuesta principalmente por dos picos localizados en 70 ° C y 140 ° C y uno poco intenso alrededor de 255 ° C, en tanto que el sistema ZrO 2 -CeO 2 presenta menor sensibilidad con una curva TL constituida por dos picos también localizados en 60 y 140 ° C. Con respecto a la estabilidad a temperatura ambiente se observó que para ambos materiales, el pico localizado a baja temperatura se desvanece en los primeros minutos después de la irradiación, el segundo tiene mayor estabilidad. De acuerdo a su sensibilidad y estabilidad de la información, el sistema ZrO 2 -La 2 O 3 resulta prometedor para aplicaciones dosimétricas en campos de radiación UV.

Palabras clave: Radiación UV, Termoluminiscencia, óxidos mixtos, ZrO 2 , La 2 O 3 , CeO 2 .

Introducción

    The interest that exists on the development of new materials for applications in the detection and measurement of the radiation has attracted the interest of some researches of the scientific community at world level. Mexico is not the exception, for such reason, in the Instituto Nacional de Investigaciones Nucleares in collaboration with Instituto Mexicano del Petróleo and the Universidad Iberoamericana we are developing investigations focused on the study of the properties and possible applications of dosimetric systems, such as zirconium oxide, that are sensitive to the UV radiation.

    Quantify fields of ultraviolet radiation with acceptable precision is important due to an increase of this radiation type that impacts on the earth surface, which is associated to the reduction of the ozone layer in the stratosphere. On the one hand, is important to know the role that can play the physics in the study of the UV radiation from the sun and its effects.

    Zirconium oxide (ZrO 2 ) is an excellent material for optical applications due to its optical transparency and high refractive index. For thermoluminescence (TL) applications, ZrO 2 has been exposed to different ionising radiation (UV, X-ray, gamma, etc). Some results showed that TL response depend on the crystalline structure for pure zirconium oxide prepared by sol-gel method [1,2]. It is important to mention that when thermoluminescent materials are exposed to ionising radiation, some of the electrons released by the ionization have enough energy to move trough the crystal. Many of these electrons are trapped in the defects of the crystal lattice and will remain there while the material is kept at room temperature, when the material is heated the trapped electrons are released emitting visible light. The light emitted during heating is measured and is proportional to the radiation absorbed for the material.

    It has been reported that pure ZrO 2 presents low phonon energy, increasing the number and the probability of radiative transitions in rare-earth doped samples [3]. This fact has increased the interest in developing rare-earth doped zirconium oxide for photonic and luminescence applications [4-6]. In the present work, the experimental results of the thermoluminescent signal induced by the UV radiation in two mixed oxide systems formed by ZrO 2 -La 2 O 3 and ZrO 2 -CeO 2 are presented; we suppose that rare-earth in oxide form improve the radiation sensibility to the ZrO 2 .

Experimental

    The mixed oxide materials were prepared by the Sol-Gel method, using zirconium propoxide (ZP) as precursor of the zirconia. The synthesis was carried out in a reactor of glass at 60 °C, in an acid medium, employing hexilenglycol as an additive (HG). The dopant salts (Ce(NO 3 ) 3 .6H 2 O and La(NO 3 ) 3 .6H 2 O, previously dissolved in Ethylic Alcohol (AE) are added, and stirring it during 3 h. After this time, the temperature is increased to 80 °C for 12 h until the gelation is completed. The materials are dried at 120 °C for 48 h and sintered first at 500 °C during 5 h and subsequently at 1000 °C during 10 h. The samples were prepared in form of solid disks of 5.0 mm in diameter and 1.0 mm in thickness. The disks were prepared by casting 0.1 g of the sintered material in a tubular mold of 5.0 mm in diameter and 1 cm height provided with a piston, the mold with the material was putted in a manual press, the pressure applied to form the disks was 2 tons.

    The solid disks were exposed to UV radiation from a commercial lamp of 40 W in power. The dose was controlled as a function of the time of exposition. The TL signal was measured using a HARSHAW 4000 TLD System where the samples were heated following reproducible controlled temperature cycles (from 30 to 330 °C). A linear heating rate of 10 °C s -1 was employed.

Results

    In the Figure 1 the TL spectrum of the samples of ZrO 2 -CeO 2 exposed to UV radiation for different times can be appreciated. It can be observed two intense emission peaks mainly located at 60 °C and 140 °C; while another peak of very low intensity located at 250 °C is also observed. It is also possible to note an increment in the intensity of the TL signal as the time of irradiation is increased conserving the form of the spectrum. This can be more appreciable in the Figure 2 , where the graphic of the intensity of the TL signal is presented as a function of the irradiation time; in this graph is appreciated an approximately lineal tendency between 10 and 120 minutes. For greater times of irradiation a saturation of the signal is observed.

Figure 1. TL response induced by the UV radiation in the ZrO 2 -CeO 2 system varying the time of irradiation (15, 30, 60, 130 and 180 seconds).The samples were measured immediately after the irradiation

Figure 2. TL response as a function of the exposure time to the UV radiation on the ZrO 2 -CeO 2 system.

    Figure 3 shows the TL spectrum of the samples made of ZrO 2 –La 2 O 3 irradiated at different times. In this figure two intensive peaks of emission can be seen at 70 °C and 140 °C as well as another peak of lower intensity localized at 250 °C. It is possible to note that there is an increment in the intensity of the TL signal as the time of irradiation is increased, the curve keep the same form. This result can be more appreciated in Figure 4 , where the graph of the intensity of the TL signal as a function of the irradiation time is presented; in this graph, a linear tendency can be seen from 10 to 120 seconds of the exposure time, in the interval from 120 to 230 seconds of exposition to the UV radiation a linear behavior is appreciated but it has a different rate of growth. For higher exposure time can be seen a response with some instability.

