Fission of polonium, osmium, and erbium composite systems

Fission cross section excitation functions were measured from near threshold to ∼10 MeV/nucleon using ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16,18}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{24,26}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$, and ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$ beams. The systems studied included ${}_{}{}^{210}{}_{}{}^{}\mathrm{Po}$ formed in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ induced reactions; ${}_{}{}^{186}{}_{}{}^{}\mathrm{Os}$ formed in ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, and ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$ reactions; and ${}_{}{}^{158}{}_{}{}^{}\mathrm{Er}$ formed in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$, and ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$ reactions. In addition, the composite systems ${}_{}{}^{204,206,208}{}_{}{}^{}\mathrm{Po}$ formed with ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ projectiles were studied. In the experiment the velocities and emission angles of two coincident fission fragments were measured using position-sensitive multiwire proportional counter stop detectors and a thin (∼200 μg/${\mathrm{cm}}^2$) gas "start" detector. The measured fission excitation functions along with previous data from ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ bombardments for the ${}_{}{}^{186}{}_{}{}^{}\mathrm{Os}$ and ${}_{}{}^{210}{}_{}{}^{}\mathrm{Po}$ systems and recent data on the ${}_{}{}^{200}{}_{}{}^{}\mathrm{Pb}$ system are compared to statistical model calculations using recent angular momentum dependent fission barriers calculated by Sierk. Comparisons of calculated and measured fission excitation functions show good overall agreement between data and theoretical predictions. These comparisons give good agreement for both a wide range of mass of the composite system and for a wide range of mass asymmetry in the entrance channel. It is concluded that the fission barriers of Sierk give a good description of both the mass and angular momentum dependence of fission barriers in this region.