J. Eric Lynn

Neutron Doppler broadening studies of tantalum and tungsten metal

Abstract With the aim of providing experimental verification of the validity of the neutron resonance spectroscopy method for measuring the moments of the phonon frequency spectra, we have determined some of the phonon spectral parameters for W and Ta in the bcc phase of these metals by measuring the Doppler broadening of neutron resonances as a function of temperature. We find the mean phonon energy (first moment of the phonon frequency spectrum) of W to be 〈 hν 〉=20.2±0.4 meV and the second moment to be 〈( hν ) 2 〉=(4.9±0.2)×10 −4 eV 2 . The Boltzmann-weighted moments from all the measurable resonances of tungsten are quite consistent with a Debye model with a Debye temperature of 315±4 K. For Ta, 〈 hν 〉=14.3±0.1 meV, 〈( hν ) 2 〉=(2.2±0.2)×10 −4 eV 2 . The fitted Debye model temperature for the tantalum data is 217±2 K. In both cases the first and second moments agree well with those determined from neutron coherent inelastic scattering. Results with reasonable accuracy were extracted from resonances up to a few tens of eV. These results give high confidence that neutron resonance Doppler broadening spectroscopy is a viable and useful method for determining the characteristics of phonon frequency spectra.

Theory in Evaluation of Actinide Fission and Capture Cross Sections

We discuss the possibilities and limitations of the use of theory as a tool in the evaluation of actinide fission and capture cross sections. We consider especially the target 235U as an example. We emphasize the roles of intermediate structure in the fission cross section and of level width fluctuations in both intermediate structure and fine structure, noting that these lead to a breakdown of Hauser‐Feshbach theory at sub‐barrier and near‐barrier energies. At higher energies (where fluctuation‐averaged Hauser‐Feshbach theory is applicable) semi‐quantitative and intuitive representations of transition state spectra and barrier level density functions have to be tested against experimental data wherever these are available. Adjustment of the fission cross section against inelastic scattering to the much better known levels of the residual nucleus should then lead to a fairly sound estimate of the capture cross section. We compare such estimates with evaluated and experimental data for 235U.