Positron Polarization in a Mirror Transition

The positron polarization in the mirror transition ${\mathrm{N}}^{13}$, a predominantly Fermi transition, has been measured by detecting the circular polarization of the annihilation-in-flight quanta. The results indicate complete polarization of the positrons parallel to their momentum, showing that Fermi coupling contributes significantly to the positron polarization in the same sense as the Gamow-Teller coupling.