Nuclear structure of neutron-deficient nuclei in the mass region A 180

To describe the nuclear structure of one or a group of nuclei several observables should be consulted. The excitation energies of states with given spin and parity is often known and gives a good first impression. Nevertheless transition strengths of excited states are a much better probe, since they depend quadratically on the wavefunction. These can be determined in a model-independend way from the corresponding level lifetime. Within this work a total of four experiments were performed to measure the lifetime of excited states in 180Pt and 178Hg. In the neutron-deficient mass region close to the shell closure at lead (Z=82) interesting phenomena such as shape coexistence and shape transitions are observed. The most prominent example is 186Pb, which exhibits three different shapes. The ground state as well as two excited 0+ states together with the corresponding rotational bands are associated to multiparticle-multihole excitations resulting in the different shapes. A similar situation is established for the Hg isotopes near the neutron N=104 midshell, where the lightest isotope for which transition strengths are known is 180Hg. Going further away from the shell closure the 176,178,180Os isotopes are good candidates for the X(5) symmetry, i.e. the transition from spherical to axially deformed. Two questions arise: How can the Pt isotopes just between Os and Hg be described and what is the evolution of shape coexistence in the Hg isotopes leaving neutron midshell? Both questions are addressed within this thesis. 180Pt shows no remains of a X(5) like shape transition and multiparticle-multihole excitations are evident. This structure reaches the ground state and a new so-called island of inversion is found. This simultaneously answers the longlasting question whether configuration mixing is needed to describe the Pt isotopes or not. From the excitation energies 178Hg is expected to be at the edge of the shape-coexistence region. The measured transition strengths confirm the shift of the structure with different shape to higher-lying states, but also a new phenomenon of intermediate deformation (between normal and superdeformation) shows up with decreasing neutron number. The interpretation is given by comparison to several existing calculations as well as particular calculations using geometrical models and the interacting boson model. The experiments were carried out at different accelerator facilities employing the recoil distance Doppler-shift (RDDS) method, except one experiment when electronic-timing techniques were used to measure long lifetimes. Further a dedicated plunger apparatus was commissioned during one experiment. The GALILEO -ray spectrometer is the successor of GASP, which necessitated a completely new mechanical design of the plunger. This fragile device newly allows the combination with charged-particle detectors, e.g. EUCLIDES

Preliminary results of lifetime measurements in neutron-rich 53Ti

To study the nuclear structure of neutron-rich titanium isotopes, a lifetime measurement was performed at the Grand Accélérateur National d'Ions Lourds (GANIL) facility in Caen, France. The nucleiwere produced in a multinucleon-transfer reaction by using a 6.76 MeV/u 238U beam. The Advanced Gamma Tracking Array (AGATA) was employed for the γ-ray detection and target-like recoils were identified event-by-event by the large-acceptance variable mode spectrometer (VAMOS++). Preliminary level lifetimes of the (5/2−) to 13/2− states of the yrast band in the neutron-rich nucleus 53Ti were measured for the first time employing the recoil distance Doppler-shift (RDDS) method and the compact plunger for deep inelastic reactions. The differential decay curve method (DDCM) was used to obtain the lifetimes from the RDDS data

Lifetime measurement of the

A $$\gamma$$-$$\gamma$$ coincidence Recoil Distance Doppler-Shift measurement has been carried out on $$^{60}$$Ni to re-measure the lifetime of the $$2_1^+$$ and $$4_1^+$$ states. The new lifetime of the $$2_1^+$$ state supports the adopted NNDC value but disagrees with the results of two more recent Doppler-Shift Attenuation Method measurements, which suggested a longer lifetime. The new result for the $$4_1^+$$ state’s lifetime is significantly shorter than the one recommended in the latest NNDC compilation while also reducing its uncertainty. It therefore resolves an unclear situation, where an unexpected drop in transition strength appeared from $$^{58}$$Ni to $$^{60}$$Ni. Both values match very well with recently applied shell model calculations using the GXPF1A interaction

Lifetimes and electromagnetic transition strength in 124Ba

Lifetimes of excited states of 124Ba were measured by the Recoil Distance Doppler Shift (RDDS) technique. The γ-ray coincidence data were analysed by the Differential Decay Curve method (DDCM). The trend of the experimentally deduced normalized B(E2) values reveals a drop at the 8+1 state, which may be caused by structural changes in the backbending region

Lifetimes and electromagnetic transition strength in

Lifetimes of excited states of 124Ba were measured by the Recoil Distance Doppler Shift (RDDS) technique. The γ-ray coincidence data were analysed by the Differential Decay Curve method (DDCM). The trend of the experimentally deduced normalized B(E2) values reveals a drop at the 8+1 state, which may be caused by structural changes in the backbending region

Structural investigation of neutron-deficient Pt isotopes: the case of

Lifetime measurements with the recoil distance Doppler-shift technique have been performed to determine yrast E2 transition strengths in 178Pt. The experimental data are related to those on neighboring Pt isotopes, especially recent data on 180Pt, and compared to calculations within the interacting boson model and a Hartree-Fock Bogoliubov approach. These models predict prolate deformed ground states in Pt isotopes close to neutron midshell consistent with the experimental findings. Further, evidence was found that the prolate intruder structure observed in neutron-deficient Hg isotopes that is minimum in energy in 182Hg becomes the ground state configuration in 178Pt and neighboring 180Pt with nearly identical transition quadrupole moments. The new data on 178Pt are further discussed in the context of the systematics along the Pt isotopic chain with respect to a possible sharp shape transition towards a weakly deformed or a quasi-vibrational ground state whereas the prolate structure increases in energy in 174,176Pt and becomes an intruder configuration similar to, for example, 180,182Hg

Structural investigation of neutron-deficient Pt isotopes: the case of 178Pt

Lifetime measurements with the recoil distance Doppler-shift technique have been performed to determine yrast E2 transition strengths in 178Pt. The experimental data are related to those on neighboring Pt isotopes, especially recent data on 180Pt, and compared to calculations within the interacting boson model and a Hartree-Fock Bogoliubov approach. These models predict prolate deformed ground states in Pt isotopes close to neutron midshell consistent with the experimental findings. Further, evidence was found that the prolate intruder structure observed in neutron-deficient Hg isotopes that is minimum in energy in 182Hg becomes the ground state configuration in 178Pt and neighboring 180Pt with nearly identical transition quadrupole moments. The new data on 178Pt are further discussed in the context of the systematics along the Pt isotopic chain with respect to a possible sharp shape transition towards a weakly deformed or a quasi-vibrational ground state whereas the prolate structure increases in energy in 174,176Pt and becomes an intruder configuration similar to, for example, 180,182Hg