Hyperfine structure has been observed in Mössbauer spectra of the 59.54-keV $\gamma$ ray of ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ in Np${\mathrm{Al}}_2$ and Np${\mathrm{Cl}}_4$. At 77°K, Np${\mathrm{Al}}_2$ gave an unsplit absorption line; at 4.2 and 2.35°K, magnetic hfs extending over $\approx 18$ cm/sec was observed. The partially resolved spectrum of Np${\mathrm{Cl}}_4$ at 77°K was interpreted as pure quadrupole splitting; at 4.2°K, combined magnetic and quadrupole interactions were found. From these spectra, the following ratios were determined between the 59.54-keV level and the ground state of ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$: quadrupole-moment ratio +1.0±0.1; magnetic-moment ratio +0.537±0.005. Nuclear polarization in the Mössbauer spectra of Np${\mathrm{Al}}_2$ at 4.2 and 2.35°K enabled the magnetic-moment ratio to be determined uniquely. The results agree with theoretical predictions of the collective model if the odd-nucleon $g$ factor is quenched to about 70% of the free-proton value. From a comparison of the present work with earlier measurements, a new estimate for the ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ ground-state magnetic moment is 2.8 nm.

• Received 17 July 1967

DOI:https://doi.org/10.1103/PhysRev.165.1319