Energy-Dependent Beta-Gamma Circular Polarization ofLa140

The $\beta -\gamma$ circular polarization $P_{\gamma }$ has been measured as a function of the $\beta$-particle energy for ${\mathrm{La}}^{140}$ and for ${\mathrm{Co}}^{60}$. The ${\mathrm{Co}}^{60}$ results confirm that $P_{\gamma }$ is proportional to $\frac{v}{c}$. The results for the 2.2-MeV transition of ${\mathrm{La}}^{140}$ show that the original prediction for the vector-matrix-element ratio is not correct for this transition. However, the results are consistent with predictions based on the conserved-vector-current (CVC) theory if the approach suggested by Damgaard and Winther is followed. This interpretation yields information on third-forbidden matrix elements.