Fission lifetimes of Th nuclei measured by crystal blocking

Abstract:

Crystal 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 ³²S 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 ³²S 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, η, must be lower than for Th and U nuclei near β-stability. The effect of the N = 126 magic number is discussed.