Production of Neutron-Rich Isotopes from ThO$_{2}$ Targets

The Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory provides radioactive-ion beams for research in nuclear and astrophysics. An essential function is to produce a variety of radioactive species to meet the intensity, energy, and purity requirements of specific experiments. The primary production method has been proton-induced fission of uranium. However, production rates in the mass 80-95 region are expected to be higher from proton-induced fission of thorium. [1] In particular, $^{82}$Ge is expected to increase by an order or magnitude [2], and $^{92}$Sr yields should increase. Our first series of online experiments using a dense ThO$_{2}$ powder target, nevertheless, produced yields roughly a factor of 10 less than for UC$_{x}$ targets. Porous targets of ThO$_{2 }$have been produced and extensive yield measurements performed. Results from the different ThO$_{2}$ targets are compared to each other and to yields from some UC$_{x}$ targets. In order to obtain some understanding of these results, holdup-time measurements have been made on some chemical elements. [1] T. Ohtsuki, et al., Phys Rev C40 (1989) 2144. [2] V. Rubchenya, private communication.