Isomer Shift and Hyperfine Splittings of the 59.6-keV Mössbauer Resonance inNp237

The Mössbauer resonance of the 59.6-keV transition in ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ was observed in a variety of chemical compounds, using the $\alpha$ decay of ${}_{}{}^{241}{}_{}{}^{}\mathrm{Am}$ as a source. The shift of the resonance as a function of the charge state of neptunium, coupled with atomic self-consistent-field calculations for the electron densities at the nucleus, leads to a change in the nuclear charge radius $\frac{\Delta 〈r^2〉}{〈r^2〉}=-3.5\mathrm{}\times {10}^{-4\mathrm{}}$. Using the collective nuclear model, this number can be expressed as a change in the nuclear deformation of $\frac{\Delta \beta }{{\beta }_0}=-6.3\mathrm{}\times {10}^{-3\mathrm{}}$. All compounds investigated exhibited electric quadrupole or magnetic hyperfine splitting, or both. These data confirmed the earlier results of the ratios of the nuclear moments as $\frac{g^{*}}{g}=0.533\mathrm{}\pm 0.005$ and $\frac{Q^{*}}{Q}=1.0\mathrm{}\pm 0.1$.