Possible Evidence for Multistep Processes in the (d,t) Reaction on Rare-Earth Nuclei

The ($d,t$) reaction has been studied for targets of ${}_{}{}^{160}{}_{}{}^{}\mathrm{Gd}$, ${}_{}{}^{162,164}{}_{}{}^{}\mathrm{Dy}$, and ${}_{}{}^{166,168}{}_{}{}^{}\mathrm{Er}$ at a deuteron energy of 17 MeV. Although the angular distributions for transitions populating most residual states are well reproduced by distorted-wave Born-approximation (DWBA) calculations, a number of transitions have angular distributions whose shape departs significantly from the DWBA predictions. This paper presents a sample of 10 such anomalous transitions which were selected because their Nilsson model band assignments are reasonably well established and their cross sections are reasonably large relative to cross sections for transitions to well fit levels of the same spin-parity. The levels which exhibit anomalous angular-distribution shapes also tend to have anomalous cross sections (spectroscopic factors) relative to Nilsson model predictions. Also most of these levels are concentrated in the ${\frac{11}{2}}^{-}\mathrm{}\left[505\right]\mathrm{}$, ${\frac{3}{2}}^{-}\mathrm{}\left[521\right]\mathrm{}$, and ${\frac{5}{2}}^{+}\mathrm{}\left[642\right]\mathrm{}$ bands.