Phonon Spectrum of Thorium

Inelastic-neutron-scattering measurements of the phonon dispersion relation in thorium at room temperature are reported. The measurements were made along the ($0,0,\zeta$), ($0,\zeta ,\zeta$), ($\zeta ,\zeta ,\zeta$), and ($0,\zeta ,1\mathrm{}$) symmetry directions. The dispersion curves are dominated by nearest-neighbor interactions but indicate a residual long-range interaction to at least seventh nearest neighbors. A seventh-nearest-neighbor force-constant model was fitted to the data and used to obtain a frequency distribution function and the temperature dependence of the Debye temperature. The transverse modes along the ($0,\zeta ,\zeta$) direction appear to show structure indicative of strong electron-phonon interaction effects.