Search

Self-conjugate nuclei in the A ��� 70���80 region have attracted a great deal of attention due to phenomena such as shape coexistence and increasing collectivity along the N=Zline. We investigate the structure of nuclei in this region through lifetime measurements using the GRETINA array. The first implementation of the Differential Recoil Distance Doppler Shift technique with fast radioactive beams is demonstrated and verified through a measurement of the well-known B(E2; 2��������� 0������)transition strength in ������Kr. The method is then applied to determine the B(E2; 2��������� 0������) transition strength in ������Rb, the heaviest odd���odd N=Z nucleus for which this quantity has been determined. This result and extended systematics along N=Z suggest the dominance of the isoscalar part of the quadrupole transition strengths in self-conjugate nuclei, as well as the possible presence of shape coexistence in ������Rb.

This work focuses on new electromagnetic decay mode in nuclear physics. The first part of the thesis presents the observation of the two-photon decay for a transition where the one-photon decay is allowed. In the second part, so called quadrupole mixed-symmetry is investigated in inelastic proton scattering experiments. In 1930 Nobel-prize winner M. Goeppert-Mayer was the first to discuss the two-photon decay of an exited state in her doctoral thesis. This process has been observed many times in atomic physics. However in nuclear physics data is sparse. Here this decay mode has only been observed for the special case of a transition between nuclear states with spin and parity quantum number 0+. For such a transition, the one-photon decay - the main experimental obstacle to observe the two-photon decay - is forbidden. Furthermore, the energy sharing and angular distributions were measured, allowing conclusions to be drawn about the multipoles contributing to the two-photon transition. Quadrupole mixed-symmetry states are an excitation mode in spherical nuclei which are sensitive to the strength of the quadrupole residual interaction. A new signature for these interesting states is presented which allows identification of mixed-symmetry states independently of electromagnetic transition strengths. Furthermore this signature represents a valuable additional observable to test model predictions for mixed-symmetry states.

The first part of this thesis covers the two-photon decay of the 11/2- isomer in 137Ba. Clear evidence of this exotic decay mode is found, which opens up the possibility to use the two-photon decay probability as a general tool to investigate the structure of atomic nuclei. Details on the experiment are given, and the major challenges to successfully perform this difficult experiment are outlined. The two-photon decay experiment was conducted with the new LaBr3-detector ball which was setup from scratch in the framework of this thesis. The properties and performance of this powerful detector system - consisting of 18 large LaBr3-detectors - are discussed. Furthermore, proton scattering at 200 MeV has been performed on the nuclei 92,94Zr, 94,96Mo and 70Zn.The measured cross sections are compared to calculations in the framework of the Quasi-Particle Phonon Model (QPM). Conclusions about the validity of the QPM results are drawn. A comparison of the excitation cross sections of the 2+1 states and the one-phonon quadrupole states with mixed neutron and proton symmetry (2+ms) allows to prove the special character of the 2+ms states independently of electromagnetic transition strengths. The possible existence of 3- and 4+ states with mixed proton and neutron symmetry is discussed. Furthermore the origin of collectivity of low-lying states in spherical nuclei is investigated in the QPM.