Evidence for a Dipole Component in the Yrast Cascade

The shapes and angular distributions of continuum $\gamma$-ray spectra for ${}_{}{}^{174,175}{}_{}{}^{}\mathrm{W}$ and ${}_{}{}^{161}{}_{}{}^{}\mathrm{Yb}$, produced through (${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, $\mathrm{xn}$) reactions, have been measured down to $E_{\gamma }\simeq 300$ keV. Large negative anisotropies are observed at the lower energies, suggesting the existence of a dipole component in the yrast cascade. A simple model gives good agreement with the results.