Método de preparación materiales termoluminiscentes

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La película delgada, amorfa o tanto como mono o policristalino son muy importantes en la tecnología moderna. Pueden ser utilizados para formar las cubiertas de protección en otros materiales y han jugado un papel esencial en la miniaturización de los componentes de los dispositivos electrónicos, pero también son útiles en la dosimetría de TLD para la radiación de baja energía. Otra característica importante es que sus propiedades son a menudo diferentes de las propiedades de un material grande, que se deriva de la gran proporción de área / volumen que poseen.

Los materiales TL dosimétricos actualmente disponibles se pueden agrupar en dos categorías principales (Azorín, 1990):

(a) Tissue equivalent phosphors, which have an effective atomic number in the range from 7 to10 and generally show low sensitivity, such as alkali halides, alkali borates and alkaline earth metal oxides. For example: LiF, Li2B4O7, BeO, etc.

(b) Phosphors with atomic numbers ranging from 15 to 18, with

high sensitivity but no tissue equivalents such as salts of alkaline earth metals, and metallic oxides. For example: CaSO4, CaF2, Al2O3, ZrO2, TiO2, etc.

In general, nominally pure compounds show weak TL signal, and are not considered efficient dosimetric materials. Much higher efficiency is obtained by doping such materials with proper impurities which act as activators of TL phenomenon. The impurity is normally chosen on the basis of the highest TL response, but other important dosimetric characteristics must be taken into account. Then, the search of good dosimetric TL materials, points to an optimized coupling of a suitable host material with a dopant of very high efficiency to be easily introduced into the desired TL phosphor.

The preparation method used depends on the physical form required for the TL material, either polycrystalline powder, single crystals or thin films (Azorín et al., 1993a). Most commonly used methods for obtaining polycrystalline powder are precipitation and evaporation; while for single crystals, most commonly used methods are those of Czochralski, zone melting and precipitation from solutions or molten phases (flow method). While for thin films, the most common methods are chemical vapor deposition, spray pyrolysis and sol–gel. Below is a brief description of the above mentioned methods:

2.1. Precipitation method

In this method, a solution of the precursor reactants is mixed with dopants in acid solution. Once the precipitate has the desired compound, the sample is centrifuged and washed repeatedly. The precipitate is treated at high temperature, then cooled to room

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temperature and dried in an inert atmosphere. Thereafter, the temperature is raised to higher temperature, holding for a time. Subsequently, the sample is moved slowly toward a zone of lower temperature to crystallize, then the sample is removed from the furnace by cooling rapidly to room temperature. Finally, the product is pulverized and sieved, selecting powder with desired grain size (Zumdahl, 2009). This method has been used by some authors for synthesizing diverse materials such as alkaline earths sulfides (Rao, 1986), metallic oxides (Kumar et al., 1994; Azorín- Vega et al., 2007), strontium aluminate (Chengkang et al.), calcium phosphate (Madhukumar et al., 2007), lithium fluoride (Vu Thi Thai Ha et al., 2007) or calcium sulfate (Rivera et al., 2010).

2.2. Evaporation method

This method consists in homogeneously mixing the reactants in acid solution by adding the dopants in the desired concentration.

The mixture is placed in a sealed system for evaporating at a high temperature for a given time, by carrying the acid with an air or nitrogen flow. Crystallization is controlled by varying both the temperature and the gas flow. After the evaporation, crystals whose dimensions depend on both the initial reagents and the type and concentration of the dopants are obtained. Crystals obtained are washed several times to remove the remaining acid and submitted to a thermal treatment at high temperature. Finally, the product is pulverized selecting powder with a desired grain size.

Evaporation is the well-known method most widely used to produce calcium sulfate singly doped or co-doped. Considerable work has been done on singly or co-doped CaSO4 phosphors in the last few decades. Rare earths, especially Dy or Tm have been used as dopants (Yamashita et al., 1971; Prokic, 1978; Azorín et al., 1980; Azorín et al., 1984; Azorín and Gutiérrez, 1989). This method has been also applied by Furetta et al. (2000); by Ege et al. (2007), for producing lithium borates. The evaporation method has also been used to produce metallic oxide TL phosphors (Azorin et al., 2002; Rivera et al., 2002).

2.3. Czochralski method

In this method a single crystal is grown from a melt of the same composition. A crystal seed is brought into contact with the surface of the melt, whose temperature is maintained slightly above the melting. Withdrawing the seed slowly goes over to the surface and the melt solidifies in the same crystallographic orientation than the original seed. The growing crystal and the crucible with the molten usually rotate in opposite directions during extraction, so as to maintain a constant temperature. Usually employs an inert gas (argon or xenon) at a high pressure to prevent volatilization losses (Müller et al., 2004). Jiri Kvapil et al. (1980) have used this method to produce corundum single crystals to be used as TL materials. Rare earths phosphates have been also prepared as TL materials by using the Czochralski method (Bold et al., 1985). Kelemen et al., 2011 have prepared lithium tetra- borates single crystals by the Czochralski method.

2.4. Zone melting

In this well-known method crystals are grown by slow cooling of the small molten zone. Under these conditions the atoms are arranged such that the crystal is formed with a preferential orientation (Pfann, 1966). Lithium fluoride has been produced by using this method by Watcher (1982).

2.5. Precipitation from solutions or molten phases. (Flow method)

In this method the crystal growth occurs from a liquid phase of different compositions to the crystal; for example,

J. Azorin / Applied Radiation and Isotopes