Beta Decay Studies of Heavy Neutron-Rich Nuclei Around A=190.

Heavy neutron-rich nuclei with A~190 have been investigated following their production by the relativistic projectile fragmentation of a 208Pb primary beam on a 2.45 g/cm2 9Be target. The secondary beam of radioactive fragmentation residues was separated using the in-flight FRagment Separator (FRS) at GSI and stopped in the RISING active stopper detector. The active stopper consisted of three 5cm x 5cm Double Sided Silicon Strip Detectors (DSSSDs), each pixellated into 16 strips in both the horizontal and vertical directions. The DSSSDs were used to measure the position of the implanted ions and correlate these with subsequent beta-decays detected in the same or neighbouring pixels. Gamma-ray decays from previously reported isomers in 188,189Ta, 190,191W and 192, 193Re, together with previously unreported isomeric states in 190Ta and 187Hf have been measured using the 15 germanium cluster detectors of the RISING gamma-ray detection array. Time and position correlations between implanted ions of 188,190,192Ta and their subsequent beta-decays to 188,190,192W were also determined. Beta decay half lives of 188,190,192Ta of 19.6(20)s, 5.3(7)s and 2.2(7)s respectively were established and 7 rays from respective states in the W daughter isotopes were measured using the B-delayed coincidence technique afforded by the RISING active stopper. The results of these investigations are consistent with experimental information reported from previously reported in-beam and isomer-decay studies of 188W and 190W respectively. Evidence is presented from the beta-decay of 190Ta to provide a first measurement for the energy of the gamma bandhead Ipi=2+ state in 190W and excitation energy of the yrast 2+ state in 192W thus providing spectroscopic information on the heaviest tungsten observed to date. The evolution of ground state structure along the W isotopic chain is discussed as evidence for a possible proton sub-shell effect for the A ~190 region and the maximisation of the 7-softness of the triaxial nuclear potential in this region at N ~116. The experimentally deduced level schemes are compared with theoretical predictions using the Interacting Boson Approximation (IBA) and microscopic/macroscopic calculations of the nuclear shape. Beta decay half lives for the neutron-rich nuclei 187,188,189Hf, 188,189,190,191,192Ta, 191,192,193,194W and 193,194,195Re are also established for the first time in this work