Polarization ofγRays inTa181

Directional correlation measurements of the $\gamma$ rays following the decay of ${\mathrm{Hf}}^{181}$ to excited states in ${\mathrm{Ta}}^{181}$ have indicated two possible spin assignments for the 482- and 615-kev states. Calculation of the expected polarization of the 482-kev $\gamma$ rays in the 133-482 kev decay showed that this property was quite different for the two spin assignments. For the spin assignment 482, $\frac{9}{2^{+}}$, and 615, $\frac{5}{2^{+}}$ the calculated value of $p$ is 1.10 where $p\equiv \frac{W(90\mathrm{}°, 90°)}{W(90\mathrm{}°, 0°)}$ in the notation of Biedenharn and Rose. For the spin assignment 482, $\frac{5}{2^{+}}$, and 615, $\frac{1}{2^{+}}$ the value of $p$ is 0.67. The experimental value for liquid Hf${\mathrm{F}}_4$ source is (0.71±0.05). To check on possible systematic errors, measurements were also made with a dry Hf${\mathrm{F}}_4$ source for which one expects an attenuated value for $p$ of 0.91. The experimental value is (0.88±0.05). We conclude that the correct spin assignments are 482, $\frac{5}{2^{+}}$, and 615, $\frac{1}{2^{+}}$. Reduced $\gamma$-ray transition probabilities are obtained. The collective model of the nucleus is used to interpret the low-lying states of ${\mathrm{Ta}}^{181}$.