A new double Penning trap structure has been built which permits one to trap a positron in a well-compensated experiment trap where measurements have yielded the geonium positron $g$-factor anomaly, $\frac{g\left(e^{+}\right)}{2}-1\mathrm{}\equiv a\left(e^{+}\right)=(1\mathrm{}159652222\pm 50)\times {10}^{-12\mathrm{}}$. The uncertainty is based on the resonance linewidths and an estimate of the remaining systematic error associated with extrapolating to zero spin-flip power. By comparison to the electron-spin anomaly, a positron/electron $g$-factor ratio of 1 + (22±64)×${10}^{-12\mathrm{}}$ is obtained.

• Received 14 September 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.1679