The high-energy $\gamma$-ray spectrum from thermal-neutron capture in natural holmium has been studied over the energy range of 5000 to 6200 keV. Low-energy gamma radiation of the same reaction has been measured from 30 to 750 keV and conversion electrons from 29 to 500 keV. Data from the reaction ${\mathrm{Ho}}^{165}\mathrm{}(d,p)\mathrm{}{\mathrm{Ho}}^{166}$ have been analyzed. The combination of the results of these experiments yields an energy of 6243±3 keV for the neutron binding in ${\mathrm{Ho}}^{166}$. The ground-state rotational band is observed with members up to $I=6-$, and the $I=7-$ level is strongly suggested. A $K=3+$ band built on the 190.9-keV isomer, and a $K=4+$ band are disclosed. The $K=3+[523\mathrm{}\uparrow -521\mathrm{}\downarrow ]$ band members are observed up to $I=6+$ and very probably $I=7+$. The $K=4+[523\mathrm{}\uparrow +521\mathrm{}\downarrow ]$ band contains the 4+ and 5+ levels, and the 6+ is indicated. The $1+[523\uparrow -523\mathrm{}\downarrow ]$ level is strongly populated in the $\gamma$-ray cascade following neutron capture. A rotational band superimposed on this state and containing the 2+, 3+, 4+, 5+, and probably 6+ levels is proposed. The heads of the $5+[523\uparrow +521\mathrm{}\uparrow ]$ and $2+[523\uparrow -521\mathrm{}\uparrow ]$ bands are suggested at 264 and 430-keV, respectively. The 7 - and 8 - members of the $[523\mathrm{}\uparrow +633\mathrm{}\uparrow ]$ band are indicated at 5±2 and 136±2 keV, respectively. The $6+[523\uparrow +512\mathrm{}\uparrow ]$ state at 294±2 keV probably decays through the 289.12-keV $\gamma$ transition to the 5-keV level. An additional level with spin 4+ or 5+ has been found at 558.56 keV. A 0- (1-) state at 373.13 keV seems to be very weakly excited during the $\beta$ decay of ${\mathrm{Dy}}^{166}$, but fairly strongly populated in the ($n,\gamma$) process. The precise energies of low-energy ($n,\gamma$) transitions permit a very accurate energy determination of most states observed or indicated. It is found that most bands where more than two members have been seen obey the simple rotational formula very accurately, i.e., they have a small value of $B$, the "rotation-vibration interaction constant." In fact, the rotation of ${\mathrm{Ho}}^{166}$ is observed to be more perfect than that of neighboring even-even nuclei in the ground-state configuration. The $\gamma$-ray transition probabilities for intraband transitions obey the normal Alaga rules, and the constancy of the strength parameter, $\frac{C^2}{q^2}$, gives additional evidence for band assignments. A calculation of the energies of low-lying levels in ${\mathrm{Ho}}^{166}$ shows good agreement with the observed level energies. The partial cross section for thermal-neutron excitation of the 1200-year state of ${\mathrm{Ho}}^{166}$ has been found to be 3.5±0.5 b by a measurement of the ratio of 1200-year to 27.74-h activities.

• Received 31 October 1966

DOI:https://doi.org/10.1103/PhysRev.155.1265