Kinetic Energy Release in Fission of ${\mathrm{U}}^{238}$, ${\mathrm{U}}^{235}$, ${\mathrm{Th}}^{232}$, and ${\mathrm{Bi}}^{209}$ by High Energy Neutrons

The ionization produced by single fission fragments was observed in an ionization chamber using electron collection. If fission is induced by 90-Mev neutrons, the distribution of kinetic energy of the fission fragment versus the number of fragments is found to have a single peak in contrast to the double-peaked curve corresponding to the fission made by thermal neutrons. Fission induced by 45-Mev neutrons gives a distribution in which two peaks appear. The dip between the two peaks is about twenty percent of the height of the peaks. The mean kinetic energy is found to be 80±2 Mev for ${\mathrm{U}}^{238}$, 83±1.5 Mev for ${\mathrm{U}}^{235}$, 82±2 Mev for ${\mathrm{Th}}^{232}$, and 71±2 Mev for ${\mathrm{Bi}}^{209}$ in the case of fission produced by 90-Mev neutrons. The mean kinetic energy is 79±3 Mev for ${\mathrm{U}}^{238}$ and 84±3 Mev for ${\mathrm{Th}}^{232}$ if 45-Mev neutrons are used. The most probable kinetic energy of the fragment is 83 Mev for ${\mathrm{U}}^{238}$, 80 Mev for ${\mathrm{U}}^{235}$, 83 Mev for ${\mathrm{Th}}^{232}$, and 75 Mev for ${\mathrm{Bi}}^{209}$ in the case of fission produced by 90-Mev neutrons. The bearing of these observations on the mechanism of fission is discussed.