The spin and hyperfine structure of 7.7-min ${\mathrm{K}}^{38}$ have been determined by the atomic-beam magnetic-resonance method. In this experiment about 100 mg of ${\mathrm{K}}^{39}$ was placed in an oven in the resonance apparatus and was bombarded with an 0.3 μA beam of 18-MeV protons. ${\mathrm{K}}^{38}$ was produced by the reaction ${\mathrm{K}}^{39} (p,d or pn){\mathrm{K}}^{38}$, and the resulting specific activity was high enough so that an adequate atomic beam could be maintained for 2 to 3 h before the oven was empty. Both $\Delta F=0\mathrm{}$ and $\Delta F=1\mathrm{}$ resonances were observed, and final values are $I=3\mathrm{}$, $\Delta \nu =1415.292\mathrm{}\pm 0.009$ Mc/sec. Comparison with other potassium isotopes yields ${\mu }_I=+1.3735\mathrm{}\pm 0.0010$ nm (diamagnetically corrected). The value for the magnetic dipole moment is in good agreement with the prediction based on the coupling of a $d_{\frac{3}{2}}$ neutron hole and a $d_{\frac{3}{2}}$ proton hole to the above spin value.

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