Atomic and Nuclear Effects in the Slow-Neutron Total Cross Section of Terbium

The total cross section of terbium has been measured for neutron energies $E_n$ from 0.003 to 1.78 eV. For neutron energies above the Be cutoff a new method employing two crystal monochromators in series was used. For $E_n\lesssim 0.005$ eV a single-crystal monochromator in conjunction with a Be filter was employed. Using earlier measurements of the radiative-capture cross section, the experimental results have been analyzed to include the contributions due to paramagnetic scattering, coherent scattering, and phonon effects. The analysis shows that the experiment and the calculations are consistent everywhere except $0.015\lesssim E_n\lesssim 0.10$ eV. In this energy range inelastic coherent scattering is not accurately accounted for with the use of Placzek's incoherent approximations. In addition crystalline-field effects give rise to further complications. The comparison between the experiment and the calculations for $0.015\lesssim E_n\lesssim 0.10$ eV suggests that studies of coherent inelastic scattering and crystalline-field effects in terbium metal are needed. The experimental results for $E_n\gtrsim 0.12$ eV yield the potential scattering cross section as 7.5 ± 0.5 b. This corresponds to a spin-independent nuclear radius of 7.73 ± 0.27 fm and to a radius parameter of 1.43 ± 0.05 fm. The results suggest that the incoherent scattering cross section, if present, is very small (≲ 1.0 b).