[en] One of the main goals of the SESAM/T_χL collaboration is to explore the structure of the full QCD vacuum within the framework of lattice calculations. In this work we concentrate on two aspects of this question: The contribution of disconnected diagrams to the η' mass and the search for signals of string breaking due the effects of virtual quark anti-quark pairs. Our analyses were done on the SESAM/T_χL QCD configurations with dynamical Wilson fermions at β = 5.6 which cover a m_π/m_ρ range of 0.57 to 0.83 with an extrapolated inverse lattice spacing of 1/a ∼ 2.3 GeV. The flavor singlet meson η' should obtain its relatively large mass by large contributions from disconnected diagrams which vanish for octet mesons. The evaluation of these noisy two loop correlators is a numerically demanding task. We discuss various methods and improvements of quark loop estimation on our configuration sample. We find that the use of O(400) smeared stochastic Z₂ source vectors per configuration makes the η' propagator accessible to standard spectrum methods. We extract η' observables which, after chiral extrapolation, are 15 - 25 per cent below the pseudoexperimental N_f = 2 values. This could of course be an O(a) effect or due to a nontrivial extrapolation from the N_f = 2 world to reality. A quenched reference analysis leads to similar results. We find large contributions to η' observables from topologically nontrivial configurations. This justifies qualitatively the theoretical picture which relates the η' mass with the U(1) anomaly. These dependencies seem to be absent for light spectrum observables. The observation of string breaking, the energetically favored decay of a static quark anti-quark pair into a static-light meson pair, above a certain separation should be a qualitative feature of full QCD simulations. In the framework of a two channel mixing analysis we estimate and improve noise to signal ratios of the static quark pair and the transition amplitudes. We determine the expected string breaking distance from the mass of two static-light mesons and the static quark anti-quark potential. It turns out that string breaking should in principle be observable on our T_χL lattices. Again, smeared sources lead to a significant noise reduction at the relevant separations. (orig.)