Measurement of the Velocity Dependence of the Helicity of Beta Particles fromCo60

A series of experiments has been carried out to study the velocity dependence of the helicity $\mathcal{H}$ of $\beta$ particles from the allowed Gamow-Teller decay in ${\mathrm{Co}}^{60}$. The energies examined were varied from 205 keV ($\frac{v}{c}=0.7\mathrm{}$) to 50 keV ($\frac{v}{c}=0.41\mathrm{}$), a region where the velocity is changing rapidly with energy, and where several other measurements have reported deviations from $\mathcal{H}=-\frac{v}{c}$. We have not observed any discrepancy with a $-\frac{v}{c}$ dependence for the helicity within a measurement accuracy of a few percent at the highest velocities and 9% at the lowest velocity. The weighted mean of the helicity measurements is $-\frac{\mathcal{H}}{\left(\frac{v}{c}\right)}=1.014\mathrm{}\pm 0.018$. Assuming, in turn, either the two-component neutrino theory with predominantly axialvector interaction or the absence of any tensor interaction admixture, this result limits the ratio $\frac{C_T}{C_A}$ to <0.0029, and the ratio $\frac{C_A^{\prime }}{C_A}$ to values between 0.90 and 1.11. The helicity was analyzed by Mott scattering after transformation of the polarization from longitudinal to transverse. A screened Mott scattering function $S\left(\theta \right)$ was used in the analysis. The behavior of depolarization effects and the other systematic errors associated with the technique as a function of $\beta$-particle energy was investigated extensively, and the influence of these effects on the results is reported.