Linear Polarization of the ContinuumγRays Feeding High-Spin Yrast States inDy151,152andEr156−158

We have measured the linear polarization, $P_{\gamma }$, of continuum $\gamma$ rays for ${}_{}{}^{151,152}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{156-158}{}_{}{}^{}\mathrm{Er}$ produced through (${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$,$\mathrm{xn}$) reactions. We find that $P_{\gamma }\approx 0.35$ for $E_{\gamma }<~1.2$ MeV in the rotational Er nuclei and, in contrast, that $P_{\gamma }\approx 0.05$ for $E_{\gamma }<~1.0$ MeV in the oblate Dy isotopes. This, taken together with the spectrum shapes of the contiuum $\gamma$ rays, suggests a rather direct feeding of the yrast states by statistical cascades. The Dy data for $1.0<~E_{\gamma }<~1.5$ MeV indicate collective rotations at spin $I>\mathrm{}40\mathrm{}\hslash$.