Hyperfine Structure of Erbium-169

The hyperfine structure of radioactive ${\mathrm{Er}}^{169}(T_{\frac{1}{2}}=9.4\mathrm{} \mathrm{days})$ has been studied in the ${}_{}{}^3{}_{}{}^{}H_6^{}{}_{}{}^{}$ electronic ground state by the atomic-beam magnetic-resonance method. The apparatus used was of sufficient accuracy to measure the nuclear dipole moment directly through its interaction with the external magnetic field. The results are $A=725.46\mathrm{}\left(31\right)\mathrm{}$ Mc/sec, $g_J=-1.16381\mathrm{}\left(5\right)\mathrm{}$, and $g_I=+5.55\mathrm{}\left(27\right)\mathrm{}\times {10}^{-4\mathrm{}}$, where $A$ is the magnetic dipole interaction constant and the electronic and nuclear $g$ factors, $g_J$ and $g_I$, are given in units of Bohr magnetons. The nuclear magnetic moment inferred from $g_I$ and corrected for diamagnetic shielding is ${\mu }_I=+0.513\mathrm{}\left(25\right)\mathrm{}$ nm. This value of ${\mu }_I$ is consistent with that obtained from $A$ using the $〈\frac{1}{r^3}〉$ value given by Lindgren.