The hyperfine structure in the ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ electronic ground state of ${\mathrm{K}}^{42}$ (12.4-h) has been studied using a high-precision atomic-beam apparatus. The hyperfine separation is shown to be $\Delta \nu =1\mathrm{} 258 876 947 \mathrm{}\left(15\right)\mathrm{}$ Hz. The nuclear magnetic dipole moment, measured independently of $\Delta \nu$, is ${\mu }_I=1.142\mathrm{} 4\left(2\right)\mathrm{} {\mu }_N$. The hyperfine magnetic anomaly is evaluated as ${}_{39}{}^{}{}_{}{}^{}\Delta _{42}^{}{}_{}{}^{}=+0.000\mathrm{} 3+0.003 36\left(38\right)\mathrm{}$.

• Received 9 May 1969

DOI:https://doi.org/10.1103/PhysRev.184.1102