Nuclear Resonance Absorption in ${\mathrm{Dy}}^{161}$ Situated in ${\mathrm{Dy}}_2$${\mathrm{O}}_3$ and Dysprosium Iron Garnet

The recoil-free resonant absorption of the 26-kev $\gamma$ ray ($T_{\frac{1}{2}}\sim 3\times {10}^{-8\mathrm{}}$ sec) emitted in the decay of ${\mathrm{Tb}}^{161}$ by absorbers containing ${\mathrm{Dy}}^{161}$ has been investigated. High Mössbauer efficiencies at room temperatures have been observed for sources and absorbers in the form of oxide and rare earth iron garnet. The line shapes obtained were very broad, of the order of 100 times the natural widths, and showed no resolved lines. The broad lines are interpreted as due to a wide complicated hyperfine spectrum whose details have been smoothed out by transitions between magnetic sublevels induced by paramagnetic relaxation. In the rare earth iron garnet, the exchange field acting on the rare earth ion should decouple the nuclear and electron spins. The effective magnetic field at the nucleus in the rare earth garnet is about 2×${10}^6$ oe.