Alpha decay studies of very neutron deficient isotopes of Hf, Ta, W, and Re

Very neutron deficient isotopes in the Hf-Re region were produced by bombarding targets of sup(107),109Ag, sup(nat, 108, 110)Pd, and 103Rh with 58Ni accelerated by the linear accelerator UNILAC at GSI. After separation from the projectile beam by the velocity filter SHIP, the fusion products were implanted with their full recoil energy into a silicon surface barrier detector. The subsequent alpha decay of the implanted ions was measured with the same detector. In some of the experiments the evaporation residues were implanted into a position sensitive silicon surface barrier detector. With a newly developed position and time correlation technique parent daughter relationships, half lives, and α branching ratios of a large number of isotopes could be determined. In the investigated reactions the eleven new isotopes 161-164Re, 160W, 157-161Ta, and 156Hf could be identified. The measured Qsub(α) values, half lives, and α branching ratios are discussed. Two new high energy α transitions of (7,408 +- 10) keV and (7.804 +- 15) keV were also found with half lives of 2.7 ms and 0.52 ms, respectively. A tentative mass and atomic number assignment gives A = 153-156 and Z = 70-72. The energies of the excited isomeric states are between 2 MeV and 3 MeV. From the systematics of alpha ground state transitions in even nuclei a hindrance of 105 can be deduced for both transitions, possibly indicating orbital angular momenta on the order of 10 h for the alpha emitting states. The isomers are proposed to belong to shell model isomers beyond 146Gd. (orig.)