Thermal-Neutron-Capture Gamma Rays inYb170,Yb172, andYb174

A Ge(Li) detector was used to investigate the $\gamma$ spectra following thermal neutron capture in enriched isotope targets of ${\mathrm{Yb}}^{170}$, ${\mathrm{Yb}}^{172}$, and ${\mathrm{Yb}}^{174}$. In addition to capture in these isotopes, $\gamma$ lines were identified as resulting from thermal capture in ${\mathrm{Yb}}^{171}$ and ${\mathrm{Yb}}^{173}$. The neutron separation energies of ${\mathrm{Yb}}^{171}$, ${\mathrm{Yb}}^{172}$, ${\mathrm{Yb}}^{173}$, ${\mathrm{Yb}}^{174}$, and ${\mathrm{Yb}}^{175}$ were found to be 6616, 8023, 6365, 7465, and 5819 keV, respectively. The experimental error is ±3 keV in all cases. For capture by the even-even isotopes, the energies and intensities of $\gamma$ transitions originating from the capture state are presented together with the spins and Nilsson assignments of the low-lying rotational bands which they populate. Our measurements are compared with those in other experiments, and the results of our measurements are discussed. Regularities in the decay of the capture state to low-lying Nilsson levels have now been observed in comparing thermal neutron capture by different even-even nuclei in the region $A=176\mathrm{}$. The regularities include relatively strong population of Nilsson bands with $\Lambda$ equal to zero or 1 as compared to population of bands with $\Lambda$ greater than 1. In addition, there seems to be a preference for the capture state decaying to particle rather than hole bands. A purely statistical version of thermal neutron capture would not seem to hold in this mass region.