This whole project would not have been possible without Jérôme Beaucour and Jose Luis Martinez, who accepted me first as a trainee for a neutron guide design internship and then as a Ph.D. student on the related topic of mirror substrate study. The main work partner I had throughout these years was Michael Kreuz who has always been available to discuss scientific studies whether they were from his field of expertise or not, thank you very much. From the same ILL service, I would like to thank Giuliana Manzin and Benjamin Giroud, who advised me whenever I needed it. From the reactor division, I would like to especially thank Marc Samuel and his team, Elvio Mannino, Cédric Laurent and Renaud Silvestre, who assisted me on the T4 irradiations of this study. Also, I cannot forget Xavier Fargues, Frédérique Fréry, Véronique Caillot, and Bruno Desbrière for their help at some point on the hard path to send samples to irradiation in the ILL High Flux Reactor. In the same vein, I wish to thank Ulli Köster, who advised me a lot on in-situ flux monitoring with Zirconium foils and the perturbed flux issue. His explanations on various topics ranging from gamma activation analysis to radioactive material transport were always useful. In terms of radio-protection, I am happy to thank the constant help from Gilles Rignon and his team,