**4. Conclusions**

Polycrystalline powder of α-Al2O3 was successfully obtained using sol-gel and Pechini process. A second phase was found in doped samples, forming nanocrystals of aluminates and lanthanides oxides on the surface of alumina grains. We did not observe the incorporations of the dopants inside the alumina structure. Strongly dependent of temperature calcinations, the nanocrystallinity of the sample was retained after calcinations at higher temperatures, and an increase in the crystallite size was already perceptible.

The average diameter of nanocrystals depended on the dopant specie: for Yb and Er doped samples, it was D = (36 ± 2) nm for the sample calcinated at 1200 °C and D= (182 ± 8) nm for one calcinated at 1600 °C, therefore increased by 5 times. The approximately size of the AlNd is 200 nm for sample calcinated at 1600 °C during 4 h and for Tb3Al5O12 crystals the size is about 300 nm in the same calcination conditions.

The TL emission mechanism in the visible region can be related to F center and to the lanthanide (Ln) relaxation. During the irradiation the Ln3+ ion is reduced to Ln2+. It is not completely known if the reduction is due to the transfer of an electron or a hole; however, the process of reduction of trivalent lanthanide ions by irradiation was previously verified in literature [18, 19]. During thermal stimulation an electron can recombine with the Ln2+ forming Ln3+\* in excited state, which emits a photons returning on this way to the ground state. In the case of Mg spinel, probably the Mg ions promoted the oxygen vacancies stabilization, improving the luminescence response in the visible spectra, causing the main peak to increase 5 times in comparison with the undoped sample. It is believed that the occurrence of the nanometric spinel layer created an interface between both materials (Al2O3/MgAl2O4) with high concentration of defects.

OSL shinedown curves, supplied by undoped samples calcinated to 1200 and 1600 °C, could be fitted by second for all the samples except to α-Al2O3:Yb, Er, which was fitted by first order exponential decay. TL intensity of 190 °C peak and OSL responses with the dose increased linear for low doses region, from 80 to 1000 mGy, and the minimum dose detected value was 5 mGy obtained for TL (UV) and 350 µGy for OSL α-Al2O3 + Tb3Al5O12.

In summary, calcination conditions are of great importance for materials production that are being used as radiation sensors, once it greatly influences the stabilization of intrinsic defects, diffusion of dopants and the occurrence of new phases, due to the incorporation of dopants alongside the matrix, and others. These new phases also seem to play an important role in the luminescence emissions, due to the creation of new trapping and recombination centers, producing materials with unique properties that can be exploited to obtain better dosimeters.
