Gamma spectra from /sup 233/U, /sup 235/U, and /sup 239/Pu during thermal neutron irradiation. [Reaction mechanisms not distinguished]

The gamma spectra from three fissionable isotopes, /sup 233/U, /sup 235/U, and /sup 239/Pu, were measured during neutron irradiation in the thermal column of the Omega West Reactor. The measurements were preceded by a neutron irradiation of at least 20,000 s to build up a near-equilibrium concentration of the shorter half-life fission products. The measurements provide the total gamma spectra and do not distinguish between photons emitted following neutron capture, fission, or by fission products. Cross sections were obtained by comparison with the hydrogen (n,..gamma..) peak from a 44.2-mg sample of polyethylene irradiated in the same flux. A 4.7-g/cm/sup 2/ polyethylene scatterer was used to remove fission neutrons from the photon beam. The spectra for all three isotopes are similar, and prominent peaks occur at the same energies. The spectra can be approximated within approximately 30% by a single exponential: for /sup 233/U and /sup 235/U, sigma(E) approximately equal to 8000 (exp -1.1 E), and for /sup 239/Pu, sigma(E) approximately equal to 11,000 (exp -1.1 E), where E is in MeV. The multiplicity of photons per absorbed neutron, the energy released as gammas per absorbed neutron, and the average photon energy were obtained by summing the photon probability over the spectra.