Figure 3. TL response induced by the UV radiation in the system ZrO 2 – La 2 O 3. varying the time of irradiation (15,45,60,100,200 and 280 seconds) The samples were measured immediately after the irradiation.

Figure 4. linearity of the TL signal as a function of the exposure time for the system ZrO2 –La2O3

    In the analysis of the stability of the TL signal in the system ZrO 2 –CeO 2 , a very marked reduction was observed in the intensity of the TL signal during the 30 minutes after the irradiation was suspended, and the first emission peak localized at 60 °C disappeared, as is shown in figure 5. The slope TL intensity was lower at higher exposure time and it is related with the reduction of the intensity of the emission peak localized at 140 °C as can be seen in figure 6.

Figure 5. TL signal induced by UV radiation in the system ZrO 2 –CeO 2 varying the relaxation time after the irradiation has been suspended (7, 15, 60 and 240 minutes)

Figure 6. Linearity of the response of the TL signal in the system ZrO2 –CeO2 .

    For the analysis in the stability of the TL signal on the mixed system of ZrO 2 –La 2O3, a very marked reduction was observed too during the 30 minutes after suspended the irradiation; the disappearance of the first emission peak localized at 70 °C was observed too as can be seen in Figure 7 .

    The slope TL intensity was lower at higher exposure time and it is related with the reduction of the intensity of the emission peak localized at 140 °C as can be appreciated in the Figure 8 , in this graph can be seen that for 120 minutes or more the TL signal is satbilizised.

Figure 7. TL signal induced by UV radiation in the system ZrO 2 –La 2 O 3 varying the relaxation time once the irradiation is suspended (0, 30, 120 and 240 minutes)

Figure 8. Linearity of the response of the TL signal in the system ZrO2 –La2O3.

Conclusions

    The systems ZrO2–La2O3 and ZrO2 –CeO2 presents a TL response induced by the UV radiation. This response is formed mainly by two maxima of emission located at 60 °C and 140 °C for the first system. For the second system the maxima are located at 70 °C and 140 °C. The results also show that the system formed by ZrO2 –La2O3 is more sensitive to the UV radiation compared with the system formed by ZrO 2 –CeO 2 as is shown in Figures 1 and 3 . This behaviour can be associates to the presence of La 2 O 3 ; E. de La Rosa et all reported two peaks located at 62 and 135 ªC for the pure monoclinic zirconium oxide [5], our systems shows peaks positioned all around these temperatures, we suggest that the T.L signal from the systems ZrO 2 -La 2 O 3 and ZrO 2 -CeO 2 is associated mainly to the ZrO 2 ; however, the intensity of the T.L signal is modified by the presence of the La 2 O 3 and CeO 2 . In the case of the system ZrO 2 -CeO 2 the reduction in the T.L signal some of the energy from the U.V radiation is absorbed by the CeO 2 without production of T.L signal

    The system formed by ZrO 2 –La 2 O 3 presents better behaviour and sensibility to the UV radiation, in accordance with the stability observed in this system once the time of irradiation has been suspended. We can conclude that this material could be a good candidate for being used as a UV radiation dosimeter.

References

1.  P. Salas, E. de la Rosa-Cruz, D. Mendoza-Anaya, P. González-Martínez, R. Rodríguez and V. M. Castaño. “High Temperature Thermoluminiscence Induced on UV-Irradiated Tetragonal ZrO 2 Prepared by Sol-gel”. Materials Letters 45 (2000) 241-245.         [ Links ]

2.  P. Salas; E. de la Rosa-Cruz; D. Mendoza-Anaya; P. González-Martínez; R. Rodríguez; and V. M. Castaño. “Thermo-luminescence Induced by Gamma Irradiation in Sol-Gel Prepared Zirconia-Silica Materials”. Materials Research Innovations, V4, No.1 (2000) 32-35.         [ Links ]

3.  C. Urlacher, J. Dumas, J. Serughetti, J. Mugnier and M. Muñoz. Planar ZrO 2 waveguides prepared by the sol-gel process: structural end optical properties. J. Sol-Gel Sci. Technnol. (Netherlands) 8 (1997) 999-1005.         [ Links ]

4.  B. Savioini, J. E. M. Santiuste and R. Gonzalez. Optical Characterization of Pr3+-doped Yttria-Stabilized Zirconia Single Crystals. Phys. Rev. B, Condens. Matter ( USA ) 56 (1997) 5856-5865.         [ Links ]

5.  E. de la Rosa-Cruz, L.A. Díaz-Torres, P. Salas, D. Mendoza, J.M. Hernández, V. M. Castaño. “Luminiscence and Thermol uminiscence Induced by Gamma and UV-Irradiation in Pure and Rare Earth Doped Zirconium Oxide”. Optical Materials 19 (2002) 195-199.         [ Links ]

6.  E. de la Rosa-Cruz, L. A. Díaz-Torres, P. Salas, V. M. Castaño and J. M. Hernández. Evidence of non-radiative energy transfer from the host to the active ions in monoclinic ZrO2:Sm3. J. Phys. D: Appl. Phys. 34, No. 14 (2001) 83-86.         [ Links ]