| Popis: |
In order to investigate the reaction mechanisms for elastic and first excited level ( I π = 2 + , E x = 4.44 MeV) inelastic n- 12 C scattering, angular distributions of several observables have been measured: differential elastic cross sections at E n = 15.0, 15.85 and 18.25 MeV, elastic polarizations at E n = 15.85 MeV, differential inelastic cross sections and spin-flip probabilities at E n = 15.0, 15.85, 16.9 and 18.25 MeV, and inelastic analyzing powers and spin-flip analyzing powers at E n = 15.85 MeV. Neutron time-of-flight techniques with a scintillating scatterer have been used. Spin-flip data have been determined by means of the (n, n′ γ ⊥) correlation. Detailed comparisons of the experimental results were made with the predictions of collective-model CC calculations using the full Thomas form for the deformed spin-orbit potential. Large and energy-dependent spin-orbit deformations ( β 2 s . o . ≳ 3 β 2 c ) were needed indicating nuclear structure effects. The fit to the 15.85 MeV polarization data was considerably improved by adding d 3 2 and f 5 2 Breit-Wigner resonance amplitudes to the optical-model amplitudes. Inelastic scattering data were analyzed in terms of a microscopic antisymmetrized DWA in which high-lying resonance states are incorporated explicitly as doorway states. Higher-order processes are described by the addition of a phenomenological imaginary part to the transition potential. These analyses reveal the presence of a quadrupole resonance at about 21 MeV excitation in 12 C. The results are also influenced to a lesser extent by the El giant dipole resonance and a dipole resonance at E x ≈ 19.5 MeV. |
