Development of 207Pb, 208Pb and 209Bi target wheels in the synthesis of 107Ns, 108Hs and 109Mt

Abstract

The development of 207Pb, 208Pb and 209Bi 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 ∼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 207Pb, 208Pb and 209Bi with deposition yields of 35–55% from tantalum crucibles. Applications of the target wheels in heavy-ion fusion reactions with beams of 54Cr and 58Fe at energies near the Coulomb barrier and intensities of ∼10 12 particles/s are reported. The target parameters for the production runs of the new chemical elements 107Ns, 108Hs and 109Mt are included.