Energy Distributions of Fragments from Fission of U235 , U238 , and Pu239 by Fast Neutrons

The energies of fragments from fission of ${\mathrm{U}}^{235}$, ${\mathrm{U}}^{238}$, and ${\mathrm{Pu}}^{239}$ by 14-Mev neutrons, from fission of ${\mathrm{U}}^{238}$ by 2.5-Mev neutrons, and from fission of ${\mathrm{U}}^{235}$ and ${\mathrm{Pu}}^{239}$ by thermal neutrons have been measured in a single Frisch grid ionization chamber. The energy distributions for fast neutrons are similar to those previously obtained for fission by thermal neutrons. The most probable energies of the light and heavy fragments for fission by 14-Mev neutrons do not change significantly from their values for slow-neutron induced fission. The valley between peaks is higher for fission induced by 14-Mev neutrons than for low-energy neutron-induced fission.A double Frisch grid chamber has been used to measure simultaneously the energies of both fragments from fission of ${\mathrm{U}}^{235}$ by 14-Mev neutrons. The main change in the distribution of fission modes from that for thermal-neutron-induced fission is the increased probability for symmetrical fission.