Résumé du preprint Irfu-09-165

Positron annihilation spectroscopy on Niobium samples.
B. Visentin, M.F. Barthe, V. Moineau, P. Desgardin
Since the baking effect discovery, a part of the Saclay R&D is dedicated to understand this effect and its correlated question about the High Field Q-slope origin. Complementary experiments on fast baking in Oxygen free atmosphere and SIMS analyses have shown that interstitial oxygen diffusion cannot be involved in baking physics. Presence of niobium vacancy near the surface could contribute to explain such phenomenon. To explore this possibility, we have performed experiments on niobium samples by means of positron annihilation radiation Doppler broadening spectroscopy. For the first time, an increase after baking of vacancy sites is disclosed at the niobium sample surface. Vacancy-hydrogen complex dissociation could be involved. This modification is observed in 100 nm depth under the sample surface, an area where the superconducting RF layer is located.