**2. Experimental part**

178 Heat Treatment – Conventional and Novel Applications

irradiation annealing between 150–400 °

commercial dosimeters.

Czochralski, Bridgmann, etc.

vacancies creating F-

emission.

1953, with LiF, Bjarngard, 1967 with CaSO4 and Ginther and Kirk (1957) with CaF2. After these works search in other materials such as natural fluorides or synthetic as, LiBO3:Mn, CaF2:Dy, CaSO4 and MgSiO4:Tb. They usually obtained as monocrystalline samples

However, Cameron, 1961, with their research on the application of LiF: Mg and Ti obtained the first thermoluminescence dosimeter (TLD-100) [5], which is still one of most popular TLD phosphor, due to tissue equivalent Zeff = 8.04, which is an important characteristic for personal dosimetry. Akselrod et al., 1990 [6] carried out studies on TL properties of carbon doped Al2O3 (TLD-500), a very sensitive material to radiation exposure, showing few TL peaks, with dose interval of detection between 0.05 µ to 10 Gy and fading rate of 3% by year (when kept in the dark). This high sensitivity of the material is attributed to oxygen vacancies created during the crystal growth procedure; the electrons can be trapped at these

In order to increase the luminescence emission response with dose of some thermoluminescence dosimeters (TLD), heat treatments procedures were frequently performed. Halperin et al., 1959, [7] noted that the thermal treatment enhanced the intensity of various TL glow peaks of NaCl by factors of a few thousands; on prolonged heat treatment. They observed that the intensity of TL peaks locate above RT decreased, while those at lower temperatures continued to grow even after 80 hours of heat treatment at 550 °C. Mehendru, 1970 [8], studied the effects of heat treatment on the TL response of pure KCl; they associated the peaks at 95, 135, and 190 °C with the F centers, these last created due to the background divalent cation impurities, and with the first- and the second-stage F centers, respectively. Kitis et al., 1990 [9], studied the sensitization of LiF:Mg, Ti as a function of irradiation at elevated temperatures, pre-irradiation annealing, and post

conditions cause an enhancement of the sensitivity; after more two works about preheating and high temperature annealing on TL glow curves of LiF:Mg, Ti [10] and in LiF TLD100 [11] were published. Holgate, 1994, [12] investigated TL and radioluminescence (RL) spectra of calcium fluoride samples doped with neodymium and variations of spectra with Nd concentrations and thermal treatments were observed. Nowadays, it is possible to find oxides, sulfates, sulfides and alkali haloids doped with rare-earths and transition metals as

The optically stimulated luminescence (OSL) was pioneered used to determine environmental radiation dose received by geological samples [13]. However, the idea of using OSL dosimetry was first suggested in 1956 [14]. The first experience was made using MgS, CaS, SrS and SrSe phosphors doped with different rare-earths [15,16]. Nanto et al., 1993 [17], investigated the OSL properties of single crystals of KCl:Eu; after, Akselrod et al. 1998 [18], proposed the use of Al2O3:C for OSL dosimetry, because the high sensitivity of the crystal to visible light. At the present time, the crystal is the principal OSL dosimeter [19-22]. Currently, there are various materials proposed for OSL dosimetry as KBr:Eu [23,24],

and F+ centers, which act as recombination centers yielding a bright

C; the results showed that the first and third

Polycrystalline powder samples of α-Al2O3:Er, Yb; Mg; Tb and Nd were obtained by sol-gel and Pechini process. In the sol-gel procedure stoichiometric amounts of tri-sec-butoxide of aluminum was dissolved in distilled water and hydrochloric acid. The dopants Er, Yb, Nd and Tb oxides were added during the sol stage, with different concentrations. Some portions of the resulting powder were calcinated at different temperature from 1200 to 1600 °C. Experimental parameters of the calcination process, as heating and cooling rates and set point values were varied, in order to verify the effect on the luminescence response.

Pechini is a chemical routine that produces, at the end of the stage, an organic polymer with metallic ions, which will be responsible for the formation of the desired material. The polymer is obtained after low temperature reaction among ethylene glycol, citric acid and aluminum nitrate. Once the polymer is ready, a number of heat treatments are carried out in order to (1) collapse the polymeric structure and allow the gradual oxidation reaction of the metallic ions with atmospheric oxygen, and (2) obtain the desired structure of the material. This technique is known to obtain uniform composition and controlled grain size distribution, due to the slow oxidation reaction and the viscosity of the polymer, which avoid precipitation.

The morphological characteristics of the samples were analyzed using a Philips CM200 TEM equipped with EDS operating at 160 keV, some Cu contamination from the sample holder can be observed in the all the EDS results, the samples were located at 400 mm from the source. The X-ray powder diffractions were recorded with the MiniFlex II model diffractometer of Rigaku Corporation.

Effects of Heat Treatments on the Thermoluminescence and Optically Stimulated

Luminescence of Nanostructured Aluminate Doped with Rare-Earth and Semi-Metal Chemical Element 181

intensity of high temperature peak simultaneously. The TL intensity in VIS region is higher

**Figure 1.** XRD pattern of the α-Al2O3 samples, a) undoped sample, b) doped with Tb, c) doped with Nd, d) Er and Yb (1 and 2 mol%) doped and calcinated at 1200 °C and (e) Er and Yb (1 and 2 mol%) and

than those found in the UV one.

calcinated at 1600 °C.

TL and OSL measurements were performed in an oxygen-free nitrogen atmosphere using Daybreak Nuclear and Medical Systems Inc, model 1100-series TL/OSL reader and RISØ TL/OSL reader Model DA - 20.

TL was detected using the BG-39 (340-610 nm) optical filter and heating rate of 10 °C/s. OSL measurements were made using an array of blue (470 nm) LEDs for sample stimulation and detected in the UV with a Schott U-340 optical filter.

The irradiations were performed at RT in a 60Co source with dose rate of 28.7 Gy/h and with a beta source (90Sr/90Y) coupled to the RISØ TL/OSL reader, with dose rate of 0.08 Gy/s.
