Nuclear Matrix Elements in the First-Forbidden 2.2-MeVβTransition ofLa140

The angular dependence of the circular polarization of the 1.6-MeV $\gamma$ radiation following the 2.2-MeV $\beta$ transition of ${\mathrm{La}}^{140}$ has been measured. The analysis of these data, together with $\beta -\gamma$ directional-correlation and spectrum-shape data, was made using exact electron wave functions and taking into consideration finite-nuclear-size effects. The nuclear beta matrix elements determined in this manner show that the deviations of the 2.2-MeV $\beta$ transition from the Coulomb approximation is caused by a cancellation effect of the vector-type matrix elements and not by a selection-rule effect. The experimentally determined ratio of the $\int i\alpha$ and the $\int \mathrm{r}$ matrix elements does not agree well with the value predicted by Fujita on the basis of the conserved-vector-current theory. The discrepancy may be due to the presence of higher order effects in the 2.2-MeV $\beta$ transition of ${\mathrm{La}}^{140}$.