Nuclear matter distribution of 56Ni measured with EXL

In the present work, the nuclear matter distribution and the RMS matter radius of 56Ni were successfully measured for the first time by exploiting elastic proton scattering. Being a doubly magic nucleus with an equal number of protons and neutrons, 56Ni is of particular physical interest. Since it is also a radioactive nucleus, the experiment has to be performed in inverse kinematics. Hence, the experiment was conducted at the ESR (Experimental Storage Ring) at the GSI Helmholtzzentrum fuer Schwerionenforschung as part of the first experimental campaign of EXL (EXotic nuclei studied in Light-ion induced reactions). The beam of 56Ni, which was produced by in-flight fragmentation of a 58Ni beam and selected by the FRagment Separator (FRS), was injected into the ESR and interacted with the internal hydrogen target. The demanding vacuum conditions of a storage ring made it necessary to develop a novel detector system. This had to be ultra-high vacuum (UHV) compatible and, at the same time, feature an energy threshold as low as possible to enable the measurement of particles scattered at low momentum transfer. To equally fulfil both conditions, a windowless detector system was developed in which the UHV is separated from an auxiliary vacuum by a silicon strip detector. In the auxiliary vacuum, additional detectors as well as other non-UHV compatible components may be placed. This way, a telescope based on silicon detectors was set up which makes the measurement of protons in an energy range starting at few hundreds of keV up to about 50 MeV possible. In the course of the present work the employed detectors were tested and further developed by extensive laboratory tests as well as in-beam experiments. The differential cross section for elastic proton scattering was deduced from the measured angular distribution of the detected recoil protons. For this, comprehensive Monte-Carlo simulations of the setup have been performed. Then, the nuclear matter distribution was extracted from the cross section with the help of the Glauber multiple-scattering theory. For this purpose, the density distribution was parametrised by a phenomenological distribution for which a symmetrised Fermi distribution and the model-independent Sum-Of-Gaussians (SOG) method was used. The latter allows to determine theory-dependent contributions to the systematic error. Eventually, the RMS matter radius of 56Ni was calculated from the matter distributions to be (3.76+-0.08) fm which is in agreement with predictions by HFB and HF+BCS calculations. The correctness of the whole method, i.e. the measurement in inverse kinematics and the applied analysis procedure, was proven in comparison to an already known nuclear matter distribution of 58Ni of which the results are in a good agreement with the literature values