Nuclear Spin ofHo166

The hyperfine structure of ${\mathrm{Ho}}^{166}$ has been examined by means of the atomic-beam magnetic-resonance technique. The atomic ground state appears to be ${}_{}{}^4{}_{}{}^{}I_{\frac{15}{2}}^{}{}_{}{}^{}$, and $g_J$ has the measured value 1.19509±0.00007, in close agreement with the Russell-Saunders value of $\frac{6}{5}$. Only one resonance is observed, and its transition frequency for $0.15<~H<~150$ gauss is proportional to the magnetic field strength to within experimental error. The simplest interpretation is that the nuclear spin is $I=0\mathrm{}$. It is shown that if $I=1\mathrm{}$, the magnetic hyperfine interaction constant $a$ must be less than 5 kc/sec.