Mainz, Universiẗat, Diss., 2000.

Universiẗat, Diss., 2000--Mainz. / Auch als gedr. Diss. Zeichendarstellung im Hauptsachtitel teilweise nicht vorlagegemäß wiedergegeben.

Universiẗat, Diss., 2000--Wuppertal.

Universiẗat, Diss., 2003--Bonn.

Universiẗat, Diss., 2004--Münster (Westfalen).

University, Diss., 2004--Bonn.

University, Diss., 2005--Heidelberg.

The present thesis describes two experiments performed in the storage ring CELSIUS at The Svedberg Laboratory in Uppsala. In the first experiment the importance of three - nucleon clustering in the six - nucelon system was investigated. The total cross section for the production of the ground state and the 3.56 MeV second excited state of 6Li in the 3He(3He,6Li)π+ reaction has been measured at two beam energies, 261.1 and 262.5 MeV, corresponding to center - of - mass energies 1.2 and 1.9 MeV above the production threshold for the 3.56 MeV state. For the ground state the result was 347 ± 84 ± 42 and 92 ± 84 ± 11 nb respectively. The result for the 3.56 MeV state, 104 ± 23 ± 12 and 56 ± 35 ± 7 nb respectively, is compared to the result of a previous study where the 3.56 MeV state was populated in the d(4He,6Li)π0 reaction. In the second experiment a clean sample of 5×105 eta mesons was prepared by means of the d(p,3He)η reaction. Eta production was tagged by the precise determination of the kinetic energy of the associated 3He ions. In the subsequent decay of eta mesons, channels with lepton - anti - lepton pairs were studied in the WASA detector. In a separate study properties of the WASA deuterium pellet target were investigated and in particular the effects on the beam of the beam - target interactions. In both sets of experiments the fused nuclear system was detected by means of a zero - degree spectrometer with a semiconductor detector telescope. Choosing the detectors to match the rather different requirements, precise information regarding the identity and the momentum of the detected ions could be obtained in both cases.

A comparative study is done on the Mini Drift Chamber (MDC) and the Scintillating Electromagnetic Calorimeter (SEC), two main components of the WASA detector. The purpose of the study is to determine the most effective way to calculate the mass of the η−meson by determining the invariant mass of the final products in the η → e+e− decay. When calculating the invariant mass of the lepton pair the distribution from the MDC had a standard deviation of σMDC = (4.212 ± 0.080) · 10−2 GeV/c2 and from the SEC σSEC = (3.563±0.011)·10−2 GeV/c2. To get a precise measurement, events with a lepton scattering at a polar angle below 30◦ have to be rejected, and to achieve highest precision, it is favorable to use the SEC for momenta above 0.28 GeV/c and the MDC below, respectively. In this study, a combined method is developed which results in an invariant mass resolution of σMix = (3.341±0.012)·10−2 GeV/c2. Thus, the combined method gives a small improvement compared to using the SEC alone, but a considerable improvement compared to using only the MDC.

The long-standing problem of possible formation of metastable states in collisions of the eta-meson with atomic nuclei is revisited. The two-body eta-nucleon interaction is described by a local potential, which is constructed by fitting known low-energy parameters of this interaction. The many-body eta-nucleus potential obtained within the folding model, is used to search for metastable states of the systems formed by the eta-meson with hydrogen and helium isotopes. It is found that all these systems generate strings of overlapping resonances. / Physics / M.Sc. (Physics)