DEVELOPMENT OF PB-207, PB-208 AND BI-209 TARGET WHEELS IN THE SYNTHESIS OF (107)NS, (108)HS AND (109)MT

The development of Pb-207, Pb-208 and Bi-209 target wheels and their applications in heavy-ion fusion reactions are reviewed. In both fabrication and use, the centers of the evaporator or accelerator beams are focussed at wheel radii of 155 mm to specially shaped frames which generate very homogeneous target layers and very constant reaction and counting rates in the experiment. Target areas of up to approximately 98% of a wheel's circumference of 974 mm can be provided. The preparation procedures for the necessary C backings and protecting layers of C are described, and details are given for the development of high-vacuum evaporations of Pb-207, Pb-208 and Bi-209 with deposition yields of 35-55% from tantalum crucibles. Applications of the target wheels in heavy-ion fusion reactions with beams of Cr-54 and Fe-58 at energies near the Coulomb barrier and intensities of approximately 10(12) particles/s are reported. The target parameters for the production runs of the new chemical elements Ns-107, Hs-108 and Mt-109 are included.