Gamma Rays from Short-Lived Fission-Fragment Isomers

Measurements of the energy spectra of isomeric gamma rays from the neutron fission of ${\mathrm{U}}^{235}$ and ${\mathrm{Pu}}^{239}$ at a number of time intervals between 50 and 600 μsec showed six prominent gamma rays for both cases of fission. The intensities and half-lives for these gamma rays indicate that there are three fission-fragment isomers, each giving rise to a pair of gamma rays in cascade. The energies of the cascade gamma rays and the half-life for each isomer are: 1260, 450 keV (80 μsec), 850, 250 keV (54 μsec), and 990, 710 keV (32 μsec). Absolute gamma-ray intensities were obtained by measuring the integral number of gamma rays above a fixed bias energy as a function of time and normalizing these data at 10 msec to the absolute intensities derived from the theoretical work by Griffin and the data of Fisher and Engle. The intensities of delayed gamma rays from photofission of ${\mathrm{U}}^{235}$, ${\mathrm{Pu}}^{239}$, and ${\mathrm{U}}^{238}$ were also measured at early times, starting at 2 μsec after fission. An intense short-lived component with an apparent half-life of a few μsec was observed for all three cases of photofission.