Scattering of low-energetic pions on the sulfur isotopes 32S and 34S

At the Paul-Scherrer Institute PSI in Villingen/Switzerland the scattering of positive and negative pions of 50 MeV kinetic energy on the sulfur isotopes 32S, 34S was measured with the new installed low-energy pion spectrometer LEPS. Elastic cross sections were measured for scattering angles of 5deg-123deg for both pion polarities and for both isotopes. Because of the good resolution of 500 keV it was also possible to measure the inelastic scattering to single excited nuclear states of 45deg-123deg. Aim of the analyses was the testing of the basing reaction model, the test of the effective pion-nucleon interaction in the nucleus, as well as the extraction of neutron density distributions and neutron transition densities in 34S. At the N=Z nucleus 32S, the nuclear structure of which is known from the electron scattering, by means of the elastic scattering the effective πN interaction was fixed. The validity of the impulse approximation at 50 MeV was proved by the quantitative reproduction of the 21+ scattering data on 32S by parameter-free DWIA calculations. For the 41+ and the 31+ excitations in 32S the expected isoscalar character was confirmed. For the nucleus 34S the analysis of the eleastic scattering yields a neutron skin of Δrnp=0.03(1)fm. The scattering to the 21+ yields a neutron moment of Mn=16.6(10)efm2 respectively a ratio of proton to neutron moment of Mn/Mp=1.19(9), which corresponds to a collective isoscalar excitation. Against this for the 22+ excitation a ratio for the moments of Mn/Mp=-0.4(2) is determined, what means that to this not collective, proton-dominated excitation neutron and proton wave functions contribute antiphasically, as it is typical for isovectorial excitations. (orig./HSI)