Hyperfine Interactions and Anisotropic Lattice Vibrations of ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ in $\alpha$-Np Metal

Hyperfine spectra utilizing the nuclear $\gamma$-ray resonance (Mössbauer effect) of the 59.6-keV $\gamma$ ray of ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ have been obtained in $\alpha$-Np metal. For the temperature range 1.7-48°K, the spectra show no magnetic effects and can be described as two quadrupole interactions having widely differing values of electric field gradient and asymmetry parameter. These two spectra can be related to the two inequivalent sites in the orthorhombic unit cell of $\alpha$-Np. The intensities of the hyperfine lines show the presence of anisotropic lattice vibrations (Goldanskii-Karyagin effect). At one site the lattice vibrations are only slightly anisotropic, while for the other site a maximum rms deviation of ≈ 0.1 Å is deduced.