[en] The present work contains two parts, both devoted to the investigation of light nuclei. In the first part of the thesis the structure of the exotic ⁷He nucleus is studied. The disappearance of the usual magic numbers in extremely neutron-rich nuclei implies a considerable modification in the spin-orbit interaction. Recent experiments yield contradictory results about a possible existence of the p_1/2^- spin-orbit partner of the ⁷He ground state with a dominant p_3/2^- single-particle character. In order to clarify this question a study of the ⁷Li(d,²He)⁷He reaction has been performed using a 171 MeV deuteron beam provided by the cyclotron at Kernfysisch Versneller Insituut (KVI) in Groningen. The experiment was carried out in April 2003. The setup at KVI offers a resolution ΔE ∼ 150 keV (FWHM) in the measured spectra, better than the line width of the ground state of ⁷He. The unbound ²He system was identified by detecting coincidences between two protons with small relative energy. The data were taken over the angular range Θ_cm=0 -11.3 . A possible resonance at an excitation energy E_x=(1.45_-0.5^+0.7) MeV with a width Γ=(2.0_-1.1^+1.0) MeV is suggested by a decomposition of the spectrum using known resonances, the breakup behaviour of ⁷He and quasifree charge-exchange contributions, taking into account the cluster structure of ⁷Li. Gamow-Teller strengths for transitions to the lowest states in ⁷He are in remarkable agreement with results from ab initio Quantum Monte Carlo calculations. The neutron spectroscopic factor S_n=0.64±0.09 of the ⁷He ground state (⁷He=⁶He x n) is extracted by an R-matrix analysis. In the second part of the thesis the deuteron breakup has been studied in the ²H(e,e^') reaction at Θ=180 . The present measurements were performed in March and April 2006 at the superconducting Darmstadt electron linear accelerator S-DALINAC at an incident electron energy E₀=27.8 MeV and 74 MeV. At low momentum transfer (q=0.28 fm^-1 and 0.73 fm^-1, respectively) magnetic transitions are expected to give the dominant contributions to the breakup cross sections. Thus, the measured deuteron-electrodisintegration cross section at threshold can be used to obtain the cross section for the np → dγ reaction applying the principle of detailed balance. The accurate information about this process is of great interest in nuclear astrophysics, specifically with regard to Big-Bang nucleosynthesis. The experimental data are in excellent agreement with theoretical calculations based on a nucleon-nucleon potential model (Bonn-B), under the inclusion of meson-exchange and isobar effects. (orig.)

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[en] Studies of the ²H(e,e') reaction of Θ = 180 were performed at the S-DALINAC at an incident electron energy E₀ = 27.8 MeV and 74 MeV. At such low momentum transfers (q = 0.28 fm^-1 and 0.72 fm^-1, respectively) magnetic dipole transitions are expected to give the dominant contributions to the breakup cross sections. An energy resolution of ΔE = 45 keV (FWHM) was achieved at the lower incident energy. Thus, the measured deuteron-electrodisintegration cross section at threshold can be used to obtain the cross section for the np → dγ reaction applying the principle of detailed balance. Accurate information about this process is of great interest in nuclear astrophysics, specifically with regard to Big-Bang nucleosynthesis. The experimental data are in excellent agreement with theoretical calculations based on a nucleon-nucleon potential model, after the inclusion of meson-exchange and isobar effects. (orig.)

[en] The light odd-even nucleus ⁹Be has the lowest neutron threshold (S_n 1.665 MeV) of all stable nuclei. Already the first excited J^π = 1/2⁺ state lies at an excitation energy of several tens of keV above the ⁸Be + n threshold. Parameters of this resonance are of great astrophysical importance. The description of this unbound level is long-standing problem. Due to its closeness to the neutron threshold the resonance has a strongly asymmetric line shape but despite a large number of different experiments there still exist discrepancies between the various deduced resonance parameters. We present high-resolution ⁹Be(e,e') experiments performed at the S-DALINAC and a reanalysis of our old electron scattering data. (orig.)

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[en] A search for the J^π=1/2^- spin-orbit partner of the J^π=3/2^- ground state in ⁷He has been performed with the Li7(d,He2) charge-exchange reaction. The experimental results are incompatible with recent claims of such a state at very low excitation energy [M. Meister, et al., Phys. Rev. Lett. 88 (2002) 102501]. A decomposition of the spectrum is performed taking into account known resonances and quasifree charge-exchange reactions on ⁷Li as well as on triton and ⁴He clusters in the ⁷Li ground state. A possible resonance at an excitation energy E_x∼1.45MeV with a width Γ∼2.0MeV is suggested when the quasifree charge-exchange process on ⁷Li is constrained by a measurement of the Li6(d,He2) reaction. Gamow-Teller strengths for transitions to the lowest states in ⁷He deduced from the differential cross sections are in remarkable agreement with results from ab initio quantum Monte Carlo calculations

[en] A high-resolution (γ,γ^') study of the electric dipole response in Pb²⁰⁸ at the S-DALINAC reveals a resonance structure centered around the neutron emission threshold. Microscopic quasiparticle phonon model calculations in realistic model spaces including the coupling to complex configurations are able to describe the data in great detail. The resonance is shown to result from surface density oscillations of the neutron skin relative to an approximately isospin-saturated core. It also forms an integral part of a toroidal E1 mode representing an example of vortex collective motion in nuclei

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[en] The neutron spectroscopic factor S_n of the ⁷He ground state is extracted from an R-matrix analysis of a recent measurement of the ⁷Li(d, ²He)⁷He reaction with good energy resolution. The width extracted from a deconvolution of the spectrum is Γ=183(22) keV (full width at half maximum, FWHM). The result S_n=0.64(9) is slightly larger than predictions of recent 'ab initio' Green's function Monte Carlo and fermionic molecular dynamics calculations