Daño por excitación electrónica en SiO2 mediante irradiación con iones pesados de alta energía

Resumen

Key words: Ion, electronic damage, SiO2, silica, quartz, point defects, amorphization, swift heavy ions. Summary Damage induce by electronic excitation is a very important process in multiples technologies and devices that works under radiation going from space technologies to nuclear waste treatment and storage technologies. On the other hand, diagnosis, remote handling and final optics devices in fusion reactors will require the use of optical components (windows, lenses and propagation fibers) which should stand high levels of radiation, mainly gamma and fast neutrons. The thesis is focused on the effect of the electronic damage induced by swift heavy ions in silica and alpha quartz in order to understand the mechanisms involved. There were two main objectives: 1. Studying point defect creation induced by ion irradiation, analysing the colouring kinetics both in silica and quartz in order to determine if the structure plays a role them. 2. Studying structural changes induced by ion irradiation through the analysis of changes in the refractive index of the damaged area and quartz amorfization. The experiments were performed in a 5 MV tadem ion accelerator at ¿Centro de Microanálisis de Materiales¿ (CMAM-UAM). Samples of pure silica and alpha quartz were irradiated with different ions and energies. A systematic series of Bromine ion irradiation were performed at energies going from 2 MeV to 40 MeV. Isolated irradiations were made with Fluorine, Chlorine and Oxygen ions. The main results obtained are listed below: Point defect creation The density of point defects in silica and quartz grows up with the increase of fluence but exist similar average saturation level (Ns) for all point defects considered around 1019 ions/cm-3 [1 - APL]. This behaviour is observed both in silica and quartz and the saturation value is quite similar and suggests that exist a dynamic competition between creation and annihilation mechanisms. The initial growth rate for colour centers is almost double in silica than quartz. On the other hand, for low energies, less than 5 MeV for Bromine, the growth rate keeps constant with energy and for greater energies grow up. The saturation density of point defects keeps roughly constant with the ion energy which indicates that the annihilation mechanism only depends of the number of point defects in the material. Structural Changes Electronic excitation induces changes in the refractive index associated to density and structural changes. Some of the irradiations gave rise to optical wave guides which were analysed using the dark mode spectroscopy technique [2]. Silica and quartz ion irradiated show a quite similar refractive index and both materials also show a reduction in the fundamental absorption edge associated with the increase of the irradiation fluence. These results demonstrate can be associated to a final similar structure. The disorder induced in the crystalline structure of quartz by the ion irradiation was measured by Rutherford Back Scattering in channelling configuration (RBS-C). The results show a complex track generated by the ion impact consisting in of an inner amorphous core surrounded by a homogeneous defective halo with high concentration of defects. From these measurements it has been obtained a threshold value for the electronic stopping power (Se) below which doesn¿t exist amorphous core. Finally a comparison between the initial point defect growth rate and the initial disorder growth rate has been made. It shows that both processes seems no to be related and follows different kinetics. References [1] J. Manzano, J. Olivares, A. Rivera, O. Peña-Rodríguez, F. Agulló-López; Appl. Phys. Lett. 101, 154103 (2012) [2] J. Olivares. Guías de onda planares en LiNbO3 por intercambio protónico. Ph.D. thesis, Univ. Autónoma de Madrid (1994).