Fission lifetimes of Th nuclei measured by crystal blocking

stal blocking lifetime measurements have been made for highly excited Th nuclei with neutron number well removed from the stability line. Thin W crystals were bombarded with S-32 ions in the energy range 170-180 Mev and the yield of fission fragments was measured for emission close to a (111) axis. The fission blocking dips are compared to the appropriately scaled ones for elastic scattering of the S-32 beam ions and no significant difference is seen between the dips. This implies that the fraction of nuclei fissioning with lifetimes longer than 10 as is less than 2%. Fission lifetimes are increased by viscosity in the nuclear mass flow and comparison with a statistical model calculation indicates that the viscosity parameter, eta, must be lower than for Th and U nuclei near beta-stability. The effect of the N = 126 magic number is discussed.