Delayed Neutrons fromU238andTh232Fission

Abstract

The half-lives of the delayed neutron emitters from ${\mathrm{U}}^{238}$ and ${\mathrm{Th}}^{232}$ fission were found to be 0.4, 1.7, 5.8, 22.6, and 55.2, and 0.41, 1.4, 4.4, 20, and 56 sec., respectively, by using fast neutrons from 15-Mev cyclotron deuterons on a thick carbon target. These, when compared with those from the slow neutron fission of ${\mathrm{Pu}}^{239}$ and ${\mathrm{U}}^{235}$, indicate that delayed neutron emitters from fission of ${\mathrm{Pu}}^{239}$, ${\mathrm{U}}^{235}$, ${\mathrm{U}}^{238}$, and ${\mathrm{Th}}^{232}$ are identical. The delayed neutron yield after saturation bombardment was about 8 per 100 fissions. A preliminary survey indicated that the 22 and 56 sec. half-lives were also present when LiF or ${\mathrm{B}}_4$C instead of carbon was used as the cyclotron target for producing faster neutrons.