Delayed Gamma Rays from Photofission of ${\mathrm{U}}^{238}$, ${\mathrm{U}}^{235}$, and ${\mathrm{Th}}^{232}$

Absolute delayed gamma-ray intensities from the photofission of ${\mathrm{U}}^{238}$, ${\mathrm{U}}^{235}$, and ${\mathrm{Th}}^{232}$ were measured as a function of time after bursts of fissions produced by the bombardment of samples with 20-MeV x rays from an electron linear accelerator. For all three target nuclei, components of short-lived gamma rays, which are attributed to the decay of isomers formed either in fission or, less likely, by other photoreactions, contributed markedly to the gamma-ray activity for times up to 800 μsec after the beam pulse. Beyond 800 μsec the gamma-ray activity decreased little for times up to about ${10}^{-1\mathrm{}}$ sec. The gamma rays emitted in this interval, the "plateau" region, and at longer times are believed to follow beta decay of fission fragments. In the "plateau" region, the absolute intensities of gamma rays ($E_{\gamma }>0.51\mathrm{}$ MeV) from the photofission of ${\mathrm{U}}^{238}$, ${\mathrm{U}}^{235}$, and ${\mathrm{Th}}^{232}$ are 0.70, 0.26, and 0.35 photons/fission-sec, respectively. After 0.1 sec the gamma-ray decay curves have the same shapes as those observed by other investigators in neutron fission of the same target nuclei, but the absolute intensity from photofission is approximately a factor of two less than that from neutron fission.