Determination of Electron and Positron Helicity with Møller and Bhabha Scattering

The determination of the helicity of electrons and positrons from beta decay by means of electron-electron (Møller) and positron-electron (Bhabha) scattering is discussed. The theoretical background, the apparatus, and the experimental procedure are treated in detail. The apparatus included, in addition to the conventional parts, a beta monochromator with a momentum resolution of 16% for the investigation of the energy dependence of the helicity. Experiments were performed with ${\mathrm{P}}^{32}$, ${\mathrm{Au}}^{198}$, RaE, and ${\mathrm{Ga}}^{68}$. In all cases, the helicities $P$ were found to be proportional to $\frac{v}{c}$. The measured helicities, in units of $\frac{v}{c}$ and averaged over the observed range of energies, are summarized as follows. (The errors given are statistical only; systematic errors are estimated to be less than ±3%).