Gradient couplings (weak magnetism, induced-second-class tensor) and recoil terms manifest themselves specifically through a parameter ${\alpha }_{-}$ in the angular distribution of $1^{+}$ → $0^{+}$ ${\beta }^{-}$ decays of oriented nuclei, i.e., of ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ to ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(g.s.). To decide between two contradictory results, we have redetermined ${\alpha }_{-}$ by a direct measurement of the alignment correlation term, $A{\alpha }_{-}E{P}_2\left(\theta \right)$, with a novel technique. We find ${\alpha }_{-}=+0.24\mathrm{}\pm 0.44$ Ge${\mathrm{V}}^{-1\mathrm{}}$, thereby confirming the Louvain result, and ruling out a "large" second-class current.

• Received 11 October 1977

DOI:https://doi.org/10.1103/PhysRevLett.40.306