Fission-fragment angular distributions from resonance fission with oriented nuclei

The angular distribution of fission fragments from oriented 235U nuclei has been measured as a function of incident neutron energy from 0.04 eV to about 2000 eV, using the Harwell electron linear accelerator time-of-flight spectrometer with an electron pulse width of 0.23 μs and a flight path of 10 m. The 235U nuclei were aligned using the electric hyperfine structure method, suggested by Pound (Phys. Rev. 76 (1949) 1410), in the rubidium-uranyl nitrate single crystal. Thirty-six crystals with surfaces enriched with 235U were attached to a copper plate with their c-axes parallel. They were mounted in a cryostat in the neutron beam and cooled to about 0.1K with a He-3/He-4 dilution refrigerator. Fission fragments in directions both parallel and perpendicular to the c-axis were detected by silicon diffused-junction semi-conductor detectors operating at a temperature of 1 deg K. Separate measurements were made to determine the detector background and the unoriented fission rate. The fission-fragment anisotropy follows a relation of the form W(θ) = 1 + A2 f2 P2 (cos θ) + higher terms, where A2 is a nuclear parameter depending essentially on K and J, and f2 is an orientation parameter. The variation of the effect with temperature at particular neutron energies has been studied down to about 0.08 deg K. The effect tends to saturation and little advantage is obtained from working below ∼ 0.1 deg K. The variation of f2 with temperature must be considered carefully, but the higher terms may be largely neglected. The parameter A2 shows significant fluctuations as a function of neutron energy, for example, changing from about -1.1 to -0.5 from 0.2 to 0.5 eV, in qualitative agreement with the work of Dabbs et al., reported at the Salzburg Conference (Proc Conf. Salzburg, 1965 1, 39). The A2 values for individual resonances below about 70 eV, and its average values over groups of resonances at higher energies are being obtained. Later the work will be extended to cover the nuclei 233U and 237Np. (author)