The 49-25.6-keV $\gamma -\gamma$ cascade in ${}_{}{}^{161}{}_{}{}^{}\mathrm{Dy}$ has been used for making time-differential perturbed-angular-correlation measurements. For ${}_{}{}^{161}{}_{}{}^{}\mathrm{Dy}$ in ${\mathrm{Gd}}_2$${\mathrm{O}}_3$ the internal fields are so large that only the hard-core correlation is seen. For ${}_{}{}^{161}{}_{}{}^{}\mathrm{Dy}$ in gadolinium metal above the Curie temperature the correlation function decays exponentially due to the presence of rapidly varying time-dependent fields. The half-lives of the 25.6- and 74.6-keV levels have been measured and found to be 30.0 ± 0.5 and 3.16 ± 0.05 nsec, respectively.

DOI:https://doi.org/10.1103/PhysRevB.16.967