The ratio $\left|\frac{M^{\beta }}{\xi i\int \overrightarrow{\mathrm{r}}}\right|$ for the first-forbidden ${\beta }^{-}$ decay has been studied to discuss the composition of $M^{\beta }$. Experimental values of $\left|M^{\beta }\right|$ and $\left|i\int \overrightarrow{\mathrm{r}}\right|$ are estimated from $\mathrm{ft}$ values and from the $E1\mathrm{}$ matrix element. The latter was obtained from the ($e,e^{\prime }p$) reaction through the isobaric analog states in ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$, ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}$, and ${}_{}{}^{148}{}_{}{}^{}\mathrm{Sm}$. The experimental value of the ratio was compared with the theory using a $j-j$-coupling shell model. The result shows good agreement for ${}_{}{}^{141}{}_{}{}^{}\mathrm{Ce}$ but not always good agreement for ${}_{}{}^{139}{}_{}{}^{}\mathrm{Ba}$ and ${}_{}{}^{148}{}_{}{}^{}\mathrm{Pm}$.

• Received 23 November 1970

DOI:https://doi.org/10.1103/PhysRevC.3.2006