The Mössbauer resonance in ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ has been observed for a source of dilute Pt in Fe at temperatures between 0.06 and 20 K, obtained by employing a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ dilution refrigerator for cooling. At these low temperatures, the source nuclei are partially oriented because of the magnetic hyperfine interaction present in the alloy. The intensities of the observed hyperfine lines allow the determination of the polarization of the 77-keV first excited state in ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$, which is populated by $\beta$ decay from the ${}_{}{}^{197}{}_{}{}^{}\mathrm{Pt}$ source. From this polarization, one can calculate the mixing ratio of the squares of the reduced matrix elements of different tensor rank present in the $\beta$ transition. For the first-forbidden $\beta$ decay of ${}_{}{}^{197}{}_{}{}^{}\mathrm{Pt}$ ($\Delta I=0\mathrm{}$, $\Delta \pi =-1\mathrm{}$), we find the mixing ratio of rank-one elements to the rank-zero elements to be ${\beta }^2=7\mathrm{}\pm 1$.

• Received 5 January 1970

DOI:https://doi.org/10.1103/PhysRevC.1.2109