Study of the behaviour of the Δ(3,3) resonance in nuclear matter by (π,π'γ) angular correlations on 12C(2+)

The detailed study of the pion-nucleus scattering in the range of the Δ(3.3) resonance permits statements on the behaviour of a strong resonance in the surrounding nuclear medium and raises by this the question for the importance of subnuclear degrees of freedom in the nucleus. In the present thesis the pion-nucleus reaction mechanism was studied by means of pion-γ-angular correlation measurements on the 12C(2+, 4.4 MeV) state. The experiments were performed at the Swiss Institute for Nuclear Research. The scattered pions were detected by the SUSI spectrometer to which in coincidence six NaI detectors for the detection of the emitted γ-quanta were operated. For the momentum transfer q = 0.47 fm-1 and pion incident energies of 116, 140, 162, 180, and 226 MeV respectively for the momentum transfer q = 0.85 fm-1 and incident energies of 116, 162, and 226 MeV values of the angular correlation function for in the mean each 16 γ emission angles within and beyond the reaction plane were determined. The measurement results are compared with DWIA calculations in the static limit and in the Δ-hole model. In the static limit the transition is described starting from the free pion-nucleon process whereby the dynamics of the intermediate Δ-resonance are suppressed. The Δ-hole model yields an additional microscopical description for the propagation of the Δ-resonance. The static model cannot reproduce the energy slope of the correlation function for q = 0.47 fm-1 while the Δ-hole model achieves a very good agreement. The experiment shows by this that the description of the pion-nucleus scattering in the resonance range requires the regard of medium effects especially from the Δ propagation. (orig.)