Coincident Time-of-Flight Measurements of the Velocities of Fission Fragments from Charged-Particle-Induced Fission

Measurements have been made of the velocities of the coincident fission fragment pairs emitted in the 25.7- and 29.5-MeV ${\mathrm{He}}^{4}$-induced fission of ${\mathrm{U}}^{233}$ and ${\mathrm{Th}}^{230}$, the 25.7- and 29.5-MeV ${\mathrm{He}}^{4}$-induced fission of ${\mathrm{Th}}^{232}$, and the 12.0- and 14.0-MeV ${\mathrm{H}}^{2}$-induced fission of ${\mathrm{Th}}^{230}$. Previously unpublished data for the 21.8-MeV ${\mathrm{He}}^{4}$-induced fission of ${\mathrm{U}}^{233}$ and ${\mathrm{Th}}^{232}$ are also included. A decrease in average total fragment kinetic energy for symmetric mass division is observed in each case. The relatively large yield of symmetric mass divisions in this work permits a reliable measurement of this effect. The distributions of fragment velocities, masses, and kinetic energies are consistent with the hypothesis of two competing modes of fission. A comparison is made of the primary mass yields obtained in the present work with the yields obtained by radiochemical methods to determine the dependence of prompt neutron emission on fragment mass. The neutron emission found, although subject to poorly known and possibly large uncertainties in the radiochemical data, appears to show a rather pronounced structure of just such a nature as might be attributed to fragment-shell effects that persist to these high excitation energies. Such a structure is not observed, however, when the present primary mass data are compared with mass distributions derived from recent fragment double-energy measurements. The time-of-flight mass distributions were corrected for the influence of the prefission neutron emission and the detection solid angle. As a part of the correction, the dependences of the yield of symmetric mass divisions on the excitation energy of the fissioning nuclei were estimated for each of the target-projectile systems.