The recent discovery of a modified low temperature baking process established an increased accelerating gradient of TESLA shaped cavities through reduction of surface losses. A possible explanation for the performance gain is the suppression of lossy nanohydrides via defect trapping, with vacancy-hydrogen (v+nH) complexes forming at the lower temperatures. Utilizing Doppler broadening Positron Annihilation Spectroscopy, Positron Annihilation Lifetime Spec-troscopy and Nuclear Reaction Analysis, samples made from European XFEL niobium sheets and cavity cutouts were investigated. The evolution of vacancies, hydrogen and their interaction at different temperature levels have been studied during in-situ and ex-situ annealing and in-situ cooldowns. Measurements of niobium samples and a correlation between RF, material properties, and v+nH distribution in cavity cutouts have been carried out. BAKING AND SRF PERFORMANCE The influence of hydrogen on rf losses ('hydrogen Q-disease') of cavities and the need of outgassing cavities is known for quite some time. The operating temperature of su-perconducting accelerating cavities is 2-4 K, and while crossing the temperature range of 200-50 K during cool down, different phases of niobium hydride on the rf surface are forming, causing the increased losses. To prevent this, cavities are baked at 700 − 900 o C at pressures below 10 −6 mbar to purify the material. After the final electropolishing, an additional 120 o C bake for 48 h has shown to reduce losses and cure the 'high field Q-slope' [1]. Lattice deformations, interstitials and vacancies are known to have high trapping potential for interstitials, especially hydrogen. Formation of so-called "nanohydrides" which are only weakly superconducting by proximity effect up to a certain threshold of applied field is assumed to be responsible for losses above the threshold causing the high field Q-slope [2]. The assumption is that the modified low T baking procedure [3] might influence the vacancy-density and their interaction with hydrogen in the relevant rf penetrated layer in a beneficial way to prevent formation of lossy nanohydrides. This new bake includes a 75 o C step before the 120 o C. At this temperature a β → α ′ NbH phase formation takes place [4, 5] which potentially influences Nb-H dynamics during cooldown. So called vacancy-hydrogen * marc.wenskat@desy.de (v+nH) complexes have been studied and found to play a role already in the standard 120 o C bake [2, 6]. POSITRON ANNIHILATION (LIFETIME) SPECTROSCOPY Positrons are easily trapped in vacancies and are very sensitive to their chemical environment. The positron annihilation spectrum (PAS) can be characterized in terms of the line shape parameter S and the wing parameter W which contain information of the low and high momentum part of the distribution, see Fig. 1. Those positrons annihilated with free electrons, which on average have a low momentum, will contribute to the S parameter. The higher the amount of vacancies, the more positrons will annihilate in vacancies with free electrons increasing the S parameter. The W parameter is the fractional area in the wing region and contains information of the annihilation of positrons with core electrons of the surrounding elements which on average have a higher momentum. Hence carrying information on the chemical loading of vacancies. At pulsed sources, the Figure 1: Annihilation spectrum of positrons in metallic material. The central area A 1 is used to quantify the S-parameter, while the areas A 2 and A 3 are used for the W-parameter. lifetime of positrons depend as well on the density and types of vacancies (PALS). Deconvolution of the observed lifetime distribution will allow to identify the different contributions. In addition, when the energy of the annihilation photons is obtained, an energy shift ∆E can be observed (Coinci-dence Doppler Broadening (CDB) shift). This energy shift depends on the chemical surroundings of the annihilation site and also on the density and types of defects.