Experimental investigation of the isomeric-shelf phenomenon in the photofission cross sections of heavy nuclei

Yield measurements are reported for photofission of /sup 232/Th, /sup 236/U, /sup 238/U, and /sup 237/Np in the deep sub-barrier energy region 3.5--4.6 MeV where anomalies in the cross section: isomeric shelfs: were previously observed (Phys. Rev. C, 12, 863 (1975)); Pis'ma. Zh. Eksp. Teor. Fiz. 22, 255 (1975), (JETP Lett. 22, 118 (1975)). The various sources of background arising in this energy region, which is only with difficulty accessible for measurements, are analyzed in detail. Analysis of the anomalies in the behavior of the integrated photofission yields in the case of the two nuclei /sup 236/U and /sup 238/U most favorable for study indicates a resonance nature of the cross sections for delayed fission and a substantially more complicated physics of the phenomenon than the simplified interpretation of Bowman (Phys. Rev. C, 12, 863 (1975)).