Search for Higher Order Effects in AllowedβTransitions

The $\beta -\gamma$ directional correlations of several allowed $\beta$ transitions with unusually large $\mathrm{ft}$ values ($log\mathrm{ft}>\mathrm{}7\mathrm{}$) were measured. Effects of second-forbidden $\beta$ components are expected to manifest themselves in small observable anisotropies if second-forbidden matrix elements are not reduced appreciably by those nuclear-structure effects that cause the reduction of the allowed matrix elements. The experimental results for the anisotropy factor $A_{22}\mathrm{}\left(W\right)\mathrm{}$ in the $\beta -\gamma$ directional correlation function $W\left(\theta \right)=1+A_{22}\mathrm{}\left(W\right)\mathrm{}{P}_2\left(cos\theta \right)$ are: ${\mathrm{Na}}^{22}(W_0=1.69, logft=7.4)$; $A_{22}(\overline{W}=1.8)=-0.001\mathrm{}\pm 0.005$; ${\mathrm{Co}}^{56}(W_0=3.88, logft=8.6)$, $A_{22}(\overline{W}=3.3)=-0.001\mathrm{}\pm 0.003$; ${\mathrm{Sb}}^{124}(W_0=2.23, logft=7.7)$, $A_{22}(\overline{W}=2.0)=+0.002\mathrm{}\pm 0.003$; ${\mathrm{Cs}}^{134}(W_0=2.3, logft=8.9)$, $A_{22}(\overline{W}=1.9)=+0.001\mathrm{}\pm 0.003$; ${\mathrm{Tb}}^{160}(W_0=2.1, logft=8.1)$, $A_{22}(\overline{W}=1.9)=+0.013\mathrm{}\pm 0.007$. Except for the highly deformed ${\mathrm{Tb}}^{160}$, no evidence of interference effects with second-forbidden $\beta$ components was found, indicating that whatever the mechanism that reduces the allowed matrix elements it somehow also reduces the second-forbidden matrix elements. The small anisotropy found in ${\mathrm{Tb}}^{160}$ may be a result of some $K$ selection rule effect or may be caused by a competing forbidden $\beta$ transition. Evidence for the existence of such a $\beta$ transition (which, however, is very difficult to fit into the established ${\mathrm{Tb}}^{160}$ decay scheme) is given.