For the first time, positron annihilation spectroscopy performed on niobium samples reveals the increase after baking of positrons trapped under the Nb surface. Presence of hydrogen-vacancy complexes and their dissociation by baking could both explain rf losses observed at high fields (Q drop) and its cure (baking effect).

Involvement of Hydrogen-Vacancy Complexes in the Baking Effect of Niobium Cavities. B.Visentin, M.F.Barthe, V.Moineau, P.Desgardin: Physical Review - Special Topics, 2010, 13[5], 052002