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Nuclear structure studies of neutron-deficient nuclei in light Pb region using radioactive ion beams

Final Report Summary - HEAVYRIB (Nuclear structure studies of neutron-deficient nuclei in light Pb region using radioactive ion beams)

One of the goals of modern nuclear physics research is to understand the origin of coexisting nuclear shapes and exotic excitations and their relation to the fundamental interactions between the nuclear constituents. These subjects can be investigated particularly well in the Pb isotopes close to neutron mid-shell, where a relatively small proton shell gap, together with a large valence neutron space, provides fertile ground for studies of shape transitions within a small energy range. In alpha-decay studies, the first two excited states of the mid-shell nucleus 186Pb were observed to be 0+ states. On the basis of alpha-decay hindrance factors, the second 0+ state was associated with mainly (2p - 2h) configuration, whereas the third 0+ state was associated with a (4p - 4h) configuration. Consequently, together with the spherical ground state, the three 0+ states with largely different structures establish a unique shape-triplet in 186Pb. Very recently, rotational bands built on these states were observed in in-beam gamma-ray measurement and their collectivity confirmed in lifetime measurements. In order to establish a complete picture of shape coexistence in this region, the knowledge of transition probabilities from nuclear states assigned with different shapes is essential. Transition probabilities are very sensitive to the details of a nuclear wave function and, consequently, information about nuclear shape and configuration mixing can be inferred. The main objective of the present project was to carry out the investigations of nuclear collectivity and mixing of the low-lying states in the neutron-deficient Pb nuclei, namely even-mass isotopes 188-198Pb, employing the REX-ISOLDE facility. The isotopes of interest are of particular importance as they lie in so-called transitional region, where a transition from a weaker deformed oblate structure to a strongly deformed prolate structure is proposed for the yrast states.

The first part of experiment was run in August 2010, in which data for 192Pb nucleus was collected. The second part was run in the beginning of June 2011. The Pb nuclei were extracted from the ISOLDE UCx target and the post-accelerated beam was delivered to MINIBALL target position. At MINIBALL, the Pb nuclei were Coulomb excited in inverse kinematics using secondary 112Cd target. The MINIBALL Ge-detector array was used to detect gamma-rays de-exciting levels under investigation. Both scattered projectiles and target recoils were detected using an annular double sided silicon strip detector (CD) positioned on the beam axis after the secondary target. For the first time, such an exotic Pb beam was post-accelerated. Data are still under analysis, but preliminary results show that the collectivity (B(E2)-values) of the first 2+ states in 188-198Pb isotopes can be deduced. Data will also allow to study the systematic behaviour of mixing between different shape coexisting structures.

One of the objectives of this proposal was to study and develop new methods for manipulation and preparation of radioactive beams at REX-ISOLDE. The fellow developed a device to monitor the shape of the REX-LINAC macro pulse. This will be an important tool when setting up the slow extraction from REX-EBIS.

In the endeavour to understand nuclear structure and related phenomena, in-beam gamma-ray spectrometers have been important tools for decades. However, a gamma-ray spectrometer can provide only partial information of nuclear de-excitation processes as it can not detect electrons originating from internal conversion. The fellow has initiated a project that aims to combine the MINIBALL array with electron spectrometer. The simultaneous detection of conversion electrons and gamma-rays at MINIBALL would not only provide information on shape coexistence in light Pb isotopes, but has also considerable interest from the MINIBALL community.