Nuclear-Orientation Study of the First-ForbiddenβDecays ofAu198

Using a ${\mathrm{He}}^3$-${\mathrm{He}}^4$ dilution refrigerator coupled with adiabatic demagnetization, ${\mathrm{Au}}^{198}$ has been polarized in a magnetized Fe foil at 5 mK. By measuring the directional distribution of the $\gamma$ rays emitted following the $2^{-}$→$2^{+}$ first-forbidden $\beta$ decays to ${\mathrm{Hg}}^{198}$, the $\beta$ dealignment parameters $U_2$ and $U_4$ have been determined. For the 0.96-MeV transition, we find $U_2=0.795\mathrm{}\pm 0.007$ and $U_4=0.347\mathrm{}\pm 0.03$. This implies that about 60% of the total decays carry off no angular momentum, about 39% carry off one unit of angular momentum, while 1% ±1% carry off two units of angular momentum. For the 0.29-MeV transition, we find $U_2=1.047\mathrm{}\pm 0.054$, implying that less than 1% of the decays carry off one or two units of angular momentum.