Determination of the lifetime of excited compound nuclei in fission using the shadow effect

The results of three experiments on the observation of the shadow effect in the fission of ²³⁸U nuclei in a UO₂ single crystal induced by 1.7 and 3.3 MeV neutrons and 25 MeV α-particles are discussed. Comparison is made of the shapes of the fission-fragment angular distributions in the vicinity of two 〈111〉 crystallographic axes directed along and normal to the incident beam. It was found that the displacement of a fissionable nucleus from a lattice site in response to the momentum of the incident particle affects the shape of the shadow. The values of the lifetime of the ²³⁹U compound nucleus were found to be (3.5±1.0) × 10⁻¹⁶sec at an excitation energy of 6.5 MeV and (2.0±0.8) × 10⁻¹⁶sec at 8.1 MeV.